Vs versus Vmca

[Tyger]

Am I being stupid?

Can a published figure for Vs ever be below Vmca?

I have just started ME training and found that the Duchess (BE76) loses controllability at 65 but does not stall until 60!

Unless I'm mistaken, a mishandled plane, or one subject to bad wx, below Vmca will inevitably stall. As this is below Vmca recovery action should a wing drop (incipient spin) - opposite rudder, nose forward - is not possible and the a/c will inevitably crash.

The result of this is that the aircraft should never be brought below Vmca, and can therefore never be used for stall training, which it patently is.

Come on you Gurus, please put me out of my misery (or have put me down!).

[FlyingForFun]

The two numbers are completely un-related. I think you may have misunderstood Vmca.

Vmca relates to the directional control you have on one engine. I'm sure someone with a better memory, or the time to look it up, will give you the exact numbers, but Vmca is basically the speed at which, with one engine inoperative and the other at full power, you have at least a certain pre-defined amount of roll and yaw capability. It is not connected to the stalling speed in any way.

I'm not aware of any aircraft where Vmca is lower than Vs, although someone might be able to correct me on this one. I would imagine that, if you were to find or build such an aircraft, it would either be very underpowered (lower power means lower yaw from the working engine, so more controlability) or have very badly designed wings (resulting in a high Vs).

Hope that helps,

FFF
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[mad_jock]

Or a huge rudder FFF. I believe the seminole Vmca is lower than Vs although its been 2 years since i flew it.

As FFF says it is the speed after which the rudder dosn't have enough humf for you to keep the plane under control with one engine out and the other on full power.

The only reason why I remember this is because my CAAFU had me sweating asking questions on the twin commit height.

Apparently on bigger twins your approach speed can be less than Vmca. So if a engine goes on the approach you need enough height to be able to reconfigure, accelerate (by dipping the nose) the plane to past Vmca while slowly applying power until you have full power past Vmca heading for blue line and the climb. So if your engine goes below that height you are commited to land because the machine can't perform the go around.

It was all a bit hazy though because your IR GFT isn't the best place to remember the examiners pet subject.

Alex, Keith and those types does this sound about right?

MJ

[Canada Goose]

Tyger,

FFF touched on it and from what I can recall (all my notes etc are happily stored away in a warehouse the other side of the Atlantic !!) Vmca is to do with a loss of control of the a/c. Vmca is based on failure of the 'critical' engine which is related to four factors. Don't quote me but I don't think the Duchess has a critical engine. Critical engine is the ........... ahh sod it, I did a quick butchers, see the following link http://www.copanational.org/non-members/safety/safetyPPDec00.htm

Enjoy .........

CG.

[Tyger]

Thanks Chaps

CGs link provided a succint summary.

"It is important to remember that at certain altitudes the aircraft will actually stall prior to reaching Vmca. As a result, it is critical to recover at the first sign of a stall since the simultaneous occurrence of a stall and Vmca loss of directional control could easily lead to a spin."

This confirms my suspicions, twins like this have their very own, additional, coffin corner. Recovery, when it diverges from controlled flight, appears however to be relatively straightforward - shut both throttles, recover the (now) glider, pitch for Vmca+, increase power on good engine and climb away, hoping you haven't met Terry Firma on the way.

Cheers

:D

[Dick Whittingham]

I don't know why the gurus have not yet commented, and if you will forgive my lack of recent contact with the subject I will have a go.

Clearly, up to now, we have been talking about non-scheduled performance aircraft. Transport aircraft certified under JAR 25 must be able to sustain a failure of the critical engine at any stage of flight and recover safely on the remaining engines. Without the tits and knobs this means that once you have passed Vmcg on takeoff you can continue if you have enough runway available, and when you lift off you must be over Vmca. For certification Vmca must therefore be less than 1.2Vstall, and by the time you have passed Vr and unstuck you will be flying faster than this.

Coming back, you have to stay above Vmcl in the landing configuration until committed to the flare. Vmcl is just as low as Vmca, so usually is lower than Vref, and all is ok. In some extra critical conditions - two of four out on the same side - Vmcl may be higher than Vref, in which case you have to fly Vmcl right down to the flare and accept the fast landing.

On top of this you have to consider the power loss, and how it affects your decision, but that is another subject.

Hope I got that right!

Dick W

[mad_jock]

What sort of twins was the caafu on about then?

Presumably pre JAR prop jobs with automatic freight unloading doors fitted.

So what is asymetric comit height all about then?


MJ

[Mad (Flt) Scientist]

Clearly, up to now, we have been talking about non-scheduled performance aircraft. Transport aircraft certified under JAR 25 must be able to sustain a failure of the critical engine at any stage of flight and recover safely on the remaining engines. Without the tits and knobs this means that once you have passed Vmcg on takeoff you can continue if you have enough runway available, and when you lift off you must be over Vmca. For certification Vmca must therefore be less than 1.2Vstall, and by the time you have passed Vr and unstuck you will be flying faster than this.

The bit in bold is not quite right.

For a part 25 certified aircraft, V2 must be higher than both 1.1Vmca [25.107(b)(3)] and 1.2Vs [25.107(b)(1) - exceptions under 25.107(b)(2)]. There is no direct requirement between Vs and Vmca as a result of this.

Additionally, Vr must be greater than 1.05Vmca [25.107(e)(1)(ii)], and this is what ensures that you are above Vmca for flight.

Vmca is required to not be greater than 1.2Vs for maximum takeoff weight [25.149(c)(4)] - but there is no requirement for this to be met for all weights.

[Dick Whittingham]

Guilty as charged - but it was a simplified answer.

Dick W

[Tinstaafl]

MJ

Light twins are not required to have any single engine climb performance in the landing or approach configurations. This means that if you decide to go around you can expect to lose some height while you accelerate to at least Vxse & reconfigure. At some point in the approach it becomes impossible to make an asymmetric missed approach without hitting the ground ie you must commit to a landing regardless. The height to do this varies with a/c type, weight & configuration, density altitude, pilot skill/currency & obstacles to be cleared on climb out.

This problem can be minimised by maintaining a low-ish drag configuration (eg wheels down but limit the amount of flap) & >Vyse for as long as possible. Allowing for a/c response time & skill, at some point you have to decide that a landing is a reasonable expectation, configure the a/c & reduce speed to achieve Vat. That point commits you to landing.

Additionally, under Oz rules, an instrument approach must not be commenced unless the a/c can maintain the missed approach climb gradient OR has the MDA/DA adjusted upwards so that the additional missed approach starting height + the achievable climb gradient give at least the same obstacle clearance. This can complicate things a bit...

[mad_jock]

Thnaks for that tinny

MJ

[letsrock2019]

the bn2 (britten norman) has no VMCA, it stalls before VMCA.
VMCA is a speed calculated at the max weight with full back CG.
if you fly "light" at high altitude, you will certainly stall first with one engine.

to go out of Vmca, and it surprise me a lot, apparently no one knows that it's possible to reach Vmca and even go to a lower speed. you just have to put both(or the remaining) throttle on idle or at low power when u start to loose control of your aircraft(Vmca demo), pitch down slowly to Vyse (blue line)and add power and continue your flight at this speed or higher. So what is this blue line(Vyse)?, it's the speed where you have max lift and min drag on one engine, or you can call it best range speed for 1 engine.

so now, why Vmca is dangerous, not because you could loose the control if you keep high power on one engine and your speed start to blow out due to high pitch angle, but when engine failure occurs right after take-off, push this yoke to blue line(Vyse) immediately and apply rudder to keep the nose straight (dead leg, dead engine)and apply full power from right to left(mixture, rpm, and trottle), then identify, verify, feather and check list...

everyyear, there are crashes of planes hitting ground just after take off .mainly due to a poor technic and mainly due to the fact that the pilot want absolutely come back to the runway after engine failure. In hot weather at high pressure altitude, the plane deliver 50% of power, but due to "non centered line thrust" you have only 20 or 30% thrust. after engine failure, when you fly back to Vyse (blueline), it's possible you have a negative rate of descent, if you turn, it will be worse. better to go straight, burn fuel or go land to a runways staight in front of you. Declare an emergency anyway and apply the procedures given by your flight instructor!!