Do jets have a critical engine

[The Hairy Log Book]

I understand the basics behind a light twin (non counter rotating) having a critical engine. Does a jet, particularly a twin turbo jet have a critical engine?

[mjbow2]

Yep....

the one thats still turning!

[404 Titan]

Certainly a four engine jet with an engine layout like a B747/A340 has two critical engines due to aerodynamic reasons. The outer engines are critical. If there is any crosswind then the upwind outboard engine is the most critical. With a twin jet the upwind engine is critical for the same reason in a crosswind. Other considerations are systems, i.e. hydraulics, electrics and pneumatics. An engine failure with one particular engine “could” be more critical than the other.

[Gear in transit]

I was thinking T.H.L.B was meaning from a yawing point of view.

[enicalyth]

The one you've just shut down can be critical too maybe... recent incident report pending on four-holer gone to three-holer. Just a hunch that the fuel transfer "problem" might've had something to do with that.

[Piltdown Man]

No, the bits are designed to be neutrally balanced and no nasty prop-wash. And the automatics handle the job perfectly!

[Speeds high]

Why is the upwind engine critical?

[*Lancer*]

The upwind engine is critical because...

You need rudder to counter crosswind on takeoff, and;

You need rudder to counter engine failure on takeoff.

If you need the same rudder to counter BOTH, you run out of it faster... An upwind engine failure will require more rudder than a downwind engine failure because you already need some of that rudder to counter the wind.

On a maximum weight 747, with maximum crosswind, and a critical engine failure (outboard upwind) at V1, it is impossible to remain on the runway.

[Capt Claret]

Speeds high

The critical engine on a jet is the upwind engine, and if four engines (wing mounted) the outboard one.

Imagine taking off on RWY18 with a 30kt easterly blowing. With the tail fin and fuselage aft of the wing/landing gear generally being a larger area than the fuselage forward of the wing/landing gear the tendancy is for the aircraft to weathercock into wind (left in this example). Much as would happen in flight should the nose yaw one way or t'other.

If the upwind engine fails on takeoff the down-wind engine (asymmetric) will combine with the weathercocking effect so you'd get (x-wind + asymmetric thrust wrking in the same direction.

Should the down-wind engine fail, the upwind engine yawing moment will be counteracted to some degree by the weather-cocking effect, ie the yawing force is x-wind minus asymmetric thrust.

[Capt Fathom]

On a maximum weight 747, with maximum crosswind, and a critical engine failure (outboard upwind) at V1, it is impossible to remain on the runway.
Oh really? Do Boeing and all the 747 operaters know of this flaw ?

[Capt Basil Brush]

I was under the belief that V1 has to be > Vmcg.

[Feather #3]

The Boeing/FAA argument is that suitable application of full rudder with an upwind failure at max x/wind will realign the a/c downwind of the centreline at best and at worst should be airborne by the time the a/c leaves the runway.

The ARB argument has sliding scale reductions which would impinge on payload.

Guess who most airlines/States follow?

To be fair, you have to have both a failure right AT V1 and a max crosswind to cause the problem. The FAA say that this is statistically highly unlikely. For the past 35 years of my career they've been right.

G'day ;)

[Speeds high]

thanks all for the good oil

[*Lancer*]

Capt Fatham, its not a certification requirement... Feather #3 summed it up. At max weight, the difference between V1 and Vr is large enough that the aircraft would probably have to rotate early to avoid departing the runway edge.

Capt Basil Brush (seem to be a lot of captains here), Vmcg is based on nil wind...

[Wizofoz]

There was a similar question some time ago on Tech Log, and a very knowledable person (John Farley, ex Harrier test pilot) pointed out there is a very small amount of "P" factor in a jet, as the eflux swirls. I think the consensus was that it was negligable.

[DirectAnywhere]

Sorry Lancer, I disagree. On a maximum weight 747, with maximum crosswind, and a critical engine failure (outboard upwind) at V1, it MAY WELL BE POSSIBLE to remain on the runway.

The directional control issue is more critical at light weights - assuming thrust is not de-rated for takeoff.

At heavy weights V1 will be significantly higher than Vmcg - 25 odd kts. in a jumbo. If an engine fails at a higher V1 less initial yaw will wil be experienced due to the higher directional stability at the higher speed and more control force is available to counteract it.

The most critical situation for directional control is at light weights where V1 = Vmcg. In this case, assuming standard certification parameters, by definition maximum rudder deflection is required to maintain directional control.

Worst case scenario is where the upwind engine fails, in a crosswind, it fails at V1 which happens to = Vmcg, and thrust has not been de-rated for takeoff. In this case insufficent rudder is available to maintain directional control. However, in this case, V1 will be fairly close to Vr and the aircraft should accelerate to Vr quite rapidly due to the light weight.

In short, I believe the directional control is initially more critical in a light aeroplane than a heavy one around V1 but is a problem (on the ground at least) for less time!