Best Glide, Minimum Sink... how about Minimum Energy?

[TMetzinger]

Here's a hypothetical situation.

You're flying along in IMC enroute on a long cross country. You have a complete loss of engine power (on all engines). Perhaps you had bad fuel, perhaps you were stupid and ran out, perhaps you had a leak - doesn't matter.

Given:

• You know where you are, and you're out of gliding range for any airport.
• You don't know how low the clouds are in this phase - again, you're far out of useful range of any reporting station.
• You have the MEFs on your chart for terrain, but no detailed knowledge, so you don't know where ground level is.
• Essentially, you may return to earth without ever seeing anything but clouds.
• Your gyros are ok - either you're getting enough suction from windmilling, or you've got electrics - but you CAN keep the wings level.

The key to survival is (as I understand it), to contact the earth:

• Under control
• With wings level
• At the slowest speed possible while meeting the first two goals

What airspeed would you trim for, and how would you configure the airplane?

I think I'd put out the flaps and the gear and trim for 1.2 Vs0.

Best glide gives maximum forward distance for vertical distance lost, but is not a super slow speed, and I don't know if I'll get enough warning to "land" if I'm in IMC all the way down.

Minimum sink is similar - I'm not sure it results in mimimum energy on touchdown.

Stall speed seems to me to be the minimum energy state, but it's also the knife edge of controllability. So I think I still need a small cushion above it.

But, perhaps 1.1 Vs0 would be sufficient.

 

[EdFred]

V1.15...unless it's fog under the clouds you should be able to flare it a little at the end. Plus the body does better with injuries front to back than it does straight down the spine. You can handle (temporarily) over 100Gs going forward...but coming down your spine - bad news.

 

[SkyHog]

Well, I'd think that minimizing sink would be more important than minimizing speed, in certain areas. For example, in the flat lands of NE, KS, CO, TX, etc, I'd want to minimize sink, because there ain't a lot to hit that sticks up.

In places like the Rockies, or even on the east coast, I'd want minimum speed, to ensure that when I hit the vertical wall, I'm going as slow as possible.

 

[TMetzinger]

True, but at least in the modern aircraft they seem to really address vertical impacts with the seats now - crush zones and all that stuff. I like the way you split the difference btw.

 

[brcase]

I would use only about 10 degrees of flaps and fly at about 1.1 to 1.2 above stall. This should give you about the minimum sink for the airplane and still give you enough energy to flare a bit if you do see the ground before you hit it.

More flaps will probably just increase the decent rate.

Brian

 

[tonycondon]

dont forget to consider vertical speed. here is the glide polar for my glider, minimum energy would be the point on the curve that is closest to (0,0). I suppose that would be a little slower than minimum sink, but not much. I think if i was in that situation i would go for minimum sink as any really survivable impact would include the landing gear or belly hitting the ground first, and id want to minimize the vertical impact as much as i could.

 

[dmccormack]

I would use only about 10 degrees of flaps and fly at about 1.1 to 1.2 above stall. This should give you about the minimum sink for the airplane and still give you enough energy to flare a bit if you do see the ground before you hit it.

More flaps will probably just increase the decent rate.

Brian

Hmmm...

"Best Glide" is the optimal speed for covering distance without power (well, with power too, I suppose, but that's not the current scenario).

Any faster or slower and you'll descend at a greater rate than optimal speed. Flaps always increase drag (so for the added lift you pay the penalty of increased induced drag).

So flaps should only be applied when you have your landing spot made OR for some reason you need to descend steeply (a hole in the clouds?).

So in the OP scenario, you likely want to use Best Glide to stay airborne as long as possible, then reduce speed to some minimum controlable airspeed to reduce impact.

The problem becomes 1.1 Vso doesn't give much margin for error -- are there gusts? Are you distracted while looking for trees?

I'm sure there are uber-pilots who will raise thier hands and let us know they've successfully negotiated a night off-airport landing in IMC at 1.00001 * Vso.

That's nice.

For the rest of us, best glide (which is somewhat near best endurance) down to the chosen landing point, then 1.2 Vso with full flaps to touchdown in order to arrive with the least energy while still in control of the airplane.

Unless you're flying a paper crate, whereupon a stall into the treetops might be an option.

 

[tonycondon]

(well, with power too, I suppose, but that's not the current scenario).

Actually since thrust is function of power and speed, that is not true

Any faster or slower and you'll descend at a greater rate than optimal speed. Flaps always increase drag (so for the added lift you pay the penalty of increased induced drag).

not true. a little slower will result in a lower descent rate (more time aloft), but also a steeper glide angle.

 

[Greg Bockelman]

not true. a little slower will result in a lower descent rate (more time aloft), but also a steeper glide angle.

Barry Schiff did an article in AOPA Pilot not too long ago about that very subject.

 

[tonycondon]

Yea Barry has done a few on the subject. Always nice to see that getting into mainstream print. Hopefully a few CFIs are reading it, actually believe it, and pass it on.

 

[CJones]

Heck.. Just kick the rudder and see how big of a hole you can make.

Just kidding of course...

Interesting discussion. I like the graph Tony. So in your glider, it would be wise to shoot for somewhere around 35-50mph, right?

 

[poadeleted20]

For configuration, I'd follow the POH -- they usually have good reasons for telling you gear up/down for off-airport emergency landings. Also, I'd probably be trying to slow the forward speed, but if you go below the min sink speed and get a big sink rate going, it's a back-breaker (got a friend who did that).

 

[dmccormack]

Actually since thrust is function of power and speed, that is not true.

Vg and Max endurance will be pretty close in most GA singles.

not true. a little slower will result in a lower descent rate (more time aloft), but also a steeper glide angle.

How much slower?

I agree there may be a difference between published best glide and actual Best Glide -- the only way to know for sure is to have some way to assess forward track while adjusting speed to lowest FPM descent rate.

That's alot to do when the fan's off in a typical GA single.

 

[tonycondon]

Heck.. Just kick the rudder and see how big of a hole you can make.

Just kidding of course...

Interesting discussion. I like the graph Tony. So in your glider, it would be wise to shoot for somewhere around 35-50mph, right?

depends on the wind, the lift, the sink, etc.

minimum sink is at 41 mph so if im flying straight ahead wings level in lift ill slow down to that to maximize my climb.

in perfectly calm smooth still air (like that ever happens) i fly 46 mph for best glide. add speed for sink, the stronger the sink the faster you go. and add speed for headwind, that is if you want to make headway into the headwind. inter thermal speeds of 60ish maybe up to 70 are pretty typical for me.

 

[tonycondon]

Vg and Max endurance will be pretty close in most GA singles.

I won't argue there

How much slower?

According to the guy who wrote the flight performance book we used in school, the V(min sink) = .76*V(best glide). That would be a good starting point.

I agree there may be a difference between published best glide and actual Best Glide -- the only way to know for sure is to have some way to assess forward track while adjusting speed to lowest FPM descent rate.

That's alot to do when the fan's off in a typical GA single.

the only way to really know is to have access to the real data (not an option in any airplane i've ever seen) or do the testing yourself.

 

[tonycondon]

IAccording to the guy who wrote the flight performance book we used in school, the V(min sink) = .76*V(best glide). That would be a good starting point.

and that 76% figure is a pretty rough starting point. In my glider, 76% of best glide speed is stall. the figure came from theoretical manipulation of equations, based on airfoil only, no fuselage and real world drag etc etc.

my glider is 35 mph stall, 41 mph min sink, 46 mph best glide

so min sink is 89% of best glide.

min sink is 1.17 Vs and best glide is 1.3 Vs.

 

[dmccormack]

According to the guy who wrote the flight performance book we used in school, the V(min sink) = .76*V(best glide). That would be a good starting point.

I can tell you from experience trying various best glide attempts in 5 different airplanes (BE36, BE35, C205, C172, BE77) that .76 of Vg will result in >1000 FPM descent rate, while 10 kias +/- Vg will range from 500-800 FPM. When I'm looking for an emergency landing site, I want to be airborne as long as possible, and then do what it takes to land at the best option.

So perhaps we should break the emergency scenario into 3 phases:

1. Vg to stay airborne until landing spot is chosen ("Single Engine Soaring")
2. Whatever it takes to get to the landing spot approach
3. 1.2 Vso or so to contact the ground with minimal energy, while still in control of the airplane.

How's that?

 

[jesse]

You all fly airspeeds in emergencies by far better than I do. I'm really not going to be focused on the airspeed indicator enough to differentiate the difference between a knot or two. I'm just going to look out the window and put the airplane where it should go while dumping excess energy in the air instead of letting the ground do it for me. Even then..I might still **** up and get killed. Some airplanes in some situations would just be really hard to land power off and walk away.

 

[tonycondon]

sounds fine to me. see my second post about the realism of that .76 term.

 

[tonycondon]

You all fly airspeeds in emergencies by far better than I do. I'm really not going to be focused on the airspeed indicator enough to differentiate the difference between a knot or two. I'm just going to look out the window and put the airplane where it should go while dumping excess energy in the air instead of letting the ground do it for me. Even then..I might still **** up and get killed. Some airplanes in some situations would just be really hard to land power off and walk away.

the original scenario was about IMC to the surface. nothing to see outside.

 

[dmccormack]

You all fly airspeeds in emergencies by far better than I do. I'm really not going to be focused on the airspeed indicator enough to differentiate the difference between a knot or two. I'm just going to look out the window and put the airplane where it should go while dumping excess energy in the air instead of letting the ground do it for me. Even then..I might still **** up and get killed. Some airplanes in some situations would just be really hard to land power off and walk away.

I don't think anyone's advocated staring at the ASI. But I'm sure you can feel/hear/sense the difference between "close to stall" and "bat outta hell."

Kepp it close to stall without stalling close to the intended landing spot to minimize F=M*A. What follows is in the hands of Luck or Providence.

 

[dmccormack]

sounds fine to me. see my second post about the realism of that .76 term.

Just read it after I posted.

That .76 number sounded kinda academicky. :smile:

 

[jesse]

the original scenario was about IMC to the surface. nothing to see outside.

Oh. Well. In that case. Pick a speed..hope to hell you see the ground before the airplane touches it.

If it ever happens to me -- I'll be sure to bust out the iPhone and write about it on my facebook wall. Perhaps the power of the PoA and Facebook community could come up with a solution in time.

 

[TMetzinger]

Well the discussion so far is interesting. Tony, thanks for chiming in - I figured a gliderguy would have some good info to share.

Dan, I agree with you but you still seem to talk about altering configurations as you approach a chosen landing spot. You can't do that in this scenario - you have no way to choose a landing spot, and no radar altimeter to tell you your height above ground. So you've got to pick a configuration and stick with it, because impact may be in more than five minutes, or less than three minutes, depending on terrain and your sink rate.

Tony's polar does a really nice job of showing vertical and horizontal speeds. Tony, can you convert the X axis to FPM instead of MPH? That would make it easier to judge the spot that really is closest to 0,0.

 

[tonycondon]

tim - 0 mph is equal to 0 fpm

 

[TMetzinger]

tim - 0 mph is equal to 0 fpm

Yes, but you know damn well you've got a scaling factor on that curve when you have two different units of measurement. It will look different with FPM on both axes.

 

[dmccormack]

Well the discussion so far is interesting. Tony, thanks for chiming in - I figured a gliderguy would have some good info to share.

Dan, I agree with you but you still seem to talk about altering configurations as you approach a chosen landing spot. You can't do that in this scenario - you have no way to choose a landing spot, and no radar altimeter to tell you your height above ground. So you've got to pick a configuration and stick with it, because impact may be in more than five minutes, or less than three minutes, depending on terrain and your sink rate.

Tony's polar does a really nice job of showing vertical and horizontal speeds. Tony, can you convert the X axis to FPM instead of MPH? That would make it easier to judge the spot that really is closest to 0,0.

I haven't flown IFR without a GPS for a while now, so I always have some HAA ("Height Above Accident") info available (I know, I know -- vertical is not very accurate without WAAS).

I avoid 0-0 conditions. My personal mins are whatever the nearest ILS mins are. I fly old airplanes, and no one is paying me to fly them.

Have I flown over 0-0 to the surface IMC? Yes. Did I like it? No.

Losing an engine over 0 vis surface means the only choice is between open or closed coffin, IMHO.

 

[Greg Bockelman]

If it ever happens to me -- I'll be sure to bust out the iPhone and write about it on my facebook wall. Perhaps the power of the PoA and Facebook community could come up with a solution in time.

"There is an App for that."

LMAO, Jesse.

 

[TMetzinger]

I haven't flown IFR without a GPS for a while now, so I always have some HAA ("Height Above Accident") info available (I know, I know -- vertical is not very accurate without WAAS).

I avoid 0-0 conditions. My personal mins are whatever the nearest ILS mins are. I fly old airplanes, and no one is paying me to fly them.

Have I flown over 0-0 to the surface IMC? Yes. Did I like it? No.

Losing an engine over 0 vis surface means the only choice is between open or closed coffin, IMHO.

Good point on the GPS, it will at least give a partial clue, and in airplanes with terrain databases too (a la synthetic vision) you may get enough info on when to "flare", or at least get closer to stall speed.

What comes to mind is that even though you can have acceptable weather at departure and destination, you may have an enroute segment where the weather is bad, or you really can't be confident of what the weather is.

My original question really is how to make the best of a really crappy situation, assuming you weren't able to avoid it in the first place.

 

[tonycondon]

like this?

 

[tonycondon]

and fwiw i did a little pythagoras action on the raw data and the closest point to 0,0 is the point at minimum speed. and on my glider, much slower than that and im buffeting into the stall. so maybe minimum controllable airspeed really is minimum energy. but minimum sink speed certainly is NOT minimum energy.

 

[TMetzinger]

and fwiw i did a little pythagoras action on the raw data and the closest point to 0,0 is the point at minimum speed. and on my glider, much slower than that and im buffeting into the stall. so maybe minimum controllable airspeed really is minimum energy. but minimum sink speed certainly is NOT minimum energy.

Thanks! That is sort of what I was thinking.

 

[redcloud]

In response to your original post:

In a cessna 172 why not just use full elevator trim, full flaps, and keep the wings level. I don't believe the airplane would stall, but then I am not a CFI.

 

[dmccormack]

In response to your original post:

In a cessna 172 why not just use full elevator trim, full flaps, and keep the wings level. I don't believe the airplane would stall, but then I am not a CFI.

IIRC, the airplane can't stall with full up trim per certification requirements (and has max force limit if full power is applied with full up trim). Someone better versed in cert reqs will chime in, I'm sure.

Usually full nose-up trim will get you pretty near best glide. I don't think you'd want full flaps. You will be increasing your descent rate to no advantage. (I know the scenario is your IMC, but you should have a clue where you are, and best glide will keep you airborne while you try to restart, look for options, prep for impact, etc).

Once you're close to the ground, you'll want to add flaps to be able to fly as close to stall as possible to reduce total energy at impact.

 

[brcase]

IIRC, the airplane can't stall with full up trim per certification requirements (and has max force limit if full power is applied with full up trim). Someone better versed in cert reqs will chime in, I'm sure.

<snip>

I remember the AOPA artical recommending this proceedure. Shortly there after I did a stage check with a student that had been taught this proceedure for a power failure.

After presenting this student (and a few others) with a few power failure scenio's I decided that while on paper this procedure may be true, I think it practice it is dangerous to practice and teach this procedure to new students.

What I found was the trim speed various with flap settings and it might work if you set the flap every time. But we should not be teaching students to throw full flaps out when the engine quits. At least not until they have commited to a landing spot.

I also found that the full trim back is usually a bit less than best glide, and teach to pull it back immediately tends to distract the student from TS the cause and picking a landing location.The plane will fly it self just fine at 120kts and it takes less time to just throw a couple turns in the trim to slow down if the realize they are fast.

I also found the in stressfull situations the students had a tendancy to pull on the stick(yoke) inadvertantly. Not a good scenerio when already slow.

So my technique is to teach to try and make you emergency landings as normal as possible. Approach at your normal approach speed (maybe a bit faster if you typically do power on approaches) if you have the altitude fly at least a base if not a downwind leg. Trim it as much as possible like you would if you were flying a normal approach.

Brian
CFIIG/ASEL

 

[PittsDriver]

My original question really is how to make the best of a really crappy situation, assuming you weren't able to avoid it in the first place.

It's a really good question but I think a good outcome isn't going to be based upon a difference of a few knots (min sink, max range, etc) but rather how proficient you are stick and rudder wise in bringing it down exactly where you want at some minimum controllable airspeed. I fly one of those aircraft that has no flaps and returns to earth pretty fast when it's still flyable/controllable. Every time I hit the aerobatic box to practice a sequence, I practice a simulated power off emergency landing to a spot on the runway. My goal is a high, steep approach to a spot with little or no flare or float. A skill that will serve you way better than exquisite airspeed control is the ability to use maximum slip to hit your spot. Why? Because landing short is really, really bad so high on final to the crash site is a good thing. If you haven't practiced hitting a spot with no power then anything else you're concerned about is academic.

 

[dmccormack]

I remember the AOPA artical recommending this proceedure. Shortly there after I did a stage check with a student that had been taught this proceedure for a power failure.

After presenting this student (and a few others) with a few power failure scenio's I decided that while on paper this procedure may be true, I think it practice it is dangerous to practice and teach this procedure to new students.

What I found was the trim speed various with flap settings and it might work if you set the flap every time. But we should not be teaching students to throw full flaps out when the engine quits. At least not until they have commited to a landing spot.

I also found that the full trim back is usually a bit less than best glide, and teach to pull it back immediately tends to distract the student from TS the cause and picking a landing location.The plane will fly it self just fine at 120kts and it takes less time to just throw a couple turns in the trim to slow down if the realize they are fast.

I also found the in stressfull situations the students had a tendancy to pull on the stick(yoke) inadvertantly. Not a good scenerio when already slow.

So my technique is to teach to try and make you emergency landings as normal as possible. Approach at your normal approach speed (maybe a bit faster if you typically do power on approaches) if you have the altitude fly at least a base if not a downwind leg. Trim it as much as possible like you would if you were flying a normal approach.

Brian
CFIIG/ASEL

I agree -- hopefully you drew that conclusion and quoted my thread for some other reason.

 

[dmccormack]

If you haven't practiced hitting a spot with no power then anything else you're concerned about is academic.

Plus it's way fun.

 

[TMetzinger]

It's a really good question but I think a good outcome isn't going to be based upon a difference of a few knots (min sink, max range, etc) but rather how proficient you are stick and rudder wise in bringing it down exactly where you want at some minimum controllable airspeed. I fly one of those aircraft that has no flaps and returns to earth pretty fast when it's still flyable/controllable. Every time I hit the aerobatic box to practice a sequence, I practice a simulated power off emergency landing to a spot on the runway. My goal is a high, steep approach to a spot with little or no flare or float. A skill that will serve you way better than exquisite airspeed control is the ability to use maximum slip to hit your spot. Why? Because landing short is really, really bad so high on final to the crash site is a good thing. If you haven't practiced hitting a spot with no power then anything else you're concerned about is academic.

What spot? I can't see a damn thing, I'm in IMC.

Note, I agree with every thing you say for a forced landing in VMC. But this thread is about surviving a forced landing when all you've got are instruments and a vague (really vague) idea of your height above ground.

 

[Erice]

Good point on the GPS, it will at least give a partial clue, and in airplanes with terrain databases too (a la synthetic vision) you may get enough info on when to "flare", or at least get closer to stall speed.

I know this thread is about how to AVIATE. If there is time, don't forget the NAVIGATE-COMMUNICATE part. If you are in touch with ATC (you were on an IFR flight plan if you were in IMC, right?), and your radio still works, declaring an emergency and asking for vectors to the lowest terrain might be wise, especially if you don't have the terrain-aware GPS. Then, if your impact is survivable, you might have a better chance of being rescued. I might even just hit the Ident button as I am talking, if I felt I was too overloaded to tune to 7700.
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G5