Height velocity avoid curve for fixed wing?

[Pilot DAR]

It occurs to me, with the recurring discussions about speeds, that there is an important speed (Vy) which is regularly ignored - at great peril. For those who are familiar with helicopter flying, Helicopters are provided with the Height Velocity Curve (AKA "avoid curve" or "deadman's curve"). For convenience for those not familiar, here's one:

http://upload.wikimedia.org/wikipedia/en/4/48/Hvcurve.png

You can notice the reference to the recommended takeoff profile. It occurs to me that where this to also be presented for fixed wing aircraft, and pilots to take it seriously, accidents could be prevented.

In essence, that curve tells the pilot the flight conditions from which a safe autorotation landing is not possible. An important thing to know! So no, you can't sit up hovering at 500 feet, and expect to land safely if the engine quits. Similarly, you can't blast along at 60 knots, and ten feet up, and expect to land safely after an engine failure.

So why does PilotDAR think this applies to airplanes too? Though I have never seen a height velocity avoid curve for a fixed wing aircraft, I think that they might be equally applicable, and important. The airplane Flight Manual will state a Vy, and a Vx, and a recommended glide speed. Think about your plane, which is the slower speed - Vx, or glide speed? Ah, Vx you say - there in lies the problem.

Pilots find that the plane will climb away at Vx, and it's cool to look like you're flying a super STOL departure, 'cause the nose is way up, and the increase in altitude verses ground covered is very good. But, hanging up there, on power, but with lesser forward speed, you're in the middle of the curve avoid zone for your aircraft. The engine goes blat, and you're gliding at that slower speed, you're going to have to trade altitude for speed - just to get going a fast a glide speed! Then you glide! Depending on your beginning altitude, you may have reached the ground before you reached glide speed. If you descend at a speed slower than glide speed, you've reduced the reserve you have with which to flare, pull to flare, you just continue down, with even less control.

People ask me what's the most scary thing about test flying modified aircraft. Well, the testing I did, which I most thought would lead me to wreck a plane was required engine failures from 50 feet, and glide back to the remaining runway in a modified Cessna Grand Caravan. 'Thing was that as I had reduced Vy from 87 knots to 80, Transport Canada required a demonstration of a land back from that slower speed - I did it, but it was scary! Now I know why Cessna errs to the faster speed for climb out. Not so much for good climb performance, but simply so you can glide and land reasonably after an engine failure at that altitude.

So, the climb speed in the flight manual might be established more based upon safe return to earth, following a low altitude engine failure, rather than the optimum performance. I would be pleased to see a height velocity curve published for planes too, so a pilot understands why that Vy speed is a little faster, and actually embraces that speed for normal operations, to improve safety.

There are many recommended operational practices, with hidden underlying rationale. I wish that pilots were given the "full story" so they could make more informed decisions about how they fly. An HV curve in the emergency procedures section of a flight manual would be one way of presenting this information to the pilot.....

[FullWings]

If you have an engine failure in a helicopter in the red bit on the graph, you’re most likely looking at a crash irrespective of pilot ability. If you have one at Vx in a single, you can’t glide as far as if you were at Vy from that height but if you recognise the failure in a timely manner and fly at an appropriate attitude, I’d have thought you would have enough energy left for some sort of flare, especially as you were re-entering ground effect. Your presence on this forum is good evidence that this can work. ;)

I agree that it’s not a good area to operate in but a fixed-wing graph might not have much actual red in it as you’ve got to have an IAS > stall speed in that config. in order to leave the ground in the first place. Personally, if it looked like I had to fly at Vx from rotation in order to clear an obstacle, I probably wouldn’t even attempt to take off...

[Pirke]

On engine failure you already have some speed as we can't gain altitude without speed. When the failure occurs, as long as you push forward hard, you keep your speed.

[Pilot DAR]

When the failure occurs, as long as you push forward hard, you keep your speed.

The speed you have, which with great skill, you might manage to keep, may be too little to allow an effective flare in the first place. But even if it is, you have retained this speed as you now enter a rather steep approach angle, now you have to accelerate the plane upward from that approach path to be near parallel to the landing surface. The steeper the approach, the more you have to accelerate "upward" away from it, the more energy you'd best have in reserve, which (in a plane) means more speed.

Your presence on this forum is good evidence that this can work. ;)

Thanks for the vote of confidence, but I could not succeed in the Caravan. I had to feed in some power on the way down (which is even more difficult to accomplish than a piston plane). The authority and I agreed that compliance with the requirement could be found, because I could have walked away from the plane had I not, but I would have bent some landing gear if I'd not used some power.

I have glided other aircraft, both modified and standard, and repeated this characteristic. Happily, with a running piston engine, you can fix it on the way down, but it is the fact that I had to, which brings the HV avoid curve to mind for fixed wing too.

If pilots were flying with this always in mind, there would be fewer injuries in EFATO events, and people would completely give up the idea of a 180 turn back!

[Mechta]

Glider pilots are taught, when being winch launched, that a gentle climb angle should be maintained until at least 200ft above the ground for the very reason that Pilot DAR describes. A loss of 'thrust' either due to a cable break or a winch failure can easily leave the glider nose up and rapidly losing energy if too steep a climb angle is attempted too close to the ground, with insufficient height to regain flying speed even with an immediate application of full down elevator. The British Gliding Association consider it such a high risk stage of the launch that low level cable breaks are 'demonstration only' for pre-solo pilots. Instructors are not encouraged to 'pull the bung' on trainees until further up the launch.

Any low energy and/or high drag aircraft would be in the same position if attempting a steep climb out and an immediate loss of thrust was experienced (e.g. propeller coming off). I suspect most pilots think in terms of a loss of thrust being something gradual, which would give them time to react before they become an involuntary glider pilot. Sadly this is not always the case.

I wouldn't want to be this pilot if his prop went solo:

7SCorwLqO9U

[Big Pistons Forever]

If pilots were flying with this always in mind, there would be fewer injuries in EFATO events, and people would completely give up the idea of a 180 turn back!

I very much agree with the above. I each my students that a normal takeoff is Vy airspeed to 1000 feet AGL. Vx is only used on an actual short field and the airplane is accelerated to Vy as soon as the obstacles are cleared.

A useful exercise is to, at altitude and in a practice area, establish a Vx climb at full power. When the speed has stabilized, abruptly close the throttle but do not lower the nose. The time from loss of power to stall will be an eye opener. Now do the same exercise and practice transitioning to the glide at the moment of failure. I think you will be surprised at how much and how quickly the nose has to come down to recover.

Finally know that this is in a controlled environment where you know the failure is coming and are primed for it. Human factors studies have shown that there can be up to a 3 second delay before a pilot responds to an engine failure because of the shock factor. So finish your practice session with an engine cut in a Vx climb but this time count to 3 before lowering the nose.

[Mike Cross]

I agree. It's a pity in some ways that we teach Vx and Vy as "best angle of climb" and "best rate of climb". If a student is told it's the best then that's what he feels he should use.

There's no reason at all to use Vx unless you want the best possible climb angle to get over an obstruction. Vy is usually also not necessary in the early stages of the climb. A friend of mine had a sudden EFATO while still low and nose-high. The effect of this is that the aircraft immediately loses speed, and hence control authority, and will not regain it until the nose is down and speed has picked up again. Just rotating the aircraft to get the nose down requires energy which has to be regained. He was not able to make a controlled landing and fortunately was not badly injured in the resulting crash. On the other hand, if the failure had occurred at a lower altitude, but with more airspeed and a less nose-high attitude he would have had sight of the ground ahead and stood a better chance.

Mechta has it spot on, if the donk fails you are in a glider, but one with a far worse glide angle than the 28:1 of a basic 2 seat glider. Hanging it off the prop at low altitude is not a good idea. An immediate pull back to maintain Vx or Vy is just as bad as the glider pilot pulling hard immediately after launch.

[Pirke]

If you really need Vx, your margins are already too low :)

[flyinkiwi]

I was just taught never to climb at a speed lower than best glide, so you always have enough energy to get to best glide speed if things go quiet.

[ChickenHouse]

There is one single reason fixed wing pilots are not really interested in such diagram - in contrast to helicopters we cannot land safely almost everywhere! Topology usually renders such diagram thinking unusable.

[The500man]

I thought the published normal after take-off climb speed was already based on Vy? Is there a flight manual/check list where this isn't already the case?

[Pilot DAR]

in contrast to helicopters we cannot land safely almost everywhere! Topology usually renders such diagram thinking unusable.

Perhaps. But, I would much rather arrive to my intended landing site, for better or worse, with enough reserve energy to accelerate upward from my descending glide path and arrest the rate of descent, before I slam into the earth.

When we see a "best glide" speed published for an airplane, "best" for what? The speed which will provide the least FPM rate of descent, or greatest distance until you reach earth is NOT the speed I would like to be doing when I decide to initiate a flare, or worse, maneuver around something as I cross the fence. Running out of energy there is the worst!

Remember, if the forced landing does not go well, you would rather that it not go well because you ran off the end of the area at 20 knots, ( I guess unless it's a cliff!) than because you undershot it at "best glide" speed and hit the hedge. Side slipping off the extra 10 knots as you flare is better than really wishing you had it!

[The500man]

The speed which will provide the least FPM rate of descent, or greatest distance until you reach earth is NOT the speed I would like to be doing when I decide to initiate a flare

That's a good point but isn't the speed at the threshold when landing normally lower than the best glide speed? I appreciate slipstream will have something to do with elevator authority but surely the elevator has to be big enough to flare sufficiently from best glide in a forced landing? I don't know much about the design/certification requirements for aircraft but would've thought it was something that was tested.

[Genghis the Engineer]

In practice, you need to be just above the stall in the final part of the flare, and you're quite right that ability to flare effectively is a test carried out - usually this is most critical at MTOW / aft CG, and I've certainly seen either approach speed, or forward CG limit, or both, set by the ability to effectively flare.

G

[Madbob]

For anyone who has flown a glider, it is common practice to accelerate from one's best glide/min sink airspeed on final approach. Typically this might be to add 15 knots (plus half surface wind speed if it is particularly windy or if there are gusts) to one's final approach airspeed. This is to give you some margin against wind shear/turbulence so that you don't end up inadvertently stalling, but also to increase the effectiveness of the controls so that you can flare sufficiently to control the rate of descent on touchdown.

The same applies to powered aircraft, both in a glide approach and a forced landing. For a powered approach it is less critical as slipstream will usually increase the effectiveness/authority of the elevators so as to allow a flare. This is not always the case, say with an aircraft with a T tail, or with a twin-engine aircraft, so be warned....

MB

[Pilot DAR]

A flare takes time, but the key is, will you complete the flare in that time, because without power to stretch it, there won't be any more time! When you begin your flare power off, you just began to turn your stored energy into the acceleration you must have to change from descent to smooth touchdown.

If you are really good at timing and executing a flare a short time with little stored reserve energy might be fine for you, but there will be no reserve if you get it wrong, which could be "thump", or undershoot crash.

Carrying that extra speed in the latter stages of a glide to a power off landing can be swiftly undone with (more) flap and/or a sideslip. And, yes, to save your life, a 172 will slip fine with flaps extended, just pay attention, and fly it!)

The theme of what I'm saying is that conventional training is falling short of explaining this, and it's importance, and fixed wing manufacturers are very quiet about describing that characteristic of their aircraft. Pilots need to understand this - for better or worse. On the other hand, helicopter manufacturers are required to test a present this information, so it's right there for the pilot. The fact that fixed wing does not put it in front of the pilot is, in my opinion, a regulatory oversight, because pilots need to know.....