Airline Flying 101: Anatomy of a Take-off.


Take-off? That’s easy, right? You fasten your safety belt, move your seat fully upright and stow your tray table. Ready. Right?

Not even.

But if that’s the full extent you prefer to be aware of, fine. Otherwise, read on as we take apart this very complex, important maneuver.

The planning starts long before you strap yourself into your seat in the back of the plane, and here’s why.

Take-offs come in all sizes and shapes because of several variables–so there’s no “one size fits all” logic or protocol. What are the variables? Well, aircraft weight, runway length, winds, runway surface condition and temperature are the basics, and each has an effect on performance.

You might think runway length is the great reliever, right? Miles of runway, like at DFW or Denver mean simple, low-risk performance, right?

And you might think a short runway or nasty weather are the “problem children” of take-off performance. But let me give you the pilot answers: no, no, and furthermore, no.

Throw out what you’ve been thinking about take-offs as a passenger, and strap in tight (is that tray table up? is Alec Baldwin playing “Words” in the lav while we all wait for His Highness to finish?) because you’re about to test drive some “pilot think:”

I don’t worry about taking off–I worry about stopping.

Why? This sounds so simple that when you think about it, you’ll have to agree: aircraft are made to fly–not drag race.

Huh?

Look, accelerating 85 tons to nearly 200 miles per hour builds tremendous kinetic energy. Not a problem for the landing gear if you take off because it’s simply rolling. But if you must stop, the brakes and wing-located speed brakes have to dissipate that energy within the length of the asphalt ahead.  The runway length is finite, the aircraft weight is unchangeable once you’re rolling. So where is the point of no return, the point after which there’s not enough runway to stop?

Brakes are key--and checked visually before EVERY take-off.

As a pilot–particularly as the captain who makes every go-no go decision no matter which pilot is actually flying–you must know when that instant occurs. That magic point is not a distance down the runway but rather, a maximum speed: “Refusal Speed.” In other words, the maximum speed to which we can accelerate and still stop within the confines of the runway if we choose to abort the take-off.

But there’s a catch, of course.

Refusal Speed is only half of the go-no go decision. Part Two is just as critical: what is the minimum speed I must have in order to take-off if one engine fails, continuing on the other. I can hear this already: why the hell would you want to continue the take-off on one engine?

To which I’d answer back, what if the failure happens above Refusal Speed? In other words, there’s not enough runway ahead to stop your high-speed tricycle.

Okay, that minimum speed–the speed you must have in order to continue the take-off in the remaining runway on one engine–is called “Critical Engine Failure Speed.”

All of the performance numbers for each unique take-off are computed, with corrections for the many variables to be made by the pilots.

Now you have the two controllers of the go-no go decision; one a minimum speed (you must have Critical Engine failure Speed achieved to continue safely into the air) and one a maximum (if you attempt to abort in excess of Refusal Speed–you ain’t stopping on the runway).

So which is the deciding factor? Well, in modern day jets under average circumstances, the “max” speed is normally way in excess of the “min” speed. In other words, you normally achieve the min required for single-engine continued take-off before you reach the max allowed for stopping. So, in ordinary circumstances, Decision Speed–which we call V1–is Refusal Speed.

In other words, we know we’ll secure adequate flying speed for a single-engine take-off before we hit the max abort speed. So we use the max abort speed–Refusal Speed–for V1.

Pilot-think lesson one: it’s easier to deal with a single-engine aircraft in the air than it is to stop a freight train on the runway. Which goes back to my earlier point: airliners fly great but make only adequate drag racers, stopping on the drag strip remaining being the challenge.

Single-engine take-off, or high speed abort?

Add to that the wild card: the captain must decide in a split second as you’re rolling toward V1 if any malfunction that occurs will affect the ability to stop the jet: did an electrical system failure kill the anti-skid system required for max braking? Did a hydraulic failure eliminate the wing spoilers figured into the stopping distance?

Some jets require very little system support to fly–but a lot of factors to stop: the MD80 will fly all day without hydraulics, electrics or pneumatics–but it ain’t stopping on a “balanced field” without electrics and hydraulics.

Hydraulically actuated wing spoilers are figured into the stopping distance.Get my pilot-prespective regarding my preference to take a wounded jet into the air rather than wrestle it to a stop on a runway?

And remember, those speeds are “perfect world” scenarios. But on your flight–like every flight–despite the engineering numbers from which the stopping distance is computed, there are the real life factors which screw them up: wet or icy runway, tailwind, old tires, old brakes, rubber on the runway because of aircraft touchdown on landings.

Not a problem on an average day, but corrections to the numbers and your pilot-think must be made if any of those variables are present.

Now, have you deduced the worst-case scenario with the two controlling speeds, Critical Engine failure Speed and Refusal Speed? That is, you will exceed the max speed for stopping before you attain the minimum speed for single-engine flight?

That’s simple: you can’t take-off. In practice, we adjust the flap setting or even reduce the gross weight: back to the gate–some cargo and/or passengers must come off. Hardly ever happens that we return to the gate because we plan ahead–and that’s why you hear of a flight being “weight restricted,” meaning some seats will be empty by requirement before you even board. Now you know why.

But really, that’s not even the worst case scenario from a pilot’s perspective (sorry about your trip, if you’re one of the passengers left behind on a weight restricted flight–but you probably got some compensation for it). Rather, it’s when the two numbers are the same.

That is, the minimum speed required for flight is equal to the max speed for stopping.

That’s called a “Balanced Field:” the runway distance required to accelerate to minimum single-engine take-off speed is also the maximum velocity from which you can safely abort and stop on the runway.

That’s a “short runway” problem, like in LaGuardia, Burbank, Washington National or Orange County, right?

Wrong–it’s everywhere, like Denver’s 14,000 feet of runway (compared to LaGuardia’s 7,000) on a hot summer day; ditto DFW; also Mexico City even on a cool day because it’s at 7,500 feet elevation. And it can occur anywhere due to rain, ice or snow.

So here’s your plan, and as pilot-in-command, you’d better have this tattooed into your brain on every take-off: once you enter the high-speed abort regime (by definition, above 90 knots), know what you will abort for–or continue the take-off. Be ready for both–without hesitation.

LaGuardia: 7,000' between you and Flushing Bay.

It’s easier to decide what you will abort for than won’t–because the “must stops” outnumber the “can stops” and remember your pilot think: it’s often safer to continue than stop. And here are my Big Four Must Stops: engine fire, engine failure, windshear or structural failure.

So rolling past 90, I’m thinking over and over, “engines, engines, engines,” zeroing in on any malfunction in order to assess if it’s an engine problem–if not, it’s likely not a “must stop” situation; I’m aware of windshear but don’t even start the take-off roll with any of the conditions present; structural damage we’ll deal with as necessary. Otherwise, we’re flying, folks.

Got all that? Good deal: now you understand the important interrelationship between Critical Engine Failure Speed, Refusal Speed and the all important concept of V1.

And now that you understand the complex, split-second conditions surrounding the go-no go decision on your next take-off, you can relax and just put all of those crucial factors out of your mind.

Because rest assured, they’re at the forefront of mine, or that of whatever crew into whose hands you’ve entrusted your life.

Special Note:
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12 Responses to “Airline Flying 101: Anatomy of a Take-off.”

  1. With Lin Smith of the 9ACCS flying, no two takeoffs were alike. I was always happy, when I was flying with Lin, if the number of takeoffs equaled the number of landings.

    • See? They should NEVER have disbanded The Flying Bunda Brothers. Glad I got out of the squadron before they decided he might make a decent instructor pilot.

      • I was in Stan Evil and he was the chief. I use to laugh at his debriefs because they were so chicken shit and the pilots he was evaluating could fly rings around him. When he was trying to become an instructor he busted his first two evals. I flew on his third and we got him through; just barely.

  2. Thanks for another great post. Got it and been there. The take-home for every SLC is to be alert and attentive during the Take off roll. When the FAs or FC remind one to double-check those seat belts, DO IT! When a large airliner aborts as take off run at or near the calculated V1 speed (with RTO auto brakes or max manual) any unsecured items in coach class will be served to first class pax, instantly.
    I won’t call it terror time for for experienced pilots, but it must be close and easily the most riskly part of the flight. I understand the go/no go with an engine out, but I think you mangled that in the last section by not mentioning the current speed. Still, I sure got it.
    Those automatic, computer and dispatch-driven speeds always include some fudge/safey factor. Do you, at least mentally add a bit more of your own?
    When I hear or suspect “Weight Restriction” when flying from a high altitude or high temperature airport, I try to be the first one in line to get OFF the airplane. (And I don’t care about the compensation. I understand the physics and I want off!)
    I recently read that a few lines (like SWA) don’t use autobreaking, in some effort to keep their feet as common as possible. I sure hope that they still use the RTO braking option when it is available. Do you know if RTO braking is available on the SWA B737 fleet?
    Thank you for a great post. Flying is nice, but stopping before flight – if necessary is even nicer. Happy New Year, Captain. – Craig

  3. I’m not sure about the autobrakes, but I know they only started using Autothrottle and VNAV fairly recently as well as switching to the newer PFD/ND layout from the EFIS/MAP that emulates the classics. Perhaps they started using the autobrakes with those changes.

    • I swear by the 737-800 ABS (Auto Brakes System). They’re particularly valuable when you are landing in a crosswind, because while you’re holding a crosswind correction with the rudder pedals, the ABS applies equal braking, something that’s hard to do manually.

  4. I really wish I’d never clicked through from boingboin.net to your DCA post. I find this stuff endlessly fascinating, and love flying in small planes. If I had the money I’d get my license. But I’m also a control freak, so I actually hate being in the back of a plane during landing, takeoff or when things get bumpy. Yes, statistically flying is much safer then driving. But I feel much more in control bumper to bumper at 70mph on the east side highway in NYC (thought it’s been years since I actually did that particular drive, I wonder if it’s still as twisty and bumpy as a roller coaster?).

  5. This is great! Okay, probably not, but i just found your blog and I find it fascinating. I’ve always wondered about all the little details about flying and now I get to know. :D

  6. Absolutely love this. I can’t stop reading.

  7. UberMitch Says:

    I’m curious about how the calculation works in aircraft with greater than two engines. I get that they will be heavier than two engine planes, but if you lose one engine in a (say) four engine plane, you have only lost 25% of your available thrust as opposed to 50%. So that would make it much more rare for critical engine speed to grater than refusal speed, right? Though my mind is starting to boggle as I think about the permutations for many-engined planes, as they must have critical engine speeds for one engine out, two out on different wings, two out on the same wing, three out. Or is the reality that with two engines out on a four engined plane, you aren’t getting off the ground anyway, i.e. there is no runway in world long enough to reach critical engine speed.

    • You’re on the right track. But CEFS (Critical Engine Failure Speed) on multi-engine planes is only for 1 engine: on a four engine plane you never continue with one out before CEFS; on three engine plane, you never continue before CEFS with one engine out.

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