Friday, February 29, 2008

Take-off Decisions in Large Multi-Engine Airplanes

The topic of thinking in advance about what could go wrong, brings me back to my progression of posts about engine failures at take off. In a single-engine aircraft, you have to stop or land, and your choice is where. In a light twin, you have to abort the take-off if the airplane is on the ground, and in flight at low speed you may or may not be able to get the gear up and maintain the speed and altitude to turn back to land on the runway. A pilot must have thought about the options in advance and then maintain airspeed and control in order to have the best chance of at least collecting insurance on what's left of the airplane. This pre-planning becomes formalized for large multi-engined airplanes.

In transport category aircraft, the speeds involved are sufficient that trying to stop with insufficient runway remaining could be as dangerous as trying to fly without sufficient airspeed. To quote Maverick from Top Gun "You don't have time to think. If you think, you're dead." So transport category airplanes are certified and flown in a way that reduces the amount of thinking the pilots have to do at such a critical moment. To begin with, transport category airplanes are required to have enough power to accelerate to take-off speed and fly with one engine failed. And according to the rules for operating them, at any stage of the take-off there is always enough runway to either continue the take-off or to stop on the remaining runway. The speed that determines that go/no-go point is called V1 (vee-one) and is determined and discussed in advance of every take-off.

A number of factors determine how much runway an airplane needs to take off. These include aircraft weight, thrust, headwind component, and runway conditions. Similar factors affect the amount of runway required to stop, with braking force replacing thrust. The faster an airplane is going, the more dangerous it is to try and stop it, but it's also very dangerous to continue full power off the end of a runway that is not long enough for take-off. It's not the sort of thing you want to be trying to figure out at 250 km/h right after one of your engines has made a very loud noise. The calculations are done using tables, graphs or a computer, and the result is V1, the speed at which the runway required to stop is equal to the runway required to continue. During the take=off run, the non-flying pilot calls out "vee-one" when that speed is reached, and typically the flying pilot takes her hand off the throttles to emphasize that the airplane is committed to flight.

Vee-one isn't necessarily the highest speed at which it is still possible to stop on the runway, but above V1 it's safer to keep going. So V1 is the highest speed at which it is safer to stop than continue, and pilots are supposed to continue if an engine failure is recognized after the vee-one call.

None of this takes into account the possibility that something beyond an engine failure has happened. If the airplane is on fire, or the tail has fallen off, it's clearly better to attempt to stop on the runway, and continue to whatever is beyond the end of the runway, than it is to take a non-airworthy plane into the sky. Here's an account of a captain who decided to abort after V1. They had experienced an engine failure, but the captain believed it to be a bomb, and aborted the take-off, overrunning the pavement. I'm not sure if this particular report mentions it, but I remember one report about this accident mentioned that the captain had once witnessed a takeoff by an airplane that had been damaged too much to fly and had broken up during the go-around. He vowed he would never do that.

There's quite a bit more to say about this, but I'm going to leave it for another entry, because I expect some interesting comments that I'll want to respond to or expand on.

25 comments:

Brian said...

I confess to only reading the first page of the linked report, but a question springs to mind. If he rejected the takeoff within a second or two of V1 being called, shouldn't there have been an adequate margin of safety such that the plane could be stopped before running out of runway? I believe the report says the plane came to rest some 400 feet past the end of the runway... seems like V1 is cutting it a little tight. How much safety margin is built into the calculations?

Thanks for another great post.

maroj said...

Murphy's Law and human reaction time often cause calculated events to go awry.

Preplanning...I think people prefer to think you are a genius than that you are simply hyper prepared. Don't tell 'em you plotted that soccer pitch out befoehand, just "aw shuckin's" your way through as you walk away to fly another day.

One of my unit's Blackhawks had to land due to engine malfunction (unlike fixed wing aviators, chopper pilots can stop in flight also, sometimes), and they kerplunked into a meadow in the Sierran foothills. Trouble is, like most natural wemadows (i.e., not just a clearing due to logging but a former pond), it was very boggy and it required a Skycraner helo to come rescue the rescue copter.
Talking to pilots about emergency landings, and remembering accoutn after account in the popular press abot how a pilot "managed to avoid the houses but slammed to earth in a..."(fill in the blank...one I remember was a cemetary) "saving scores of lives", I am convinced (and not sarcastically) that such saves are a side effect of a pilot's mind seeking the spot most likely to afford some chance of a landing of anuy sort, versus hitting a big solid house, church, or elementary school. My hat's off to them anyway.

janra said...

I think the "managed to avoid the houses" in the media is largely due to the popular perception that once you lose (an/your) engine you don't have control of the airplane anymore.

I know I've gotten surprised reactions when I tell people about practising forced landings and simulated engine failures, and have to explain that the plane is an overweight glider but still flies just fine without an engine. Can't go up, but it's still perfectly controllable.

Once in a while, somebody does lose control of a plane for whatever reason and does crash into a building, and that ends up all over the news for weeks, and probably reinforces the whole thing about uncontrollable planes.

Soaring Student said...

I'm still toen between flying powered aircraft or gliders, and have a foot in both camps (most recently, powered flight). However, one of the chaps I was chatting with at the glider club, when I was lamenting still air and that I was doing a lot of gliding and nt a lot of soaring, suggested that an expensive method to having longer soaring flights was to take a Cessna, power it so it was descending at about 100-200 fpm, then go look for thermals... powering up if the search for thermals was unsuccessful.

Glider experience also changes your outlook about off-field landings.... one guy at the club had two in 2 weeks last summer. Of course, another pilot released from tow at 12:30 on a Friday, and we didn't hear from him for 7 hours, when he landed at the aerodrome after a 500km tour of eastern Ontario.

Glider pilots call an off-field landing "another opportunity to make friends with a farmer". I thin the French call it "going to visit the cows".

Aviatrix said...

Soaring Student: I've heard of people using small Cessnas as powered gliders, actually shutting down the engine and going thermalling for hours. That's outside my safety limits, as I never count on an engine starting when I want it to, but I admire the coolness of logging a flight in a Cessna that is well beyond the most generous theoretical fuel endurance.

The low power Cessna idea sound like a good compromise.

Aluwings said...

I'm enjoying these comments, but just to jump back to the original topic for a sec... V speeds are an interesting "science" in themselves.

To carry more payload, twin engine airliners often increase V1 above the standard definition of that speed - when runway length permits of course. Why? It allows the pilot to trade excess speed for better rate of climb to clear an obstacle in the event of an engine failure. In more technical language, to achieve better second segment climb performance.

Techniques like this are feasible now with detailed runway data available, and with computers that handle the more complex calculations.

Anonymous said...

Soaring Student's comment reminded me of this supposedly real radio exchange:

ATC: You're unreadable, say again.

Motor-glider: I've turned off the engine, is that better?

ATC: (looong pause)

Anonymous said...

@brian: The next page of the report talked about how many times V1 did not take into consideration reduced braking ability on wet runway conditions. I couldn't tell if the runway was wet for this incident but it is possible that a wet runway surface caused the plane to slow down slower than they thought.

Also, consider the fact that the plane is moving quite fast at V1, and that it is constantly accelerating. Even a couple seconds of time moving quickly (and accelerating) is quite a lot, and add the time it takes to brake back down to V1 speed. 400ft could easily have passed by then- the 400 extra feet that the plane slid past the runway end.

On a slightly related note: You see on those technology shows the experiments where they put down stretches of soft material at the ends of ruwnays a long time ago (there are pretty common videos of like an FAA test 727 plowing through on high-speed camera or something). Supposedly it was some kind of sand or breakable concrete or something, a pad of it maybe a foot deep. Airplanes that overrun the runway plow into this stuff, and the material absorbs a lot of the energy from the plane, hastening its deceleration. The result is a shorter overrrun and ideally a safer stop for the overrunning aircraft. Probably been around since the 90's. I wonder, has this technology been developed further or implemented anywhere?

Norman

Xaqman said...

Sure, it's called an Engineered Material Arresting System, or EMAS, and it's been installed in, for one, Burbank, among other airports. SWA's infamous overrun of runway 08 and subsequent trans-highway trip to a gas station prompted that installation.

Xaqman said...

Whoops, I meant to include this link about EMAS:

http://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=6279

Georg said...

Its an old saying: Take off is voluntary, landing is mandatory.

Yet in those days (we have severe storm in Europe) i wish it was the other way round: http://www.liveleak.com/view?i=ddb_1204404185

Now that's a close call... I was to be flying outbound with this Aircraft...

Aluwings said...

Wow ... I thought for sure he'd struck the runway with his left wingtip, but I guess it was just water spraying up. Must have been close.

DC-9's have landing lights that come down under the wingtip. It's not unusual after a strong crosswind landing to find this lamp hanging by the wires. If the A320 had similar lights, this one would have been knocked off for sure.

Too close.

Samwise said...

He did, here's a still image.

According to some news reports I found, the plane lost its left winglet in the event. The controversy now is why ATC cleared the plane to RWY23 and only to RWY33 after the event. RWY33 had a much smaller crosswind component during the storm.

A count of soiled pants in the cabin and cockpit would be an interesting statistic...

Anonymous said...

"...why ATC cleared the plane to RWY23 and only to RWY33 after the event. "

Probably has to do with the airport configuration and moving traffic. Such a situation exists at CYYZ (Toronto). Using parallel runways 23 moves a lot of traffic. Having to switch to the 33s slows the airport down considerably.

When conditions are changing rapidly it's often up to the pilots to "insist" on the change. I was once the first to execute a missed approach on 23L at CYYZ due to crosswinds and turbulence beyond my comfort level. At 200 AGL I was running out of crosswind aileron control during the gusts and that little voice in my head said "Do you really want to bang an engine or wing tip on the runway trying to 'make' this landing?"

I pulled up and after a five minute delay, landed uneventfully into wind on runway 33. I suspect the captain of this Lufthansa flight wishes he'd done the same. Live and learn - sometimes we pilots, being very goal-oriented, just try too hard to "make it work" when we really should abandon the approach.

Ask me how I know... (sometimes I've ignored that little voice and survived, wishing I'd paid attention...)

Xaqman said...

Wow, great photo of a really poor job! That's exactly what we pilots are paid to prevent/avoid, and that crew failed. They'll get no sympathy from me, nor would I expect any myself, in their shoes. ATC has very little to nothing to do with the accident, since, despite the impression some of them give, they can't reach the controls from down there.

Aviatrix said...

Someone sent me a link to the story where a "spokesman" was quoted calling it an "absolutely professional manoeuvre."

Reminds me of the time a pilot I worked with ran out of fuel. One of the customers involved praised him to me for his skill at the emergency landing.

Xaqman said...

I just came out of the sim on the day before yesterday, and got to do some fun V1 cuts. The scene: 300 RVR, with a 10 knot split between V1 and Vr, with an engine failure at V1. All of which means you get to stomp some rudder just to keep the thing on the runway centerline, then rotate, adding still more rudder to full deflection as your nosewheel becomes airborne. And, yes, a little over 5000hp per side makes for a definite yawing moment.
Climb out, work all your checklists, talk to the FAs, pax, dispactch, ATC, and come back around for a hand-flown CAT3A HGS approach, (the weather always seems to come up just to 600 and 400RVR when the instructor wants)!
Good times!

nec Timide said...

There is an old saying "a superior pilot uses her superior knowledge to avoid situations that would require the demonstration of her superior skill". It looked to me that the go-around was initiated way too late, but not having an ATPL in my pocket who am I to say.

On the other hand the video and still remind me of my first touch-and-go in a tail dragger. I'll have to point out to my conversion instructor that what he witnessed was an "absolutely professional manoeuvre" rather than pilot induced oscillation from being hopelessly behind the airplane ;-)

I'm just happy I didn't manage to scrape the wing tip.

CYOW + CYVR said...

@Aviatrix,

Customers are clueless. It's a constant in every industry.

But look on the bright side: at least this guy is unlikely to screw up his fuel loads again and he's got some practice under his belt for a forced landing should he have a total engine failure. Every accident and incident has a silver lining if a) no one is seriously injured and b) something can be learned from it.

(This is the point where I'm hoping you're not going to say he made the same mistake twice and earned himself a Darwin Award.)

Anonymous said...

@xaqman:

It was a poor job in that the pilot did not insist on using the safe runway. But if you look at the video, I do not think that the wing striking the ground was the pilot's fault. It looks as if just after touchdown a really strong gust suddenly lifts the right wing and pushes the plane off the center line of the runway. Good thing they were able to pull that thing off the ground again ...

Mike said...

I don't buy the "gallant pilots" pitch from Lufthansa.

Viewing the Youtube version of this incident, at 34 seconds into the video this guy appears to have begun losing the centreline big time. Where, I wonder, does it say in Lufthansa's SOPs that not being aligned with the centreline constitutes a stabilised approach?

Why didn't he do a GA from 50 feet or whatever he was at that moment. Yes he would have continued sinking, maybe even touched down, but surely a better outcome than he got?

Throwing rocks from the sidelines I know...

Mike

Xaqman said...

Anon,
We pilots, and I'm guessing you're not a professional one by your comment, sometimes joke about the 'gust ruining our landing.' But when somebody tries to land an airplane at or over the maximum crosswind component for the airplane, and it's not shaping up well in advance of touchdown but he proceeds anyway, and dings the airplane, needlessly risking the health of his passengers? Well, yeah, that's completely his fault. Hopefully you'll find it reassuring that we aren't up there saying, "Well, I hope we get lucky! Whee!"

Marty said...

Getting off the topic again;
If you were renting an aircraft by engine hours, and switched the engine off and went gliding for a while - do you get free time in the air?

Ulrich said...

Hi to all
In one report on TV I heard, that the crosswind during approach was 5 to 10 knots below the Airbus and Lufthansa limits. So I think to give the approach a try was O.K. One reason for using the 23 was that this is the only runway in Hamburg with an ILS.

Xaqman said...

To attempt the approach sounds fine, to continue the approach until you strike a wingtip? Not so much.