Does VMCG VMCA very with aircraft weight?

[tinmouse]

Does VMCG VMCA very with aircraft weight?

This is a possible interview question, Can anyone shed some light on this?


Thanks

[expedite08]

From what I understand about these two speeds, they are both control related. They are based on the critical engine failing.

VMCG: The minimum control speed on the ground during the take off run.

VMCA: The minimum control speed in the air that an aircraft is controllable.

Both of these speeds decreas with increasing airfeild elevation and Pressure Altitude. Reduced take off thrust and a large fin will also reduce VMCG.

I cant see any relation to weight, but it would be interesting to know any different...

:ok:

[tinmouse]

Thanks

Yes, I can’t find any relation to weight ether, the only relation would be with the arm between the CofG and the stabiliser (fin). I.E a short arm (aft CofG) would require a larger rudder input and perhaps a higher VMC. But other than that I can’t find any relation.

It’s an interview question that has been used, so I’m concerned that there may be a trick to it?

Cheers

[Nozwaldo]

Do you have a copy of Ace the technical pilot interview by G Bristow? If not, it's definitely worth the investment. It covers all topics.

Noz

[olliew]

No trick. In short a change of weight will not change VMCA/VMCG but a change of CG will. So if a change of weight alters the CG it is that which will change VMCs.

[SD.]

This is something that confused me during my brush up at BGS. I had been taught in the states that an increase of weight, decreases the Vmc. During the brush up, I had a feedback question that asked this question and the correct answer given was that Vmc is uneffected by weight. I asked one of the BGS instructors to clarify this.

From what I was told, the JAA does not teach the idea that Vmc decreases because it could lead to a false sense of security. For example, Joe Bloggs loads up of his Seneca to max take off weight thinking that his Vmc will be lower. Only to find when an engine quits, his single engine perfomance is ****e and doesn't climb.

From the FAA Multi-Engine oral guide by ASA............

Why does a change in weight affect Vmc?

Vmc is unaffected by weight in straight and level flight. Vmc will be affected by the aircraft's weight in turning (banked) flight. When an aircraft is banked, a component of of the aircraft's weight acts along with the horizontal component of lift to create a more effective slideslip towards the operative engine. For a given bank angle, the greater the aircraft's weight, the lower the aircraft's Vmc.


From what I can see, we are told to bank into the operative engine after an engine failure. If the aircraft is banked, then Vmc will change with an increase of weight. I'm far from an expert in the field and only passing on what I've been told by others. :ok:

[tinmouse]

Thanks,

That may partly answer the next question i'm unsure of?

If VMCG/A is limiting for the weight of the aircraft what can you do?

[SD.]

Unload a punter or syphon off some fuel ? :P

[Alex Whittingham]

SD's response is correct. The JAA examiners take the view that weight neither affects VMCG nor VMCA, the FAA think it does. You should search tech log for this, there are are number of carefully written and technically correct responses in the archives.

To the second question, what would you do if you were VMCA or VMCG limited, one answer that BA used would be to reduce the thrust. The -500 TriStar fitted with RB21-524B4s had an option to pretend that the engines were only the lower thrust -22B models and use the VMCG values for that fit. It meant that, if you lost an engine on take-off, you couldn't increase thrust on the live engine but it produced the bizzarre situation where payload was increased by reducing thrust.

[tinmouse]

Thanks everyone for your help on this, I think that definitely answers the question.

Much appreciated.


Cheers

Tinmouse.

[1Bingo]

select reheat!

[Alex Whittingham]

Tech log thread (http://www.pprune.org/forums/showthread.php?t=234326&highlight=weight+affect+vmcg)