Best Glide

drgwentzel

drgwentzel

Pre-takeoff checklist
Joined
Sep 7, 2008
Messages
287
Location
NJ
Display Name
Display name:
Kobra
Dear Aviation Guru's and Academia Junkies,

Here is a question I don't think I've seen before, but my instructor and I couldn't agree on an answer.

Ok...you're flying along at 1500' AGL and cruising at 145 knots over a densely populated urban area. You suddenly have a catastrophic engine failure and according to your GPS the nearest airport is 2.1 miles away at your 12 o'clock. This is your only safe landing option.

Your plane has a 9:1 glide ratio and you know your best glide for your weight is 70 knots. You determine that you can just make the airport. Do you:

a) hold altitude and let the speed bled off to 70 knots and re-trim?
b) pitch up and climb, gaining as much altitude as possible and lowering the nose just at 70 knots and re-trim?
c) not care less which method you use because the ground distance covering energy you gain in altitude is equal what you lost in the climb

My thoughts below: V








I ask this because I THINK immediately climbing would be the answer because by climbing you not only get the altitude, but you immediately and more quickly decrease parasite drag. Yes you increase induced drag at the same time, but if you look at a typical Total Drag chart you will see that parasite drag drops much quicker that induced drag and therefore Total Drag is shed more quickly by climbing than letting bleed off holding altitude. Holding altitude exposes you to more parasite drag for a longer time.

We all know the formula for Drag. It is: Drag increases as the square of the airspeed, so in my mind, get rid of you excess airspeed as fast as you can and trade it for altitude.

Gene
 
You should immediately establish best glide speed upon having engine failure. This will most likely result in a climb in the situation you describe, and altitude is your friend; it also immediately puts you at the lowest point on the drag curve, enabling you to get farther horizontally in the time you have remaining with the energy you have available.

The performance numbers in the book are predicated on flying at best glide speed. If you don't slow down immediately to that speed, you won't go as far as the book says you will.
 
there is probably a trade-off point, but i bet the gain in distance is minimal in light aircraft. If I had a lot of excess speed i might gradually pull up while bleeding off the speed. excess control deflection is going to generate excess drag too. I think that at one time Barry Schiff published an article discussing this topic but i cant find any references on the web. my gut feeling though is that in most situations its not going to make a significant difference, and the pilot who will get further will probably be the one who was smoother on the controls.
 
I say start looking for an alternate as soon as you reach best glide speed. A controlled crash landing will be better than a stall at 50 feet just outside the airport fence
 
I agree with Tony, A gentle pull up to gain some altitude and smoothly decrease airspeed would probably provide the maximum glide distance. By not loading up the wing the increase in induced drag will be minimal.

Gliders often have Speed to fly computers that calculate the best speed to fly based on if you are flying is rising or descending air. The general consensus is that smoothly transitioning from fast to slow or slow to fast is more efficient that trying to aggressively maintain the best speed to fly.

Brian
CFIIG/ASEL
 
drgwentzel said:
I ask this because I THINK immediately climbing would be the answer because by climbing you not only get the altitude, but you immediately and more quickly decrease parasite drag. Yes you increase induced drag at the same time, but if you look at a typical Total Drag chart you will see that parasite drag drops much quicker that induced drag and therefore Total Drag is shed more quickly by climbing than letting bleed off holding altitude. Holding altitude exposes you to more parasite drag for a longer time.
Click to expand...

I kind of doubt you'd get much opportunity to climb, as a practical matter. Going from sufficient thrust to maintain 145 kt to zero thrust is going to cause the aircraft to decelerate fairly quickly. Probably by the time you'd get your head wrapped around the fact that the engine failed and wasn't going to come back (after checking fuel, pumps, mags, carb heat, etc.) you probably wouldn't be that far from best glide speed...


Trapper John
 
drgwentzel said:
Dear Aviation Guru's and Academia Junkies,
Here is a question I don't think I've seen before, but my instructor and I couldn't agree on an answer.
Ok...you're flying along at 1500' AGL and cruising at 145 knots over a densely populated urban area. You suddenly have a catastrophic engine failure and according to your GPS the nearest airport is 2.1 miles away at your 12 o'clock. This is your only safe landing option.
Your plane has a 9:1 glide ratio and you know your best glide for your weight is 70 knots. You determine that you can just make the airport. Do you:
a) hold altitude and let the speed bled off to 70 knots and re-trim?
b) pitch up and climb, gaining as much altitude as possible and lowering the nose just at 70 knots and re-trim?
c) not care less which method you use because the ground distance covering energy you gain in altitude is equal what you lost in the climb
--snip--
Gene
Click to expand...
In a no-wind situation, the answer would be C. In general I'd do A.
If you have a headwind, A would be best, and you wouldn't make the field anyway :mad: Your best glide over the ground would be faster than 70 knots, the exact number depending on the wind.
With a tailwind, there might be a slight advantage to B, but probably not enough to make a real difference. As others have noted, by the time you think about it and decide, you're already there...
That's why it's a bad idea to fly over a densly populated area at 1500' AGL :yikes:
 
I'd do A -- if you overdo B, you really hammer yourself, since glide ratio goes down real fast below best glide speed, but not nearly as fast above it. Also, pulling the nose up kills your view ahead at a time when you need to be looking for that airport and seeing if it's moving up or down in the windshield (telling you if you're going to make it or come up short).
 
DGlaeser said:
That's why it's a bad idea to fly over a densly populated area at 1500' AGL :yikes:
Click to expand...

Scenario sounds like a two mile final with an oops, I almost had enough fuel. :devil:

Based on an article in Aviations Safety a year or so ago.

Trim for best glide, varies with wind and weight.

CS prop to course, out, reduce drag.

Throttle Full, in, reduces drag but not sure I understand why something to do with resistance in the engine from the windmilling prop.

Gear and flaps up.

At the end of the glide you can gain a little by putting in that first notch of flaps.

Gear down as late as practical.
 
RMCN172RG said:
Throttle Full, in, reduces drag but not sure I understand why something to do with resistance in the engine from the windmilling prop.
Click to expand...
Windmilling prop, throttle closed: The engine is a vacuum pump, and each intake stroke of the engine is pulling against a (theoretical) 0" MP. Windmilling prop, throttle wide open: the engine is still a vacuum pump, but now each intake stroke is pulling against 30" of MP. That's easier! So the induced drag of the windmilling prop is smaller. But I bet the overall delta is quite small.

-Skip
 
Skip Miller said:
Windmilling prop, throttle closed: The engine is a vacuum pump, and each intake stroke of the engine is pulling against a (theoretical) 0" MP. Windmilling prop, throttle wide open: the engine is still a vacuum pump, but now each intake stroke is pulling against 30" of MP. That's easier! So the induced drag of the windmilling prop is smaller. But I bet the overall delta is quite small.

-Skip
Click to expand...

It's not clear to me whether you are saying that there's less engine drag with the throttle closed or open but I believe that throttle open gives the least drag and I've tested this in flight. The engine is a positive displacement pump so the load is proportional to the pressure difference across the engine. Throttle closed has a much lower pressure on the intake than at the exhaust making the load greater. With a centrifugal pump or a ducted fan the opposite is true, i.e. lower pressure at the intake makes the load lighter (this is why your vacuum cleaner motor speeds up when you plug the suction hose).
 
Skip Miller said:
Windmilling prop, throttle closed: The engine is a vacuum pump, and each intake stroke of the engine is pulling against a (theoretical) 0" MP. Windmilling prop, throttle wide open: the engine is still a vacuum pump, but now each intake stroke is pulling against 30" of MP. That's easier! So the induced drag of the windmilling prop is smaller. But I bet the overall delta is quite small.

-Skip
Click to expand...

How long are you assuming that the propeller is going to stay windmilling for?
 
LearDriver said:
How long are you assuming that the propeller is going to stay windmilling for?
Click to expand...

Pretty much until the engine seizes or you land. It is possible to "stop the prop" on some airplanes by pitching up into a deep stall but unless you have a lot of altitude to lose the stall will decrease your glide range more than the stopped prop will lengthen it.
 
gismo said:
It's not clear to me whether you are saying that there's less engine drag with the throttle closed or open but I believe that throttle open gives the least drag and I've tested this in flight. The engine is a positive displacement pump so the load is proportional to the pressure difference across the engine. Throttle closed has a much lower pressure on the intake than at the exhaust making the load greater. With a centrifugal pump or a ducted fan the opposite is true, i.e. lower pressure at the intake makes the load lighter (this is why your vacuum cleaner motor speeds up when you plug the suction hose).
Click to expand...

You are saying the same thing I was trying to say. :cheerswine:

-Skip
 
Back to the original question -- if you're only 1500 AGL, you ain't gonna have time to do much more than pick a spot and aim for it. You won't have time for all the thinking discussed above.
 
Ron Levy said:
Back to the original question -- if you're only 1500 AGL, you ain't gonna have time to do much more than pick a spot and aim for it. You won't have time for all the thinking discussed above.
Click to expand...
I had this same thought.

On top of that, all too often, folks want to pick something away from them when the best spot is beneath their nose.

Edit: Also, I don't think anyone is going to be so perfect in holding attitude for best glide while completing other procedures (restart and/or forced landing) that they are going to max out that glide ratio. I think a more realistic glide ratio to expect is 5-6:1 or about a mile to mile and a quarter for every thousand feet. That barely gets you established for a more desirable and reachable landing spot.
 
Last edited by a moderator:
Ron Levy said:
I'd do A -- if you overdo B, you really hammer yourself, since glide ratio goes down real fast below best glide speed, but not nearly as fast above it. Also, pulling the nose up kills your view ahead at a time when you need to be looking for that airport and seeing if it's moving up or down in the windshield (telling you if you're going to make it or come up short).
Click to expand...

This is what I was taught (A).
 
Ron Levy said:
Back to the original question -- if you're only 1500 AGL, you ain't gonna have time to do much more than pick a spot and aim for it. You won't have time for all the thinking discussed above.
Click to expand...


Exactly. Best glide, place to aim for, and maybe an alternate.
 
I'm betting that if the engine coughs, quits, explodes.. what ever.. you will loose 1/2 the airspeed between cruise and best glide before you can even think about it.

That's like "rope break" training for glider pilots.. they are geared to pitch for speed and turn when they hear the snap pop bang of the rope release under tension.

A true rope "break" can be very quiet.. and their first realization they have a problem is when the tow plane gets smaller and climbs above them. Or they stare at the parted rope falling away before they realize what that means.

By then they could be in a world of hurt.

One of our "briefed training events" (Briefed between the tow pilot and the CFI) is to at a preset altitude, REDUCE tow power and start wing rocking. It takes a few seconds for the "student" to realize what is happening, and by then he is high on the tow pilot that is descending and pushes over to maintain tow position instead of pulling the release.

The hapless glider student just lost a few feet of prescious altitude while he gets his wits together and reacts correctly.

Same thing for that sudden engine failure at 1500ft AGL. You will loose 1/2 your speed before you realize it and even think of a "zoom" maneuver.. then it's too late.
 
gismo said:
Pretty much until the engine seizes or you land. It is possible to "stop the prop" on some airplanes by pitching up into a deep stall but unless you have a lot of altitude to lose the stall will decrease your glide range more than the stopped prop will lengthen it.
Click to expand...

A Cessna 172 will stop a prop at about 55-60 knots if you're patient. You don't need to stall it. It takes about 130 knots to get it spinning again without the starter. No idea about other airplanes.
 
Ron Levy said:
Back to the original question -- if you're only 1500 AGL, you ain't gonna have time to do much more than pick a spot and aim for it. You won't have time for all the thinking discussed above.
Click to expand...

right, which is why we should do the thinking now.
 
jesse said:
A Cessna 172 will stop a prop at about 55-60 knots if you're patient. You don't need to stall it. It takes about 130 knots to get it spinning again without the starter. No idea about other airplanes.
Click to expand...

It varies from 172 to 172, some require flying much slower, some don't. Also unless you are very high you will probably loose more Glide Range by flying to slow to stop that prop than the stopped prop will gain you. I think this has already been mentioned.

Brian
CFIIG/ASEl
 
Stick to flying gliders. That way you never have to worry about engine failure or stopping the prop. With no engine over a populated area you'd have a MUCH better chance of making that airport. Yup, no contest,
 
brcase said:
It varies from 172 to 172, some require flying much slower, some don't. Also unless you are very high you will probably loose more Glide Range by flying to slow to stop that prop than the stopped prop will gain you. I think this has already been mentioned.

Brian
CFIIG/ASEl
Click to expand...
Who knows. I wasn't suggesting you stop the prop--I was simply mentioning that it is easy to do.

Ultimately--you shouldn't be worrying about stretching the glide range all that far in the first place. Much better off finding something you actually have a chance at--even if it involves bending some metal.
 
I'd go for smoothness and trying to find a landing spot I can actually make, even though I might bend some metal in the process. This is also why I'd rather be high up. Then when I hit the "nearest" button on my GPS, I'll have a better shot of making the airport it brings up.

If you can get the prop coarse pitch or feathered, you're definitely better off. A few days ago in the Aztec we did a simulated engine out without feather, engine just windmilling. Even with the other engine at takeoff power we couldn't maintain altitude, whereas with the engine feathered in otherwise the same conditions you can still climb 500 fpm. Going to full throttle in such a situation I'd suspect would help, but we didn't do that in this case. Something for another lesson.
 
jesse said:
Who knows. I wasn't suggesting you stop the prop--I was simply mentioning that it is easy to do.
Click to expand...
Published tests of the practice suggest otherwise, as you have to get almost to stall to do it. Also, that costs more altitude than you can even come close to making up from 1500 AGL, making it impractical (and maybe counterproductive) as well as difficult in the postulated scenario.
 
Ron Levy said:
Published tests of the practice suggest otherwise, as you have to get almost to stall to do it. Also, that costs more altitude than you can even come close to making up from 1500 AGL, making it impractical (and maybe counterproductive) as well as difficult in the postulated scenario.
Click to expand...
Once again Ron--I'm not suggesting you stop the prop. I was simply mentioning that stopping the prop in a Cessna 172P that is stock with a O-320 isn't difficult and does not require you get anywhere near stall. I'll take observation over "published tests". It's not hard to stop it. If you want to get it spinning again without the starter, this is more difficult, and requires at least 130 knots.

If you've ever taken the little window stop deal off a 172 so that the window opens fully (all the way up) for pictures you'll know there is a speed where the window will close by itself. This speed is about where the prop will stop on a factory stock 172P with O-320.

Now, let me repeat my message, All I'm saying is stopping it isn't incredibly difficult on an airplane like a Cessna 172P (O-320). I have no idea if doing so will ultimately extend your glide and I wouldn't be trying it.
 
Last edited:
Ted DuPuis said:
A few days ago in the Aztec we did a simulated engine out without feather, engine just windmilling. Even with the other engine at takeoff power we couldn't maintain altitude, whereas with the engine feathered in otherwise the same conditions you can still climb 500 fpm. Going to full throttle in such a situation I'd suspect would help, but we didn't do that in this case. Something for another lesson.
Click to expand...

I'm missing something - Is "takeoff power" not full throttle?

Or did you mean full throttle on the dead engine?
 
I was referring to full throttle on the dead engine to minimize pumping losses caused by pulling the vacuum in the intake manifold.

I know that on my old Jag with the 5.3L V12 I used to do experiments with this and there was a noticeable difference between between full throttle and closed throttle. I'd suspect that when you're getting into these engines with significantly more cubic inches to feed you'd notice that much of a greater change, but I haven't tried it to see.
 
Do any emergency checklists in airplane operating manuals say to increase throttle to full in an engine failure?
 
skyflyer8 said:
Do any emergency checklists in airplane operating manuals say to increase throttle to full in an engine failure?
Click to expand...

not that i know of. most that i've seen just say "Throttle-Set" whatever that's supposed to mean
 
skyflyer8 said:
Do any emergency checklists in airplane operating manuals say to increase throttle to full in an engine failure?
Click to expand...

Nope, and the difference in glide ratio won't hardly be noticeable on most airplanes. Few POHs mention pulling the prop control to the low RPM limit and this does have a marked effect.
 
More reasons why having your own checklist is a good thing. I wrote my own for the Aztec. Far more thorough than what the factory recommends.
 
LearDriver said:
What are you adding that the factory checklist doesn't have in it?
Click to expand...

Here is a sample of what I did my checklist is left the Cessna one right after the vertical bar |

First more detail / sections but these titles have the same functions. My checklist follows RUNUP with a BEFORE TAKEOFF when you are ready to go.

RUNUP BEFORE TAKEOFF

NOSE WHEEL CENTER
BRAKES -SET | PARKING BRAKE -- SET
DOORS AND WINDOWS - LOCKED | CABIN DOORS - CLOSED AND LOCKED
SEATS AND SEATBELTS - SECURED
FLIGHT CONTROLS - FREE AND CORRECT | SAME
FLIGHT INSTRUMENTS - CHECKED & SET | SAME
FUEL SELECTOR VALVE -- BOTH | SAME
FUEL QUANTITY - CHECK
ELEVATOR AND RUDDER TRIM - TAKEOFF | SAME
COWL FLAPS -- OPEN
MIXTURE -- RICH MIXTURE | RICH (Below 3000 Feet)
AUXILIARY FUEL PUMP ON - (Rise in Pressure) OFF | SAME
THROTTLE 1800 RPM ALL THE | SAME
Propellor
Magneto's
Carb Heat
Engine Instruments
Ammeter
Suction Guage
THROTTLE IDLE THEN 1000 RPM | THROTTLE 1000 RPM
PRE TAKE OFF BRIEFING
The following are in my Before Taxi not runup
RADIOS -- SET
AUTO PILOT -- OFF
STROBE LIGHTS -- AS DESIRED (Lights in my BEFORE TAKEOFF)
THROTTLE FRICTION LOCK -- ADJUST
PARKING BRAKE -- RELEASE
 
Last edited:
LearDriver said:
What are you adding that the factory checklist doesn't have in it?
Click to expand...

All kinds of stuff!

I've never seen a factory checklist for a piston-powered airplane that said anything about checking the breather tube on preflight, for example. In addition, their emergency procedures generally are very incomplete - They tell you what to do *with the airplane*, not what to do. Yes, you should know in the event of an engine failure to immediately reduce to best glide and look for a spot to land, but it's not on the checklist and in the midst of freaking out you may well forget something important like that. Yes, it's a memory item, but yes, it's on the checklist now as well.

I actually have removed factory checklist items as well if they're extraneous. For example, here's a segment from one factory checklist:

MASTER................ON
FUEL GAUGES........CHECK
MASTER................OFF

Okay, how about we get rid of the first and third lines and reduce it to simply checking the fuel gauges? If the fuel gauges say 0 but the tanks weren't empty when I looked at them, I'll say "Duh," smack myself in the head, turn the master on and check the gauges. Not having those two items on the checklist cannot cause a safety of flight issue.

However - And this is the other problem with most factory checklists - Too many extraneous items like "master on... master off" in the above example leads to there being a ton of items on the checklist, and then the print has to get smaller, and then you're much more likely to accidentally skip an item on the checklist.

That's why I've made my own checklists. I start with the factory checklist and add/remove as necessary. I also add a bunch of IFR-specific stuff, like the VOR check, instrument checks, and a review of the obstacle DP prior to takeoff. My personal 182 checklist is attached, comments welcome.
 

Attachments

At work we make up our own checklists as well. I have been doing this from flying 152's as a CFI, in 135 operations and 91 corporate operations.

One thing to keep in mind with your checklist versus the factory checklist, which is advice we received from our attorney. It is ok to add to a factory checklist. However we were cautioned against removing factory items. We were told that this could put us in a bad position if there was an accident or incident which would result in any insurance involvements.

Our attorney told us to just leave the items in and just skip them if they didn't apply.
 
You must log in or register to reply here.
Back
Top