Max glide vs. min sink speed

Ibra wrote:

The only advantage of best glide is being far from stall under stress

Which is a very good thing. You have to maneuver as well. The idea is to get the airplane in a position so you have a small final at the right alt to hit your chosen touch down point. I don’t see how flying at minimum sink will help you doing that. Often there are thermals for instance, and down winds. These things will have a much larger effect on the total time in the air. You minimize the effect of this by flying faster, not slower.

mh wrote:

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

I don’t. I’ll look to the “best glide” speed published in the flight manual as a minimum speed. I will then look at the stated speed to achieve shortly after takeoff, and if it’s faster, I’ll usually choose that speed for a forced approach glide. Now if I’m way high, and want to make a good distance, I will aim for “best glide” speed for a while, and then likely speed up a little for the final phase of the forced approach. Doing that will afford some cause check time, which twice has saved me from a forced landing, but there may come a point where the landing is a certainty, I want the stored energy.

When you think about it, flaring is accelerating away from your downward descent path, hopefully to fly parallel to the surface for a few well timed seconds. Accelerating requires adding energy, if you’re gliding, the only energy you have to add, to flare, is what you’re carrying as excess speed. If you’re not carrying that excess energy when you flare, you’re not going to accelerate away from your descent.

Many events of gliding power planes has taught me that yes, you can glide at the [slower] speed nicely, but you may have a terrifyingly short interval in which to flare. if you get it wrong, you’re going to stall high. This became glaringly clear to me which doing approval test flying on a modified Cessna Grand Caravan. I was required as an element of the approval process to demonstrate a land back after engine failure from 50 feet. At the boot after takeoff climb speed of 87 KIAS, I could just do it. At the desired climb speed of 80 KIAS, I could not, and several times darned near wrecked the plane trying. While flight testing a C 182 amphibian with many mods, including a gross weight increase, I found that although I could glide at 60 to 65 KIAS, there was nothing left to flare with. When I glided at 80 KIAS, I cold kiss the plane onto the runway or water every time. I wrote the flight manual supplement to state an 80 KIAS glide – just don’t be looking to the far horizon for your landing site!

I’m not normally an advocate of flying a too fast approach, but if you’re doing an actual forced landing, I do. If you’re going to mis judge something during an emergency power off landing in a power plane, I would rather it be too fast, and take you off the far end of your selected forced landing area, rather than too slow, and you crash short, or drop in stalled – better hitting something at the far end at 20 knots, than at the near end at 60 knots!

I teach to seek a close area which is suitable, and work the plane into it, slipping if needed, a little fast if needed, rather than to hope to “make” the farther spot in a steady “best” glide. A mentor of mine taught to look close around, and see where you might best crash, go there, and don’t crash when you get there. He told me of having flown a successful power off landing in a Hawker Harrier! Any pilot who can achieve that has my ear! I’ve only had four full on, actual forced landings, and happily, I have not hurt a plane yet doing one!

I don’t think instructors slavishly instruct to fly Vbg. At the very least the instructors I’ve had start by telling you that your typical cruise speed is significantly faster than Vbg so in order to maximize both glide range (which you need to give yourself maximum choice in fields) and to maximize time aloft (to troubleshoot the problem) you need to reduce your speed. To what speed? Well, Vbg is a published speed and as good as any. So pitch up, trim for Vbg and then forget about it. You are now at a much better speed than cruise speed, and a fair margin from the stall. This gives you time, distance and mental capacity to focus on the rest.

One instructor went even further. In a PA28, from cruise to Vbg requires two turns of the trim wheel. So he told me to not even look at the ASI, but simply trim two turns up and start handling the emergency. As long as you don’t do anything wild with the yoke the aircraft will settle on something near to Vbg automatically.

None of the instructors I’ve had have made any comment about a 5 knot deviation from Vbg. As long as I pitched up and trimmed for something close to that speed, and then focused on handling the emergency, they were happy. Unless I slowed down significantly below Vbg, and then the comments were about the impending stall, and not the reduced glide performance.

Have to add that in NL we mostly fly at 1000’ due to airspace restrictions. So the focus has to be about finding a field and setting up the approach to that field. Actually finding the cause of the engine failure and solving it is not the first priority at that altitude. But I appreciate that if your engine failure happens at FL100 your priorities lie differently.

If you fly bellow 2500ft, you don’t have to look for fields, you should have few in mind along the route well before the fan stops (that goes into planing the route to start with), this is easy in the local area but tends to be difficult when you are flying in unfamiliar places

Yes, agree the whole theory of best glide, min sink does not make much difference on 1/6 glide ratios or 3min flying time, also once you have a landing spot in mind that is accessible you should fly on “constant aspect” to land which work on any speed….

Here is another scenario to consider …….

Best glide on my aircraft is 90kt.

Class rating instructor at Rotterdam says ….. OK let’s say we have a 1500ft cloud base …. you’ve reached the area that you want to be in ….. what speed now?

So I say 90kt.

He says that coming out of a 1500 ft cloud base would it not be better to carry a bit more speed, say 130 kt which gives you much more options?

I would have to agree.

Obviously this wouldn’t work in mountainous areas.

In an SEP, fly fast enough to keep the prop turning. I’ve never done a forced landing, but have lost and regained power several times. With a windmilling prop, there’s an instant response as you fault trace.

For the very rare situation of the failure of the only engine that puts the pilot under tremendous stress and therefore limited ability to take rational decisions it is good to have one glide speed on the emergency checklist.
If you have more than one speed on such a list, most pilots are likely to randomly select one rather than making a rational choice in such a stress situation.

In a single given situation, best glide might not be the best speed you can chose – but for many of the reasons in prior posts if you have to find one speed that comes closest to a “one size fits all”, best glide is not a bad choice.,

BackPacker wrote:

As long as I pitched up and trimmed for something close to that speed, and then focused on handling the emergency, they were happy. Unless I slowed down significantly below Vbg, and then the comments were about the impending stall, and not the reduced glide performance.

If you are gliding slowly such that impending stall is a risk, your speed is well below the speed which will allow you to arrest your descent rate, and hold off in a flare to touch down like a landing. One of the things I see is pilots flying Cessnas which have had STOL kits installed. This mod allows a higher angle of attack, and slower stall speed, and the airplane can be glided more slowly. But (and I’ve done it), you can glide at that slower speed, amusing yourself at how well your delaying your arrival to earth, to find that when you pull to flare, there’s just nothing left. You pull, the aircraft slows immediately, and continues to settle stalled. The arrival will be a big thump.

it is very important, that pilots practice full landings from at least 500 feet up, with no power. Listen to my words, and fly your first few a little fast. land well as you normally would. Then start flying the identical approach more slowly, knowing that at some point, flaring will require an application of power to aid lift to arrest the descent. When you have to do that, you were gliding too slowly on approach. I’m not saying what the speeds are, I’m saying that the best way for a pilot to know is to learn by doing, practicing faster first. The “best glide” speed published in the flight manual is more likely the speed to fly for the greatest distance covered per altitude lost – hence the chart which often accompanies this stated speed. So try to land power off from an approach at that speed. It’s obviously possible, or the manufacturer would not have published it, but does it leave you the margin of safety for a nice flare that you expect? Ont thing to say about the well flapped Cessnas is that if you glide at “best glide speed” flaps up for distance, and then maintain that speed as you extend flaps, you will create the margin you would like to have to flare, but timing will be important! A Cessna gliding with full flaps slows down very quickly when you pull to flare!

…the Antonov AN2 has an emergency landing in IMC technique of full flaps, minimum sink and apparently the undercarriage/airframe is strong enough you can walk away. Probably also works for the Rallye tin parachute.

Agree that a simple best glide speed (adjusted for headwind) provides a sensible compromise of maintaining energy in the flare. Minimum sink is high up on the induced drag curve, and as stated, close to the stall speed. Vx is nearby, but I believe for most propeller types it is around 3-4 knots faster.

For a constant speed SEP propeller type going to full coarse will have a dramatic effect on gliding range.