747 critical engine

[mixomax]

Hy there


Is ther such a thing as a critical engine on the 747( all models)?
and why?
Also in regard to Vmcg and Vmca,witch one is higher and does the same rule apply when 2 engines are inop?

Thank you all

[Dan Kelly]

Usually on a 4 engined jet aircraft the critical engine is the outboard engine on the upwind side. This is because should it fail, asymmetric thust (yaw) towards the dead engine will be assisted by the crosswind component's yawing effect on the fin and aft fuselage area.

[Medwin]

You got it backwards Dan, the outboard downwind engine is the critical engine because this is the engine that compensates for the yaw. When you have right x-wind the aircraft yaw to the left so its better to lose the upwind engine.

[catchup]

When you have right x-wind the aircraft yaw to the left

Sorry, are you sure?

regards

[Capt Claret]

Medwin

Are you sure Dan's wrong?

It seems to me that you're confusing the effect of a crosswind in pushing the aircraft downwind, with the effect of the crosswind acting on the aft fuse/keel/fin in conjunction with an asstmetric power situation.

[WindSheer]

I think your probably all right but mis-wording what you are saying OR what you are saying is being mis-interpreted.

:confused:

[Viscount Sussex]

Medwin

I don't really get involved in this kind of thing , normally, but Dan Kelly is right.
The outboard engine into wind is the critical engine on a four engine aircraft.
VS.

[Bellerophon]

Medwin

You are the one who has got it backwards.

Dan Kelly is correct, and has given the correct reasons.

Regards

Bellerophon

[Medwin]

Ok Ok I admit I was reading a wrong book. If you guys have the book that Gary Bristow wrote, throw it away.

[FE Hoppy]

No.
There is no critical engine on a jet.
At least not for certification.

[grind king]

Dan Kelly you are correct.
If a departing a/c had a x-wind from left to right , it would have a tendancy to want to weather cock into the wind thus yaw to the left. This is impart due to the airflow impinging on the tail. Thus right rudder would be required to keep the nose pointed straight down the runway. If an outboard up-wind engine (left) were to fail this would produce the most amount of unwanted
yaw with the least amount of rudder available due to a portion of the rudder deflection already used for x-wind directional control.

As for which is higher on the 747 Vmcg or Vmca. It is my understanding that Vmcg (on ground directional control) is higher. This is because the moment arm from the main landing gear to the rudder is shorter than from the CofG to the rudder. (Used in directional control in the air.) In addition when airborne you have the additional assistance of five degrees bank into the live engine.
Hope this helps,
Grind King

[john_tullamarine]

Several points, some of which have been made above -

(a) this discussion is not related to certification, rather to a practical operational consideration

(b) same thing happens 2-, 3-, and 4-engine aircraft. Keep in mind that this crosswind effect on Vmcg can be significant. DC9 comes to mind at an increase of around 0.5*crosswind value and, I think, someone referred to a 4-engined aircraft having an effect in excess of the crosswind value - sobering thought, is it not ?

(c) no necessary relationship between Vmcg and Vmca (or Vmc, depending on your preferences). Mad (Flt) Scientist wrote up a good brief on the subject in a similar thread (http://www.pprune.org/forums/showthread.php?s=&threadid=136625) which is worth a read.

[Arctaurus]

On the 747, with the large diameter of the front fan, is there any additional effect on which is the critical engine, due to the direction of rotation of the engines ? :confused:

[nitro rig driver]

i thought they are all critical on takeoff
thats why they are there:confused:

[Willit Run]

At the weights we fly, trust me! they are all critical. Been flying freight for near 20 years, and the fully loaded 747 sees more red lights at the end of the runway than i care to see.
The sphinctor factor!