Single engine normal climbout: Vx or Vy?

[SeanGG]

I tried to search for a topic regarding this matter, but to no success. If this has been discussed before then I would appreciate a link to the appropriate thread.

In a normal climb out (no obstacles) in a small typical single engine airplane (say a C-172), what is the safest airspeed to use in regards to having an engine failure after rotation: Vx or Vy?

I have always been taught that Vy is the best speed, simply because it will get you to the safe turn-back altitude faster. Therefore, you minimize the "prayer-zone" on the take-off where you have no other choice than flying straight ahead and hope for the best.

Some very experienced fellow instructors of mine argue that Vx is more efficient because it will get you closer to the runway for a given altitude, and that it will therefore be easier to make the runway.


Please share your thoughts!

[Intruder]

A higher airspeed in the climb will give better forward visibility with the less nose-up attitude, and help keep the engine cooler. Vy will also get you to cruising altitude faster, reducing fuel burn.

While Vx might keep you marginally closer to the runway, it also gives you less margin of error when the engine does quit, therefore less time to get the nose down to resume glide speed. You might give up any advantage you had when you push over to regain that speed.

Use Vy, unless the airplane operating manual suggests something different.

[V1... Ooops]

I think that the answer to the question arises more from a 'pure logic' analysis than from an airmanship (aviation specific) analysis.

If you knew ahead of time that you were going to have an engine failure after takeoff - for example, 30 seconds, 60 seconds, or 5 minutes after takeoff - you would want to maximize the amount of energy you had in reserve prior to the engine failing. In an aircraft, we have two types of energy, kinetic (forward speed) and potential (altitude below us). If the engine is working, it provides a third source of energy.

It appears to me that climbing at best rate (Vy) would allow you to put more energy in the bank prior to your engine failure than climbing at Vx.

[aditya104]

Your only concern is about engine failure after take off, the probability of that is very less.
When flying at Vx, and any delay in recognising the engine failure will result in stalling of aircraft and the situation becomes venal.

If u climb at Vy, the chances of making a safe landing after turning back to the runway are higher than Vx(in this case the aircraft will tend to overshoot).

Last week I came across an engine failure of a SE aircraft turning crosswind, at my aerodrome. The pilot turned to the field and landed in the opposite direction, he was able to stop just at the end of the sealed runway. He was too high and had to do sideslip in order to get down. The normal procedure for that aircraft states to climb at Vy. IF they had climbed at Vx, surely they would have overshot.

[werbil]

Assuming no obstacles generally I'd go with Vy for a number of reasons:

1. Climbing at Vy will have you higher for a given time after take-off, increasing the area of the footprint that you can reach if the engine stops.
2. If the engine fails at Vx you will have either have to convert height to speed to get to Vg or accept whatever glide performance there is at the speed that you end up at.
3. Engine cooling

Turnbacks are only an option if trained to do them (is usually done for ASEPTA - SE turbine ops). In this case use the speed as per the training.

[low n' slow]

Another point to make in this discussion:

"Some very experienced fellow instructors of mine argue that Vx is more efficient..."

Just because someone is very experienced or "allways has done it that way" doesn't make it right. Especially in flying club environments, some nasty habbits may develop with senior club members without ever being corrected (perhaps because they themselves are the most respected instructors around...).

/LnS

[BelArgUSA]

Vx or Vy -

I fly my own L-21B (an ex-military Super Cub) 150 HP - O-360 engine.
My personal "policy" - really none applies - as I fly in the open country...
Vy initially, to 500 AGL, then Vx+10 max climb power or less.
If in urban areas - Vy to 800/1000 AGL (if no suitable fields straight ahead).
This in case a 180º turn to departure runway is considered.
Obviously - many options - consider your environment and obstacles.
xxx
Super Cubs 150 HP are sometimes operated at "J-3C65" power - Engine economy...?
Seen that note in old PA18-150 owner manuals.
Throttle set to 2200 RPM at takeoff for training.
I think it should not be recommended in urban areas with obstacles.
xxx

Happy contrails

[SeanGG]

Thanks for all the responses guys!

Some things are relevant to this discussion which has not been mentioned by any of you so far.

Normally, depending on atmospheric conditions, the safe altitude in a C-172 is around 500-600 feet. At 500-600 feet I am able to turn the airplane around and land on the opposite runway after a simulated engine failure.

As I already explained earlier, if I fly out at Vy, I will get to this altitude quicker, and therefore I will be "unsafe" a shorter period of time. If my engine fails exactly at safe altitude while climbing at Vy, a climb at Vx would not allow me to safely turn back to the runway. Therefore I would be forced to land straight ahead whereas I would be able to turn back if I would have climbed at Vy.

Some arguments my fellow instructors use to explain how Vx is better is to say that once you get to safe altitude Vx will make you closer to the runway. This is true, but the way I see it having an engine failure is not a function of altitude, but rather a function of time. Either way, unless you take off from a really short runway and with a tailwind, I am fairly certain that safe altitude at Vy will allow you to reach the runway after the turn back (assuming a C-172 or any other light single engine airplane).


Either way what I just wrote and all the other arguments for using Vy rather than Vx obviously makes Vy seem like the best choice.

Any further thoughts or comments to what I just explained, or would you mostly agree?

[Cardinal]

Vy, for many of the above reasons. Also, have you considered how long it would take to get the nose down after an engine failure? If climbing at Vx, airspeed will decay well below Vx before you react / pushover, burning lots of energy in the process.

[airfoilmod]

Question

First of all, Sean, if you're in a 172 taking off from a short strip with a tailwind, I don't think you are an instructor. Secondly, an engine failure at rotation makes Vy or Vx quite moot, you're landing straight ahead or you're an idiot. (All due respect). I climb at 70 in the 172, 90 in my Skylane.

AF

[SeanGG]

First of all, Sean, if you're in a 172 taking off from a short strip with a tailwind, I don't think you are an instructor. Secondly, an engine failure at rotation makes Vy or Vx quite moot, you're landing straight ahead or you're an idiot. (All due respect). I climb at 70 in the 172, 90 in my Skylane.

AF

I'm not sure how to interpret your post, but maybe you should re-read my last post, or perhaps the whole thread.

I have never said that I take off from short strips in tailwinds, neither are we talking about engine failures on rotation. We are talking about engine failures on climb out with no runway remaining.

EDIT: and by the way, one question for you AF:

Why do you cllimb out at 70 in your C172? I am assuming that is somewhere between Vx and Vy, correct?

[airfoilmod]

Sean

In post #8 you reference taking off on a short airstrip with a tailwind. Taking off on a short strip is fine, but with a tailwind?

70 is a nice speed for a tight pattern.

If you anticipate an engine out, why are you taking off? What does Rate or Angle have to do with "timing" an engine failure? If you want to be safe and use the runway you just departed as your emergency field, Vx. Plot the glide path for each, including a safe 180. Best glide speed will always favor proximity over altitude. (No Obstacles, Right?).

AF

[SeanGG]

In post #8 you reference taking off on a short airstrip with a tailwind. Taking off on a short strip is fine, but with a tailwind?

If you actually take the time to reread my post and use some common sense, you should be able to realize that the reference I made to the tailwind takeoff was to prove my point that taking off at Vy is superior to Vx simply because one would never take off from a short field in a tailwind, and therefore the scenario where Vx would be superior to Vy would be non-existant. I never said that I have ever taken off with a tailwind from a short strip.

If you anticipate an engine out, why are you taking off?

Obviously you missed the whole point of the thread again....

Let me explain myself (once again):

I do not anticipate an engine out on take off. However, as a professional pilot it is actually my job and also in my greatest interest to always be ready for the worst case scenario. That is why we are discussing what the best procedures are to have the best chance of survival if an engine failure were to happen.

What does Rate or Angle have to do with "timing" an engine failure?

I am not sure exactly what you mean by "timing" an engine failure, but the answer to your question is probably "nothing".

[SeanGG]

If you want to be safe and use the runway you just departed as your emergency field, Vx. Plot the glide path for each, including a safe 180. Best glide speed will always favor proximity over altitude. (No Obstacles, Right?).

There you go, that's what I am trying to discuss in this thread.

And I do disagree with what you say for the following reasons (which has been explained previously by myself and also others in this thread):

1. Vy should still be able to get you enough altitude to make it back even though you are further away (unless you have certain atmospheric conditions like strong tailwind on a short runway)

2. With Vx, you might not even reach the safe turn back altitude by the time you would with Vy, and therefore climbing with Vx might force you to have to land straight ahead instead of being able to safely make the turn

3. Vx makes you more susceptible to an inadvertent stall

4. Vy leaves you more both kinetic and potential energy

5. If you lose your engine at Vx, the altitude lost by increasing your speed to best glide speed will most likely equate to an altitude loss equal or greater to the advantage of being closer to the rwy vs Vy.

EDIT: and to answer your question: yes, we are assuming no obstacles for this discussion.

[FE Hoppy]

What do you think is best glide speed Sean?

I would imagine that would be C/L max. Which is also going to be Vx.

[bookworm]

What do you think is best glide speed Sean?

I would imagine that would be C/L max. Which is also going to be Vx.

Only in an engine with a powerplant that delivers constant thrust with changing airspeed. A C172 doesn't quite fit that description, unless you have the turbojet STC.

In practice, the difference in climb angle between Vx and Vy in a C172 is likely to be marginal. The likelihood of that making the difference between reaching the runway or not after a turnback is tiny. I suspect that the optimal policy for long-term safety management in an aeroplane you own is to climb at a speed that keeps your engine running at a moderate temperature, even if that's a little more than Vy

[SeanGG]

What do you think is best glide speed Sean?

I would imagine that would be C/L max. Which is also going to be Vx.

I really do not understand how C/L max would be the best glide speed in any airplane. The lift to drag ratio should be at a maximum (L/D max) to provide the best glide speed. At C/L max, even though parasite drag is low, induced drag is very high, and therefore this does not give you the most lift per amount of drag.

Only in an engine with a powerplant that delivers constant thrust with changing airspeed. A C172 doesn't quite fit that description, unless you have the turbojet STC.

What did you mean by this bookworm? That in these airplanes that you describe C/L max is the best glide speed, C/L max is Vx, or both like FE hoppy said?

[V1... Ooops]

Hello Sean:

Several posts up, you wrote

Normally, depending on atmospheric conditions, the safe altitude in a C-172 is around 500-600 feet. At 500-600 feet I am able to turn the airplane around and land on the opposite runway after a simulated engine failure.

I'm wondering if this premise - the idea of turning a single engine aircraft around to go back to the runway following an engine failure immediately after takeoff - is a bit of a red herring in this discussion, which originally started off as a question about whether it was best to use Vy or Vx after takeoff in a single engine aircraft.

I've already posted my thoughts about the choice of speed (I favour Vy), however, I'm a bit concerned about your perseverance with the 'turn back to the runway' manouver.

I have not flown singles for a long time, for that reason, I'm not up to date on what present 'best industry practice' suggests if you have an engine failure prior to reaching circuit altitude. I think, though, that you would have much better odds of carrying out an uneventful landing following an engine failure at low altitude if you elected to land the aircraft straight ahead or within perhaps 45° of the runway heading.

This is because the 180° turn required to get back to the reciprocal heading is going to use up quite a bit of your potential energy (the altitude beneath you), and when you roll out from the turn, on the reciprocal of the departure runway heading, you may not have enough altitude left to get you to the threshold - and if that is the case, your options, and your time remaining to choose an option will be very limited.

If, instead, you elect to land more or less straight ahead or ±45° from the runway heading, you will have a great deal more time to properly configure the aircraft, establish your desired glide (and, later, touchdown) speed, and fine-tune your choice of touchdown point as you descend. Obviously, you won't be landing on the runway, but I think you will have a much better chance of walking away from the aircraft without injury.

It's a question of priorities - is your greatest priority to avoid injury to the pilot and passengers, or is your greatest priority to avoid minor damage to the aircraft that might arise from an off-airport landing?

Obviously, my proposal won't make sense if you are taking off over water - something that is not unusual in Norway - but otherwise, I think that the 'land straight ahead' option is the safer choice, especially for a pilot with relatively limited experience on type.

Michael

[airfoilmod]

What

folks may have figured out by now is that at no speed after takeoff assuming a runway heading departure is it safe to return (in one piece) to the departure runway in this aircraft after OEI. You will always be further away than your glide speed (any) will be able to match the distance you need to return. Under power at any climb is always greater than glide relative to distance covered. Land straight ahead (or at some angle to RH) and walk away, God willing. Keep in mind the "turn back" isn't really a 180. It is more like a 270, considering the distance the turn circle carries you "further away" from the threshold of the reciprocal. Sean, if you insist, I would recommend that you at least try to stay as close to the Departure Field while gaining altitude if your fear is engine loss, perhaps stay in the pattern until you work up the courage to leave it for your destination. That would mean Vx and an immediate xwind pattern turn to stay in tight, while praying for all 4 (or six) to keep banging. I also think that planting in your students' brains the thought of turnback is dangerous.


AF

[SeanGG]

AF and V1, I understand your ways of thinking.

However, I have always been taught that one should make a turn back to the runway providing one has enough altitude to do so. There is a certain altitude at which you can safely make the turn back and land on the opposite runway.

You will always be further away than your glide speed (any) will be able to match the distance you need to return.

This is probably correct, but remember that when you take off (C-172) you take off at around 1000 feet, and lets say the rwy is 6000 feet, you still have another 5000 feet left + overrun area (if applicable) which will make it easy to make the runway on the way back. The main concern is of course that making the turn back to the runway makes us lose a lot of altitude. This is exactly why one should be familiar with the safe turn back altitude of the airplane.

I have practiced a lot of turn backs from 500-600 feet, and unless the atmospheric conditions are very unfavorable it is no problem getting back to the runway. Just immediately roll in to a 35-45 degree bank into the wind and keep the speed up and one should be able to land safely on the opposite runway. Of course one must always consider the atmospheric conditions and adjust the safe altitude accordingly.

But yes, I have done this countless of times and if I know that I can safely make the runway, why would I ever keep going straight ahead? The take-off emergency briefing I was taught always to do by my instructor in the C-152/172 is as follows:

1. If engine failure on take off roll - throttle to idle, brakes apply
2. If engine failure after rotation with remaining runway - full flaps, land on remaining runway
3. If engine failure after rotation with no remaining runway below safe altitude - full throttle, land straight ahead or slightly to the sides
4. If engine failure after rotation with no remaining runway above safe altitude X feet, immediately turn left/right (into the wind), pitch down and land on opposite runway


In airplanes that don't glide too well (like our school's piper arrow) there is no safe altitude. How do I know this? Well, I've experimented with it. It sinks like a rock, so even though you make the turn back you lose so much altitude you won't make it back to the runway. It's all about knowing your airplane.