OEI Stalls

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Apache123

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Hey, Steve!
This is one thing that makes me more nervous than anything (inexperience, I'm hoping). In twin training literature/videos/etc. there's the constant "Don't let the airplane stall while on one engine, as the aircraft may enter an unrecoverable spin."

It would seem most likely to occur during a Vmc demo practicing-for/performing the practical.

I understand that the one engine induces the yaw just like the rudder on a single engine during intentional spins, but what are the odds of entering a flat-spin in this maneuver? Is it an old-wives tale/exaggeration, or is this something to be respected/feared?
 
Not that big of a deal, actually the altitude most Vmc demos happen at the instructor has to limit your rudder so the stall doesn't occur first. The real reason you don't want to let it stall OEI is that you don't want to give up altitude you may not get back! Plus you're flying around below Vyse which you shouldn't be doing unless you're nose down within a couple hundred feet of landing, once you have the runway made you can proceed below Vmc.

OEI stalls & Vmc demos aren't a big deal to be scared of. Once you pull that other throttle it all happens like any other plane. It's not like anything jumps you, especially not in any of the light twin trainers. A Baron does a pretty nifty eye opening Vmc demo especially with the left caged, I will give it that, but it it's not unrecoverable in any fashion.
 
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First let me say I am NOT a multi pilot but lets think about the physics here

If the stall is power off then you have no thrust from either engine, so the stall should be a non event. Now recovering by shoving one throttle forward...:nono:

If the stall is attempted power on I'd image you will go below blue line and begin to loose control of the aircraft into a VMC roll before you reach a stall.

my thoughts anyway.
 
Apache123 said:
It would seem most likely to occur during a Vmc demo practicing-for/performing the practical.
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Yes, which is why the FAA beats on ME instructors so hard to be careful not to allow that to happen. For example, the PTS requires that recovery be initiated upon first indication of yaw divergence or the first indication of impending stall, which ever comes first.

I understand that the one engine induces the yaw just like the rudder on a single engine during intentional spins, but what are the odds of entering a flat-spin in this maneuver? Is it an old-wives tale/exaggeration, or is this something to be respected/feared?
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There's no one-size-fits-all answer on this. The flight characteristics and resposes of the various twin trainers vary. However, if you do like the book says and initiate the recovery as described above, it's not going to enter a spin, no less go flat.
 
Henning said:
Not that big of a deal, actually the altitude most Vmc demos happen at the instructor has to limit your rudder so the stall doesn't occur first.
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I don't know how many different twins you've given training in, but very few require that. The Grumman Cougar is notably one of them, especially since stall speed is below Vmc. However, reducing the bank angle from the five degrees many folks use for this maneuver (which is not required by the PTS) to the two degrees which provides the best OEI performance in nearly all light twins (and "best performance" is what the PTS requires for this maneuver) raises Vmc enough that you see the yaw divergence first, and that's what we're trying to demonstrate to the trainee.

The real reason you don't want to let it stall OEI is that you don't want to give up altitude you may not get back! Plus you're flying around below Vyse which you shouldn't be doing unless you're nose down within a couple hundred feet of landing, once you have the runway made you can proceed below Vmc.
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Agreed, especially the second sentence there.

OEI stalls & Vmc demos aren't a big deal to be scared of. Once you pull that other throttle it all happens like any other plane. It's not like anything jumps you, especially not in any of the light twin trainers. A Baron does a pretty nifty eye opening Vmc demo especially with the left caged, I will give it that, but it it's not unrecoverable in any fashion.
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I've done the Vmc maneuvers in a 58 Baron, and it wasn't that eye-opening unless you let it go too long before recovering, but that would be true of most any light twin. First sign of yaw divergence or stall indication, and it's chop-and-push, and then recover.
 
Skylane81E said:
If the stall is attempted power on I'd image you will go below blue line and begin to loose control of the aircraft into a VMC roll before you reach a stall.
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Depends on the plane and especially the bank angle, but it's a long way from blue line (Vyse) down to Vmc (red line), and you can generally control the yaw in the intermediate regime. Also, the book Vmc (red line) is predicated on a very specific set of conditions -- density altitude, cg location, bank angle, etc. For training purposes, the instructor can adjust those to get the response the instructor wants, e.g., reduce entry bank angle so the plane reaches Vmc before stall, or increase entry bank angle so the plane reaches stall before Vmc.

By doing that, the instructor can avoid having to block the rudder, which has the potential to create other problems which interfere with not only the training, but even control of the aircraft.
 
Ron Levy said:
I don't know how many different twins you've given training in, but very few require that. The Grumman Cougar is notably one of them, especially since stall speed is below Vmc. However, reducing the bank angle from the five degrees many folks use for this maneuver (which is not required by the PTS) to the two degrees which provides the best OEI performance in nearly all light twins (and "best performance" is what the PTS requires for this maneuver) raises Vmc enough that you see the yaw divergence first, and that's what we're trying to demonstrate to the trainee.
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So you are saying this relationship does not change with altitude and that there is not an altitude at wich the Cougar will stall OEI before reaching Vmc?



Ron Levy said:
Agreed, especially the second sentence there.

I've done the Vmc maneuvers in a 58 Baron, and it wasn't that eye-opening unless you let it go too long before recovering, but that would be true of most any light twin. First sign of yaw divergence or stall indication, and it's chop-and-push, and then recover.
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I never flew the 58 much, just a couple times. The 55 will come over pretty quick around 3000', the 56 would really lever you around, but it was a OEI flying machine. It took a lot to get it to stall or Vmc, but once it cracked, you better have that throttle back quick or you're gonna do an impromptu aerobatic routine.

BTW, I have never seen a true Vmc demo done.
 
Henning said:
So you are saying this relationship does not change with altitude and that there is not an altitude at wich the Cougar will stall OEI before reaching Vmc?
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No, I'm not.

BTW, I have never seen a true Vmc demo done.
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Then how did you get your multiengine rating? This is a required item on the practical test, and has been for longer than you've been flying. However, this does explain some of your lack of knowledge about conducting the maneuver. Next time you're out my way with your 310, stop by and I'll go through it with you.
 
Ron Levy said:
No, I'm not.

Then how did you get your multiengine rating? This is a required item on the practical test, and has been for longer than you've been flying. However, this does explain some of your lack of knowledge about conducting the maneuver. Next time you're out my way with your 310, stop by and I'll go through it with you.
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So what altitude do you do your Vmc demos in the Cougar?

Vmc is defined at gross weight, how many demos or ME training you see done at gross?
 
Henning said:
So what altitude do you do your Vmc demos in the Cougar?
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Typically 3500 AGL -- plenty of room if something goes wrong.

Vmc is defined at gross weight, how many demos or ME training you see done at gross?
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Your understanding of the PTS Vmc demo appears flawed. It is not a confirmation of the book Vmc value, but rather, is a demonstration of how the aircraft reacts when yaw control is lost due to reduced speed in an OEI situation, and then how to recover from that condition. It is not performed, and in fact cannot safely be performed, under the same conditions as those in the regulation governing the establishment of "book" Vmc. For one thing, "book" Vmc is established for sea level/standard day, and even in the lowest terrain, that would be dangerously close to the ground.

You really should dig into the PTS description of this maneuver and the discussion on "ENGINE INOPERATIVE—LOSS OF DIRECTIONAL CONTROL DEMONSTRATION" in Chapter 12 of the Airplane Flying Handbook. The article on bank angle, performance, and Vmc by the late Prof Mel Byington would also be good reading, as would be (if you can find it) his article “Principles to Bank On,” in the AOPA Air Safety Foundation Flight Instructors' Safety Report Vol. 15, No. 2, April 1989 (not in their on-line archives, but available as described in the "Principles" article).
 
As I said I've never seen a TRUE Vmc demo done, they are all limited PTS approximations of what a Vmc roll does.
 
Henning said:
As I said I've never seen a TRUE Vmc demo done, they are all limited PTS approximations of what a Vmc roll does.
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So, you're redefining the term. If what you want is to see how they determine Vmc during aircraft certification, fine, go visit the factory, but that has nothing to do with the issue of the Vmc demo as a training and pilot certification maneuver.

As for "what a Vmc roll does," if you get the plane down below Vmc for the existing conditions, unless you've stalled it first, it does the pretty much the same thing regardless of actual gross weight, density altitude, cg location, etc. If you reach stall first and don't promptly recognize and recover from that, there's not much taking it further down to Vmc will accomplish other than very possibly kill you -- and that's not a desirable training outcome.

Now, will you stop typing and start reading the materials I suggested? I know you didn't read them all in nine minutes, and you really might learn something which could keep you alive in that 310 of yours the day one engine quits.
 
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Henning said:
As I said I've never seen a TRUE Vmc demo done, they are all limited PTS approximations of what a Vmc roll does.
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I was going to reply earlier in this thread but you have provided an opening that I cannot ignore. I was trained in a Twin Comanche in 1968, when the Advisory Circular that preceded Practical Test Standards required that the wings be kept level and the ball be kept in the middle when an engine failure was simulated. I remember vividly the times when my instructor had me pull the airplane up into a nose-high altitude below Vmc and then killed an engine; the goal was to recover before the bank angle reached 45 degrees. I survived, but a lot of instructors and students did not...our school lost a plane, student, and instructor to this madness.

In 1976, FAA test pilot/engineer Les Berven convinced the folks in the ivory tower that actual loss of directional control occurred as much as 15 knots above red line unless the pilot was allowed to bank into the operating engine and let the ball out of the center. The text of the Flight Training Handbook was changed from "A bank angle of no more than five degrees" to "A bank angle of at least five degrees."

I'm proud to say that Les Berven helped me write THE COMPLETE MULTIENGINE PILOT before he died far too young.

So a real Vmc demonstration is not a myth, it is just not allowed these days.

Bob Gardner
 
bobmrg said:
The text of the Flight Training Handbook was changed from "A bank angle of no more than five degrees" to "A bank angle of at least five degrees."
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The current wording in the AFH is "A bank angle of 5° (a right bank, in this case) should also be established." However, the PTS calls for "a bank toward the operating engine, as required for best performance and controllability," which is about 2 degrees in virtually all light twins. In addition, in some twins with stall speed near Vmc in most conditions (or at high DA's in many others), zero bank may be necessary to provide the increase in Vmc necessary to demonstrate the yaw control loss (the "true Vmc demo") before stall symptoms force recovery.

As regards the engine cut at/below Vmc, since nobody should ever be in the air below Vmc in a light twin, and light twin Vmc-related accidents virtually all occur during deceleration from a speed above Vmc after one engine is lost, not before, that old Vmc cut maneuver seems to me to be a case of practicing bleeding, not a "true Vmc demo." I believe the PTS maneuver is the most realistic example of how folks lose control of a twin after one engine quits -- trying to fly it after an engine failure when it doesn't have enough remaining power to do so, and watching speed and eventually control slip away.
 
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Gotta side with Henning on this one. I have never been able to do a VMC demo in any trainer type twin (PA30, BE-76, BE-95) without the instructor limiting the rudder travel. At the altitudes we tend to use for ME training, the stall warning always comes first if you don't limit the rudder.

Now the B-25.....that is a whole 'nother can of worms.....
 
bobmrg said:
I was going to reply earlier in this thread but you have provided an opening that I cannot ignore. I was trained in a Twin Comanche in 1968, when the Advisory Circular that preceded Practical Test Standards required that the wings be kept level and the ball be kept in the middle when an engine failure was simulated. I remember vividly the times when my instructor had me pull the airplane up into a nose-high altitude below Vmc and then killed an engine; the goal was to recover before the bank angle reached 45 degrees. I survived, but a lot of instructors and students did not...our school lost a plane, student, and instructor to this madness.

In 1976, FAA test pilot/engineer Les Berven convinced the folks in the ivory tower that actual loss of directional control occurred as much as 15 knots above red line unless the pilot was allowed to bank into the operating engine and let the ball out of the center. The text of the Flight Training Handbook was changed from "A bank angle of no more than five degrees" to "A bank angle of at least five degrees."

I'm proud to say that Les Berven helped me write THE COMPLETE MULTIENGINE PILOT before he died far too young.

So a real Vmc demonstration is not a myth, it is just not allowed these days.

Bob Gardner
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I used your book for my multiengine instrument rating. I found the book to be concise and helpful. Probably one of the best aviation books I have on my shelf. cheers!
 
Fearless Tower said:
Gotta side with Henning on this one. I have never been able to do a VMC demo in any trainer type twin (PA30, BE-76, BE-95) without the instructor limiting the rudder travel.
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You need a better instructor, because it's always possible to show a Vmc yaw departure without limiting rudder travel by adjusting either bank or power, and for the PTS, you don't have to get to the yaw divergence -- "recovery should be made at the first indication of loss of directional control, stall warning, or buffet."
 
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Fearless Tower said:
How do you demonstrate slow flight and stall recoveries in a twin then?
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I'm below Vmc every landing. I'll slow to blue until a mile final then start slowing for redline. A soon as I know I'll have pavement under my wheels I'll start slowing below for stall.
 
Ron Levy said:
You need a better instructor,
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I think if you search PoA you'll find this quote comes up on every single flight instruction thread. Okay not every single one, but almost every thread assumes the instructors aren't doing the right things.

Doesn't it logically indicate there's something horribly horribly wrong in the CFI certification path?
 
Henning said:
I'm below Vmc every landing. I'll slow to blue until a mile final then start slowing for redline. A soon as I know I'll have pavement under my wheels I'll start slowing below for stall.
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This is the same as what I do. Seems to work well. Basically I'm blue until I know I could get in with minimal power, then red when I know I could glide in if I had to.
 
denverpilot said:
I think if you search PoA you'll find this quote comes up on every single flight instruction thread. Okay not every single one, but almost every thread assumes the instructors aren't doing the right things.

Doesn't it logically indicate there's something horribly horribly wrong in the CFI certification path?
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IMO the checkride has little to do with actually being an instructor.
 
Apache123 said:
This is the same as what I do. Seems to work well. Basically I'm blue until I know I could get in with minimal power, then red when I know I could glide in if I had to.
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Yep, no reason to carry more energy than required, it just increases the chances of overloading the gear if I screw up.
 
jesse said:
IMO the checkride has little to do with actually being an instructor.
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True, but the checkride is only the last small factor in the overall process. As with any learning process primacy is the rule and it has it's roots all the way back to your first experience. A minority percentage of CFIs had a solid initial exposure; if you weren't born into an aviation family, the slimmer the chances of someone going through the system to be hooked up with a high experience CFI. The further from rural FBOs and physically closer to the 141 mills you get, the greater the chances that your instructors instructors instructors instructor were each and every one experienced only within the flight training environment all the way until the time they quit instructing for their Regional Airline job are.

The training system lacks passing on wisdom of experience for the greatest degree. You have the fortune that you were born into aviation.
 
Henning said:
True, but the checkride is only the last small factor in the overall process. As with any learning process primacy is the rule and it has it's roots all the way back to your first experience. A minority percentage of CFIs had a solid initial exposure; if you weren't born into an aviation family, the slimmer the chances of someone going through the system to be hooked up with a high experience CFI. The further from rural FBOs and physically closer to the 141 mills you get, the greater the chances that your instructors instructors instructors instructor were each and every one experienced only within the flight training environment all the way until the time they quit instructing for their Regional Airline job are.

The training system lacks passing on wisdom of experience for the greatest degree. You have the fortune that you were born into aviation.
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It's like pilot inbreeding.

EDIT: That's 95% of the instruction experience out here. I HATE it. I consider myself so fortunate to have been brought up in an aviation environment, and that I was lucky enough to have my dad as my CFI who got me through my private. I did my instrument 141 and actually wouldn't really change much about that. The syllabus gets you through the checkride, but it's up to you as a responsible pilot to go a few steps further in truly becoming proficient and safe enough to actually use it, again, something very few people do around here and I pride myself on doing so.

My mom just sent me this picture a few days ago, heh

141v9qx.jpg

YTdStgg0x8w

YTdStgg0x8w
 
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Ron Levy said:
You need a better instructor, because it's always possible to show a Vmc yaw departure without limiting rudder travel by adjusting either bank or power, and for the PTS, you don't have to get to the yaw divergence -- "recovery should be made at the first indication of loss of directional control, stall warning, or buffet."
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I think I see what you are saying....I've never seen an MEI limit the bank angle.
 
Fearless Tower said:
I think I see what you are saying....I've never seen an MEI limit the bank angle.
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As I said, too many people focus on the regulatory conditions for establishing "book" Vmc rather than on obtaining the full range of responses in real-world situations.
 
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