Vmca and flaps [Archive]

zakka

28th Feb 2007, 23:29

There are a lot of factors contributing to effect of flaps on Vmca. On some airplanes it will increase others it will decrease. Let me try and give some examples of some of the factors:

1. Type of flaps: Split flaps for example, will generally have a lowering effect on Vmca (BilliBob, I know Vmca is always the same, but for the sake of the argument blah blah blah) if deployed as they primarily produce drag, and almost zero lift, therefore they will have a stabilizing factor. If you have flaps that generates high amount of lift you might experience increased roll towards the dead engine due to slipstream. On some airplanes you might experience selecting flaps 20-25 will add 1 or 2 knots to the Vmca, but going for full flaps, it might decrease a couple knots as the last step of flaps is mainly drag on many flap types.

2. Position of flaps: If your engines are located on the tail, there won't be much of a slipstream over the flaps and therefore lower Vmca with flaps out. So to put 1 and 2 together, you will see that amount of power, position and type of flaps are some factors.

3. Counter or Co-rotating engines: Most types of flaps enables you to fly at a lower angle of attack at a given airspeed - hence you can say, that p-factor or asymmetric blade effect will decrerase with flaps down. With co-rotating propellers, that will be an advantage (lower Vmca because thrustline moves closer to the center), but with counter-rotating propellers it will be a disadvantage, as there is no critical engine, and with a lower AOA the thrustline will move AWAY from the center.

The conclusion is that the effect of flaps has many variables. In any case it will not change the Vmca drastically on most airplanes. Only very few POH's state Vmca with and without flaps. Sometimes it is stated in the Type certificate data sheet. I have copied this from the type certificate data sheet of a BAE HS748:

Airspeed limits: Vmo (Maximum Operating)
From sea level to 15000 feet 225 kts.
Above 15000 feet 215 kts.
Va (Maneuvering) 155 kts.
Vfe (Flap Speeds)
Flap deflection 7 1/2° 180 kts.
Flap deflection 15° 180 kts.
Flap deflection 22 1/2° 140 kts.
Flap deflection 27 1/2° 120 kts.
Vlo (Landing Gear Operation)
Operation 160 kts.
Extended 160 kts.
Vllo (Landing Light Operation)
Operation 140 kts.
Extended 140 kts.
Vmc (Minimum Control Speed)
82 kts. (Flaps 0 < 22 ½ O)
81 kts. (Flaps = or >22 1/2°)

Notice only 1 knot difference.

I hope this answers your question, otherwise I suggest you ask the CI at your flying school, as I am sure he will know :rolleyes:

formulaben

5th Mar 2007, 03:09

That is an unanswerable question and shows a complete lack of understanding of the definition of what Vmca is. For a specific aeroplane type, Vmca never changes and can be determined only with the flaps in the take-off position.

Since he's busy wearing out another mirror, I'll answer the "unanswerable" question. He's correct that published Vmca never changes because its calculation is specified for certification.

For practical use, actual Vmca is constantly changing as the aircraft goes through its flight profile. To answer the question (which IS answerable), flap position shouldn't have much effect on Vmca, ATBE. Flame away...

Charles van Haren

5th Mar 2007, 15:12

I'm sorry formulaben, but I don't agree.
VMCA is a MINIMUM controll speed, i.e. the speed above which you are certain during flight that you can maintain directional controll during an engine failure.To establish this minimum number, the manufacturer takes a worst case scenario, i.e. configuration just after t/o, and then sees at what speed you can no longer maintain direction. This configuration has been given above. This means that for a given type the VMCA is always the same. However, given your configuration and flight profile, it is possible that you are able to maintain direction at a lower speed, but that is not what the definition is about.
The rest of the story goes for a light twin prop, where a big part of the lift is generated by the propwash. If you would have flaps set for t/o, and you would have an engine failure, this would mean that the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement.
As seen before, if this difference is really noticable, remains the question, but this would be the theoretical approach.
I hope you can all live with this explanation, if not, just fire at will:8

FlyingForFun

6th Mar 2007, 21:09

Charles,

Please forgive me for taking apart your post piece-by-piece and pointing out your errors.
I'm sorry formulaben, but I don't agree
Formulaben's post is, on the whole, correct. The only bit of his post I disagree with is where he says: "actual Vmca is constantly changing". I agree with his sentiment in this statement, but his terminology is very slightly wrong. What he means is "Vmc is constantly changing".

However, if you are going to criticise someone for a minor terminology error, you'd better understand the terminology. You say:
VMCA is a MINIMUM controll speed, i.e. the speed above which you are certain during flight that you can maintain directional controll during an engine failure.To establish this minimum number, the manufacturer takes a worst case scenario, i.e. configuration just after t/o
That is not correct. It is true that Vmca relates to configuration just after take-off, but there is no assumption anywhere (except by you) that this is the worst case scenario. There are sure to be worse cases, and it is not true that you are guaranteed to be able to maintain directional control during an engine failure above Vmca if you are configured in one of these worse cases. I'm being quite picky here, more so than I would normally, but only because your criticism of Formulaben's post was even more picky than I am being now. And a small error in terminology such as Formulaben has made is unlikely to have any impact on his ability to fly a multi-engine aircraft safely, whereas a lack of understanding of what a number given in a POH actually means just might impact safety, albeit it's very unlikely in this case.
the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without
So what about selecting slightly more than take-off flap? Let's not use landing flap, because landing flap often has a high element of drag - but say 20 degrees of flap? On a type like the Duchess, where no flap is used for take-off (and therefore Vmca is measured with flaps fully up), using 20 degrees of flap would, by your own argument, give more lift on the wing with the working engine, increase the effect of the asymmetry, and increase Vmc to something above Vmca - hence we have a possible situation (a go-around, maybe?) where a speed higher than Vmca is needed to maintain control.

To repeat - nowhere is it stated that Vmca is a "worst case" figure, or a figure above which there is a guarantee that you will be able to maintain control. It is a specific case relating to the take-off configuration, but that's about all.
Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement.

Although your argument about increased lift and increased Vmc sounds good, and I'm sure it's correct, it's not the full story. Zakka has posted, just a couple of posts above, a quote from the POH of an aircraft which states the oposite - for the type he's discussing, Vmc actually goes down when flaps are lowered. So a blanket statement about Vmc increasing with flaps is not true in every case. That's why myself, Zakka and others have specifically avoided giving a blanket answer.
I hope you can all live with this explanation, if not, just fire at will
I think I just did! :}

FFF
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formulaben

6th Mar 2007, 21:56

The only bit of his post I disagree with is where he says: "actual Vmca is constantly changing". I agree with his sentiment in this statement, but his terminology is very slightly wrong. What he means is "Vmc is constantly changing".

Oops, yes, I meant Vmc...

Charles van Haren

9th Mar 2007, 12:16

return fire!!;)

I'm sorry, but I have to defend myself here...
VMCA is a minimum controll speed. That meens that the actual controll speed during flight will change, I agree fully, but it will always be lower than VMCA, so if you keep over this speed, you will be able to maintain directional control for sure, no matter what configuration you are in. Maybe when you let your speed drop below VMCA you get lucky, but I wouldn't base my life on the luck of the draw...:)

And as this is the heart of the discussion and the definition of VMCA, that is why I was "picky"...

Further I have not the slightest wish to say anything about the flying capabilities about anyone here on the board, so if I gave you that impression, I humbly apologise.:O That never was my intention, I save my opinion on your flying untill I have actally seen you perform, but then I will be strict and firm in my verdict!!!:E :p

Howerver, I stay with my opinion that VMCA is based on a worst case scenarion, although I give you points for the fact that it is never stated anywhere this explicit. And the Seneca in which I did most of the training normally also has 0 flaps for take-off, so that point is also out of the discussion. But, just after rotation, low speed, high angle of attack, one engine windmilling, one t/o pwr, a.s.o., give me an example where there is actual a worse case? I will prove you that any case is better than this ;) (purely looking at directional controll, that is).

eagerly awaiting your response...;)

englishal

9th Mar 2007, 15:18

Don't forget ISA conditions and Sea Level pressure....

What happens to Vmc at altitude? Maybe you'll stall first ? ;)

FlyingForFun

9th Mar 2007, 19:12

just after rotation, low speed, high angle of attack, one engine windmilling, one t/o pwr, a.s.o., give me an example where there is actual a worse case?
Quite gladly. Two, in fact (since Englishal's post got me thinking about one that had escaped my notice!)

1) As per your example, but with a different amount of flap selected. (In the case of the Duchess or the Seneca, I don't know whether selecting a small amount of flap would, indeed, increase Vmc, but you said yourself that it would and I agree that it might).

2) As per your example, but at a sea-level airfield, on a colder-than-ISA day, and with high pressure. In these conditions, the good engine will develop more power and more thrust than it would in ISA conditions, hence more asymmetry (thanks, Englishal!).

Both of these are realistic scenarios - the first relates to a go-around, the second speaks for itself.

FFF
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formulaben

9th Mar 2007, 21:27

If you really want worst-case scenario, think T/R deployment on one engine after rotation! :eek:

...but that stuff only happens at Flightsafety.

zakka

9th Mar 2007, 21:37

The rest of the story goes for a light twin prop, where a big part of the lift is generated by the propwash. If you would have flaps set for t/o, and you would have an engine failure, this would mean that the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement

As I just explained, it all depends on type of flaps+prop location in relation to flaps. For a Piper Aztec with floats for example Vmc is 14 kts LOWER with full flaps than with 0 flaps. On the Reims Cessna F406 it is 10 kts lower.

Zakka

Charles van Haren

10th Mar 2007, 18:44

@FFF: To the best of my knowledge nowhere is stated that these values are measured at or related to ISA conditions. In the Seneca, which has turbo's and thus constant pwr until app. 12.000' this is correct. There is a graphic in the manual in which you can see what happens whit VMCA after this alt. I don't know if there are comparable graphics in non-turbocharges aircraft, but I would like to know what they would say to ISA.
However, if you let your engine have more power at higher density, I will make my rudder more effective at higher density to counter for that!;)
Second, to the more flaps argument: The only reason I can see to fly with more flaps than recommended for t/o by the manufacturer (in normal flight operations i.e.) would be on the approach or during landing. Therefore, if we are to make a go around, the first thing in my crew concept is "g.a., set pwr for take off, set flaps... where you would fill in the normal number of flaps during t/o or at least a reduction of flaps. In other words, you would reduce your flapsetting ASAP to t/o cnfg. This would again lead to my worst case scenario, which of course relates to all "normal" flight conditions, whith all "normal" emergencies.

@zakka: In my original post I tried to give a correction on the, i.m.h.o., misuse of the VMCA definition by formulaben, and thereafter I tried to give an explanation to why VMCA would be higher with flaps lowered, as I thought this was an assumption that had to be cleared. This last part of my post was just my 2 cents worth, but as I re-read it, I can understand that you would have thought this to be my definition of the truth. I was reacting to one of the thoughts posted earlier in the thread, and missed severall of your comments in your post. Therefore to my statement that flaps would by definition raise VMCA, I stand corrected.:ok:

FlyingForFun

10th Mar 2007, 18:59

However, if you let your engine have more power at higher density, I will make my rudder more effective at higher density to counter for that!
I beg to differ.

As a twin instructor (you say in your public profile that you are a FI and CRI, so I assume you instruct on twins), I am horrified that you don't understand this, and the consequences of it.

The rudder will have exactly the same effectiveness, regardless of altitude, at a given IAS. This is because IAS is a measure of dynamic pressure, and control effectiveness is directly related to dynamic pressure. I think you may be confusing IAS with TAS here. If you increase pressure but maintain TAS, then yes, rudder effectiveness will increase. If you increase pressure but maintain IAS, then TAS will decrease, but rudder effectiveness will not change.

I accept that you teach on turbo-charged aircraft, therefore this doesn't apply to your current type. But it is a fact that, as pressure altitude increases (in a normally-aspirated aircraft), so Vmc reduces. The implication of this is that, as we continue to climb, Vmc eventually reduces to a point where it coincides with Vs. As instructors, it is vital that we understand this, since we regularly (at least once with each student) have to demonstrate Vmc - and if we this at too high an altitude, without being aware of the implications, we risk running out of rudder at the same time as we stall, resulting in a spin. :eek:

This is a well-known fact. Not directly relevant to our discussion, until you reverse it. The reverse situation is that, as pressure altitude decreases, so Vmc increases. Contrary to your last post, I believe that Vmca is measured at sea-level in ISA conditions, but I don't have documentation to hand to prove it.

But, whether that's true or not, Vmca must be measured in some kind of atmosphere. And, whatever atmosphere it is measured in, it's only necessary to add a millibar or two to that atmosphere to increase Vmc to something above Vmca (albeit only marginally).
Therefore, if we are to make a go around, the first thing in my crew concept is "g.a., set pwr for take off, set flaps...

That's interesting. My company's procedures are quite different. For an all-engine go-around, we raise the drag flap immediately, then wait for a positive rate of climb. Then we raise the gear. Then we raise the flap. Plenty of time for an engine failure to occur in there! (Although, if an engine failure were to occur at such a critical time, the EFATO drill would involve raising gear and flaps immediately after selecting full power, so the situation would only exist for a matter of seconds.)

FFF
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englishal

11th Mar 2007, 17:32

This is the FAA's certification requirements for Vmc:

1) Aft CG (at rear CG limit)
2) Trim in takeoff position
3) Out of ground effect
4) Maximum takeoff weight at sea level (or at a weight that produces most unfavorable Vmca)
5) Maximum available power or takeoff power on operating engine
6) Flaps (wing) in takeoff position
7) Flaps (cowl) in takeoff position
8) Landing gear retracted
9) Standard atmospheric conditions (29.92”, 15 deg. C, sea level)
10) Bank up to 5 degrees into operating engine
11) Prop windmilling (or feathered if equipped with autofeather)
12) No more than 150 lbs. of rudder pressure required
13) Must be able to maintain heading within 20 degrees

Flaps up and Vmc will increase
Gear down and Vmc will decrease
Fwd CofG and it will decrease
Cowl flaps closed and it will increase
> standard temp and it decreases
No bank and it increases
In ground effect and it decreases

etc.....

Regarding the Seneca II, this actually generates most power at 12,000'. Rated per side to 200HP at sea level, 215 Hp at 12,000 (which is why in FAA land you need a High Performance endorsement to fly one ;) )

italia458

10th Oct 2011, 06:28

I'm surprised at the lack of knowledge here and the ridiculous quotes people are using.

Granted I'm bringing the thread back to life years later but the correct Vmc testing standards were published in 1996, years before 2007, so why are incorrect versions of the FARs being quoted?

The current version is: § 23.149 Minimum control speed.

(a) VMCis the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.

(b) VMCfor takeoff must not exceed 1.2 VS1, where VS1is determined at the maximum takeoff weight. VMCmust be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with—

(1) Maximum available takeoff power initially on each engine;

(2) The airplane trimmed for takeoff;

(3) Flaps in the takeoff position(s);

(4) Landing gear retracted; and

(5) All propeller controls in the recommended takeoff position throughout.

Now, when aircraft were certified they most likely were using previous versions of 23.149 and possibly where it said that it was to be completed at MTOW, among other things. However, that's not the current version and I would be teaching my students the current version as well as letting them know that aircraft that have been certified before the current version date would have been tested differently.

Touching on the point of being the "worst case"... Vmc is determined for a set configuration. Nowhere does it say that it is required to be the worst case for the aircraft. You'd probably be able to pick that up just by looking at the previous versions where they say that it should be at MTOW and the first versions don't even mention weight and then the current one says most unfavourable weight. They can't all be right kids!

As FFF has stated, there are times where actual Vmc would be higher than what the POH states.

The reason I came here was to find some solid evidence about what happens with regard to Vmc when flaps are either retracted or extended. I personally think that there are too many variables to state what will happen as a rule because each airplane is so different. It makes sense that fowler flaps would increase the lift and drag symmetrically along the wings. Both the lift and drag would increase from 0 degrees to full deflection. The lift part coupled with the blown side would create more lift, banking the aircraft towards the dead engine. The opposite aileron used to counteract this would produce drag on the dead wing side which would increase Vmc.

As surprising as this might be, wings produce induced drag and parasite drag. The flaps produce the same. At small deflections the induced drag is in the majority and when increasing towards 40 degrees or even 60 degrees, parasite drag is now a big player. What's the difference between flap at 60 degrees at the trailing edge of the wing or a flap opening up to 60 degrees at the middle bottom of the wing? Both will slow the airplane down due to parasite drag.

Induced drag will decrease with an increase in speed so the induced drag on the blown wing will be lower than the dead wing, increasing Vmc. However, the parasite drag at large flap settings will produce more drag on the blown side than the dead side because of the higher velocity on the blown side. The question is where is the equal point and how much do these factors actually affect the Vmc speeds of the aircraft? That depends on a whole bunch of things. That's my proposition.

It would be good to note that there are multiple factors with regard to single engine flight. Three basic ones I can think of are stability, control and performance. Vmc is only talking about performance. The reason the 5 degree limit is there is so that manufacturers don't "abuse" the rule and state an airplane has a very low Vmc by using 10 degrees of bank into the live engine. The 5 degrees has nothing to do with performance. If you want performance you want zero sideslip which is different between aircraft but as a rule of thumb it's ~2 degrees for non critical engines and ~3 degrees for non critical engines. So, decreasing the bank from 5 degrees will increase the Vmc but will increase the performance, i.e.: climb rate. And what about weight? It's said to decrease Vmc. That's true if the angle of bank remains the same, the side component of lift will be greater which helps oppose the asymmetric thrust. But what if you decreased the bank so as to maintain the same side component of lift? What happens to the zero sideslip? Well, if your Vmc is calculated at 5 degrees of bank, it would go towards the zero sideslip, improving performance. But the extra weight would be a penalty in your climb performance. I think the overriding factor would be the weight, therefore reducing your climb performance but like above, it really depends on the aircraft.

When you're thinking about Vmc, think about the "control" of the airplane. Disregard performance factors such as zero sideslip until you're ready to start thinking about the performance of the airplane.

EDIT: When I'm reviewing material or studying I always try to ask "why" something is the way it is. Or why the regulation states something. Going back to the point of this not being the "worst case" scenario... when is the airplane at full takeoff power, gear retracted and flaps in the takeoff position? Right after takeoff, which is also where you're passing through Vmc at full power. I can't think of any other time in normal flight that you'd be passing Vmc at a high power setting. It's also where an engine failure has a significant chance of happening since you're working the engine quite hard. And "significant" meaning more so than the other phases of flight. I didn't talk to the person who made the regulation but it makes sense to test Vmc in this configuration here.

Desert185

10th Oct 2011, 15:54

Takeoff flaps is the key to this. That might mean flaps up or not up. Vmc has to be above stalling speed. If not, there is really no Vmc(a). I believe the B-N Islander falls into this category and of course the C-336/337, for obvious reasons.

Keeping in mind the conditions and aircraft configuration established for measuring Vmc published in the certification requirements, actual Vmc can be at a higher airspeed given the higher DA, lower power, higher temp, lower pressure, CG, etc. Vmc can also be at a lower airspeed at a lower DA, very cold temp, etc.

In doing an unplanned Vmc demo in the Twin Otter simulator, exceeding the 50# torque limit will result in a higher demonstrated Vmc and loss of directional control, which is why there is a 50# torque limit when upgrading the engines from -27's to -34's. Vmc in the Twin Otter is with flaps 10, not up, BTW.

Big Pistons Forever

10th Oct 2011, 15:55

Personally I think too much emphasis is placed on VMC, and not enough on Vyse, in the training for the ME rating. Every light twin will have a negative rate of climb at, or any where near, VMC. If you are close to the ground and any significant speed below Vyse, than you are in a unrecoverable situation.

Whether the VMC goes up or down slightly with configuration changes is irrelevant to the safe operation of a multi engine aircraft. If you are actually flying close to VMC in an actual one engine inoperative situation then you have grossly mishandled the aircraft.

From an operational stand point the critical point in every takeoff is that period when the aircraft is below Vyse and with the gear and flaps still down. Until the aircraft is cleaned up with Vyse on the ASI and healthy positive rate of climb, the only sane course of action is to close both throttles and land straight ahead.
VMC has nothing to do with this.

Desert185

10th Oct 2011, 16:06

Big Pistons Forever

10th Oct 2011, 17:00

Big Pistons Forever:

You are correct, although VMC has to be understood as part of the process. A good example of why you are making a good point is a 3-engine ferry takeoff in a Herc. 2-engine Vmca is 136 KIAS with high rudder boost and 166 KIAS with low rudder boost. Rotation speed is less than that, so there is a period of time when the loss of a second engine could result in a failure to continue controlled flight.

Even though I haven't flown a Herc since 1987, those speeds are still ingrained. Like I said above, Vmc has to be understood as part of the process.

Since I am type rated on the DC6 and L188 I am quite familiar with the 3 engine case and should add that Vmcg is also a very significant limitation, particularly for the L188. However I would suggest that these very specialized situations are not very relevant to flying a light piston twin during initial training for the Multi engine rating.

italia458

10th Oct 2011, 17:13

actual Vmc can be at a higher airspeed given the higher DA, lower power, higher temp, lower pressure, CG, etc. Vmc can also be at a lower airspeed at a lower DA, very cold temp, etc.

Desert, you've got it backwards. Higher DA, lower power, higher temp, lower pressure will all get Vmc to be lower since Vmc is an indicated (calibrated) speed and all of those things will reduce engine power.

BPF, I agree Vyse could actually be considered the more important value here. However, it's important to be teaching students correctly the first time. There is a lot of misinformation out there about Vmc. Just because it's possibly not as important as Vyse doesn't justify continuing to teach it incorrectly. As an instructor I've come across MANY areas that are incorrectly taught and it's frustrating when you're trying to explain to your senior instructor that they're wrong... most of them will quote things they were told with no research or reference or even any logical reasoning! My instructor who was teaching me for the flight instructor rating told me that I should teach that all forces are balanced while the airplane is in a level turn. He justified it because it's "easier for some students to understand". :eek: Btw, the forces in a turn are NOT balanced, if they were then the airplane wouldn't turn. The correct diagram shows less forces... wouldn't that be easier to understand?! I'm getting off topic a bit but this is one of the main reasons why we have so much misinformation out there!

Desert185

10th Oct 2011, 17:58

Quote:
actual Vmc can be at a higher airspeed given the higher DA, lower power, higher temp, lower pressure, CG, etc. Vmc can also be at a lower airspeed at a lower DA, very cold temp, etc.
Desert, you've got it backwards. Higher DA, lower power, higher temp, lower pressure will all get Vmc to be lower since Vmc is an indicated (calibrated) speed and all of those things will reduce engine power.

You're right. Shouldn't have been in such a hurry.

Desert185

10th Oct 2011, 18:08

BPF:

Originally Posted by Desert185 http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/viewpost.gif (http://www.pprune.org/flying-instructors-examiners/262217-vmca-flaps-2.html#post6743418)
Big Pistons Forever:

You are correct, although VMC has to be understood as part of the process. A good example of why you are making a good point is a 3-engine ferry takeoff in a Herc. 2-engine Vmca is 136 KIAS with high rudder boost and 166 KIAS with low rudder boost. Rotation speed is less than that, so there is a period of time when the loss of a second engine could result in a failure to continue controlled flight.

Even though I haven't flown a Herc since 1987, those speeds are still ingrained. Like I said above, Vmc has to be understood as part of the process.

Since I am type rated on the DC6 and L188 I am quite familiar with the 3 engine case and should add that Vmcg is also a very significant limitation, particularly for the L188. However I would suggest that these very specialized situations are not very relevant to flying a light piston twin during initial training for the Multi engine rating.

Any example to enhance understanding and transfer of knowledge to the student is relevant, IMO. I would rather impart related experience to reinforce the importance of Vmca (to include Vmcg, if applicable), and the relationship to Vyse/V2 speeds than parse the information. As usual, the devil is in the details.

Big Pistons Forever

10th Oct 2011, 18:24

Any example to enhance understanding and transfer of knowledge to the student is relevant, IMO. I would rather impart related experience to reinforce the importance of Vmca (to include Vmcg, if applicable), and the relationship to Vyse/V2 speeds than parse the information. As usual, the devil is in the details.

Actually I do use the case for 4 engine transport category aircraft as an example of a case where Vmcg and Vmca actually has operational relevance before I return to emphasizing the factors which have operational relevance to the aircraft being actually operated for the training flight (usually a Pa 44 in my case), where Vmc is only a theoretical consideration in a normal takeoff, or any other phase of flight for that matter.

I am all for students having a sound understanding of the relevant theory, but not all theory is equally important. In the case of Vmc I think entirely too much time is spent on configuration trivia and pedantry over definition and far too little time on the practical operational significance of the published Vmca and the concept of Vmc in general. In other words it is assigned an importance in the training syllabus that exceeds its value.

I find there is a discouragingly large number of ME instructors who can site chapter and verse on Vmc but also think it is possible to fly away from a engine failure just after liftoff in your average piston twin......

italia458

10th Oct 2011, 18:49

I am all for students having a sound understanding of the relevant theory, but not all theory is equally important. In the case of Vmc I think entirely too much time is spent on configuration trivia and pedantry over definition and far too little time on the practical operational significance of the published Vmca and the concept of Vmc in general. In other words it is assigned an importance in the training syllabus that exceeds its value.

Even though I believe that teaching the correct/current definition is important, I definitely agree with your comment. What Vmc means while operationally flying needs a bit more spotlight time.

Pull what

14th Oct 2011, 21:32

I'm surprised at the lack of knowledge here and the ridiculous quotes people are using.

You havent learned much in 12 months then!

Pull what

14th Oct 2011, 21:35

I find there is a discouragingly large number of ME instructors who can site chapter and verse on Vmc but also think it is possible to fly away from a engine failure just after liftoff in your average piston twin...... And more to the point quite a lot of very inexperienced PPLs are flying around thinking the same.

Pull what

14th Oct 2011, 22:03

The reason I came here was to find some solid evidence about what happens with regard to Vmc when flaps are either retracted or extended.

Anything that increases the drag will increase VMC-putting flap down always increases drag so it will always increase VMC

Desert185

16th Oct 2011, 00:30

Pull what: Anything that increases the drag will increase VMC-putting flap down always increases drag so it will always increase VMC

Well then how do you explain why Vmc in the Twin Otter, for example, is Flaps 10, 64 KIAS and Vyse is Flaps 10, 80 KIAS (reduced 4 KIAS/1000#)?

Obviously, drag has increased with flaps 10, but generally stating that a drag increase will increase Vmc maybe isn't totally accurate. I know that in a four-engine jet manuevering speed is much higher with flaps up. Of course, rudder travel is generally limited at higher airspeeds, so there is obviously a balance between flap position, rudder travel and Vmc for those aircraft. In the large aircraft I have flown, Vmc is always lower with flaps in the takeoff position, which could be as much as 50% flaps in one particular type.

italia458

18th Oct 2011, 17:16

Anything that increases the drag will increase VMC-putting flap down always increases drag so it will always increase VMC

Pull what... I'd love to see where you got that information. Vmc is regarding controllability. Drag is not a direct factor in controllability so you can't state a general rule like that.

Pull what

25th Oct 2011, 10:41

My apologies Italia and Desert-actually I think you right-thanks for challenging that.

Pilot DAR

26th Oct 2011, 08:01

Perhaps I have some learning to do... I am well aware that Vmca maneuvering close to the ground is very high risk, and the cause of too many accidents, particularly when un planned. However, when carefully planned, at altitude, why the worry?

have to demonstrate Vmc - and if we this at too high an altitude, without being aware of the implications, we risk running out of rudder at the same time as we stall, resulting in a spin. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/eek.gif

This caught my eye, and I find myself wondering about it's basis. I have been taught that the safest twin, is one where Vmca and Vs are reached at about the same time.

Any light twin to which this discussion applies, would have demonstrated compliance to the following:

Sec. 23.205

Critical engine inoperative stalls.

(a) A multiengine airplane may not display any undue spinning tendency and must be safely recoverable without applying power to the inoperative engine when stalled. The operating engines may be throttled back during the recovery from stall.
(b) Compliance with paragraph (a) of this section must be shown with--
[(1) Wing flaps: Retracted and set to the position used to show compliance with Sec. 23.67.]
(2) Landing gear: Retracted.
(3) Cowl flaps: Appropriate to level flight critical engine inoperative.
(4) Power: Critical engine inoperative and the remaining engine(s) at 75 percent maximum continuous power or thrust or the power or the thrust at which the use of maximum control travel just holds the wings laterally level in the approach to stall, whichever is lesser.
(5) Propeller: Normal inoperative position for the inoperative engine.

I've done this test a number of times, and always found the subject aircraft very compliant. The odd time, a wing has dropped, but was very recoverable, particularly when the power was reduced on the operating engine.

Does the fact that the rudder is hard over as the stall is approached suggest a spin, should things continue, if the aircraft is otherwise symmetrical because of asymmetric power?

Wouldn't it be a reasonable expectation that the candidate pilot be able to keep things under control by reducing power on the operating engine, and managing the yaw and pitch of the aircraft to prevent either a stall or a spin, while flying at the Vmca limits? The aircraft will give a warning of an impending stall, and if the stall is prevented, a spin cannot occur.

Providing that there is adeqaute space between the aircraft, and the ground, are we that concerned about directional control at Vmca? In the worst case, we don't always need to fly straight, if allowing a gentle turn allows control to be maintained otherwise? For my experience, when attemping to fly an assymetric twin slower than Vmca (when it is higher than stall speed) the aircraft does not suddenly flop over on it's back, it just starts a gentle turn. Obsticle clearance aside, a gentle turn is not that big a problem, if it keeps you in control otherwise while you sort the plane out?

I offer the following photos, taken by me while testing a Piper Navajo. Near gross weight, forward C of G, Flaps were at 15 degrees, wheels up, left engine stopped and feathered, right engine 75% power. The aircraft was in stable, straight flight, with an intermittent stall warning, but no impending change in controllability. (it was stable enough that I could take photos while flying!)

http://i381.photobucket.com/albums/oo252/PilotDAR/Jims%20DAR%20Testing/IMG_2367.jpg

http://i381.photobucket.com/albums/oo252/PilotDAR/Jims%20DAR%20Testing/IMG_2368.jpg

I also stalled the aircraft in this configuration right after I took these photos, and it had no tendancy to spin (though I did reduce power right away, and keep it straight with rudder).

I know that Vmca training is percieved as high risk, and many accidents have resulted from poorly executed single engine flying. Is it wise, however, to avoid training into these regimes of flight, when the conditions can be well controlled?

Big Pistons Forever

26th Oct 2011, 15:18

I know that Vmca training is percieved as high risk, and many accidents have resulted from poorly executed single engine flying. Is it wise, however, to avoid training into these regimes of flight, when the conditions can be well controlled?

Well for starters what is emminetly safe and reasonable for a 6000 + hr test pilot may not be so safe and reasonable for a 150 hr PPL on his/her way to a CPL student.

I have about 2000 hrs of instructing with about 50 hrs of that instructing for the ME rating. All the scary moments of my instructing career occurred in that 50 hrs. To assume that the student will always do the rational thing is simply unrealistic. The biggest example was when I was doing a engine failure in the overshoot exercise at altitude. The student established the aircraft (a Pa 34 Seneca 1) in the landing configuration at ref speed and I called "go around" As soon as the student had advanced the throttles to full power I retarded the throttle, the student then panicked as the aircraft started to descend and abruptly applied full back stick:eek:, the airplane instantly and I mean instantly snap rolled and when I took over we were inverted and 40 degrees nose down.:uhoh:. If I had not also been a aerobatic instructor I would have probably died that day.

The student wasn't stupid he was just young and nervous in a much bigger and heavier aircraft than had ever flown......and had a massive cranial rectal inversion, something which he was profoundly remorseful for and cognizant of the implications of his mistake.

That and other examples of student brain fade has made me build in a good margin where appropriate to protect both of us as the student gains initial experience operating ME aircraft.

But back to the question of VMC training. I liken the VMC question to how and why of spin training . Yes I show my students how to recover from a spin but I spend most of my time concentrating on spin recognition. The accident record is pretty clear, almost all spin accidents are close to the ground. Even if the pilot initiated a proper spin recovery the aircraft would still been too low to recover. The failure to recognize that the aircraft was in the slow flight regime and then failure to recognize the aircraft was stalling and then failing to control the yaw at the stall was the cause of most "Spin" accidents. The ability to recover from a fully developed spin. or not is pretty moot, the point of the training should be to not let the aircraft spin in the first place

Similarly when teaching VMC the point is not to determine what the actual VMC for that airplane in those conditions, it is to recognize a loss of directional control during single engine flight and to take immediate and correct action to regain/retain control of the aircraft. This can just as effectively and more safely be done by limiting the rudder to half travel thereby ensuring the loss of directional control occurs at a higher and safer speed.

But I emphasize that for most light piston twins, the aircraft will be over 20 knots below blueline speed if it is near VMC. It will have a considerable negative rate of climb and therefore if the speed is allowed to deteriorate to the VMC value close to the ground the aircraft will likely still crash despite the fact that the VMC situation was recognized and correctly dealt with.

That is why in addition to VMC training I also demonstrate the effect of airspeed below and above blue line on the single engine rate of climb. For most light twins a speed of 10 knots lower or higher than blueline will wipe out the SE rate of climb. This IMO is a more important exercise than the VMC demo.

Pilot DAR

26th Oct 2011, 18:02

Thanks for your informative response Big Pistons (by the way, watch for a PM). I have to concede that aside from the DA-42-L360 (which does not lack for power!) the lightest twin I've flown would be an Aztec, perhaps I have a skewed idea of what a light twin should be expected to do on one engine.

for most light piston twins, the aircraft will be over 20 knots below blueline speed if it is near VMC. It will have a considerable negative rate of climb

Accepting that my flight test type observations are based on flying Navajos, and bigger - not smaller, I would expect that the aircraft would still climb at speeds slower than blue line, just not at the best rate. Reviewing my Navajo testing, wayyyy slow with one feathered, I was still indicating a 250 FPM climb. I do accept that perhaps the "light twins" won't do this.

Getting back to your aerobatic ME student, I can see the problem - the poor student was just completely out of their element. I remember the first time I flew the turbine DC-3 - completely new thing for me. It honestly took 30 minutes just to get used to it at the basic level, before I started "trying things".

From your perspective (perhaps hind sight being 20/20) is it better for the instructor to demonstrate the whole maneuver first, so the student can warm up to it, and model it?

While taking helicopter instruction, my incredible instructor would often do a maneuver, sometimes even just a quick reposition. I would say to him "I'm going to redo exactly what you just did". The intent being to test if I could make the machine do what I had just seen it done (it must be safe, he just did it!). My instructor soon learned that I would as closely as possible duplicate his maneuver, so if I varied, it was a shortcoming in my skill, and he could spot it - there were no surprises (he also learned to not demonstrate things he did not want me to attempt!). For the few times (one sticks out glaringly) where I imagined what to fly next without a briefing, I was very surely told why I must never do that again, and why! VERY memorable!

I agree very much with the analogy of flying at Vmc and spinning. Somewhere there is a middle ground of EVERY pilot must experience these, then only certain pilots should ever actually attempt them unsupervised. How does a mentor pilot know who's who - every pilot should be able to safely accomplish these maneuvers. After all, the certification requirements generally say "must not require unusual pilot skill or attention".

My mentor (retired Transport Canada test pilot) tells me that I'm to demonstrate stalls in the Navajo, with one engine feathered, and the other at 75%, and I say Huh? He says "you can do it, you'll be fine". That was the first time I'd ever done that, with a total of 5 hrs flying Navajos. The plane was just as he said it would be - fine.

It must be frustrating for instructors to feel to themselves, "I have to give this candidate a ME rating, because they have met the minimum standard", but all the while thinking, "this pilot needs ten more hours training in this plane".

When I test something, and declare that it flies within the standard, can I be sure that a competent pilot can fly it safely?

italia458

26th Oct 2011, 18:17

Perhaps I have some learning to do... I am well aware that Vmca maneuvering close to the ground is very high risk, and the cause of too many accidents, particularly when un planned. However, when carefully planned, at altitude, why the worry?

The danger of getting below Vmc while close to the ground is the lack of altitude to recover. The danger of getting below Vmc while high is encountering a stall first. That could be more detrimental as you could enter an unrecoverable spin. Having the proper actions done immediately is imperative if you inadvertently enter a single engine stall. As of 1996, multi engine aircraft do not have to be able to recover from a single engine stall! It's extremely dangerous and the FAA AC 61-67C says that single engine stalls must not be demonstrated or practiced because of possible "catastrophic consequences". I'll explain why there are catastrophic consequences to hitting a stall first, below.

Look at this graph: http://airplanegroundschools.com/Transition-to-Multiengine/Figure%2012-21%20Graph%20depicting%20relationship%20of%20VMC%20to%20VS.J PG

You can see that at first you'll hit Vmc. As you climb up you'll reach an altitude that will make the stall the first thing you reach. I'm assuming you know that Vmc will decrease with an increase in altitude because of the lower power created by the operating engine. The stall is the same at any altitude because it's "indicated airspeed".

Hitting Vmc first: this is dangerous! What happens is the aerodynamic requirements of the rudder to counteract yaw will increase exponentially as you approach Vmc. If you reduce speed to 10-15kts above Vmc and then slow at a steady rate (1kt/sec) toward Vmc, you'll notice that you need to add the rudder faster and faster. At Vmc you'll begin rolling towards the inoperative engine and should be at full rudder around that point. You can recover by pitching down, holding full rudder, and rolling the airplane back to level. Nice and simple and the safest! If you approach Vmc at say 5kt/sec, you will be upside down before you know it. As above, the yaw increases exponentially requiring an exponential increase in rudder towards Vmc. As you hit Vmc, you'll be continuing to slow down and hence make the rolling and yawing tendency even greater. This is why the airplane will "snap" towards upside down. If you get a "snap" towards the inop engine, you must reduce the throttle, pitch down, roll out and then apply power as you pitch back up.

Hitting the stall first: a very bad day for you! This is far worse than hitting Vmc first. At a high enough altitude you will hit the stall first as Vmc reduces below the stall speed. When you stall you essentially "fall out of the sky"... work with me for a bit! If you stall both wings at the same time or one slightly more than the other (getting a wing drop) it's a pretty standard recovery and really not much to be worried about especially at high altitude. Now, imaging stalling one and having the other one still flying. You'll roll and yaw towards the stalled wing because of the lack of lift and increased drag. Now strap on a 180hp engine to the flying wing and put that to full power. You will have an extraordinary amount of yaw towards the stalled wing now! It also has the effect of keeping the unstalled wing even further from the stall as it 'blows' the wing creating more lift. What happens is essentially the stalled wing "falls out of the sky" while the flying wing (mostly because of the lift created by the engine) is now accelerating you into a spin. If you think a power on entry to a spin in a single engine airplane makes the spin more "fun" or "intense", what do you think would happen to that spin when you have the engine running at full power, and located a few feet from the longitudinal axis?!

Regardless of where Vmc is, hitting the stall first is incredibly dangerous. At the point on the graph where Vmc intersects the stall speed would be the worst case scenario. As you increase in altitude from that point the effects aren't as bad, even though the graph might make you think it is so. The "recovery might be difficult" pointing towards the area where the stall is encountered first is true when compared to the area where Vmc is hit first, however, make sure you know that the worst case is at the intersection point!

I want to mention some things with regard to published stall speeds. The BE95 has a published Vmc of 80mph and a stall speed of 85mph. Those numbers are at sea level. To begin with, Vmc is lower than the stalling speed, and even though hitting a stall while single engine can be catastrophic, I prefer to have the stall first. A basic reason why is because I know what the stall speed is regardless of altitude, Vmc is always changing. With regard to the twin engine stall with power on, you'll get a stall speed lower than published, quite significantly lower actually. The POH states that with 25" and 2700rpm, the twin engine stall speed is 61mph with gear and flaps up. A difference of 24mph! It's more like a fun fact though, as being that slow will be below Vmc significantly at the majority of the airports around North America. But what about the single engine stall speed?! The answer is, it doesn't really change. It will be essentially the same as the published stall speed for the configuration. When an engine fails, it's similar to pulling the throttle to idle. If you were to be very technical, in some cases, the single engine stall speed could be slightly higher than the published stall speed because it is producing zero lift and more drag than at idle (assuming a non feathered position). So, like we discussed above, the inop engine's wing will stall at the published stall speed while the operating engine's wing will still be flying.

Pilot DAR, you asked if this training could be carried out safely and I think it can and I think it should be carried out. However, due to all the complexities and dangers of Vmc training, it's imperative that the candidate and especially the instructor, have a thorough understanding of what can happen and why.

italia458

26th Oct 2011, 18:28

Accepting that my flight test type observations are based on flying Navajos, and bigger - not smaller, I would expect that the aircraft would still climb at speeds slower than blue line, just not at the best rate. Reviewing my Navajo testing, wayyyy slow with one feathered, I was still indicating a 250 FPM climb. I do accept that perhaps the "light twins" won't do this.

You will still climb at your Vxse for the altitude you're at, up to the absolute ceiling for single engine. Vx will increase with an increase in altitude and Vy will decrease with an increase in altitude. You generally won't be able to get as high as you would if you "floated" down to the absolute ceiling than if you had climbed to it while single engine.

It must be frustrating for instructors to feel to themselves, "I have to give this candidate a ME rating, because they have met the minimum standard", but all the while thinking, "this pilot needs ten more hours training in this plane".

I feel that way even with some PPL and CPL students! I'm happy to be only instructing multi engine and multi-IFR now. Dealing with ab initio students is sometimes quite ridiculous. And another thing that annoys me is that some think that the minimum standard is an "average standard" so they don't need to meet it all the time. That's not the mentality of a good pilot!

Regarding your stall in the Navajo: What made it manageable is that you were probably at an altitude that is significantly higher than the Vmc/stall intersection and were at reduced thrust, further reducing Vmc. Even at the point where Vmc/stall intersect, I think a skilled pilot could easily recover assuming that he is ready and waiting for the first indication of a stall/roll/yaw to retard the throttles and complete the rest of the recovery. The reason I don't openly recommend it and why most documents these days do not recommend it is because of how quickly it can get out of hand if you're not ready at the instant it requires input to recover.

I have no idea what your skill level is so I would just recommend that you be careful. Being able to recover easily from the single engine stall at 75% power in a Navajo might have been a bad experience for you if you now think that it's a non-event. The difference between an easy recovery and being out of control is a somewhat thin line.

Big Pistons Forever

26th Oct 2011, 21:20

Accepting that my flight test type observations are based on flying Navajos, and bigger - not smaller, I would expect that the aircraft would still climb at speeds slower than blue line, just not at the best rate. Reviewing my Navajo testing, wayyyy slow with one feathered, I was still indicating a 250 FPM climb. I do accept that perhaps the "light twins" won't do this.

From your perspective (perhaps hind sight being 20/20) is it better for the instructor to demonstrate the whole maneuver first, so the student can warm up to it, and model it?

From bitter personal experience I can assure that a Pa31-350 with a GTOW of 6971 lbs (ie 29 lbs under gross) on a 27 deg C day will when the engine fails, climb at barely 100 FPM if the speed is held exactly at blue line. In my case even a 5 knot deviation from blueline wiped out the climb.

I can also assure you when introducing a new training manoever, I don't just tell the student "hey why don't you give the engine failure in the go around a try and see how you do" :rolleyes:

The manoeuver was properly and fully briefed on the ground and demo'd by me in the air. In fact it was the students second try at the manoever. I want to be clear I was PIC so this upset was ultimately my fault and not the students. Obviously there was a big post incident debrief with the CFI and the other instructors. The take aways were as follows.

1) On the first try I had briefed that the "airfield elevation" was 3500 feet AGL and that the go around would be called at 3700 feet. He was too tentative rotating the aircraft to a nose high attitude and we lost 300 feet. I pointed out he went 100 feet below ground which doesn't work very well. This I think lead to the over aggressive pull back on the wheel as he was responding to my feed back, also

2) The Seneca has a very heavy elevator and I had not noticed that the student had applied a lot of up elevator trim with the electric trim button. He was expecting to need the usual big heave on the wheel and so the much lighter control pressure contributed to the over control and excessive up elevator. and

3) Instead of immediately taking control I tried to talk the student into correcting the problem. This was a bad mistake as he was task saturated and what I was saying was not penetrating, and finally,

4) I was unprepared for how fast the aircraft lost airspeed with a strong pitch up at Vref (90 mph) gear down and flaps full on one engine. The stall horn/actual stall/and violent roll/yaw into the dead engine all happened in maybe 2 seconds.

The only good news is I can personally attest that, on the Seneca at least, the gear retraction time is cut in half if you select gear up when the aircraft is inverted :E

italia458

26th Oct 2011, 22:05

The only good news is I can personally attest that, on the Seneca at least, the gear retraction time is cut in half if you select gear up when the aircraft is inverted

Hmm.. next time my gear doesn't go up I'll have to try selecting gear up while inverted :ok: ...maybe with a little negative G to help!

Pilot DAR

27th Oct 2011, 10:43

Yes, Big Pistons, I was pleasingly surprised by the performance apparently available to me in the Navajo I was testing. Having read the flight manual for it, I completly agree that there are other conditions of flight, where it would be nowhere near as good! Fortunately, I'm ususally able to control many variables to make them favourable for my testing - I'm not relaly very bold in planes anymore!

I can imagine the startled student syndrom. Though I am not an instructor, I do find myself doing checkouts is certain waterborne aircraft from time to time. As you well know, the water is unforgiving! I dread the day when I have to check out a new owner with limited taildragger experience on the amphibian I'm selling for it's owner. You gotta let the pilot get a little out of control, so they see it happening, but this plane does not have far to go, before it's gone too far! It's a greater challenge to instruct than most pilots realise. While taking confined area trianing north of Pitt Lake, my mentor pilot apologized in advance for guarding the controls so closely, say that there just would not be time to reach if he had to - fine with me! But I drift....

It is always of interest to me, as to what I should be writing in flight manual supplements, so as not to be too wordy (I save that for PPRuNe), yet convey the intent that a pilot approach certain maneuvers with caution, or preparedness for an unexpected outcome. No one expects a Seneca to be on it's back. Getting there is probably not difficult, getting back would be much more exciting