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Gliding, but more precisely, Unintended power off flight

Another fellow aviator, and two of his three passengers have left us, the result of a crash, which speculation is proposing may be associated with an engine failure at night. There are no facts presented yet. This pilot was amply capable on this aircraft type. That's enough about this very unfortunent event.

It has me reminding myself about the skills of powerless flight. The type of aircraft in my mind now is relativelty high wing loading, though glides and handles very nicely, as long as you keep the speed up. Being an older type, the Flight Manual does not go into a lot of detail about gliding techniques.

Training teaches us to get the nose up, trim to the "best" gliding speed, and select a suitable landing place. As inspired by John Farley's book, and his excellent discussion on gliding, I think that there might be more than one "best" speed.

The common "best" speed seems to be the speed which will get you the farthest from your altitude. That sounds nice, but behind that is the reality that you are gliding with less momentum. Now I'm not the guy who advocates faster than specified approach speeds for aircraft which are functioning normally, but I do advocate it for emergency gliding.

This came up during some mentoring flying I was doing with a newer pilot recently. I posed to him: Would you rather undershoot or overshoot your forced approach, assuming what you would hit is the same at either end. After great consideration he did say overshoot, but he did not seem to have a reason.

I agreed, and told him that I would rather go off the far end and through the hedge with a few knots I could not get rid off during a ground roll, rather than go through the near hedge at flying speed.

Thus, I will fly emergency glides faster, and risk going off the far end, so I retain the added momentum with which I can overcome a last minute downdraft, or other surprize. This becomes particularly true for aircraft which are equipped with STOL kits. They are capable of slower flight, and thus a slower glide, but when you pull to flare from an abnormally slow power off approach, there is nothing left, and you thump into the ground.

It seems this happend to a German military transport the other day....

Do other pilots practice their glides at the published speed, and aim for the far landing spot, or do they carry the extra speed, and select the nearer place, when conditions appear equal at either?

Home runway, in central Ontario, Canada

If they crashed at night, then perhaps choosing the right speed would not have made much difference.

Otherwise, opinions are bound to vary. On an engine failure, I would aim to reach a suitable group of fields.

If I had a failure at night and could see nothing at all, then I would go full flap, trim for lowest speed and hope for the best... not a good scenario, which is why I rarely fly at night (the other reason being that almost no GA-usable UK airport is open very much after dark )

Administrator
Shoreham EGKA, United Kingdom

It's often struck me that a single best-glide speed is a simplification if there's any significant wind. If you're flying a hang-glider into 20 knots of wind, you get much further if you fly at 40 knots rather than 25, and if you're flying downwind you get further if you fly closer to min-sink than best-glide. This is less significant for powered flying as few of us do enough gliding in a C152 to instinctively pick the best speed. Also, the ratio of max-glide to windspeed tends to be higher in powered aircraft. However, it does strike me that if you fly at a higher speed your glide angle relative to the ground will be less dependent on weather conditions, and that the difference between engine-off glide and engine-idle glide will be smaller, meaning that performance during pfls will be more representative of what happens during actual forced landings. Ability to judge wind direction will be reduced at higher speeds, but perhaps you'd also make pilots who suddenly find they need to stretch a glide that bit less likely to suffer a stall-spin accident.

I generally try to retain an excess of energy, even during normal circuits, in case I need to deal with an engine failure. After all, an idling engine is reasonably likely to develop issues such as carb icing and sideslipping is fun. However, I still intend to land on the runway, reasonably close to my intended touchdown spot. I always think of this as good practice for any approach. I normally do this by flying my approaches high rather than fast, though, and would hardly call this an 'overshoot'.

I've always thought that max-glide was simply the speed that got you the furthest and therefore was likely to give you the most options. Assuming it's close to min-sink it also gives you more time to radio a mayday call. I never thought of it as the best speed to fly all parts of the forced landing though, particularly at night.

EGCW

Absolutely agree.

Vbg is relevant only in still air and only if you actually need to glide as far as possible e.g. over water to reach a coastline.

Over land, one would usually be aiming for some specific field.

Not that I have ever done it myself, thankfully

If you want a fun debate, consider an engine failure above an overcast. Should you glide "fast" so that when you break out below you have the most options, or should you glide "slow" so that if you don't break out you have a (slightly) improved chance of survival?

Administrator
Shoreham EGKA, United Kingdom

When practicing 'engine failures' most instructors cut power with the throttle. One I have been with cuts with the mixture, so there is no residual tickover power in the engine. It makes a HUGE difference having a great big brake up the front.

I solo'd in a glider, to experience no power/glidespeed and having to get it right first time. Not quite the same but a good experience.

I would have thought an engine failure at night is a complete lottery, apart from aiming at the dark bits is there any other strategy than making contact with whatever you end up hitting to be as slow as possible?

I agree that too much emphasis is put in training on flying at Vbg. If you have an engine failure, your aim is not to break a gliding record. Your aim is to arrive on the ground safely, and preferably so that the airframe can be used again. The exceptions to this would be a glide to reach land, or Peters consideration of an engine failure above an overcast or in IMC. But in those situations you are generally flying relatively high and that gives you time to think, and to trim the aircraft properly.

But in a normal VFR (relatively low level) engine failure, the speed at which you glide should be dedicated by safety interest first, and will lie somewhere between your normal cruise speed and Vbg. But the exact speed is not all that important. Looking out for a suitable field, and flying an approach that allows you to reach that field with energy to spare (so you can use flaps, sideslip, S-turns and whatnot in the final stages of the approach) is far more important.

One of my instructors got that idea right. Her matra (in a PA28) was simple: From the cruise setting, turn two notches backwards on the trim and let the aircraft settle on that new speed. That will give you an excellent compromise speed with very minimal effort, and without having to look in.

(And furthermore, as a licensed glider pilot, I agree that there is no single Vbg. Your best glide speed depends both on the wind and the up/downdraughts you experience and expect, and even on the actual aircraft weight. There's a lot of knowledge about this in the gliding world, and it will be encoded in things like polar diagrams, McCready rings and such.)

To get minimum descent rate with a windmilling prop you need to fully open the throttle and pull the prop lever to full coarse, it makes a big difference.

If you want a fun debate, consider an engine failure above an overcast. Should you glide "fast" so that when you break out below you have the most options, or should you glide "slow" so that if you don't break out you have a (slightly) improved chance of survival?

One for the physicists to correct if required:

potential energy = mass x height x g

kinetic energy = 1/2 x m x v^2

70 knots = 36 metres per second 100 knots = 51 metres per seecond

(1/2 x v^2) / g = height

at 70 knots, you have the same energy as if you were dropped from 64 metres at 100 knots, you have the same energy as if you were dropped from 130 metres

The difference is 66 metres, or about 200 feet. So if you come out of the cloud at 1000 feet at 100 knots rather than 70, you have approximately the equivalent of 200 feet of extra altitude with which to find a landing place. How well you can use this extra energy will depend on the plane and pilot.

It was something I was considering the other day, flying along the coast at about 700 feet altitude, and 500 feet away; parameters chosen in order to remain legal. I kept my speed up so that if I did have an engine failure, I could turn in and have enough energy to turn and land on the fields just past the coastline. There were very few seagulls about, which I guess is the other consideration there. It felt as if the coast was within reach, though I'm tempted to do some tests at altitude.

EGCW

200ft is quite a lot

Actually I was thinking of descending at say 150kt, which would give you a whole load more options - providing you do actually become visual at all

Doesn't sound very wise, unless you have a very definite idea of the cloudbase, and if the cloudbase is high then there is little point in having too much speed anyway.

Administrator
Shoreham EGKA, United Kingdom

Didn't you invite the Kat to the forum, Peter? He has an interesting tale to tell on engine failure at night.

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