What is the relationship between VMCA and V1?

[foxsports]

Hello, I am doing some revision of my ATPL subjects and I have found 2 questions that have me completely stumped, I have been on a 2 day web search for answers and inspiration but have come up empty :ugh:

What is the relationship between VMCA and V1?
Is VMCA higher or lower than V1 and explain your answer?

I have spent a couple of days on this and have come to a couple of conclusions.

VMCA must be less than V1 because, VMCA must be 1.05 times VR and VR can be equal to V1.

and/or

After V1 you are committed to taking off so VMCA must be lower than V1 so you can maintain control and takeoff.

But if there is a wide gap between V1 and VR surely Vmca could be somewhere in-between, or is there a rule that says V1 must be more than VMCA. And does VA have any relationship to V1?

I was wondering if anybody would be able to shed some light on this subject, am I in the right ball park or am I way off. Thank you for your help!

[oz in dxb]

Vmca has to be equal to or lower than Vr as Vmca is your minimum contrable airspeed airborne. If you had an engine failure after Vr you must be able to control the aircraft when you are airborne.

Vmcg has to be equal to or lower than V1 as that is the minimum contrable airspeed on the ground. If you had an engine failure after V1 then you would be able to control the aircraft on the ground and rotate.
Engine failure prior to V1 would mean the aircraft would not be controllable if below Vmcg.

Oz

[Mad (Flt) Scientist]

There is no required direct relationship between V1 and Vmca. 25.107 (for the FARs) has the various relationships spelled out:

(a) V1 is constrained (at the low end) by Vmcg and at the high end by Vr

(b)&(c) V2 is constarined at the low end by Vsr, Vmc(a) and Vr

(d)&(e) Vr is constrained at the low end by V1, Vmc(a), Vmu and V2


On some types it may be that the minimum value of V2 or Vr is determined by the Vmc(a) criteria (i.e. that Vr be not less than 1.05Vmc(a) or that V2 be not less than 1.10Vmc(a) ). In such cases, if that minimum value is then used for the scheduled values, then the resulting Vr will constrain the upper value of V1. And if V1 is then elected as equal to Vr, it may be that V1 is above Vmc(a).

But there's nothing requiring you to make the choices that end up with that result. I could easily have chosen a lower V1, or I could have a case where Vr or V2 are determined by other criteria. So V1 could just as easily be below Vmc(a)

[safetypee]

MFS, precision as usual. However I suspect that the ATPL exam might not have any answer relating to your post. I don’t recall that detailed knowledge of part 25 is in the syllabus.
Indeed, what is the purpose of asking a question where the answer is “no relationship”, would there be any real learning or future safety value from it?
The more important aspects, to the pilot, might be the Vmcg – V1 and Vmca – V2 relationships.

[Mad (Flt) Scientist]

MFS, precision as usual. However I suspect that the ATPL exam might not have any answer relating to your post. I don’t recall that detailed knowledge of part 25 is in the syllabus.
Indeed, what is the purpose of asking a question where the answer is “no relationship”, would there be any real learning or future safety value from it?
The more important aspects, to the pilot, might be the Vmcg – V1 and Vmca – V2 relationships.

I would agree wholeheartedly; this seems to be yet another exam type question (whether interview or licence) which serves only to confuse, or to encourage a parroted answer which may even be detrimental to understanding.

The problem is that giving a useful answer involves not actually answering the question!

[kwachon]

V1 Takeoff decision speed. V1 is the minimum speed in the takeoff, following a failure of the critical engine at VEF, at which the pilot can continue the takeoff with only the remaining engines. Any problems after V1 are treated as in-flight emergencies. In the case of a balanced field takeoff (http://en.wikipedia.org/wiki/Balanced_field_takeoff), V1 is the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the aircraft within the accelerate-stop distance and the minimum speed at which the takeoff can be continued and achieve the required height above the takeoff surface within the takeoff distance. In this context, V1 is the takeoff decision speed.


VMC or VMCA minimum control speed with the critical engine (http://en.wikipedia.org/wiki/Critical_engine) inoperative. The speed below which control will be lost, normally due to roll or yaw divergence. In initial aircraft type testing and certification, this is tested at a safe height above ground and, when established, is factored in to V2 that by regulation has a set margin over Vmca and also over Vs.
KW :ok:

[ssg]

V1 as stated is takeoff decision speed...predicated on many factors, but generaly speaking you accellerate to V1, and there is a given runway distance calculated into this speed, such that when you reach V1, and are at this given runway distance, should you have some sort of a malfunctin, not just engine issues, that the pilot feels might jeapordize continuing the flight safely, then he can pull the throttles, apply brakes, TRs, Speed brakes ect, and if he did all his numbers, will have enough runway to stop safety...enough pavement to do so.

V1 also means, simply, that at after having attained this speed, and the pilot has an engine failure, he now has a couple of things...he has enough speed to safely accellerate to VR, continue the flight, accellerate to V2, and clear all the obsticles, on one engine.

So V1 is a desicion speed based on weight and balance, runway conditions, obsticles, weather conditions, that along with all the other number should ASSURE that the pilot has the option to either abort the takeoff and have enough runway to stop, or continue the flight and still be able to fly...

When these numbers are not calculated correctly, for any number or reasons, then safety is not assured. See the Concord crash, why it couldnt continue the flight.

Vmca is simply an aircraft design speed, typicaly for twin prop aircraft...that upon losing an engine, asymetrical yawing forces, trying to make the plane flip over tward the dead engine, requires a certain amount of counter flight control forces to counter this problem. Flight controls need a certain amount of air over them to make them work. Vmca is a certain minimum amount of speed needed to provide enough airflow over the control surfaces to keep the plane flying normaly under a single engine scenario in a twin: Hence why it's called a minimum control aircraft...mininum amount of speed to CONTROL the aircraft.

A transport category plane couldn't be certified to fly with one engine after V1 if it was slower then VMCA or it would crash. In smaller twins though, VMCA can be higher then the stall speed of the aircraft...so it's important to keep the speed to VX/ VY to maintain control. Read up on Twin Commanche crashed.

To counter the threat of a flying below VMCA, some people install vortex generators that make the plane's VMCA slower, then stall, hence the pilot would stall the aircraft and recover more easily then a wing over. You should read on how to correct for flying at or below VMCA...

I hope this helped.

[safetypee]

ssg, I think that you are confused in one or two areas.
First you should re-read some of the standard definitions of V1 and its use.
Second the Concorde accident involved two engines failing / shut down on the same side, thus the aerodynamics aspects of that situation involved Vmca (2), an feature of 4 engine aircraft. If only one engine had been shut down then the aircraft should have been able to maintain controlled flight, all other things permitting.
Aerodynamic ‘fixes’ such as VGs might aid Vmca, but probably only in those aircraft where Vmca is defined by the stall, in other cases where Vmca is due to lateral / directional control limits, the use of VGs is unlikely to help.

[galaxy flyer]

MfS:

I'm sure glad you are on my team and not the opposition. ;);)

[Alex Whittingham]

MFS is exactly right (again). V1 must be not less than VMCG, VR must be not less than 1.05 VMCA. V1 must not be greater than VR.

The examiners think, erroneously, that VMCG is always greater than VMCA. This allows them to conclude that V1 must also be greater than VMCA. They took their data from the L1011-1, for this aircraft the statement is correct. These are progress test questions from your course material, not JAA exam questions but there are one or two exam questions which can only be answered if you make this assumption. We've argued it, but to no effect.

[ssg]

Safety: Care to elaborate?

No intention of writing the definative definition of these subjects but merely an easy to understand synopis. If I err, please explain. V1 is pretty easy to understand. Not sure how you define VMCA as being part of a stall speed in any plane.

With regard to the Concord. It was on fire prior rotation. Crew didn't abort, after they hit a piece of metal(thump) after it hit the wing (bang) and when they got the fire light (woop woop) all before rotation. Hard to believe all that happened after V1 but it did happen prior to rotation.

This might bring up a good discussion on the ' we always go after V1' and try to fly a burning aircraft around the pattern mentality rather then simply stopping the aircraft if you have plenty of runway.

Not sure if there was a definative on whether they actualy lost power on one or two engines, but they had two good ones left. Not sure whether a Concord is rated to lose half it's engines and still be able to fly, like most planes. Eitherway...the plane was criminaly and negligentely over max gross weight, and was the primary reason why it couldn't really get any altitude...and hence the end of the Concorde program....

Sad really, for the passengers and for a great aircraft that won't see the skies again...

[chornedsnorkack]

Not sure if there was a definative on whether they actualy lost power on one or two engines, but they had two good ones left. Not sure whether a Concord is rated to lose half it's engines and still be able to fly, like most planes.

Like twins.

Are any quads rated to lose two engines and still fly? I thought that transport multiengine planes are only required to survive one engine out, and that they are permitted to crash with two engines out whether the total number of engines is three, four or more.

[Heavy Cargo]

In the freighter you pull back and the houses get smaller and if you push forward they get bigger. Don't know about the pax wagons ?:8

[Chris Scott]

Quote from chornedsnorkack:
Are any quads rated to lose two engines and still fly? I thought that transport multiengine planes are only required to survive one engine out, and that they are permitted to crash with two engines out whether the total number of engines is three, four or more.
[Unquote]

I think your second sentence is correct, assuming you are referring to engine failure on take-off. This raises the case of engines that are in close proximity to another; such as the centre engines in the B727 and Falcon 50 (?); but more particularly the pairs of engines in 4-engine types like the Comet/Nimrod, Lockheed Jetstar, VC10, and Concorde. Sadly, none of these types remains in airline service. Perhaps one of our RAF forumites can comment?

The vulnerability to double-engine failure on these 4-engine types became a concern in the early 1970s, following an incident which eludes me at the moment. [More recently, there has been at least one case of an uncontained compressor disc on an aircraft with remotely-mounted engines causing the failure of another engine, but the chances are presumably tiny.] On the VC10, which had more power and a lower MTOW than comparable B707s, my airline started practising double-engine failure on take-off for line pilots on recurrent simulator checks. From then on, we always placarded Vmca[2] on the T/O Data card.

One compensatory feature of these types is that double-engine failure produces asymmetry comparable to an ordinary twin-jet; so Vmca[2] was not much of a problem, except perhaps at low weights (unlike the B707). I don’t know if the same applies to Concorde, with such a high power-to-weight ratio, but her lack of flaps and consequent high V-speeds would suggest perhaps not.

The hope was that the second engine would not run down until gear retraction was nearly complete. If it failed prior to gear selection, the recommendation was to leave the gear down until a safe height; because of the drag involved in opening the gear doors during retraction. [There may also have been potential hydraulic system considerations, but my tech notes are not to hand at the moment.]

At low weights, departing airfields with no engineering facilities, we often used the minimum V1/Vr ratio permitted; giving a very low V1. Whether this created a problem for the double engine-failure case, I don’t remember; but we would have been aware that aborting the take-off between V1 and Vr would be a safer option than trying to keep straight and rotate with 2 engines failed. Thus, I don’t remember any mention of a Vmcg[2].