Comment by mnw21cam | Hacker News Reader

Comment by mnw21cam | Hacker News Reader

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> The purpose of telling pilots thrust was to save money and fuel.

No, it's more that using lower thrust during take off saves on engine wear and noise levels around the airport. It doesn't really have that big an impact on fuel use, at least that's not the primary purpose.

The article concentrates far too much on the thrust setting. The important bit is the speed where the plane should be rotated to take off, which is known as V_r. That depends on the weight of the plane. On the takeoff roll, the pilots watch the airspeed indicators and pull back to lift off when that speed has been reached. If they have been told a V_r that is lower than it should be, then pulling back will rotate the aircraft without it taking off, and that's when you have this danger of the tail hitting the ground. When that happens, the plane must be inspected, because a tail strike has the potential to weaken the pressurised container that is the fuselage, and this could lead to an explosive burst and depressurisation when flying at high altitude. See https://en.wikipedia.org/wiki/China_Airlines_Flight_611 for why this is a Bad Thing.

> saves on engine wear

Which is saving on money

I'm cracking up at the thought of an airline telling their pilots "fuck it, run em however hard you think you need to."

There's a difference between dumping toxic waste into a river to save money and keeping a plane engine in safe operating conditions to save money.

It seems like a slower rotation might be useful, to wait until you feel it leave the ground before aiming higher. I'm sure there are reasons for the procedure as is.

Trying to "force the airplane off the ground" by rotating prior to Vr has a few safety downsides. One of them is that it makes tailstrikes more likely, as you'll have to carefully moderate the angle before lift occurs. Another is that it removes loading from the wheels more quickly, which extends the "awkward zone" in which the airplane is still rolling on the ground but has so little weight on the wheels that it will skid very easily. This can lead to aircraft being blown sideways off the runway since you can't yet roll into the wind.

A somewhat less direct concern but a related one is ground effect lift. the aircraft can remain a short distance off the ground (roughly a wingspan of altitude) at lower speeds than it can actually "fly," due to ground effect. Smaller aircraft might routinely spend some time in ground effect gaining additional speed before they begin climbing, but airliners have so much thrust they usually rotate pretty directly to their climb speed. This makes it more of an issue though that if rotation occurs too far before Vy climb speed the ground effect period will be prolonged and increase the amount of time the aircraft spends at risk, flying but slow with poor control authority and too close to the ground to have much of a recovery opportunity. The ground is a pretty safe place to be, the sky is a pretty safe place to be, but that first couple thousand feet between the ground and the sky is rather hazardous and jetliners get out of it as quickly as possible.

>> The ground is a pretty safe place to be, the sky is a pretty safe place to be, but that first couple thousand feet between the ground and the sky is rather hazardous and jetliners get out of it as quickly as possible.

Then the problem here is trading safety for something - cost apparently.

BTW, in a small plane I found soft field takeoffs quite fun. Nose up early, wheels up early, fly in ground effect to gain speed. I suppose that is a safety tradeoff and might not want to do it in some conditions.

Rotating at a slower speed is a bad idea. Indicating a lower weight, resulting in a lower calculated rotating speed, is the root cause of the problem. There is no "slower rotation", one does not just pull the yoke slowly, raise the nose wheel and wait for the plane to take off by itself when it will be fast enough, that you can do on a light prop plane where rotate speed is so low some planes will take off by themselves, but not on an airliner where you have a high chance that raising the nose too early results in speed not increasing or even decreasing.

I interpreted "slower" as "later," ie wait longer before rotating.

That means rotating at a higher speed than calculated Vr, this is also bad as you put more wear in the tires and use more fuel (rolling on the tires vs flying in the air) to take off. The Vr is an optimal speed, below is dangerous, above is wasteful.

I think the more important factor is that it eats runway and takes longer to reach safe speeds. If there is a problem at rotation you want as much runway available as possible. If you delay rotation not only are you further down the runway before "really rotating", but you're traveling faster. A 10-knot increase in speed means 20% more energy to disipate. And you're even further above the brake's certified performace. And you've dawdled in getting to safe climb speeds; you accelerate more slowly rolling than flying.

Edit: Actually, that probably doesn't even come into it. Not a pilot, but don't imagine "barely rotate so the nosewheel is in the air but still have negative angle of attack so no lift" is very easy, or very stable if possible. Even if you could do it, you'd risk slamming the nosewheel back into the runway.

V_r is almost always above V_1, which is the decision speed. If you're going to abort a takeoff, that decision needs to be made before V_1, otherwise you're likely to overrun the end of the runway.

So, once you get to the stage where you're ready to rotate, or even late in rotating, you're committed to getting the plane off the ground.

On a big airliner yes. V1 depends on the runway length so a smaller aircraft will easily reach Vr before V1. Because most runways are made for the biggest planes.

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