Saturday, December 15, 2007

Take Off Decisions in Single Engine Airplanes

Triggered by my review course, I was going to write an entry on V1, introducing it with a comparison to take-off decision making in single-engine airplanes, and multi-engine airplanes incapable of accelerating to rotation speed after an engine failure, but I quickly discovered I had enough to say about each to make this into three blog entries. So here's the first.

I'm returning to a common theme of this blog when I say that pilots spend much of our training and mental energy preparing for what might go wrong. One of the obvious things that might go wrong is for an engine to fail, and one of the worst places that could happen is just as we're taking off. We'd be at too low a speed to safely fly an aircraft, but at an unreasonably high speed to taxi, and the runway ends abruptly not far ahead. Any time spend pondering the best course of action could be very costly, so pilots work out in advance what the options are and at what point the options change, and then practice the hell out of them, so that should the worst occur, the hesitation before taking correct action would be no longer than the time to recognize the failure plus the reflex time required for mental intention to turn into muscle action.

An engine could fail at any point in the take-off sequence, from first applying take-off power to reaching a safe altitude after getting airborne. Which situation a failure places you in depends on what kind of airplane you are flying, where you were in the take-off sequence at the time of the failure, and the environmental conditions.

Clearly a single engine airplane that experiences an engine power loss on the take-off roll is in situation must stop the take-off attempt, no matter how little runway remains, or how close the airplane was to flying speed. The pilot's job is to quickly recognize the failure, retard the throttle to eliminate any remaining engine power, and apply the brakes as heavily as necessary in order to stop before departing a safe stopping surface. The checklist probably also asks the pilot to secure the engine at this time, too: shutting it down completely as well as closing the emergency fuel shut off valve and turning off the electrical master. In short: if the engine stops, then immediately stop the movement of the airplane, fuel and electricity. If the airplane is already airborne, then the checklist is almost identical, except that the first item on the list is "land straight ahead," which may involve extending flaps.

For the single engine airplane (or any airplane that loses all engine power after take-off) there comes a point in the climb out where it becomes possible to turn around and return to the airport rather than gliding to a landing straight ahead. This point is going to depend on aircraft weight, pilot skill, wind, runway configuration at the airport, and the terrain available for the straight ahead (or nearly so) landing. Some pilots memorize an altitude above ground level at which they can turn back. A good way to determine this is to actually practise it at altitude. Start in flight well above terrain and aligned with a geographical feature like a road or stream. Apply climb power and pitch to your take-off attitude. At an even thousand altitude, retard the throttle to idle. Immediately lower the nose to best glide speed and start a one-eighty degree turn. When you are re-aligned with the geographical feature and in a configuration that would allow you to land on it, check your altitude. Adding some as an allowance for any obstacles you'd have to maneuver around to actually get back to your departure airport, you now have an idea now of how much altitude you need to turn around. Obviously if you determine this with just you in the airplane at 4000' asl, the result isn't going to be sufficient with your family and baggage on board(*), or at a density altitude of 7000'.

It's not a simple problem and no one rule will work for every airport in all conditions. If straight ahead is only jagged mountains then what do you do? Some choose "don't use that airport." A controlled landing on almost any kind of terrain is safer than a stall-spin resulting from an unsuccessful turnback, and if you aren't going to be landing at the airport anyway, landing into wind gives you a slower, safer touchdown speed. Single engine pilots are also advised to pick a go/no-go point along the runway, such that if they are not flying by that point, they abort. That would allow them to recognize an underperforming engine or insufficient runway length while it was still an anecdote and not an accident.

There may be cases where a powerful single engine aircraft loses some engine power, but has enough power to, and the pilot deems it safest to continue rather than abort. I can think of two instances where people I know experienced a partial power loss immediately after take-off and chose to limp around the circuit on three cylinders. It's impossible to make all the decisions in advance. You have to follow a general rule that if there is any doubt in ability to take-off and climb over obstacles, abort the takeoff. If airborne, maintain flying speed above all else. If unable to maintain altitude and speed, land at the safest place you can reach without risking flying speed. Airspeed, as the old saying goes, is life. Altitude is life insurance.

*Yes, yes, glider pilots: I know that glide range isn't a function of weight, but stall speed is, and that's how single engine pilots die in this maneuver.

Friday, December 14, 2007

So, You Want To Be A Pilot, Eh?

Reader James Ball sent me his recently-published guide to Canadian aviation careers, So, You Want to be a Pilot, Eh? to see if I would review it for you guys. After reading it I'm happy not only to review it but to recommend it. Judging from my own experience and the e-mail I receive, there is certainly a need for such a book, and I can't think of a comparable career guide for prospective Canadian pilots. The closest might be Landing the Big Job, but back when I bought that one, So, You Want to be a Pilot, Eh? would have been a much better choice, had it existed.

James starts where the student pilot should start, with the question "do you really want to be a pilot?" He honestly explains the parts of the career that are tough, holding out no false hope to the waverer, while maintaining a sense of humour. "It's difficult to keep track of all the different licences and ratings that are available to pilots. It can be even more difficult to pay for them all." He gives an excellent jargon-free and Canada-wide overview of the industry, from dollar-a-jumper paradropping jobs up to major airline captains drawing six figure salaries. As he discusses different hiring policies and corporate histories he refers obliquely to companies as, for example, "one operator based in Norman Wells, NWT." Those made me laugh as James and everyone else who has been around the circuit a few times can recognize the operator just by the base.

I liked the good Canada-specific advice he gives regarding joining the military to learn to fly, working the ramp and working the dock. Those are areas in which Canada is quite different from the US, and their airline job-hunting or career guides don't apply. James also gives many useful website URLs. These will unfortunately change in less time than it take to read the book, but James is providing updates and errata on his blog. Perhaps he will group all the recommended URLs on one page there, to spare readers from having to type them in.

James gives good guidance, much of it straight from the Transport Canada website, without trying to take the place of more comprehensive guides on topics like passing written examinations and flight tests.

The resume guide is very worthwhile, as aviation resumes are different than those in other industries. It is vital that your hours be clearly visible and reference contact information be actually given. James says this, but I'm going to underline it here so that anyone hunting around the net for pilot resume advice finds it and buys the book.

There were a few things I didn't like, such as James' advice to student pilots out on a solo, "After you've completed the checklist, take some time to explore your local area." Flight instructors recommending this book should censor that part. Also one of the books he recommends is of such poor quality that my local aviation store has dropped it (but they do carry So, You Want to be a Pilot, Eh?).

There are items I wanted to add, such as the possibility of other more stable and lucrative careers in aviation, but that would make it a different book. There's nothing missing within the scope of the book and I was frequently surprised to see excellent but not widely-known tips. And there was even one that I had never heard of, "abstain from sexual activity for a few days" before an aviation medical. Is this folklore or based on some kind of fact? Unless it gets you pregnant, I don't know of any physical changes as the result of sex that persist long enough to drive to the doctor's office. Perhaps this is a male thing, and one of my readers will enlighten me.

I caught a few editing glitches, the military requirement for Basic Officer Training and Foreign Language Training mentioned twice in consecutive paragraphs in the same section, but there is no fluff here. James has a concise style and still fills over two hundred pages with useful information.

This would be a great purchase for the family of a prospective pilot. Amend the cover with a sticker to make the title read "So, Your Kid Wants to be a Pilot, Eh?" and give it to your mom. I would recommend this book not only for Canadians thinking about a flying career, but to student pilots, pilots looking for their first job, and instructors looking to move on. Also if you're a foreign commercial pilot interested in working in Canada, this book contains what you should know about the Canadian industry and process. It's also written simply enough for ESL students, and laid out so you can refer to one part or another, but it's readable and interesting enough to go right through, as I did.

The list price is $24.95 and there's a Buy It Now link on James' website but it seems that the publisher and distributor have changed their links, so go straight to Chapters to buy it online. It's even on sale at $16.46 which equates to about eight minutes of dual instruction time, converted to student pilot dollars.

Monday, November 12, 2007

Remembering

I'm interrupting my story for what I really did today, Sunday November 11th. It happens that I was back in Canada.

I went to a Remembrance Day ceremony at a public cenotaph. I realized as I dressed to go, that the symbolic poppy I had donated for had fallen off somewhere, so I went without one. The cenotaph was about fifteen blocks away, so I walked. As I arrived at the park, a child who had found a poppy on the ground was giving it to a uniformed veteran. The soldier saw I didn't have one, so gave it to me. I thanked him, explaining that mine always seem to fall off. He joked that they grease the pins so that they will.

Last year some readers told me that they didn't wear poppies, they wore flags, or yellow ribbons, and seemed to think they meant the same thing. There's nothing wrong with those symbols, but they aren't the same symbol, so I will reiterate.

Wear a flag to show that you support your country's efforts in an international war. (Or in a sports event, or just to show pride in your country.)

Wear a yellow ribbon because someone you care about is away at war and you hope and pray they come home safely.

Wear a poppy to remember all those who have gone to war for their countries. All countries.

I think about those who went to war and never came back, those who went to war and came back with parts missing, those who came back seemingly whole but broken inside, and those who somehow not only survived but became stronger for the experience; they came back and went straight to work making the country they had fought to defend a better place.

I think of those who signed up because they wanted to defend their countries, those who signed up because their friends were, those who signed up because their government promised them education or other opportunities in return, and of those who went because the government sent them a piece of paper saying it was their turn to go. I read in the paper of a soldier who went to war because he accidentally shot a neighbour's cow, and was given a choice between jail and enlistment.

Wearing a poppy and remembering those who served in the armed forces is not a political statement. It neither supports nor condemns any war nor any soldier's choice. It just acknowledges that they went, and the donation funds help veterans.

It's always cold and windy on Remembrance Day. The children whine and complain from cold and boredom, but I am glad their parents thought it was important enough to bring them to the memorial.

Wednesday, October 31, 2007

Very Scary Pumpkin

A few days before Hallowe'en, I was on my own in a hotel across the street from a pumpkin patch. Hallowe'en is a big deal in the United States. Every business I go into has fake spiderwebs all over the place, or skeletons, or witches. It spoils the professional look of some of the FBOs, but it seems to be expected. I'm not sure if Americans just celebrate every holiday to the max, or whether Hallowe'en is far enough removed from religion that everyone can celebrate it without having to worry about political correctness, or maybe just because it involves masses of candy. I'll have to be here for Easter and see if those celebrations are as exuberant.

orange and white pumpkins arranged in rows

Thanksgiving is still to come in the US (Canadian Thanksgiving was a couple of weeks ago, as Thanksgiving is essentially a harvest festival, and anything not harvested before mid-November is going to be lost to frost). The pumpkin farm sells all different sizes of gourds and squashes for eating and table decorations. There is also a pumpkin maze and hayrides and so on.

Bix box of decorative gourds

Five dollars cash got me the largest pumpkin I cared to carry across the street. I lugged it back to the hotel room, washed it off, and put it on the patio while I thought about what to carve. I Googled for pumpkin decorating ideas, but most involve stencils or demand elaborate tools. I've got a small utility knife, a plastic knife, a coffee cup and an ice bucket. And none of the ideas online was very scary. I wanted a really scary jack o' lantern.

pumpkin waiting to be carved

Everyone in the rest of the world knows North Americans do this, right? To celebrate Hallowe'en, we take gourds, scoop out the seeds and some of the flesh and carve designs into the skin, through the flesh. Then we put candles inside so that the designs glow. If you scoop out enough flesh so that the walls of the gourd are translucent, the whole pumpkin glows too, but a lot of people are lazy and only remove the seeds and goop, leaving too much thickness of flesh for the light to shine through. You can also use the scooped out flesh to make pumpkin pies with, but pumpkins are cheap, so again, many people don't bother. I'm pretty sure that Hallowe'en jack o'lanterns were responsible for both the first time I was allowed to use a sharp knife, and the first time I was allowed to use fire. There's a strong thread of memory going back to childhood through jack o' lanterns, for me.

And then I got an idea for the scariest jack o' lantern I've ever made. Don't look at it too long, or you might get nightmares.

Carved pumpkin depicting an airplane lifting off from the end of a runway by moonlight. The right propeller is turning. The left propeller is stopped. Scary!

Sunday, September 02, 2007

My Job

I am a commercial pilot, but probably not the sort of commercial pilot that comes to mind when I say those words. I'm not a clean cut male strolling briskly through the main passenger terminal, rolling bag in tow, and I don't wear a crisp white shirt with little stripes on the shoulders, or a captain's hat. "Commercial pilot" means simply that I am am paid to fly aircraft. You've probably never heard of the company that employs me. I do in fact have a flight case and one of those rolling suitcases, but you'll only see me in the passenger terminal if I'm commuting to a job where the airplane is already on site. I work in t-shirts, work pants and sturdy boots. Add a sweater and parka as necessary for the climate.

I fly an airplane, sometimes IFR and sometimes VFR. Sometimes it's in the day and sometimes at night. It's often on short notice, so I have to be ready and have the airplane ready to go.

My responsibilities include finding suitable places for us to base the airplane, and ensuring that the airplane is ready to go when needed. That involves keeping it clean, and arranging for it to be fuelled and maintained when required. I plan flights, make any arrangements required with air traffic control or other organizations responsible for special use airspace, and make sure I have all the proper charts on board. Before flight, I check the weather, calculate weight and balance of the load, detail an operational flight plan, secure any baggage, inspect the airplane, brief any passengers on the safety procedures, contact flight following, and then fly the airplane as required. And of course there's the usual paperwork. Aviation requires lots of paperwork. I need to track time flown by the airplane, my own logbook times, duty times, expenses, maintenance due, and anything that breaks. I also send regular reports to my bosses so they don't think I'm just goofing off out here, wherever it is that I am on any day.

I have amazing co-workers, appreciative management and a real sense of achievement from the work I do. The airplanes are safe and the company safety culture is excellent. I'm not getting rich, but I'm not starving, and I never know where I'm going next. I don't give really specific details about what my company does or for whom because sometimes it's confidential and because sometimes people get funny ideas about what I write and I don't want to say anything that would cast my company in a bad light, even to people outside the industry. These are my adventures.

So next time you hear that someone is a commercial pilot, impress them by asking "what kind of operation?" or "what aircraft?" instead of "what airline?" And if you interact with someone who is flying for a living, don't ask "would you ever want to be a commercial pilot?" As little as her employer may pay her, she already is. It's an irritating question, a bit like asking members of a garage band if they'd ever like to make the cover of the Rolling Stone, but if you must ask it, the words you are probably looking for are, "would you ever want to fly for a major airline?" Most pilots do have that desire at some stage in their careers, but not everyone reaches the pointy top of the pyramid, whether they want to or not.

Monday, July 30, 2007

Lets Do Lunch

Now that I'm travelling all over the place, it seems a shame not to visit some of my readers when I have the chance. I don't get a lot of notice of where I'm going, and sometimes I don't have time to socialize, but sometimes I'm in the same place for a week or so, and would have time to visit people.

I have met a few readers recently, and they've all been fun, interesting people, so I'm in favour of meeting more. I was trying to concoct a good way of announcing where I am without announcing where I am. Although you can see I've become a little more relaxed about details of my location, I'm not sure I'm into a top-of-blog "Where I Am Today" flag. And I'd forget, or be unable to update it.

I concocted the idea of a feature that could exist on LinkedIn or Facebook or whatever contact-tracking website cared to implement it. The user has already registered a name, home town, and e-mail address. You add in a "where I am now" location, and have some more options for who should be able to view that information. But that's not the important part. The important part is that when you're planning to be in Chicago for a couple of days, it gives you the option to e-mail everyone on your contact list who is in Chicago, to tell them you'll be in town. You don't actually have to know their e-mail address, or even that they live in that town, just have them on your list of friends. There would be all kinds of customizing options to determine whether you wanted e-mail to go out to everyone on your list, or just those whom you tag as people you want to meet in real life. This would take care of both privacy and the problem of keeping track of where the heck your friends from two companies ago live now. You could also use it to help you e-mail all the right people when you change your permanent address.

While I was fantasizing about this possibility, a reader suggested a much more practical one. I already use Google Mail, and it allows the creation of an infinite number of groups. Now whenever someone sends me e-mail of the form, "Let me know if you're ever going to be in XYZ," I will put them in the group "XYZ." Then when I find out I'm going to XYZ in the morning, I can fire off a group e-mail and if our stars and schedules align correctly, we can do lunch, or see your fish hook museum, or irritate the local wildlife together.

I think this could work pretty well. So if you'd like to meet up if I have time to spare in your town, send me an e-mail including the identifier of "your" airport. If you split your time between more than one, or are equidistant from several, tell me multiple airports that you are near enough to want to know when I'm there.

I'll try this out for a bit, and see how it works. If the rest of you are nearly as interesting as the people I've encountered already through the blog, then it should be a lot of fun.

Also, for the folks who like round engines, I don't think I'll be going here, but I would if I were in town.

Saturday, June 16, 2007

Adiabatic Lapse Rates

A few weeks ago I discussed expansion cooling, and the relationship of the stability of an air mass to its environmental lapse rate. You can pretend you remember that if the temperature decrease with altitude is less than the rate of cooling of a parcel of raised air, then the air is stable, but if the atmosphere decreases in temperature faster than raised air cools by expansion, then the air is unstable. I said that raised air always cools through expansion at three degrees celsius per thousand feet. But I hinted that I was leaving something out. I was.

I left out the consequences of condensation. Once air cools to its dewpoint, water vapour starts to condense into water droplets. These droplets make up clouds or mist, and as I explained in a different earlier post, the act of transforming from vapour to liquid actually releases heat. This release of heat partially cancels out the cooling by expansion, such that once visible moisture starts to form, raised air cools at an effective rate of one and a half degrees per thousand feet. You can think of it continuing to cool at 3 degrees/1000 ft, but then warming back up again by a degree and a half, for a net decrease of 1.5 degrees/1000 ft. Apply that knowledge to the idea of stability and instability and you see that moist air will be unstable with a shallower lapse rate than dry air.

Air that has an environmental lapse rate of more than three degrees per thousand feet is going to be unstable. As rising air is cooled to its dewpoint the temperature differential between raised air and its environment will increase even faster, and that's how giant cumulus clouds form. Air that has an environmental lapse rate of between one and a half and three degrees will be stable if the air is dry, but unstable if the air is saturated. so the air is termed conditionally stable. Or damn, maybe it's conditionally unstable. One of the two. But if the environmental lapse rate is less than one and a half degrees per thousand feet, the air is stable regardless.

Stable air is smoother to fly in and the clouds that form are flat and featureless. Unless flat is a feature. Rain falling from flat clouds is steady. Flat clouds are called stratus clouds, because everything is always cooler in Latin, so you get STable air with STratus clouds and STeady precipitation.

Unstable air is bumpy. And I suddenly remember an elementary school class in which Caroline, asked to provide an adjective to describe a cloud, supplied bouncy and refused to back down when the teacher rejected it. Caroline, you were right all along, and if I ever see that teacher I will take him for an airplane ride through a bouncy cumulus cloud. Cumulus is Latin for heap, I think. You can remember that the cloud acCUMULates upward. Precipitation falling from cumulus clouds is showery, meaning intermittent, not steady, but probably heavier than the precipitation from stratus clouds.

Monday, April 30, 2007

Phases of Water

My last weather theory blog posting faded uncommented into the blogosphere so I probably went too far. These postings were precipitated by someone who e-mailed me more than once lamenting the lack of weather discussion on the blog, so I'm trying to oblige. Today will be easier to understand than last time. Today will be so easy to understand you'll wonder why I bothered, but it will all tie together in the end.

Water comes in three phases: solid, liquid and gas. Solid water can be in the form of snow or ice or frost or high altitude clouds. Liquid water is present in lakes and puddles and rain and clouds and mist and squirrels. A small percentage of the atmosphere all over Earth consists of gaseous water. And water in any phase can convert to any other.

When I arrived in the north, most of the water I saw was in the solid form. I remember being in a northern town in April and watching little kids gleefully jumping on frozen puddles to shatter the ice. I remember being on final for a runway and reflexively double checking the water under my approach path, to confirm wind direction, then laughing at myself because the water was frozen, its apparent ripples indicating perhaps the wind direction at the time of the latest snowfall on top, but not the current winds. As the weeks went on, open water appeared and I watched the transformation from solid to liquid, which any kid who has ever had an ice cream cone knows is called melting. It takes energy to melt ice, energy that can be provided by the sun, or by the alternators and brushes driving the propeller deicing system on my airplane.

Had I stayed through the fall I would surely have seen the reverse transformation, open water disappearing and the ice finally becoming solid and thick enough for the ice roads to go in. Demand for flying drops off then, but so does flyable weather. As water freezes, it gives off some heat, exactly the same amount of heat energy that will be required to melt it again. Energy is conserved in such a transformation.

Similarly. in order for liquid water to sneak out and hide inside the air (an explanation I once gave to a small child who wanted to know where the puddles had gone), it needs to absorb some energy. You're familiar with the cooling effect of evaporation from sweating or if you've ever worn wet clothes: the water takes heat from your body in order to effect its transition. That also explains why sweating or wearing wet clothes is not a very effective cooling mechanism if the air is muggy. Muggy air is saturated, with a very high relative humidity. The air contains the maximum amount of moisture it can at that temperature, so there is little tendency for sweat or moisture on your clothing to evaporate, hence no evaporative cooling.

When that water that has been sneaking around inside the air as water vapour reappears as liquid, the energy will be released again. I can't think of clearly observable examples of the heating phenomenon caused by condensation, but you can observe condensation itself as beads of water appearing on the outside of a glass of cold liquid, or on the inside of windows on a cold day.

There are two more possible transformations between different phases of water, but a lot of people have never acknowledged their existence. Lets start with the freezer compartment of a refrigerator. We'll assume that you are careful and never spill your icecube trays when you're putting them, full of water, into the freezer. Even if you did, you know you'd get a puddle at the bottom of the freezer compartment and some would run out onto the floor and some would freeze there at the bottom, permanently attaching the frozen broccoli to the freezer. So how does there get to be ice stuck to the inside top of the freezer? There's never any liquid water dripping there. The answer is that water vapour present in the freezer compartment deposits directly onto surfaces it finds there, transforming directly into the solid phase. And unless you and your party animal friends use a lot of ice cubes, you've probably noticed that ice cubes left in the freezer gradually shrink. They aren't melting: the water is going directly to vapour, called sublimation. As you might guess, it takes energy to sublimate, and the amount required is equal to the amount required to melt plus the amount required to evaporate. There's no shortcut. It's not like on the airlines where a ticket from Toronto to Halifax costs more than a ticket from Toronto to London, England.

So that's my whole point today. Water can be solid, liquid or vapour. It can transform up or down that sequence, one step at a time or two steps at once. It costs energy, taken from the environment, to go up or down that sequence, and the energy required is the same whether or not you stop off at the intermediate phase. When you go back down the sequence, the same amount of energy is returned. So even though you probably associate the formation of little droplets of water--as mist, on your plumbing, or on a cold drink--with cold things, try to remember that that little droplet of water brought a little teeny bit of heat with it as it appeared.

Thursday, April 19, 2007

Stability

As discussed earlier, huge lumps of air roam freely over the surface of the earth. Some lumps are warmer, some are colder. Some are wetter, some are drier, and some are piled higher than others. And they are the way they are because of where they formed. Let an air mass sit over a warm ocean and you'll get a warm, moist air mass. Of course even an air mass that is a tropical thirty-five degrees at the surface is colder aloft, with the temperature decreasing by anywhere from about one to five degrees celsius for every thousand feet you go up. The rate of temperature decrease is called the lapse rate. There can be odd local variations in lapse rate, but by the time you reach the tropopause (the end of the first layer of air) at 30,000-60,000 feet, the temperature is -56C. At any one altitude within the same air mass, the temperature is about the same.

In addition to temperature and moisture, we are interested in the stability of an air mass. Stability is not with regards to lateral motion of the air mass, but rather to vertical motion within the air mass. If an air mass is stable then air displaced vertically tends to return to where it was, while in an unstable air mass, vertical displacement results in continued vertical motion. Kind of like a stable person who goes to Mexico for a vacation goes home and back to work, while an unstable one might get a new job as a llama herder and end up six months later calling you from Tierra del Fuego, asking you to wire money. Well maybe not much like that. But that's the terminology. I'll be using it in a few paragraphs.

Air within air masses is getting displaced all the time. As the air mass moves over uneven ground, some of the air is displaced upwards. An airplane flies by, swirling the air around. Some of the air is heated, becomes less dense and thus starts to rise above the denser air around it. There are lots of reasons for air to move.

As soon as some amount of air, some textbooks call it a "parcel," moves upward, it is in a new location. The air newly surrounding it is different than the air in its old neighbourhood. For starters, the pressure is lower. The only thing that was keeping the parcel of air at a higher pressure was the presence of air at that pressure all around it, so as it rises and the pressure around it drops, it is no longer as contained and it expands until its pressure matches the pressure around it. That expansion results in cooling, as I mentioned last time. Thus the raised air parcel has a lower pressure, a greater volume, and a lower temperature. The surrounding air hasn't changed as a result of the move, but the temperature of the surrounding air is going to be less than the temperature of the air that surrounded the parcel at its old altitude, simply because the atmosphere is colder at a higher altitude.

So which is colder, the parcel of air that has been raised, or the air that now surrounds it? They are both colder than the old temperature of the air parcel: the parcel of air cooled off as a result of expansion when it moved upward, and the surrounding air just happens to be colder than the air that surrounded the original parcel. The answer is, it depends on whether cooling by expansion was greater or less than the lapse rate, the change in temperature with altitude.

The trick is, cooling through expansion is predictable. A parcel of air that is raised one thousand feet will cool by three degrees. Done deal. So you need only look at the lapse rate of the surrounding air to predict whether the raised parcel will be warmer or cooler than the air in its new environment. If the lapse rate is steeper (i.e. greater) than three degrees per thousand feet, then the surrounding air will be cooler than the raised parcel. If the lapse rate is shallower than three degrees per thousand feet then the the raised parcel will be cooler than the surrounding air. (There's an exception to that last sentence, but I will explain it later).

Next question, why have I spent so many words wrangling with whether one bit of air is warmer or colder than another bit? Well what happens when a parcel of warm air is surrounded by colder air? (Hint: see the title of the last weather theory post). The warmer air rises. So if a parcel of air is disturbed in surrounding air that has a steep lapse rate, the parcel will continue to be warmer than the surrounding air and will continue to rise. If the lapse rate of the surrounding air is shallow, the parcel soon cools below the temperature of the surrounding air, and sinks back to its original level.

And now you can see that if the lapse rate of the surrounding air (known as the environmental lapse rate) is less than the rate of cooling with expansion of lifted air (known as the adiabatic lapse rate) then the air is stable. If the environmental lapse rate is greater than the adiabatic lapse rate, then the air is unstable.

And on that terribly technical-sounding but somewhat simplified sentence I will end this blog entry. If you know about the dry and saturated adiabatic lapse rate don't complain that I didn't mention them, I'm getting there, I promise.

Friday, March 30, 2007

Hot Air Rises

In order to continue explaining air masses I must first explain density and pressure. Density is a measurement of how close together the air molecules are, or, put another way, a measure of what weight of air exists in any given volume. More weight per volume is the same thing as higher density.

If you pack socks into a box there are two ways to maximize the sock density. One is to use thin, non-fluffy socks and the other is to cram your socks in as hard as you can, stomping on them to get them to fit before you seal the box. (You know I just moved house, right?) These two factors are the same as the ones that affect air density.

'Fluffiness' of the air corresponds of temperature. The higher the temperature, the greater the volume the air wants to occupy. It's actually because the speed of the air molecules is greater at high temperature, but you can think of the volume they thus occupy as them being all fluffed up hot out of the tumble drier. All else being equal, warm air occupies more volume than the same weight of cold air. Therefore, all else being equal, an equivalent volume of cold air weighs more than warm air.

Stomping on the pile of socks corresponds to pressure. Is pressure, really. Pressure is defined as force per unit area, and in the atmosphere it is a result of all the air stacked up on top of the bit of air you're considering. Imagine you're looking at a cubic litre of air(*). Its pressure is equivalent to the weight of all the ten by ten by ten cubes of air that are stacked on top of it, all the way up to the top of the atmosphere. Sure, one little cube of air doesn't weigh much, but stack enough up and it adds up. So air down near the bottom of the atmosphere is at a higher pressure than air further up in the atmosphere, where it has fewer boxes stacked on top of it. Kind of like the box of drinking glasses underneath four boxes of aviation textbooks is under more pressure than the one that is ony one box into a pile.

You can see pressure and temperature kind of work against one another with respect to density. If the pressure increases and the temperture stays the same, the density will increase. If the temperature increases and the pressure stays the same, the density will decrease. Cold air at a high altitude is less dense than warm air at a low altitude, because the effect of the low pressure at high altitude more than balances the effect of the temperature difference. There's even a formula:
  P x V = T x constant
P = pressure, T = temperature, V = volume.

So, we have a bunch of air hanging about. Air in the same vicinity within the same air mass is pretty much interchangeable. It's all mostly nitrogen, and contains some amount of moisture, and at the same pressure because its under the same pile of air. And its at the same temperature. If some of it were to be heated up to be warmer than the surrounding air, look at what would happen. Firstly, it doesn't warm the air around it. If air were good at sharing its heat with the molecules around it, down-filled parkas wouldn't be such treasured possessions in the north. (The puffy feathers create little air pockets and heat doesn't travel well through air, so that keeps me warm.) If a little bit of air is warmer than the air around it then it is also less dense than the air around it. And that means that the gravitational force holding it down isn't as great as the pressure differential between the air above it and the air below it, so it is pushed up, and rises.

And yes, I did just spend six paragraphs explaining that hot air rises. Just think of it as a demonstration of hot air. The reason I did it that way is that warm air doesn't always rise. If that were true it would be warmer at the tops of mountains than at the bottom. Air rises if it is less dense than the air around it. If the pressure is the same, then temperature determines density. So it rises if it is warmer than the air around it, sinks if it is colder than the air around it, and stays in the same place if it is the same temperature as the air around it.

There's one further trick to the rising air, as it rises, the pressure around it decreases, so according to the formula, if the temperature stays the same, the volume has to increase. And it does. The rising air expands. It actually also cools as it expands, so the result is that the same air occupies a greater volume at a lower pressure and temperature.

What it does next is for next time this multi-threaded blog returns to weather theory.

(*)If you didn't go to elementary school in Canada after metrification you missed out on carefully measuring ten centimetres by ten centimetres by ten centimetres and building a little cardboard box. That's about four inches cubed, for the aggressively non-metric. Once you'd built and folded your cardboard cube, and mended any measuring or folding errors with vast quantities of cellophane tape, you had a concrete way to visualize a ten centimetre length, one thousand cubic centimetres, one litre capacity, and, if you imagined what your cube would feel like if it were filled with water, one kilogram. I'm not making this up. They used to hand these things out at fairgrounds, in modern 1970s colours like pink, yellow and lime green. Someone back me up here. I'll trade you a working flashlight for a genuine 1970s MetriCube.

Sunday, March 25, 2007

Great Lumps of Air

I keep promising weather theory, but I get distracted. It's also hard to start in the middle as I have to assume something. So I'll start at the beginning and weave more weather into the continuing story, continuing to be distracted on and off. My life is alrady a soap opera, so now I'll run multiple story lines. In any one week everyone should be able to find something of interest. And that will distract you from the fact that I haven't confessed what I'm doing yet. Today you have my take on some of the basic components of weather.

The Earth, as those of you who breathe regularly will have noticed, is surrounded by air. All the air contains the same gases: nitrogen, oxygen, argon, water vapour and a number of lesser components like carbon dioxide, helium, and even krypton (no, it's not green). The proportions of the non-water gases are almost completely uniform from place to place, so in dry air, that's 78% nitrogen, 21% oxygen and 1% argon (the other gases are present in a few parts per million, sharing that one percent with the argon). Air temperature varies from place to place, both horizontally and vertically. Plus the air is not distributed perfectly uniformly about the earth. There are bigger piles of it some places than others.

Some of you won't believe me about the bigger piles thing, thinking that making a bigger pile of air would be like making a bigger pile of water, and that differences in pressure thus created would fill in the gaps and and even out the piles. Of course that does happen, and that plus the behaviour of the water vapour makes weather.

I didn't mention water vapour yet, because its variation would have made it awkward to include in the general composition of air, and it's important enough to merit its own paragraph. Water vapour is the gas form of the wet stuff we normally call water. It is a colourless, invisible gas. (The steam you see coming out of the kettle is not actually water vapour, it is liquid water droplets. If you want to 'see' water vapour, crouch down to eye level with the spout of the boiling kettle. Be careful not to burn your nose, and you will be able to observe a space in the first centimetre above the spout in which there is no appearance of steam. That's air with a high concentration of water vapour, rising from the kettle. As the liquid water boils, it turns to hot vapour and rises. As it leaves the spout of your kettle it mixes with the much cooler air of your kitchen and condenses, turning back into liquid water. Because the liquid water is in the form of very small droplets, the warm rising air can support its weight and it continues to rise as steam. Until it condenses on the underside of your cupboards and makes them all soggy so they won't hold plates anymore. But I digress.) So there is water vapour in the air all around you, but you can't see it any more than you can see the nitrogen. The proportion of water vapour may be up to about 4% of the total air, but can be 1% or less. So where you sit right now the actual proportion of gases in the air might be something like 76% nitrogen, 20% oxygen, 3% water vapour and 1% argon and other.

So we have these great piles of air. Each pile, called an air mass, starts at the surface and wherever it ends, somewhere between around 30,000' and 60,000' up, is called the tropopause. There's more air above the tropopause, but that's called the stratosphere and stratospheric weather is a different subject. Air masses are formed by air lying around in one place for a while. Air masses are big, so by "one place" I mean "Antarctica," "subtropical Africa," "the Pacific Ocean" or "the far north of Canada." The air takes on the relative characteristics of the place it hangs out. Well not all of them. We don't get pointy air or high-crime air or fundamentalist Christian air. We just get moist air versus dry air and cold air versus warm air. It's all relative, so an air mass that forms over the Canadian prairies/American midwest in winter is cold compared to the air mass that formed over the southern states, but warm compared to the one that formed over the bleak arctic tundra and frozen seas. Yes, frozen seas. But it's a dry cold.

Of course everything has special names so that you don't think this weather stuff is easy. Moist air masses, like the kind that form over non-frozen seas, lakes, and jungles is called maritime, and dry air, like the kind formed over deserts or frozen landscapes is called continental. If you've ever had a "continental breakfast" at a Holiday Inn you can remember this by the dry, cellophane wrapped pastry. Or you can just remember it, because continents that don't have the Great Lakes and Michigan/Manitoba in the middle of them tend to be drier in the middle and wetter at the (maritime) coasts. That second way would really be a better way to remember it, because it is actually true, but isn't as funny as Holiday Inn breakfasts.

The cold and warm air masses mostly just go by "cold" and "warm" but they do have fancy-schmancy names, too. From north to south in Canada we are influenced by two different Arctic air masses, Polar, and Tropical air masses. South of the tropical air lurks an Equatorial air mass, but I must confess to being largely unfamiliar with its whims.

Tuesday, February 20, 2007

How to Post a Website in a Comment

Frequently my readers know of resources or documents relevant to my postings and you post them in the comments. It's great, like an intelligent search engine. Some of the URLs are long, and sometimes they appear truncated or overflow the column. There's an easy way to prevent that. I usually start by explaining how something works, and then get to the result, but today I'll do it backwards. The following line is all you need to know.

<a href="http://www.website.com">text to click on</a>

If you want to post a comment with a URL in it, all you need to do is highlight and copy the above line, and paste it into the comment box. Then you can edit it to insert the real website URL, and the text to click on. Be careful to leave the quotation marks in place around the URL. That way you can type a really long URL but all that displays in the comments are the words you link to, the text to click part of the formula. The text to click on will automatically be displayed like that: underlined and/or in a different colour so that people know they can click it.

In case you want to know what that means, I'll tell you. But you can ignore this paragraph if you don't care or already know. When you tell a webpage to display something you use tags. The tags tell the computer what to do with the part in between, whether it should be displayed as a picture or a paragraph, how big it should be, what colour, what style and if it is connected to other text. The tags go before and after the stuff you want to display, like the opening and closing credits of a movie, except instead of theme music you get angle brackets. The less-than sign is the opening angle bracket. A link comes under the category of "anchor"--you're hooking your text to the text on some other webpage. So a is the opening tag. After that comes a code specifying the kind of anchor, a hypertext reference, so href. And then you specify the reference itself, the URL in quotation marks. And that's the end of opening tag, so you close the angle brackets, giving <a href="http://www.neatwebsite.com">. Don't put a space after the >, just type the text you want clickable right up against it. (It will still work if you leave a space, it will just look dumb). And then after the clickable text, you type the closing tag. The closing tag is simply the opening tag preceded by a slash: that's the bottom-to-top, left-to-right slash that is on the same key as the question mark. </a>. Done.

There's another strategy for making long URLs managable, and that's a service like tinyurl. You can paste any URL you like into the window at that site, and it will give you back a gobbledegook URL that is very short. That is very useful if you have to write down a long URL or dictate it over the telephone. I prefer to see the real URLs on websites, so I have some idea where I am going before I go there. I can't tell whether http://www.tinyurl.com/ad45g is cartoons, porn or airplanes. Also I can't click on it. I have to open a new browser to copy and paste into. Whereas if someone links to something directly, like this I can check it out in advance if it seems to be a good neighbourhood on the internet by placing my mouse over it and looking at the bottom of my window before I click. You'll see the one above takes you to "nastystuff.ca" which probably doesn't exist, but doesn't sound like a good place to go for information on fireproof lifejackets.

Nothing to do with airplanes, but at least it's not the same story as last week.

Oh and you're all welcome to post your websites or other favourite URLs in the comments just for practice.

Thursday, February 15, 2007

Wednesday, January 17, 2007

Coffin Corner

My apologies to nervous fliers (believe it or not some read this blog), but there are lots of things in aviation with morbid names. We do take our jobs seriously so when we talk of a "graveyard spiral" or the "coffin corner" it's because we know the associated risks. They aren't theoretical risks. We know the names and families of people who have taken those risks and lost. But, that said, the term coffin corner summons to my mind the shape of a graph, not of the box that probably wouldn't even be needed to dispose of the remains of those who disregarded the meaning of that graph.

The graph in question shows airspeed on the x-axis, the horizontal, and altitude on the y-axis, the vertical. It depicts two lines, one showing the minimum flying speed and the other showing the maximum flying speed. I'll work up to explaining their slope and intersection, but I'll explain stall speed, and Mach in order to get there.

Stall Speed

An airplane is supported in flight by the pressure difference that develops between the upper and lower surfaces of the wing because of its forward motion through the air. The slowest forward speed that develops sufficient pressure difference to counteract the weight of the airplane is called the stall speed. Attempt to fly slower than that and the airflow over the top of the wing starts to break away, resulting in ineffective controls, altitude loss, and usually (but not necessarily) a nose down pitch. This is undesirable in air transport operations, so pilots try to maintain a good margin above the stall speed.

As the air becomes thinner with altitude or high temperature, the speed the airplane must travel through the air mass to get the same airflow over the wing increases. Two days ago I said that that same pressure over the wing represents constant indicated airspeed. That is true for slow airplanes (200 kts is slow in this context!), because for them we can ignore the effect of compressibility of air. When your airplane gets above 10,000 feet and 200 kts, that is no longer true. The ram effect of the speed compresses air inside the pitot tube, causing a higher indication than would otherwise be seen. You can subtract a correction factor to get equivalent airspeed (EAS), but even quoted in EAS stall speed does not remain constant with altitude because of the changes that occur near the speed of sound. So when we are talking about high speed, high altitude aircraft, the speed at which the aircraft stalls actually increases with altitude, whether the speed is given as indicated, equivalent or true. That's the first line on the graph: stall speed. Starting from sea level at the bottom, it slopes slightly towards the right, more so at higher altitudes.

Speed of Sound

The speed of sound in air depends pretty much solely on the temperature of the air: the colder the air, the slower the speed of sound. Air gets steadily colder with altitude, thus the higher you go, the slower the speed of sound. That means without going any faster, an airplane gets closer to the speed of sound as it gets higher. It's sort of like you get closer to the speed limit as you drive towards town, not because you're accelerating, but because the speed limit decreases towards your speed.

While there are airplanes designed to fly at and beyond the speed of sound, most airplanes become unstable as the airflow over the control surfaces approaches supersonic speeds. Because of the way the air travels over surfaces, the airflow in some places is faster than the airplane, so adverse effects may start at an airspeed of about eight tenths the speed of sound, expressed as a "Mach number" of about 0.8. For safety, transport aircraft comply with a maximum operating Mach number (Mmo) specific to that airplane. For example, a Gulfstream V jet has an Mmo of 0.885, and if I keep getting distracted by looking up specs of other aircraft I'll never get this entry written, so that's the only example you're getting. At sea level, the speed of sound is so high that most airplanes would exceed structural limitations based on the airframe before they approached Mmo. So at low altitudes, the maximum operating speed (Vmo) is not related to the speed of sound.

Vmo does not change with altitude, so as you go up in altitude, the maximum operating speed line is vertical, pretty much paralleling the previously mentioned indicated stall speed line. But eventually, the vertical Vmo line intersects the negative slope of the speed of sound with altitude. From that altitude up, the governing maximum speed becomes the Mmo, the safe margin for that aircraft below the speed of sound. The higher you go, the lower the speed of sound, and that Mmo line slopes all the way back to meet the stall speed line.

Understandably, a pilot is always trying to maintain a safe airspeed above the stall and a safe airspeed below the Vmo/Mmo. But as the altitude increases, the difference between the stall and the max narrows into the space inside the pointy apex of that graph. And that is coffin corner. The pilot must fly accurately because pitching down may increase speed towards Mmo, pitching up may decrease speed towards stall, and banking actually increases the stall speed.

As usual, I've discussed a graphical topic without a graph, and you can't see me waving my hands around. The only one I could find online was in this this thread. The thread itself is alternately intriguing and amusing, as the one poster keeps interjecting the fact that the decrease in air density with altitude is the reason for the increase in stalling speed with altitude. He would be correct if the graph were in true airspeed, but it's in indicated airspeed, which already depends on density. I don't have information on the other poster's theory on momentum.

For the sake of completeness, I should mention that in aviation coffin corner also refers to the top left hand corner of an approach plate, where warnings such as mountainous terrain all quadrants or add 200' to all altitudes when using Dog River altimeter setting are located. It's an easy alliterative phrase that turns up on sports, other industries, and is probably the namesake of many a black diamond ski run. It might originate as the name of an alcove in Victorian staircase landings that allowed one to maneuver a coffin down the stairs. I'm not sure why they wouldn't bring the deceased down the stairs sans coffin, but then there's a lot about the Victorians that doesn't make a lot of sense.

Here's a final footnote on the stall speed. A graph I found in Handling the Big Jets by D.P. Davies plots low speeds against the probability of achieving those speeds in the course of an air transport jet flight. (The data comes from flight data recorders and other research). The probability of flying at 1.25 times the stall speed is 1:1, i.e. always, as that is the speed at which the airplane lifts off the runway. The jet quickly accelerates to a higher margin above the stall and usually does not return to that region until just before landing. But the graph shows that this is not always the case. There's a slightly better than one in ten chance of reaching 1.2 times stall speed, a one in a thousand chance of 1.1 times the stall speed, and one in one hundred thousand transport jet flights for some reason hit the actual stall speed of the airplane. Apparently these are almost the same odds as losing an engine near V1, a crucial take off speed. Davies uses the data to demonstrate the need for air transport pilots to know the handling characteristics of their aircraft at the stall.

Tuesday, January 16, 2007

The Incredibles

I'm working on a coffin corner post, but I'm not done yet, so you're getting a few notes from a movie I saw instead.

One day I watched the DVD extras from the movie The Incredibles (an animated show about the home lives of superheroes). One of the animators was talking about how realistically everything must be animated because any errors irk or distract the audience from the story. He summed it up as:

"If we do a fantastic job, nobody notices."

I guess that applies to a lot of professions.

There's also a funny scene where the mom is flying a jet to the bad guy's secret island hideout, and she's overhead, trying to check in with the control tower. She announces that she is "VFR over the top," which I've never heard in a movie. I wonder where she got the tower frequency from. I've never noticed any secret island airport frequencies published in the CFS or CAP. Apparently she has the right frequency, though, as they hear her, and ... well that would be spoiling the movie for you. It's a fun movie.