Analysis of "stopping the prop" after engine failure

[Immelman]

This is a topic that I became interested in several years ago, both for my own education and for discussion in flight instruction. I captured some data on it and thought it would be interesting to share with the Mooney community:

http://redfeatherpilot.blogspot.com/2014/05/should-you-stop-prop-for-forced-landing.html

 

 

[Mooneymite]

Thanks for posting that.  I've never seen it before.  I think we all suspected the outcome, but now we have empirable data to support our hunch.

[WardHolbrook]

Good stuff.

 

I had an engine failure in a Cessna 172 several years ago. I was up with a beginning instrument student doing some basic attitude instrument flying stuff. We had been airborne only about 30 minutes – long enough to depart SLC and head out to the backside of Antelope Island in the Great Salt Lake where the designated practice area was located and to begin his training session. We had been flying a racetrack pattern while I was giving him various altitudes, airspeeds and headings to fly. It was about our 2nd or 3rd orbit when all of a sudden I had this feeling that I should vector him tightly around and head back towards the island. All of a sudden the engine quit. Period. No sputtering, no missing, no surging, it just flat stopped running. I immediately took the airplane from the student and ran the engine failure checks, but the engine wasn’t responding. I aimed for the beach and established best glide speed, but it was almost instantly apparent to this glider instructor that we weren’t going to make it and would end up in the salt water off the beach. I then decided that I had nothing to lose so I slow down and got the prop stopped and then went back to best glide speed. That was enough to make the difference and we did make the beach with precious little to spare. To this day, I know that we would have gone into the lake if I wouldn't have got the prop to stop and used all of the glider pilot tricks in my bag.

 

[As it turned out, the engine failure was due to a magneto switch internal failure that caused the engine to stop. On the bright side, I got my first (and only) ride in a Utah Air National Guard Bell Huey helicopter and I learned that if there’s a lot of congestion on the radio frequency, all you have to do is say “May Day” and the frequency becomes yours and yours alone.]

[EDNR-Cruiser]

Thanks a lot! Valuable stuff... - in theory learned and memorized but always with some questions left in mind...
These questions/concerns are now answered!

My most valuable learnings:
1) Don't kill yourself by trying to stop the prop, just pulling min revs is almost as good...

2) Best glide varies significantly with total weight... - I added an automatic calculation for best glide speed based on current weight to my Excel W&B spreadsheet... - one for take-off weight that day and one for estimated landing weight.

[kmyfm20s]

The nice thing is the test shows that practicing with the engine at idle and high RPM is equal to a no power, windmilling prop at low RRM. I think it is realistic for people in this situation to not feel confident in stopping the prop but easlily being able to decrease the RPM for a better glide. I was always under the assumption the a dead engine with a windmilling prop at lower RPM would give you a much better glide ratio and this artcle shows that not to be the case. Thanks for posting!

[Immelman]

Thanks for the kind words fellows and, Ward, that's a heck of a story. I'm knocking on wood that I haven't lost one yet, but you never know...

 

Hey one thing about the last comment on the glide ratio with power idle vs. cut-off. I posted in the blog article that my idle on the ground is 750 RPM. I did that for a reason, but I didn't explain myself. I believe that is slightly on the high side.

 

The throttle rigging on the fuel servo/carb will, I think, vary airplane to airplane a bit. Because of this and variations in the engines we have in the fleet, I think that the look and feel of a 'power off' glide with the engine at idle power, versus the engine shut down will be one thing on my airplane as it is currently setup and another on somebody else's IO-360, and perhaps something all together different with a 6-cyl engine as we have on the M20K and on up from there. The takeaway was not to get too used to the perceived "idling glide" performance.

[N601RX]

Will lowering the nose and gaining airspeed restart the engine, or is it necessary to use the starter?

[jetdriven]

I read another article that said often the prop will often start windmilling near glide speed, resulting in the whole almost stall, stop prop, resume best glide maneuver all over again. I'm not convinced stopping the prop is worth it.

[Immelman]

Well, I'm here to tell you that once best glide was re-restablished following stopping the prop that, no, the propeller remained stopped. I did not try to dive for higher speed to see what it would take to start it turning again. It does not, however, take much effort from the starter to get it spinning.

 

Restarting was very easy.. just cracked the mixture and throttle to ground idle positions, bumped the starter, and away she went.

[jetdriven]

Ok so is that valid for 3 blade props or the C212/214 McCauley prop with wide chord blades? And how much altitude did you lose by going below best glide speed and then regaining it? I'm not saying it's not possible or more efficient, but I have noticed a surprising lack of articles saying its a net win. Same for POH's.

Another thing. When it's windmilling, if you get spark, fuel, compressing in the right place it might relight. Obviously if there's a hole in the case or a diagnosable major deal it won't restart. But what about water ingestion and you switched tanks? Stopping the prop effectively gives up on all restart options.

[marks]

In 2004 I lost my engine with an engine fire five minutes after take-off in hard IFR.  While I find the above info very interesting, I'd have to say that sticking to the basics is your best chance for survival. If you know that you aren't likely to make it to a field then maximum glide distance is less important than making sure you get the aircraft facing into the wind and putting her down at the slowest possible controllable airspeed.  Being truly prepared for a crashing landing at the slowest controllable groundspeed can save your life even if you are forced to land in the woods.  Very often when pilots lose the engine the pilot will first think of getting to the closest airport and many will choose the "death turn" and take a tail wind back to the airport.  Other times the moment the engine is dead the pilot will contact ATC and the plan will be vectors to the closest airport when there's no way the pilot can make the field.  Valuable time is lost and tail winds accepted when "almost" just won't work.  Being able to glide farther is very nice, but I go to church on Sunday and we say, "Lead us not into temptation".  Many lives could have been saved if pilots didn't maintain tunnel vision as they strive for the "almost" solution.  Some stall and spin very close to stretching out the glide.  Others try to get back to the field and roll over with that last final turn and others are tempted to stretch out the glide while headed for an airport as they fly past farm land or golf courses, when if they had pointed the plane into the wind with gear up and flaps down at the slowest safe groundspeed they would be alive today.  I know one ex-military pilot who was ten times the pilot I am who built his own Lancair and on his first day flying it lost the engine and is now dead because he made the terrible mistake of trying to save the airplane as well as himself, rather than just trying to make a straight ahead safe little crash landing.  -  Knowing the safe maximum glide airspeed is part of the basics, but beyond that my advice would be to determine ahead of time that if you lose the engine you are determined to take the first opportunity to have a safe ending and not pass by flat fields looking to stretch the glide to the perfect ending.     

[Ratherbflying]

Immelman, That is a great blog post, thanks for sharing your field research.

[tpapilot]

Hi all. being new to the mooney " family " I had received my 10 hr transitional training from a very wise pilot that reminded me that the aircraft becomes a survival tool kit when an emergency comes up no matter how much we like the airplane .

With that said I am a fairly low time pilot, and I don't think I

would be able to stop the propeller during an emergency and aviate, so I intend to pitch for best glide and locate the most suitable landing area that I can glide to.

I hope to never test this theory but I think if I can't get to the nearest airport that this should be a viable second option.

Sent from my iPhone using Tapatalk

[DaV8or]

Thanks so much for going out and doing this research Immelman. I have wondered the effects of the idling engine myself on glide performance. I have also wondered if a truly dead engine windmilling would produce enough oil pressure to make pulling the prop control back to low RPM setting effective, or not. You have answered these questions for me.

 

My take away from your tests are this-

1. I will skip trying to stop the prop. It's too close to stall for my taste in a stressful, emergency situation. Better for me to focus on the emergency at hand rather than create an even worse emergency by accident.
2. I will definitely pull the prop back to full course pitch in a real engine out.
3. Practicing engine outs with the engine at idle and max RPM roughly approximates glide performance of dead engine at minimum RPM.

My last questions are though, is there a formula to calculate best glide speed for all weights? It seems to me that if it is an established fact that 107 mph is best glide distance at max gross weight, then there should be a way just to use a calculator and come up with best glide speeds for lighter weights. Also, does anybody know what the speed is for minimum sink rate? I wonder why they didn't publish that in the POH?

[WardHolbrook]

Stopping the prop makes a very significant difference. I have heard that a windmilling propeller has the same drag as a flat plate of the same diameter. Is that true? I don't know, but the difference in drag once the rotation has stopped is quite pronounced. It's something that I demonstrate to all of my students.

 

Remember what they say about tools - having the correct one makes the job easier. I believe that a pilot should have as many tools as possible in his tool box. Not every job you'll be asked to perform in a cockpit can be adequately handled with just a "hammer, screwdriver and a pair of pliers". Stopping the prop isn’t something you’ll always want or need to do in the event of an engine failure. It s only one technique or tool. I'm just glad that I had that particular tool in my tool box that particular day. It came in quite handy. I also don't think this is something that you'll need to "practice". Seeing it demonstrated once is always enough. All you're looking for is to see and experience the difference in drag (descent rate) with the prop stopped and windmilling. Then just file that knowledge and understanding away in your "tool box" and hope the time never comes when you have to draw upon that knowledge.

 

As far as how much does it cost, performance wise, to stop the prop, again from a glider pilot POV, you're only slowing from Max L/D speed to somewhere around minimum sink speed and then accelerating back to L/D max, so the penalty or loss is going to be minimal and as long as you've got some altitude under you, it will make a difference. No one is advocating that this be done in each and every case of an engine failure, but only as a way to add to your engine out glide range. In my case, I was going to get wet so there was nothing to lose. Fortunately, it was all I needed. Like I said, it's just another tool in your pilot toolbox.

 

Remember, when it comes to max L/D, it is actually a specific angle of attack and not a specific airspeed. It is also constant. What varies is the speed at which max L/D is achieved. The heavier you are, the faster you have to fly to achieve it. At lesser weights, it is achieved at slower speeds; but regardless the max L/D is constant. (That's the big reason competition sailplane pilots use water ballast in their gliders - they still have the same glide ratio, but the extra weight allow them to achieve it at a faster speed which gives them better penetration. If soaring conditions deteriorate, all they have to do is jettison the water.)

 

As far as what speed to fly after an engine failure, it may or may not be appropriate to go directly to your actual best glide speed.  If you’re directly over your landing spot it might not be necessary. However, I always tell guys that unless they are directly over their landing spot, they need to go for max distance over the ground - you can always loose altitude, but it's impossible to get back once you've given it up. If you’ve got to cover some ground to get to a suitable landing site then you’ll probably want to factor in the winds aloft. If you've got a tail wind then you'll want to fly at a speed that give you maximum endurance (minimum sink) and take advantage of Mother Nature’s push. That speed varies as well, but it is normally close to the stall speed. If you've got a headwind, the speed to fly will be somewhat faster than your best glide speed. The reason for this is simple - let’s say your best glide speed is 100 kias and you're flying into a 100 knot headwind. Although you are at the airplane's most efficient airspeed, you end up descending vertically over a fixed point on the ground - in other words, you're going absolutely nowhere fast. By increasing your airspeed by about 1/2 of the speed of the winds aloft even though you will be descending faster that you would be at your best glide speed, you will at least be making forward progress across the ground. The rule of thumb for this is to add 1/2 of the wind speed to your best glide speed if you're flying into a headwind and decrease 1/2 of the wind speed from your best glide speed if you've got a tailwind. (However, don't fly any slower than your minimum rate of descent IAS.)

 

This may all sound quite complicated, but it’s really straight forward and easy to understand with a little thought and effort. (14 year old kids are legally able to solo gliders and they are quite good at it. This is basic glider stuff.) The payout can be dramatic as it was that afternoon when this stuff kept me from landing in the Great Salt Lake.  This is also the reason why I recommend to all of my fixed-wing pilot friends to go spend a little time in a glider. The lessons and skills you will quickly learn transfer readily over to your powered flying and you’ll never know when they will come in quite handy.  Like the philosopher said – “Engine? We don’t need no stinking engine!”

[marks]

Great post Ward!  I agree with every bit of it.  But remember that once you get there, and especially if the best choice isn't such a great place to land, how you land in a controlled crash will make all the difference.  Note my landing spot among the trees with a fire on board.  I glided through the top of a large maple tree before I floated over the green, into the wind and onto the fairway.  The whole time ATC was giving me vectors to the airport two miles ahead and I knew I couldn't make it.

[WardHolbrook]

My last questions are though, is there a formula to calculate best glide speed for all weights? It seems to me that if it is an established fact that 107 mph is best glide distance at max gross weight, then there should be a way just to use a calculator and come up with best glide speeds for lighter weights. Also, does anybody know what the speed is for minimum sink rate? I wonder why they didn't publish that in the POH?

Both Max L/D and Min Sink are actually specific angles of attack, not air speeds. For those of you who have been wise enough to install an AoA indicator the question really is what AoA results in Max L/D and what AoA results in minimum sink? As far as determining what is the minimum sink speed, it's simply a matter of comparing airspeed to rate of descent, but it's typically quite close to stall speed. (Glider pilots get pretty good at flying around at minimum controllable airspeed.) AoA gauges are great tools to have, they are more than expensive stall warning indicators can are useful for more than determining 1.3Vso.  

[WardHolbrook]

Talking about tools in your toolbox, I find this particular app quite interesting. Anyone tried this? 

[marks]

Jim -  I had hopes that the Sierra would fly again, but she never did.  The wings were held together with a huge box of criss-crossing metal strips and the collective part was unavailable.  The engine was toast as well as the flaps and radios too.  -  I did not buy the Mooney for speed.  I bought her because if I ever lose an engine like that again I can expect to glide about half again as far as the Sierra.  The Mooney has a slower roll rate, but both planes have the IO360 and in many ways the Mooney was a natural choice after my kids grew up and the college trips were over.

[WardHolbrook]

Great post Ward!  I agree with every bit of it.  But remember that once you get there, and especially if the best choice isn't such a great place to land, how you land in a controlled crash will make all the difference.  Note my landing spot among the trees with a fire on board.  I glided through the top of a large maple tree before I floated over the green, into the wind and onto the fairway.  The whole time ATC was giving me vectors to the airport two miles ahead and I knew I couldn't make it.101504sh13.JPG

Sounds like a text book example of flying the airplane until it comes to a full and complete stop. In my opinion, too many people have the wrong idea of what constitutes a successful off airport landing. Well done. 

[WardHolbrook]

Well, I'm here to tell you that once best glide was re-restablished following stopping the prop that, no, the propeller remained stopped. I did not try to dive for higher speed to see what it would take to start it turning again. It does not, however, take much effort from the starter to get it spinning.

 

Restarting was very easy.. just cracked the mixture and throttle to ground idle positions, bumped the starter, and away she went.

That's my experience too. It seems to take near Vne airspeeds to get the prop turning again on many of the Lycoming-powered trainers that I've done this exercise in. However, if all else fails, there is always this...

 

[marks]

Thanks guys, and Jim, my first plane was a '79 Tomahawk. Bought it for $8,300 in '84 and sold it for $11,000 three years later.  Very economical.  I never gave a thought to how far that airplane would glide, but I liked how slowly it would land into the wind at light weights.  Living near Cape Cod there was always wind.

[carusoam]

Look to the Missile to see what the prop can do for glide ratio.

The Missile's prop can be feathered (because of it's twin history)

Seth, any advice?

Best regards,

-a-