Making Sense of Best Glide and Glide Ratio

[Jetpilot86]

Had a crankshaft snap on an A36 I flew Part 135 in over 30 years ago.  As a result, the prop flat pitched at high RPM. IIRC, I was 8 miles from an airport at 4000 feet. I barely covered 4 miles and managed to land on an emergency grass strip for crop dusters instead. 

[Hank]

3 hours ago, Jetpilot86 said:

Had a crankshaft snap on an A36 I flew Part 135 in over 30 years ago.  As a result, the prop flat pitched at high RPM. IIRC, I was 8 miles from an airport at 4000 feet. I barely covered 4 miles and managed to land on an emergency grass strip for crop dusters instead. 

Finally! A real example, with numbers, of "glides like a sewer lid"!

[A64Pilot]

11 hours ago, Jetpilot86 said:

Had a crankshaft snap on an A36 I flew Part 135 in over 30 years ago.  As a result, the prop flat pitched at high RPM. IIRC, I was 8 miles from an airport at 4000 feet. I barely covered 4 miles and managed to land on an emergency grass strip for crop dusters instead. 

I had a similar experience with the Prototype S2R-H80 coming back from Oshkosh at 9500 ft when the Gas Generator let go, as in blew apart. The Avia 106” three blade is a dual acting prop, which means it takes oil pressure to both increase and decrease pitch, unlike a Hartzell that a spring drives it to feather, so if oil pressure is lost it goes to feather.

It had a Walter E-11 engine as the GE was in Certification and the Walter was identical in form, fit and function.

So as the oil pump is driven off of the compressor section my prop stuck at cruise pitch. I called Grand Strand approach I think it was and declared an Emergency and asked where the nearest airport was, he said you just over flew it.

I was able to do a 180 from 9,500 ft, fly one mile, enter downwind and land, even made the turn off at mid field.

So rate of descent of I guess over 3,000 FPM? Anyone who has flown turbines knows if you pull one to idle the prop goes so flat it’s a hard decel, and a Crop Duster with a 106” prop is worse.

Disassembled it, put it on a truck and shipped it home.

 

Edited February 27 by A64Pilot

[jlunseth]

@A64 In response to your climb rate note, there was a really good article in AOPA magazine a few years ago. The writer actually went out with a safety pilot and did some tests to determine when and if the impossible turn might work. According to their results - and if I recall the article correctly they had some other results on climb rate also- neither Vx nor Vy are very good climb rates immediately after takeoff. Vx is too nose high and in their testing, stall came too fast. Normal pilots, not expecting the engine to quit, would generally react too slowly to avoid a stall. Vy resulted in the plane getting too far away from the runway, so the impossible turn would not work out. What they landed on was a speed about half way between Vx and Vy, which works out to, guess what? Best Glide. So I always do my initial climb at 85 kts., which if you look at the Best Glide chart in my POH is in that range. I am a little inconsistent, if I am trying to stretch out a glide I generally use 82 kts. but when I am climbing out post takeoff I use 85, which gives me a little better cushion if the engine were to fail. In their testing for the article they required the pilot flying to wait for, as I recall, 3 seconds before "realizing" the engine had quit and pushing the nose over. They found that the pilot had the time to respond if the climb rate was at Best Glide, and even better, the plane was not so far from the airport that it was no longer possible to return. 

Sometimes I use a faster climb speed when I am out in a rural area where there are endless areas to land in the event of an engine failure, but my home base, KFCM, is pretty well hemmed in so whatever I can do to get to an altitude where a return to the airport is possible, that is the best choice.

[Hank]

27 minutes ago, jlunseth said:

AOPA magazine a few years ago . . . neither Vx nor Vy are very good climb rates immediately after takeoff. Vx is too nose high and in their testing, stall came too fast. Normal pilots, not expecting the engine to quit, would generally react too slowly to avoid a stall. Vy resulted in the plane getting too far away from the runway, so the impossible turn would not work out. What they landed on was a speed about half way between Vx and Vy, which works out to, guess what? Best Glide.

As I learned early in my engineering career, this is a "case by case" decision, Best thought through in advance, on the ground. 

But the summation "halfway between Vx and Vy works out to Best Glide" is not always accurate.

For my C:

• Vx = 85 mph 
• Vy = 100 mph - Altitude 
• Vbg = 105 mph 
• Halfway between Vx and Vy ~ 92 mph, a much steeper climb than Best Glide would produce.

So I routinely climb at Vx to clear obstacles then lower the nose to Vy to altitude. But Vbg is always there in the back of my head if something happens. 

Many people here recommend a Cruise Climb around 120 mph for engine cooling.

[jlunseth]

1 hour ago, Hank said:

Many people here recommend a Cruise Climb around 120 mph for engine cooling.

Just to be clear, the climb speed I was referring to is intended to get you to 1,000 feet AGL, or whatever altitude in that vicinity allows the pilot to make the 270 degree turn to return to the runway. One of the things they did in creating that AOPA article was to practice the impossible turn, at a safe altitude, in order to determine how much AGL altitude the particular aircraft requires to make the turn, and it is a 270 turn to get back.

I use 1,000 AGL.

[PT20J]

I did a lot of practicing the "impossible" turn in a sim and then verified my findings at the airport. With no wind, I can just barely make it back at 800' AGL using my normal takeoff sequence where I lift off at about 65 KIAS, pitch to 7 deg, raise the gear at positive rate, raise the flaps passing 80 KIAS. Wind is the biggest variable. 1000' is more comfortable. 

[A64Pilot]

If you practice the maneuver be sure to “freeze” the controls for say 2 sec after pulling the power as even Superman has a reaction time.

Secondly if you just barely make it, it’s unlikely you really would due to a real windmilling prop sucks a lot of energy where an idling engine won’t or sucks a lot less anyway, it may even provide a bit of thrust, reason so many have the idle set so low to help prevent float. 

Problem with climbing at best glide is that if the engine quits, you will be below best glide by the time you react you have lost speed.

Personally I have a field in mind just to the left of a house on one end and just a field on the other end. Years  ago the house had a V tail crash in their front yard and the two inside burned, so I’m sure they don’t want another crashed airplane, so I don’t overfly their house.

Very often if an engine quits soon after take off the pilot stalls and of course lose control often being severely injured or worse. I just accept that the impossible turn is just that and hang my hat on that most forced landings in fields etc the people walk away.

Edited February 28 by A64Pilot

[Pinecone]

AOPA Impossible Turn Video

 

 

[Geoff]

On 2/23/2025 at 8:15 PM, Max Clark said:

 

The Hartzel 3 blade prop drops “approximately 7%” on glide ratio - but trying to figure out what the starting number is based on that image…

Keep in mind that if the reason you are gliding is the engine has stopped without oil pressure in the governor the prop will go to full fine pitch and that 7% improvement may not be available to you.  

I suggest being conservative on glide distances.  The whole idea is to make it to your selected landing location at an altitude that allows a normal stable approach with a minimum speed touchdown (if off field).

This can be done by setting the "glide ring" on your avionics and test flying it at various weights.  In cruise put yourself with an airport just inside the ring, pull the engine (prop full fine) and see if you can glide and arrive at the first key position at 1,000 ft above the field.  If you make it with extra altitude enlarge the ring (slightly) and try again.

If you are super confident, try and land from that position engine out.  Always a good idea to take an instructor along for this part.

 

[Max Clark]

I had an interesting discussion about this recently. I pointed out how the POH had Vbg at different weights (80 - 91.5 KIAS), and the emergency procedures specified 85 KIAS so wouldn't that be a better number to sear in my mind.

Their point was caring more about stall speed vs angle of bank and making sure I was always at an airspeed above a stall.

The more I've thought about it, the more happy I am for adding an AOA to the plane.

 

[PT20J]

1 hour ago, Max Clark said:

I had an interesting discussion about this recently. I pointed out how the POH had Vbg at different weights (80 - 91.5 KIAS), and the emergency procedures specified 85 KIAS so wouldn't that be a better number to sear in my mind.

I believe you are exactly right: The purpose of the speed listed in the emergency checklist is to give you a go to speed to remember (you are not going to be reading that checklist or referring to glide charts if the engine quits shortly after takeoff for example). The GAMA Spec 1 for POH states that in addition to the checklists, the following information be included:

3.9(e) Glide
Procedures and information shall be provided for a
gliding descent. including:
(1) The Recommended Airspeed
(2) The Associated Configuration
(3) The distance(s) from (a) specified height(s)
above ground level that an airplane will glide, or
the glide ratio in nautical miles per thousand
feet.

The drag curve has a fairly broad minimum, so a few knots either way doesn't make a lot of difference, but if you have time (altitude) is seems optimizing the speed might be worthwhile. Maneuvering at low altitude, I would be more concerned about getting too slow and stalling.

There was some blank space on my new panel and I had tables of best glide weights/airspeeds and maneuvering weights/airspeeds silkscreened on the panel for reference.

[Geoff]

Granted it varies with weight, I have always used 85 KIAS in the long bodies as the "memory" Vbg.  85 KIAS is also what I use for Vx in the long bodies it's one less number to memorize.

In the "generic" airplane in the theory books for a given weight you are at best glide at max L/D.  The rate of deterioration (slope) in L/D is less as you move to a faster airspeed than a lower airspeed.  So you are penalized less for being a little faster than slower.  Some one on the board may have the specific Mooney version of this chart but expect it is similar.

If I must err, I prefer to err on the high side of Vbg for memorized targets, knowing I have fine tuned my glide ring with empirical trials.

Also keep in mind that every Mooney is handmade and differences in fuselage length, rigging etc may have more of an impact that a few knots off the theoretical best answer.  Go fly and gather the data for your aircraft.

[76Srat]

This blew me away and is well worth the 25 minute time suck it requires to watch:

 

 

[Fly Boomer]

56 minutes ago, 76Srat said:

This blew me away and is well worth the 25 minute time suck it requires to watch:

Prepare to get blasted by the Dan Gryder haters.

[Pinecone]

I just looked at my 252.Encore POH and it has 85 knots (flaps up, 75 with any flaps) for the Engine Failure After Lift Off checklist.

On the glide chart, it has speeds from 76 knots at 2300 pounds to 90 knots at 3130 pounds.   85 would be the speed for about 2800 pounds.

[76Srat]

20 hours ago, Fly Boomer said:

Prepare to get blasted by the Dan Gryder haters.

Haha. Thanks for the heads-up.  Have no idea who he is other than someone IMHO who's doing his best to offer yet another way to fly as safely and as competently as possible.  After 40+ years in GA and transport-category aviation in one for or another (but never as a CFI, btw), I hope I've learned the most valuable lesson of all:  never stop learning.  Hell, I can learn a lot from a bartender, even if I don't like his or her ultimate cocktail in my glass.  I can and will always be learning, all the time.  

Reminds me the rare time my wife was with me at our local FBO years ago in the late '90s.  She asked me "does that happen often out here??" after watching our local village (airport) idiot literally ramrod his homebuilt at about a 50kt rolling high-speed turn onto the runway, midfield, and take off.  My reply:  "not for very long and not very many times in a row".  As fate would have it, that same idiot killed himself and 6 other innocent souls (yes, 7 total) when he attempted to takeoff in high DA in a summertime, over-gross Aerostar flight.  

While that story has nothing to do with the Dan Gryder video I posted above, it has everything to do with my lifelong obsession with being the best pilot I can be:  never stop learning and always keep your options open, no matter the situation.  That village idiot locked the door on learning years prior to his death and doing so ultimately did him in.  The sadness of all of that is less about losing an idiot from the pilot corps and all about losing the 6 others (all from the same family) who had no idea he was such a f*****g idiot. 

The video is a great reminder that we can all learn from everyone else, both good and bad.  I love the approach he shows on how to quickly and easily establish the most important part of any engine-out situation:  quick awareness and reaction that gives you options.  If we stall in that situation, all options are removed and there is likely only one inevitable result:  your family, assuming they're not in the plane with you when it happens, will not be dining at the same table as you that evening.  I never want that . . .

 

[Pinecone]

The problem I have with that video is his diatribe that GA pilots are wrong about maneuvering speed.

They are two similar terms that mean different things. One is minimum speed to maintain control in low speed situations and the other is a maximum speed to prevent structural damage

 

[PT20J]

I was fooling around today and pulled the throttle back and punched the Smart Glide button. It engages the autopilot, heads for the nearest airport and sets 85 KIAS. The rpm at idle power and 85 KIAS gets into the yellow arc on my M20J and the vibration reminds me to push in the prop control. If at a different weight, it's easy to change glide speed with the thumbwheel on the GFC 500. It took me over an airport at 2500' where I disengaged the autopilot and made a normal dead stick landing. Pretty slick.

[76Srat]

18 hours ago, Pinecone said:

The problem I have with that video is his diatribe that GA pilots are wrong about maneuvering speed.

They are two similar terms that mean different things. One is minimum speed to maintain control in low speed situations and the other is a maximum speed to prevent structural damage

I hear you, Mr. Cone.  My take on his diatribe is less about all GA pilots being wrong about it, and more about being wrong about overlooking establishing habits that focus on maneuvering speed versus religiously and rotely (is that a word?) going for glide speed in engine-out situations, instead of perhaps better options that give you more airspeed options (such as the aforementioned maneuvering speed instead of best-glide).  

Quick side story:  My son is going through his PPL training now as a 16 year old and has been doing so since age 15 at my buddy's local 141 and he won't even be able to take his check ride until this summer when he turns 17 (I know; cue the "why are you doing that" questions).  I occasionally go up with him and his instructor and I just watch and listen.  I'm amazed at what they're doing now in 141 curriculum versus the part 65 when I did mine back some 7 presidential administrations ago.  

To my shock and dismay, there isn't a standard "engine-out" curriculum in the C172S 141 syllabus.  Instead, there are all kinds of "engine out recovery" situations.  Now I'm not here to debate the ills or extol the virtues of these many stair-stepped syllabus steps to learning emergency recoveries in a 141 environment--they all have their pros and cons.  What does make we amazed and astonished is how rote and routine they try to make all of them.  To me, there doesn't seem to be much thought or questions around why they do them or whether there are better ways to react and recall how to recover--it's literally become a check-the-box learning and achievement environment.  

Case in point:  in the "engine fire in-cruise recovery" portion, guess what the very first requirement is?  "Turn the aircraft toward the direction of the engine fire".  Seriously?  That's the stupidest thing I've ever heard.  How about establishing where you're going to put the aircraft down safely, first?  If one stops long enough to think about the recovery steps in that situation, shouldn't we ask "how on God's green earth, or more accurately, how in God's blue sky are we supposed to determine 'which side the engine fire is coming from'?  If you're supposed to "turn in the direction of the engine fire", shouldn't you be able to figure out which side of the cowl it's coming from first?  So please tell me, even at C172S cruise speeds of ~110kt, give or take, how are you doing to determine where the flames/smoke are?  You can't.  Ram air physically prevents it.  So what good does that kind of recovery training teach a learning pilot-to-be?  I detailed all of this with my son afterward and he hadn't even thought about it that way.  Most, if not all such student never would think of it that way because it isn't "in the syllabus", so they aren't supposed to think of it that way.

All of that sidebar to say that as long as videos and other elements of learning that are logical and get us to actually think about an emergency situation logically, we'll all arrive safely at our destination, whether such destination is planned or unplanned.  After all, that's always our ultimate goal of flying:  make sure our number of landings equals our number of takeoffs, period.

 

   

[Pinecone]

My point was, there are two different things for two different circumstances.  And BOTH are important.  We just don't teach the one in GA aircraft.  But the training in GA has always been pretty much land straight ahead with engine failure.

Hmm, turn to the side of the fire.  WHY?  If you are in coordinated flight, it makes no difference.   Now you could slip/skid to keep the flames away for other bits.