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Power off landings - glide slowly, or glide fast?

dublinpilot wrote:

I don’t follow the logic that flying slower than that commits you to a crash.

To land an aircraft damage free, will require accelerating it away from earth as you approach the ground. You were going down, now you’d like to stop going down, and fly parallel to earth, so the airplane settles onto the runway with the least bumping. The necessary upward acceleration will require energy being applied to the airplane. This energy most commonly comes from a well executed flare, during which some airspeed is traded for the required upward acceleration, and the descent stops just above the runway. Engine power may play a part in this, but does not need to, every aircraft can be landed power off.

So it’s necessary to have some energy stored, so you can spend it to flare (accelerate upward a little, to arrest the descent). for an airplane with no engine power, it can be only be stored as excess airspeed. (In a helicopter that energy can be stored as excess speed, and rotor RPM, but that’s another story). If you arrive to the top of the flare with no excess airspeed – flying slower than “best glide speed” or just a little faster than stall speed for that configuration, if you pull for some upward acceleration to arrest the descent rate, the plane will stall, and just keep going down with a thump.

For most GA aircraft (not intending to apply to jet fighters, nor sailplanes) even approaching the top of the flare power off, at the stated “best glide speed” will require some skill in correctly judging the flare. If you want some insurance, enter the top of the flare with a few extra knots, to allow yourself just a little more stored energy. You’ll find that even a high wing loading plane can be kissed onto the runway, if you’ve stored some extra airspeed with which ti finesse a landing. If you find you’ve stored too much speed, no problem, that can be easily slipped off.

Home runway, in central Ontario, Canada, Canada

I’m confused, on the DA40NG, for instance, the approach speed (flaps LDG) is ~75 kts (stall ~60). Vbg is 88 kts. Are you suggesting that one should approach at Vbg or faster? Even with full flaps, approaching slightly fast already makes you a very long floating trip on the runway.

Noe wrote:

I’m confused, on the DA40NG, for instance, the approach speed (flaps LDG) is ~75 kts (stall ~60). Vbg is 88 kts. Are you suggesting that one should approach at Vbg or faster? Even with full flaps, approaching slightly fast already makes you a very long floating trip on the runway.

I think you will find that the Vapp you quote is for full flap, whereas the Vbg you quote is clean configuration.

I would think that Vbg WITH FULL FLAP is very much closer to the Vapp speed you mention.

Darley Moor, Gamston (UK)

Yes, it was full flaps but didn’t have at hand the one with no flaps. No flaps Vref is ~82 at higher weights and ~72 at lower weights. I don’t have a figure for Vbg with flaps but that’s understandable, if you want to glide well, you remove the drag.

While I understand the argument for keeping some kinetic energy in reserve, and therefore expending a bit of potential energy to achieve that has some merit, but there is also an argument to say that the longer you can stay in the air, the better the chance of:

  1. TDODAR in general
  2. Fixing the problem and climbing away
  3. Contacting rescue services
  4. Giving rescue services an accurate position and likely position on crashing
  5. The rescue services setting off to get to you the minimum time after the crash
  6. Picking a good place to crash
  7. Configuring the cabin for the crash
  8. Briefing and preparing pax for the crash
  9. Getting ELTs and PLBs bleating
  10. Preparing rafts
  11. Thinking
EGKB Biggin Hill

Pilot_DAR wrote:

If you arrive to the top of the flare with no excess airspeed – flying slower than “best glide speed” or just a little faster than stall speed for that configuration, if you pull for some upward acceleration to arrest the descent rate, the plane will stall, and just keep going down with a thump.

Tell that to my aircraft. Best glide speed is 50 mph indicated, but I can flare and land softly from a power off approach at 40 mph indicated.

Last Edited by alioth at 21 Feb 16:24
Andreas IOM

To avoid usual complaints about obscure abbreviations I have added TDODAR here. That said, I have never heard of it in my 48 years in the UK. Why use it?

Administrator
Shoreham EGKA, United Kingdom

Presumably if you are gliding from FL200 to sea level, the speed doesn’t really matter (except to buy more time in the air, etc, by descending slower) until you are nearly there.

One extreme case was a UK aerobatic champion whose mag article I read a few years ago and who said that if he got stuck on top of an overcast in his Extra, he would flat-spin it all the way down

Administrator
Shoreham EGKA, United Kingdom

Peter,

TDODAR is grist to the mill of professional flying and is gaining traction in GA and private flying. It is used in most airlines, including BA. I teach it on my seminars and when training. I am surprised that it is considered obscure.

It is a bit of a shame that the reference you chose was to a non-aviation website, with an article title of “think like a pilot.” Um, we are pilots, aren’t we?

But let us not argue about whether the acronym can be used on your forum. It is more important that people who don’t know it have a think about it. Essentially, it gets you to slow down and think what the problem might be.

The most recent time I have used it was on Saturday, three days ago. I was instructing in an Arrow flying back to the UK from Calais when, just before mid-Channel (isn’t it always mid-Channel!?) there was a sudden onset quite noticeable vibration.

It would have been very easy to have reduced power, put out a Pan-Pan on 121.5 (which we were on anyway) and pointed at the nearest land.

But instead I talked the owner/student through TDODAR, we diagnosed that the engine was OK and that the most likely problem was a slightly dropped nose gear, so proceeded to feet-dry and then cycled the gear, whereupon the problem went away.

That is a minor example, but shows the real life advantage of slowing you responses down to a known algorithm, like TDODAR.

EGKB Biggin Hill

Timothy do feel free to post a better link in the Abbreviations thread. I have no agenda; I just posted the one I found on google. We have 1 or 2 individuals here who enjoy complaining about abbreviations… Also please note that EuroGA is not “my” forum; it is an community of over 3000 members (plus another 1-1.5k/day who just read it) and I am a mod/admin here (one of 2) and do a job which is occassionally made quite difficult.

Administrator
Shoreham EGKA, United Kingdom
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