Saturday, October 18, 2008

What and Balance?

The latest issue of Aviation Safety Letter (or at least the latest one that I have managed to be home to receive) reprints an article on weight and balance that surprises me. His thesis is that only five percent of private pilots do a weight and balance calculation before a flight, and that eighty percent couldn't do the calculation correctly if their lives depended on it. His narrative implies that W&B isn't even taught as part of the licence, and is not tested as part of the flight test. A Canadian pilot licence candidate would fail a flight test if she couldn't produce a very accurate weight and balance for the proposed flight.

Now I'm not expecting that a private flyer would whip out an operational flight plan every time he came out in the evening to do a few circuits between dinner and sunset. If you own or regularly rent the same kind of airplane, you know, for example, that your own weight plus full fuel is well within limits. You'll know perhaps from repeated calculations, or careful examination of the graphs, that a passenger in the seat next to you cannot put the airplane out of balance limits, so you only need to count weight. Or perhaps you've determined that with 200 pounds in each passenger seat you can carry 50 lbs of bags, but they have to go in the forward compartment. I wouldn't fault someone who knew his airplane taking on loads like this without doing any calculations.

The example given in the article of the pilot not asking the heaviest person to sit in the front with him seems bizarre to me. If he was shy about mentioning weight to a female passenger, then just say, "Hey Susan, Sara has sat up front lots of times, why don't you sit up here." Or offer here the front seat for more "hip room." Myself I would have just sized up the passengers and assigned seats, no excuses, no explanations. My own family might know that I was doing it to get the weight in the front, but no one needs to be told if they are going to be embarrassed by it.

The thing is, an airplane loaded out of the manufacturer's limits probably will fly. It probably won't even crash. But in some configuration there is some otherwise reasonable maneuver that your airplane is no longer able to perform. Maybe it becomes unstable in the slow flight regime. Maybe it makes it harder to maneuver with a distraction. With an overweight airplane, turbulence puts more stress on the airframe and could lead to an in-flight break up. Maybe it won't climb out of ground effect or won't give you the published rate of climb. Maybe there's some attitude or or maneuver from which it won't be recoverable. it happens to commercial pilots too. This airplane has flown a couple of legs with an unknown elevator rigging problem, but it only killed people on the leg that they had an aft C of G.

The last example above departed with paperwork showing it within limits, but that is because the paperwork took into account standard passenger weights, based on a fifty year old survey. People weigh more now, and that accident was the last straw. Americans don't use quite the same standard weights Canadians do, but within twenty days after that accident, Transport Canada had issued new standard passenger weights to be used by all operators. If your passengers aren't standard, don't pretend they are. One method a reader mentioned for discreet but accurate weight and balance on small private airplanes is for the pilot in command to enter the moment of the empty airplane on a calculator, hit the plus key, then to tell each passenger to enter his or her honest clothed weight multiplied by the arm (obviously you tell the passenger the arm of their seat), hit the plus key and hand the calculator to the next passenger. You'd want to assume reasonable calculator skills on the part of the passengers, and ensure your calculator used the proper precedence of operations.

You'll notice that in almost all of these fatal accidents, the out of envelope condition was not the only factor in the accident. It's a little like "speed was a factor" in automobile accidents. In the car, once you're going too fast, any other thing that goes wrong, or any other mistake you make, is more likely to lead to a crash. In the airplane, once you're loaded out of weight and balance you've deliberately given yourself a penalty for anything you have to do. Not being within weight and balance limits is like not sending all six players onto the ice for a hockey shift. Yes, you may successfully skate out the penalty, and sometimes you even score a goal shorthanded, but in sudden death overtime, everyone should know enough not to take unnecessary penalties.

I'm guessing that pilot readers of Cockpit Conversation, being biased towards the smart and geeky, can all do the simple arithmetic required to determine the centre of gravity of a loaded airplane, and mostly do check the balance limits if they are putting an unusual load in their airplane. if any non-pilots (or any of those hypothetical eighty percent of pilots) is now curious how to do a weight and balance calculation, leave a comment and I'll be happy to explain in a future post.

23 comments:

Dispatcher said...

Interesting subject, I don't have any experience with smaller light aircraft but I'm still amazed and the effect, or at least calculated effect that such small numbers can have on the larger jets.

I know with some of our flights with perhaps only a half full load of passengers on a 140-180 seat jet weight 50,000kg + that moving one or two passengers can affect it enough to put the aircraft out of trim according to the computer. Obviously the computer system can work much more accurately than the manual dropline trim sheet, and moving 80kg can set it just outside of the aircraft's limits meaning we have to start moving bags/cargo or people around to get it within limits.

I've been working on a post about the way we load aircraft that should be posted soon, I'll add in some examples of having to move loads around.

Oshawapilot said...

It's been several years since I did a hand calculated W&B (the last time was my flight test, actually) and I really don't think that I could do one manually any longer - I'd have to sit down and re-learn the whole procedure. All that said, I have an application on both my Palm and iPhone which do the W&B calculations (based upon the aircrafts specific figures) automatically, and I wouldn't contemplate flying without having done the W&B ahead of time. It was drilled into my head as a student the importance of being inside the envelope, and what can happen if you are not.

When I was primarily flying 152's W&B was a constant issue and I never treated it lightly. I wouldn't fly if I was so much as a few pounds over MGW be it due either to pax, fuel load, or the weight of the aircraft itself not working out given either of the first two.

I have, however, seen lots of people take to the air without barely contemplating a W&B, however.

There's a few choice videos on the internet of the results of others who did the same and didn't fare well as a result.

Anonymous said...

This non-pilot would like to know how a W&B calculation is done!

Also, a question for aviatrix, if I may ask: you've taken down those old work stories, but now you're posting about work occasionally. Why did you stop, and what's the impetus for starting again (if I may)?

Thanks...

Aviatrix said...

1. Request noted. I'll work on that for a future post.

2. I first took down work stories that might go in a book someday, and then I started a kind of rolling blackout for past stories, so that any one story was only up for a month or so, sort of like PostSecret does week by week. Blogspot makes it an incredible pain to change the status of posts between draft and published. You have to do them all one at a time and you have to edit and scroll down for each one, so I stopped doing that because it was just annoying. By the same token it would be that annoying to put them back up again, so I won't.

My willingness and detail when it comes to blogging about work ebbs and flows.

Wayfarer said...

Don't forget the impacts on ground handling and loading procedures when thinking about w/b....I was sitting in the adjacent aircraft and watched this incident happen.

Insufficient fuel loaded to maintain forward CoG during cargo loading....
http://cdn-www.airliners.net/aviation-photos/middle/1/8/8/0220881.jpg

jinksto said...

Glad I had a chance to read everything before you took any of them down. I seriously spent a couple of days reading nothing but this just to get caught up. :)

I don't know that the comments about flight training are quite true. I know my instructor was VERY strict about having them done and it was absolutely something that was on the US tests (both written and flight). Your description of flight reviews and tests in Canada seem very similar to the US tests. It's possible that the article included statistics from other countries but in the US you're not getting a license without knowing W&B.

jinksto said...

And... oh goodness, did you mention a future book? Hopefully you have real plans for that and it's not a "someday I might like to" kind of thing. Just the mention of an Aviatrix book gets me excited. Yes, I'd pay money (and gladly) to read all of your posts again in book form! :)

A Squared said...

His narrative implies that W&B isn't even taught as part of the licence,

It does seem to imply that, and perhaps he wasn't. However, for an absolute fact, it is required to be taught, it *will* be on the knowledge (written) exam, and applicants are required to demonstrate the ability on a checkride, even a private checkride, although the examiner in question seems to have omitted that requriement.

The last example above departed with paperwork showing it within limits, but that is because the paperwork took into account standard passenger weights, based on a fifty year old survey.


If you read the CVR transcript, it is pretty clear that the crew falsified the W& B to show in under weight and fwd of CG, even with the outdated passenger weights.

Julien said...

A few months into my flying training my instructor asked me to do a W&B calculation for a C152A(MTOW 757kg) with full fuel, an 85kg student and a 95kg instructor.

I was very surprised to find out that this put us over the maximum take-off weight by 34kg. And we had been flying in this configuration most of the time.

He went on to explain that this was OK as long as we didn't stress the airframe and accepted a lower rate of climb.

What do you guys think?

PS: The calculation goes as follows:
Basic empty weight: 545kg
92 litres usable fuel: 66kg
student and instructor: 180kg
Total: 791kg

majroj said...

Again on the reverse side, pilot and copilot of the Cessna T-37 "Tweet" (a two seater twin engine little jet trainer) or its attack cousin, the A-37, would routinely fuel the plane without armament or pax/luggage, get out and lift the tail to get a few more gallons in, then load er up. Amazing little craft, you could open a little hatch, undo a zipper, and reach into the cockpit to unlock the canopy manually from outside.

http://en.wikipedia.org/wiki/Cessna_T-37

Aviatrix said...

Julien: In Canada that would be illegal and would void the certificate of airworthiness for the airplane, thereby voiding the insurance coverage and putting the pilot's licence at risk.

It's not okay to fly an airplane outside of its certified envelope, and no instructor should be saying that. It doesn't mean that it won't fly, or that you won't probably be okay, as you've seen, but if it provided an acceptable safety margin at 791 kg, it would have been certified at that weight.

Oshawapilot said...

Julien:

IMHO, find a new instructor. As mentioned, your C-of-A is invalidated by being even 1lb overweight.

For your instructor to then go forth and somehow try to justify/validate it as being "OK" in any way should cause you to run away screaming.

I remember before my very first flight as a student my instructor did a W&B and continued to do so until I learned how to do it myself, and then *I* was required to do it before every flight, and he checked it to make sure it was actually correct.

In retrospect, anything else would have seemed totally unacceptable now.

Blake said...

Whenever I fly solo, I never complete a w&b before a flight. Why? Because I know that with full tanks and my weight in a 172 I will be within limits.

That said, I do carry my w&b worksheet with me in my flight bag in case I get stopped for a ramp check by TC. If they ask, I can whip it out and do the calculation on the spot.

If I take passengers, then that is a different story. When 3 people in the airplane, I need to know which of my two passengers have to sit in the front or back. I can only calculate that by doing a w&b. It's important to know if you're flying an airplane that's in the utility or normal categories.

One thing to note. When I was getting my multi rating in the Seminole. The school would always keep 100lbs in the cargo area. They weights that were specifically designed for this purpose (sand filled bags that were strapped down). This was to keep the CoG more in the middle of the craft. Even though we would still be within limits without the weight, for training purposes it was better to have the CoG more in the centre than forward.

jinksto said...

For Julien: as others have said, it's NOT OK in my opinion. The "M" in MGW means "Max[imum]" for a reason. Sure, if you're an aircraft engineer you can do your own calculations and take into account any number of variables that would make this "ok". Even if you're not an engineer you can use the laymans calculation of, "well, it worked last time" and probably be mostly right. That said though, as far as the FAA (and apparently the canadian equivalent) is concerned it's not in any way correct. If an accident happens the aircraft insurance is void as is your own life/health insurance.

It's just the wrong thing to do.

Soaring Student said...

In my earlier student days, I did a W&B before each flight. Always got the same result, we were within limits at the start and at the end of the flight (hopefully, the only shift will be due to fuel burn!).

After a gap in training in 2007, I've done two W&B in all of 2008 - one for each of the two instructors I fly with (the two aircraft are identical models, and within 10 pounds of each other). Each W&B assumed full tanks at the start of the flight, and empty tanks at the end. Each W&B showed us within specs for both total weight and CoG.

If anything changes, or if my target destination is other than Carp, I'll keep the paperwork Gods happy and write up another one.

Now that I think if it, now that I am flying solo I should two more W&B. total weight will, of course, be OK, but need to have documented proof that the CoG is within spec without the self-unloading allast on board.

syrad said...

Obviously working as a commercial passenger pilot with constantly changing passenger, cargo, fuel, and freight numbers requires that a W/B be completed each flight. I'm even pretty quick at doing a manual version with a calculator in the event the aircraft's computerized version is MEL'd.

However, even as a GA pilot I can't say it was something taken lightly. As a CFI I only didn't require W/B if I had flown before with the same fuel and student. Even then, a change in density altitude warranted a check of the charts. Every single checkride, review, or new student was an automatic W/B requirement. I don't know any CFI who wouldn't ensure that a PPL student could do and understand a thorough W/B. I can believe that many people who have been flying their same airplane for a long time start to believe that they don't need to do W/B because they "know their airplane". I have a hard time believing that any CFI who tests them or signs them off wouldn't ensure that they have the knowledge and tools to do a W/B calculation.

Also, I don't think that any exceedance of the maximum weight is okay. I've occasionally waited at the end of the runway when I had a fully loaded aircraft to ensure that we had burned off enough of our planned taxi fuel so that we would not be one pound over the maximum gross takeoff weight.

Gary said...

I do a W&B anytime I fly with more then my Bride aboard. I have our W&B calculated on full and to the tabs fuel settings ingrained in the brain. I ALWAYS do a W&B anytime the previous condition changes, from taking a child with us to a couple climbing in the back.

My W&B is printed out on my pre-flight wx excel spreadsheet along with a note area for NOTAM's and TFR's. It's the way I was taught and anything short of that wouldn't feel right. Besides, my wife always asks about the W&B and follows me around on the pre-flight.

Callsign Echo said...

Oh my God Julien. It is NOT okay. Your flight instructor is being irresponsible. "Pushing the envelope" is for test pilots. Fly outside CG, overspeed flaps or gear, or exceed Vne and that's exactly what you've become. There is margin of safety built into most of the aircraft's limitations, but you never know how much.

Get a new flight instructor. He's taking risks with your life.

Yellowbird said...

Like Blake, I don’t normally do a W&B calculation when the loading is within known acceptable ranges. I know from experience that my Cardinal can carry two adults (hefty ones at that) and full fuel and be within limits. With 3 - 4 people aboard, I'll figure the weight and adjust fuel load (and baggage, if necessary) to compensate.

Balance is another matter. The Cardinal has such a wide envelope that it's practically impossible to load it out of balance. I ran a number of sample scenarios when I first bought the plane and was amused to find that I would have to go to extreme, even absurd lengths to get the balance out of limits: A pair of 300lb gorillas in the front seats and nothing at all in the back or the baggage compartment (behind the rear seats) would exceed the forward limit by a few inches. To get anywhere near the aft limit, you'd have to put both gorillas in the back seats, max out the baggage compartment loading, and have a 70lb pilot alone in the front seats.

Aviatrix said...

That's awesome, Yellowbird: both your aircraft's envelope and the fact that you did the calculations. That's exactly what everyone should do to know their airplane. It helps you catch errors, too.

Guy said...

I did something pretty similar to Yellowbird. I don't run a weight and balance unless I think I'm close to limits, but right after I got my Comanche I spent a while running the various combinations of 2, 3, and 4 passengers with and without luggage.

I found that for the most part I just had to stay under weight, but two heavy front passengers and no weight in back could put the aircraft out of its forward CG. Other than that 2 passengers and full fuel is no problem, 3 passengers are OK as long as one of them weighs around 100 lbs, and past that I need to have the aux tanks empty.

When I was a student I ran the W&B every time I went to a new plane, and a few times with the same plane. After that I just kept the same W&B with me.

Rhonda said...

Oh. My. God.

That article, and the attitude behind it, was terrifying.

Even on an airplane I've flown before with a loading I've flown before, I'll fill in the W&B form. Even though I know full well that me + my instructor + standard tanks are 3-10lbs under MTOW in all but one of the school's C152s, and I know which plane needs to lose 20 lbs :-)

Anonymous said...

I once spent a fair amount of time flying one particular light aircraft. Other than my first couple flights I almost never did a weight and balance, I'd just add rough numbers up to ensure we weren't near gross weight and use what I knew about the balance to ensure we weren't near CoG limits (I did a similar exercise to what yellowbird did when starting out). I also did runway length and density altitude calculations based on educated guesses and mental approximations and I never scared myself once. That said, I took pretty conservative estimates and when the numbers got near the limits in my head, I'd do a proper full calculation on paper. It's also worth noting that the CARs don't require you to prove that a W+B was done before the flight, they only require that the pilot be able to prove the aircraft was within W+B for the flight (i.e. they can do the calculation at the end of the flight, when they're being ramp checked, but it better show a legal result).

So yeah, for a significant portion of my GA flying experience I'd say I did a W+B only 5-10% of the time. I don't feel it was unsafe.