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?
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.
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?
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.”
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.
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.
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.
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.
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.
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