ATPL question - VMCA

[Finals19]

Which of the following is true with regard to VMCA (air minimum control speed)?

a) The aeroplane will not gather the min required climb gradient?
b) VMCA only applies to four engine aeroplanes?
c) The aeroplane is uncontrollable below VMCA?
d) Straight flight cannot be maintained below VMCA when the critical engine has failed?

Is it just me not understanding the principles here or are the answers worded in a very strange way - notably c) and d)?

I went for c) - below VMCA you risk VMC rollover and loss of control. By suggesting "straight flight cannot be maintained..." are the CAA suggesting that controllable flight CAN be maintained below min control speed? If so they're a brave bunch! The only sense of "control" I could think of would be a controlled recovery...

I note that c) does not specify engine failure, but surely we are only talking about VMCA in an engine out scenario in the first place? I.e. Vs can be below Vmca, but this is a two engine situation only, because your limiting speed OEI would be Vmca if it was higher than Vs?

[littco]

Can't be B as it can and does relate to 2 engine aircraft also.

Can't be D as if you are in straight flight and the plane is trimmed etc then there is no reason why it shouldn't stay in straight flight as long as it doesn't slow to much and reach stall speed. As you say wouldn't want to try it!

Unlikely A as VMCA is not do with Climb gradient.

I would have said C anyway but by Elimination I would say C also.

[Finals19]

Nope, its D) Straight flight cannot be maintained below VMCA when the critical engine has failed...

D) is only correct in its assertion that straight flight cannot be maintained...but C) in my opinion is a better answer. Goes without saying that straight flight will not be maintained below Vmca....thats because the aircraft will be out of control...:ugh:

[BEagle]

RTFQ!

It cannot be anything other then (d):-

(a) and (b) are obviously wrong.

Of (c) and (d), the only option which mentions engine failure (when Vmca is only relevant) is (d).

If all ATPL questions are that easy, you should all get 100%!

[Finals19]

I hear you. However, what I was saying is that by asking a question about VMCA, you must automatically assume engine failure Therefore, that would also make c) correct...

Typical CAA wording I guess. Below VMCA (which is a speed that only applies to a failed engine scenario) the aircraft is uncontrollable... Straight and level flight cannot be maintained below VMCA...yes agreed, but the use of uncontrollable and VMCA in the same sentence seems to make sense also..

[Rene Pedersen]

I would go with option D.

You can still control the aircraft below vmca you can just not fly straight and level anymore since you would need more than 5 degrees of bank toward the live engine and that's more than the vmca criteria allows for.

[Artie Fufkin]

Who was it who said assumption is the mother of all f*ck ups?

This is a good lesson in ATPL exam technique; is a stupid system, but learning the way they deliberately try to trap you rather than test your actual knowledge helps avoid being trapped again.

Those exams are 90% exam technique/ feedback and 10% knowledge.

[Finals19]

You can still control the aircraft below vmca

Hmmm..ok..maybe I am showing my ignorance here as I have never really ventured below vmca..but the statement "min control airspeed" ??

This is a good lesson in ATPL exam technique; is a stupid system, but learning the way they deliberately try to trap you rather than test your actual knowledge helps avoid being trapped again

So true Artie...isn't it a sad state of affairs that the powers-that-be have arrived at?? :sad:

[Artie Fufkin]

Have thought some more and actually the answer is patently D.

If the engine failed between VMCG and VMCA you would continue the take off roll and get airborne under control. Therefore C cannot be right.

[bekolblockage]

I would agree that D is the answer they're looking for, however you could maintain "straight" flight by reducing power on the live engine. Off course, you may not climb.
If you pick C and think that "rollover" is inevitable, I think you've been listening to too many people in the Aero Club bar.

[Dick Whittingham]

Easy, Beags, D is also wrong. You can maintain straight flight below Vmca etc by throttling back the live engines. I can't think why you might want to try gliding in the Vs to 1.2Vs range, but with skill it could be done.

I guess this question is a translation from some other JAA language, and has lost something in the translation.

We fight a continual battle to get clear and unambiguous questions and this is not one of them!

Dick

[Rene Pedersen]

I'm not sure D is wrong. From ICAO Annex 6:

2.3 Minimum control speed
2.3.1 The minimum control speed, VMC, is determined not to exceed a speed equal to 1.2VS1 , where VS1 corresponds with the maximum certificated take-off mass with:
a) maximum take-off power on all power-units;
b) landing gear retracted;
c) wing flaps in take-off position;
d) cowl flaps and radiator shutters in the position recommended for normal use during take-off;
e) aeroplane trimmed for take-off;
f) aeroplane airborne and ground effect negligible.
2.3.2 The minimum control speed is such that, when any one power-unit is made inoperative at that speed, it is possible to recover control of the aeroplane with the one power-unit still inoperative and to maintain the aeroplane in straight flight at that speed either with zero yaw or with a bank not in excess of 5 degrees.
2.3.3 From the time at which the power-unit is made inoperative to the time at which recovery is complete, exceptional skill, alertness, or strength on the part of the pilot is not required to prevent any loss of altitude other than that implicit in the loss of performance or any change of heading in excess of 20 degrees, nor does the aeroplane assume any
dangerous attitude.
2.3.4 It is demonstrated that to maintain the aeroplane in steady straight flight at this speed after recovery and before retrimming does not require a rudder control force exceeding 800 N and does not make it necessary for the flight crew to reduce the power of the remaining power-units.

[Ayla]

Poorly worded answer but I agree D is the most correct.

[Keith.Williams.]

"You must automatically assume engine failure"

No, If you want to pass a JAR ATPL examination, you must automatically assume that the examiner wants you answer the question that he has asked you.

You must not convert it into the question that you wanted to have been asked.

There can be only one correct answer in this question and that is option d.

But as Dick has pointed out, we could argue that even that is not stricktly correct.

But we do not have an option that is marked "None of the above", so we are stuck with d.

[Artie Fufkin]

Could it not be argued that flight, whether straight or not, cannot be "maintained" if you reduce thrust on the live engine? There's only one way you're going and that's down.

But then of course, it doesn't say how long flight is to be maintained...

I completely forgot how much "fun" those ATPLs were.:\

[Rene Pedersen]

But Vmca is with take-off power on the remaining live engine(s) so you cannot reduce power to maintain straight flight. If you do you no longer fulfill the requirements for Vmca. And remember that Vmca will be most critical just after getting airborne where you dont really have any height to play with but still must be able to control the plane.

[KingAir77]

Hi guys,
it seems to me that Tene got it right. You are getting into a technical discussion about controlability of an aircraft in various conditions (throttling back etc.) Nevertheless, there is only ONE definition of VMCA laid out in aircraft certification requirements, and that's the one Rene dug out, and that's what the CAA want to know. It sets certain conditions under which VMCA is measured. Sort of compare it to all your performance tables, where it always says "associated conditions"

As a comparision, for those who heve not flown twins yet, take the VSO for a cessna 172. it is defined as the stall speed in straight and level flight at max gross weight and full flaps. This also is the lower limit of the white arc on the ASI. Now that was the definition. take a 172 up, alone, full flaps, on a cold winter day, and she'll stall way below that. that is the ACTUAL stall speed for your conditions. But VSO is still the one as defined.

Hope that was clear, exams are about the legal definitions, unless otherwise stated. All exam technique... You'll get used to it before the big day comes, don't worry, we've all been there.

[Dick Whittingham]

No, I'm sorry, the answer D is not clearly right. Agreed, we all think that the answer the JAA expect is D, but the question has no correct answer. This is not an aerodynamic problem but a language communication problem.

Rene and others, Vmca is not a set of conditions, it is an IAS. It is defined, or demonstrated under certain conditions to show you that under these worst case emergency conditions you can maintain straight flight, but you can fly at, above or below the IAS known as Vmca if other conditions apply. Answer D only says "Straight flight cannot be maintained below VMCA when the critical engine has failed". So long as you remember that Vmca is just an IAS you can see that this is not true.

If this question came up in an exam I would expect the FTOs to appeal it. Questions ought to be clear, precise and unambiguous. This one is confused and badly written.

Dick

[dartagnan]

the answer is "D",
it 's the longest answer.