EFATO at Vx

[PilotDAR]

Some here will recall my expressing disdain for pilots who needlessly (no obstacle to clear) climb at Vx, rather than Vy after getting airborne. For this phase of the early climb, aside from possibly needing to clear an obstacle, there's no need to achieve good angle - So why put yourself at risk? What's the risk of climbing away at Vx rather than Vy? some may ask themselves...

Vy will be much more close to a suitable glide speed than Vx. So if you're at Vx, nose high in the air, hanging off the prop, and it quits, you're going to have to lower the nose, accelerate to Vy to get a good glide going, to then have the airspeed to flare at the bottom for your forced landing. But, you don't have the altitude to give up to glide accelerate to glide speed, and then flare, before the ground is already there. So, when the POH (I'll use a 172S, just 'cause it's common) gives you a normal procedure to [Normal Takeoff] "Climb Speed -- 70 -80 KIAS", they're not as worried about you getting up that first couple of hundred feet quickly, or in a short distance, but rather keeping you in the speed range ( Airspeeds for emergency operation - Engine failure after takeoff - Wing flaps up .... 70 KIAS, or "Flaps down, (30)... 65 KIAS) from which a safe gliding return can be made from any altitude you have reached. If you have used the 172S Short field takeoff technique, you used 10 flap, and climbed at 56 KIAS. At best doing that, you simultaneously lowered the nose, accelerated (power off) to 65 KIAS, and extended full flaps, at which point you are properly configured to begin your flare for a forced landing. But how much altitude did you use up doing all that? I've done the testing - many hundreds of feet!

Otherwise, you've left the ground at Vx, and continued to climb at Vx, 'cause it looks cool to those on the ground who might be watching - Look how I can make it climb!! At 100 feet it quits, and as you have not lowered the nose just yet, you get a stall warning right away, so, you lower the nose, but now you have a good view of alarming close remaining runway, so you pull again to avoid, but having not accelerated at all yet, it just gives you a stall warning again, then a stall, and you keep going down unarrested - but, not through the ground.

This video which presented itself to me just now, shows exactly this happening...

https://www.youtube.com/shorts/tpLY6SjH-2U

All if this is akin to the "Height Velocity Avoid Curve" which is required be published for every helicopter. The same rules of physics also apply to airplanes (which also cannot store a little extra reserve energy as slightly excess rotor RPM to use at the bottom). I've never understood why the regulations say that it must be published for helicopters, but not airplanes!

[digits_]

I remember receiving instruction from a highly experienced instructor at some point, flying a high performant and high drag airplane. He was adamant it was much safer to climb at Vx until you were passed the edge of the runway (we only used 20% or so of the runway to take off). I just don't see how that would be beneficial or better than climbing at Vy. It gives you the option to crash on the runway instead of gliding into a field I suppose...

[pelmet]

I would be interested in more info on this.

One particular airport stands out in mind where I would use Vx for the initial climb. My reasoning was that the places to land straight ahead and with any turn other than a return to the airport were likely to cause a lot of damage. Therefore, I would climb at Vx with the intent that while I would not be quite as high as with Vy, I would be closer to the airport which would improve my chances for a turnaround for landing in the opposite direction, whereas Vy would have me a bit higher but further away and possibly not able to make it back.

The question is.....Does this make sense.

[PanEuropean]

I think the answer to the question can be found by looking at what your energy reserves are when the (hypothetical) engine failure occurs,

My background is twin-engine turbine aircraft, keep in mind that I'm not an expert on issues affecting singles but I think the concepts apply to both.

When considering an EFTO (Engine Failure at Takeoff), you will have two sources of energy to fall back on: potential energy (altitude above ground) and kinetic energy (speed). Although "in theory" both are useful, altitude is a lot more comforting, because high kinetic energy but low potential energy just means a bigger hole in the ground when you hit the ground. You can, of course, convert your kinetic energy into potential energy, but just like doing currency conversions with money, there's a small commission (loss) involved when doing so.

I would therefore argue in favour of accumulating as much potential energy as possible following every takeoff, which means climbing at Vy, the speed that gives you the greatest amount of potential energy in the shortest period of time. I know from personal experience (a couple of real-life engine failures shortly after takeoff, as well as teaching many thousands of EFTOs in a full motion simulator) that if you suffer an EFTO when you are at Vy, all you have to do is lower the nose promptly in order to maintain either Vyse in a twin, or best glide speed in a single. if you have an EFTO at Vx you will have to stuff the nose quite dramatically to achieve Vyse or best glide, then make another attitude adjustment shortly afterwards to maintain Vyse or best glide.

We might as well take our guidance from looking at what big twins (737s, 767s, 777s & the like) do when they take off. Although their performance calculations are more complex (multi-stage climb gradients, etc.), they basically target Vy after takeoff, you can see this from looking at the attitude of these large aircraft once they rotate.

Michael

[PilotDAR]

if you suffer an EFTO when you are at Vy, all you have to do is lower the nose promptly in order to maintain either Vyse in a twin, or best glide speed in a single. if you have an EFTO at Vx you will have to stuff the nose quite dramatically to achieve Vyse or best glide, then make another attitude adjustment shortly afterwards to maintain Vyse or best glide.

This. Except for: when you stuff the nose down to achieve best glide, you could be at the ground before you have achieved best glide, and a flare won't work, 'cause the airplane has no energy with which to be flared. SO what is seen in the video is what will happen, no reduction in descent rate as you contact the ground.

Sure, there are airports which temp one to "be higher, just in case". But, until you're high enough to stuff the nose down, then accelerate to best glide, then still have altitude to pull. flare and judge a landing, you're going to crash. In a helicopter, the flight manual kindly provides that minimum altitude [vs airspeed] as the "height velocity" avoid curve. Fixed wing flight manuals are not so generous.

So, you'd like to know that altitude for your favourite plane.... Here's how you safely find it:

Get yourself with room around you, and a few thousand feet of vacant airspace under you. Secure the cabin as if you're going to do some stalls ('cause you might). Give yourself an imaginary whole thousand foot "hard deck", with a few more thousand feet under it. Start a climb from that hard deck altitude at Vx. At 100 feet higher on the altimeter, chop the power. Then lower the nose, accelerate to best glide speed, and flare to "land" again at your hard deck. You succeeded in "landing" if you can make the altimeter and VSI "pause" at that altitude. If you continue down through your hard deck with the stall horn blaring, it did not work. Go up to 200 feet, and try it again. Keep stepping up until you find that altitude from which a Vx engine chop can be recovered to a pause at your hard deck, and your feeling comfortable that you could have flared and "landed" there.

For my experience, draggy airplanes (like a 185 amphibian) may take 300 or 400 feet up before a Vx power chop can be recovered to a safe landing. I'm sure that airplanes like a DA40 would be much more forgiving, and maybe only 100 - 150 feet (though I have not tried it in a DA40). In any case, this seems to be one of those "look at me!!!" things that pilots do in airplanes, until they learn that it's unwise. Sure, if you have to get over an obstacle, you're going to have to use Vx, but then, was it wise to operate into that runway in the first place? 'Makes me nervous doing it - I avoid! We learn to only take an additional risk for an additional benefit. There's no benefit to clearing an imaginary obstacle at Vx, but there sure is an additional risk!

[just clearing the trees]

Sure, if you have to get over an obstacle, you're going to have to use Vx, but then, was it wise to operate into that runway in the first place?

This.
IMO, good decision making should more or less eliminate the need to ever fly at Vx, barring abnormal/emergency situations.

[digits_]

I would be interested in more info on this.

One particular airport stands out in mind where I would use Vx for the initial climb. My reasoning was that the places to land straight ahead and with any turn other than a return to the airport were likely to cause a lot of damage. Therefore, I would climb at Vx with the intent that while I would not be quite as high as with Vy, I would be closer to the airport which would improve my chances for a turnaround for landing in the opposite direction, whereas Vy would have me a bit higher but further away and possibly not able to make it back.

The question is.....Does this make sense.

It depends a bit on what you're worried about. Generally, to feel safe, you'll need a certain altitude and a certain airspeed. For the sake of argument, let's say you need to reach 500 ft AGL and Vy to be able to make an emergency landing without crashing. Let's call it the energy level required for recovery. That leaves a few possibilities: your engine might die above 500 ft but at a slower speed. If you're high enough, you can exchange speed for altitude and still make the landing area, for example Vx at 700 ft, which converts into Vy at 500 ft. Or, you could have an engine failure at 300 ft but be super fast that you can zoom up to 500 ft and be at Vy. I believe military jets use that technique, mentioned somewhere in the snowbird topics.

Anyway, trying to minimize our risk during the flight, we'd likely want to be at the recovery energy as soon as possible. To gain altitude, flying at Vy would be most efficient, perhaps compensated for use of flap. But let's ignore that for a moment. If you increase your speed above Vy, you'll gain altitude less quickly, but your airspeed will be a bit higher, so you'll gain more kinetic energy. So maybe there might be other situations above Vy where your airplane might find an optimum. I have a suspicion that Vy would still be the most efficient if we really calculate it in detail (photofly, where are you?). However, what we can be absolutely sure of, is that flying at Vx will increase your time to reach the altitude, while also flying more slowly. Which means you'll need to climb even higher to reach the recovery energy.

Time wise, there is absolutely no reason why you would want to fly at Vx.

What if you're worried about landing on the same runway you're taking off from, because it's the only option around at low altitude? In that case there's PIlotDAR's technique that will demonstrate that below a certain altitude at Vx, you might/won't be able to land. So yeah, the first 200 ft (estimate) will be a guaranteed crash, even if you still make the runway. Even climbing at Vx, you'll still be moving over the runway pretty quickly, and eventually the runway will be behind you. Now you need to make a 180. Tricky. Note that if you want to land on the runway you took off from, you want to have *some* distance between the runway and your position. You need to turn about 270 degrees in order to land on the same runway (180 degree of turning and a bit of maneuvering). in that case, it would help you to be further away from the runway.

If we compare these situations:

Engine failure below 200 ft:
- At Vx: likely a stall crash on the runway
- At Vy: likely a smooth landing, not necessarily on the runway

Engine failure between 200ft and 500 ft:
- At Vx: a smooth landing, not necessarily on the runway
- At Vy: a smooth landing, not necessarily on the runway

Engine failure between 500 ft and 700 ft (estimated)
- At Vx:a smooth landing, not necessarily on the runway
- At Vy: a smooth landing on a pre planned suitable surface

Engine failure above 700 ft:
- At Vx and Vy: a smooth landing on a pre planned suitable surface

The biggest difference will be in the first 2 phases. Between 200 ft and 500 ft you might have a bit more chance of being able to land on the runway, depending on the exact situation. But if the engine failure happens below 200 ft, you'll stall crash on the runway. Will you accept a certain crash below 200 ft only to potentially slightly improve the landing outcome between 200 ft and 500 ft? It just doesn't make sense to me.

Perhaps there's a very unique runway where Vx would be beneficial. But I just can't imagine it. Below 200 ft you'll crash anyway, no matter what the runway conditions or environment is. Above 200ft you might benefit if the runway is a bit longer so you can land straight ahead if something happens. But if the runway is super long, then you can do that same thing flying Vy. Perhaps there's a certain sweet spot runway length where it would theoretically be better to fly Vx?

Perhaps we can change the theory and fly Vy the first 200 ft anyway, and then slow down to Vx if unique circumstances require it? It does seem like a lot of hassle for something that might not have many benefits.

So, you'd like to know that altitude for your favourite plane.... Here's how you safely find it:

Get yourself with room around you, and a few thousand feet of vacant airspace under you. Secure the cabin as if you're going to do some stalls ('cause you might). Give yourself an imaginary whole thousand foot "hard deck", with a few more thousand feet under it. Start a climb from that hard deck altitude at Vx. At 100 feet higher on the altimeter, chop the power. Then lower the nose, accelerate to best glide speed, and flare to "land" again at your hard deck. You succeeded in "landing" if you can make the altimeter and VSI "pause" at that altitude. If you continue down through your hard deck with the stall horn blaring, it did not work. Go up to 200 feet, and try it again. Keep stepping up until you find that altitude from which a Vx engine chop can be recovered to a pause at your hard deck, and your feeling comfortable that you could have flared and "landed" there.

Don't forget to add a few seconds for the startle factor. Might make a huge difference if you start out at Vx. You might even stall before you realize what's happening in real life, depending on your airplane.

[Redneck_pilot86]

Just fly Islanders - all the numbers are the same.