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Max glide vs. min sink speed

I’ve wondering about this for some time…

Why do we instruct best glide speed in case of an engine failure? At the altitudes most VFR private pilots fly, it is very unlikely that the difference between some airbitrary glide speed and the best glide speed will make the difference between making some decent landing spot and not making it.

I think that on most occasions, people are short of time (to sort out all the things to be done in case of an engine failure) rather than distance. Time is usually more valuable than distance. So why don’t we instruct to use minimum sink speed? It appears to me best glide is just one of many things we do in training to get across a theoretical principle, but which has very little significance in practice…

In fact, many instructors, during an engine failure exercise, want you to establish best glide speed, but then a second later they suggest to look for a field close-by, and to avoid choosing one which might be too far away. Doesn’t really make much sense.

It might make a difference for a Malibu flying at FL250, but that’s not the target during PPL training.

I will also add that after an engine failure at say 2000 feet AGL, all precision will be thrown out the window anyway, and the last thing one would care about is whether the speed is 5 knots lower or higher than any specific V-speed. One would just want to make sure it’s a reasonably safe speed and then concentrate on actions and decisions..

Mainz (EDFZ) & Egelsbach (EDFE), Germany

Why do we instruct best glide speed in case of an engine failure?

Do we?

mh
Aufwind GmbH
EKPB, Germany

While it may indeed be infrequent to have the only decent landing spot at the very limit of possible gliding distance, I certainly would actively discourage gliding at the minimum sink rate speed, as it is dangerously close to a stall – in fact, it actually is the very beginning of the stall. If you are flying straight at the minimum sink rate, entering a turn without increasing the speed will get you into a real stall. In fact, even at best glide speed, it is advisable to go faster even in 30° bank turns. I remember being reprimanded by the examiner for not doing that. Aside from that, being startled by an engine failure, an inexperienced pilot may be instinctively inclined to go as slow as possible, and having “best glide speed” hammered into his brain serves to counteract that.

Last Edited by Ultranomad at 14 Dec 21:16
LKBU (near Prague), Czech Republic

What I remember from my gliding days is that you lose much more in glide ratio by flying slower than Vbg compared to flying the same amount faster than Vbg. (And that is true of all aircraft.) If you have to fly with a headwind the effect is even more noticeable – you should speed up to get the best glide ratio relative to the ground. So as long as you’re trying to reach a field you should not slow down below Vbg. If you’re simply trying to sort things out then it makes sense to fly at minimum sink.

ESKC (Uppsala/Sundbro), Sweden

Exactly like Airborne Again said. I found this interesting:
http://www.ifr-magazine.com/issues/33_2/features/Best-Glide-Speeds_1145-1.html

always learning
LO__, Austria

Do we?

Yes.

I took “we” as “basically all the instructors/examiners I have ever flown simulated engine failures with”.

I certainly would actively discourage gliding at the minimum sink rate speed, as it is dangerously close to a stall

Well…, about 20 knots for a few common SEPs that spring to mind. I wouldn’t really call that “dangerously close to a stall”. Otherwise we shouldn’t ever do a Vx climb…

an inexperienced pilot may be instinctively inclined to go as slow as possible, and having “best glide speed” hammered into his brain serves to counteract that.

Sure, and of course I thought about that. But then it’s indeed more about the increased stall margin due to the higher speed, rather than the increased glide distance. So I’d say it’s reasonable to do this, but then one shouldn’t say to do it to get “best glide speed” but “a safe glide speed”.

So as long as you’re trying to reach a field you should not slow down below Vbg. If you’re simply trying to sort things out then it makes sense to fly at minimum sink.

Of course. And my thesis is that in a typical engine failure scenario at say 2000 feet AGL, it is likely more beneficial for the outcome to have a bit more time than having a bit more glide distance. But even this guy says that “Most often you want to maximize the distance you can glide” which I don’t agree with. In order to get where?

Mainz (EDFZ) & Egelsbach (EDFE), Germany

I just trim for best glide speed because someone, usually a better pilot than me has tested it and written it in the aircraft manual. It is one less thing to think about or calculate if the only engine fails.

France

Agreed. It’s a number, just like Vx vs. Vy. If we try to think too hard about these things, we’ll be subtracting 1.456238 kts from Vs for being 23.6251kg below MTOW.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

Fly min dink untill you find a field then fly best glide? None will not matter if you dont know the wind?

The only advantage of best glide is being far from stall under stress….one can just fly VA=1.3×VS0 as rule of thumb on min sink/best glide

I stopped flying Vx after hearing the sound of the stall horn one day ;)

Last Edited by Ibra at 14 Dec 23:16
Paris/Essex, France/UK, United Kingdom

boscomantico wrote:

Well…, about 20 knots for a few common SEPs that spring to mind. I wouldn’t really call that “dangerously close to a stall”. Otherwise we shouldn’t ever do a Vx climb…

The Vx–Vs0 margin varies very significantly from type to type. For Cessnas it’s significant indeed, but it’s 4 to 10 knots on the Socata TB series, 5.4 knots on a Socata Rallye 100, 9 knots on PA-38, etc.

Last Edited by Ultranomad at 15 Dec 01:21
LKBU (near Prague), Czech Republic
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