Engine Failure in Xwinds

[Macgyver]

Hey guys,

Reading over "Ace the Tech Pilot Interview" right now and one of the questions kind of puzzled me.

Which engine is critical on t/o in a crosswind situation?

I always figured the upwind engine was critical. ex) Taking off with a xwind from the left, the a/c would tend to weathervane/yaw into the wind, to the left. If the left engine fails, the a/c will also want to yaw to the left. So you have the combined effects from the xwind and engine failure.

In the Ace Book, however, the author says its the opposite. That it's better to lose the upwind engine than the downwind one, and I dont really buy his explanation.

Any clarification??

Also, assume you were landing now. Is it better to land with a crosswind on the dead engine or operating engine's side?

Thanks in advance!

[ClippedCub]

Critical engine on jets is the upwind/outboard one. Have less rudder power available losing the downwind one, since some rudder is allocated to counter the crosswind as you said. Would have to give more thought to props with P factor, counter vs. co-rotating, which side, geometry in relation to prop wash effects on the vertical stabilizer, lose of lift on the upwind vs. the downwind wing if engine quit at rotation, ... Would still think upwind engine loss would be critical for prop twins, unless he's talking about losing powered lift at rotation.

Engine out crosswind landings would be a non-event unless you allow for go-around after touchdown. Then the takeoff case applies.

[Mad (Flt) Scientist]

Originally Posted by Macgyver

Hey guys,

Reading over "Ace the Tech Pilot Interview" right now and one of the questions kind of puzzled me.

Being puzzled by something in that book is a sign of understanding. It has been repeatedly noted here as being at best unreliable and at worst downright nonsense.

Do a search and check a few threads ... here's a couple to get started:

http://www.pprune.org/fragrant-harbo...interview.html

http://www.pprune.org/tech-log/31249...interview.html


With regard to the specific question, in general the upwind engine is critical for the basic aerodynamic reasons given. However, there may be circumstances where another engine may be critical, due to system configurations or unusual aerodynamic characteristics.

For example, if your NWS is powered by the hydraulics from a given engine such that you'll lose NWS when that engine fails, it may be that case which poses the greatest challenge for directional control.

Also, the basic reasoning assumes that the aircraft will weathercock into wind. This may not be the case under all circumstances.

[SNS3Guppy]

Reading over "Ace the Tech Pilot Interview" right now and one of the questions kind of puzzled me.

There's the problem. Put down the book. Finding something correct in that book is more a matter of happy coincidence, than an expectation.

Think about it. Which way does the airplane want to turn, with a crosswind, before airborne? Into the wind. Which way does an airplane want to turn with an engine failure? Into the failed engine. Marry the two concepts; during the ground roll if an upwind engine is lost, you have the worst of both worlds, and the worse case combination.

[Old Smokey]

Sadly, a number of Airline Interview panels take their questions straight out of the "Ace the Tech Pilot Interview" publication.

A "No Win" situation for the interviewee who does have all of the right answers!

Regards,

Old Smokey

[Flaperon777]

Mac,
Here's what you're looking for.
The weather vane effect or 'weather cock' effect is only and I repeat ONLY applicable when the airplane is on the ground.This is due to the main wheels acting as a pivot somewhere pretty close to the centre of gravity of the airplane.Therefore if hypothetically,you had a V1 of 99 kts and a Vr of 200 kts and a left engine out at 100 kts IAS combined with a nasty crosswind from the left,i'm quite certain that you'd have a darn tough time keeping the airplanne straight on the runway for takeoff.Combine that with a slippery runway and I could bet my last dollar that you'd be off the runway!
HOWEVER......things are quite different when in the air.Out 'there',in the air,its the drift associated with the engine fail that matters.And with the absence of a 'pivot' fulcrum point ie the main wheels on the runway,the plane does not weather cock,rather it 'drifts'.And this is where the simplest of laws of physics will apply.Wind from left,combined with failure of the rhs engine will simply drift the airplane to the right due to:
1)Loss of thrust on the rhs thus resultant vector to the right
2)More lift from the lhs due increased wind component from the lhs.Therefore resultant lift vector will make airplane roll AND yaw to the right...
To cut a long story short,the downwind engine IS the critical engine in case of EO(systems dependability notwithstanding of course!).
Same goes for Engine out landings...
Hope I could be of some help(albiet minuscle,but nevertheless)....
Happy X winds...

[kenparry]

Same goes for Engine out landings...

While airborne, yes. Once you land and apply asymmetric reverse, the situation changes. X-wind from the left, if you are using the left engine (of a twin) then that engine in reverse thrust will yaw you further to the left.

Years ago when doing base training on various twin Boeings, for the engine-out exercises, if x-wind was significant, in my company we would "fail" the downwind engine to do the V1 cut, then change to having the upwind engine fully available prior to the single-engine landing. This minimised the total yawing moments on both take-off and landing. As I recall, the exercise, for licensing purposes, required the use of reverse on the 1-e landing, so this was the safe way to go about it.

[ClippedCub]

1)Loss of thrust on the rhs thus resultant vector to the right

It's a moment to the right, not a vector. We're talking about the difference between Vmcg and Vmca. While airborne, the airplane sees no crosswind. The statement;

More lift from the lhs due increased wind component from the lhs.Therefore resultant lift vector will make airplane roll AND yaw to the right.../

Implies the airplane is in a slip, not a crosswind with wheels on ground.

[Flaperon777]

Ken,
Exactly my point.The airplane WILL behave differently (read opposite)to when on ground.Thats cause of the pivotal effect on the wheels on the runway.
Clipped,
I'm sorry but with all my imagination still in place,I have absolutely no idea what you're mumbling about.Sorry mate....my bad...

[ClippedCub]

Must be me if I'm not communicating.

Thought you were making a case of the downward engine being critical while airborne. My point was that there is no downwind engine when airborne, because the ball is centered.

[Dani]

Sadly, a number of Airline Interview panels take their questions straight out of the "Ace the Tech Pilot Interview" publication.

Especially in Singapore...

[bookworm]

The slip angle experienced during a crosswind take-off is going to have two major aerodynamic effects.

The upwind (i.e. into wind) yawing moment caused by the directional stability is one of them, and it's counteracted by rudder, or steering, depending on speed. But there's also a downwind slip force. If you're in the air, it's easy -- you bank upwind to provide an equal and opposite force, and you have a sideslip. But on the ground, with a constraint to keep the wings level until you rotate, the only thing stopping the aircraft from moving downwind is the lateral force from the wheels. Thus in a normal situation, crosswind might be limiting either through lack of rudder authority to keep it straight and avoid leaving the runway upwind, or through lack of weight on wheels to keep it from drifting off the runway downwind.

Engine failure will mitigate (downwind engine fails) or exacerbate (upwind engine fails) the total yawing moment that the rudder has to handle, and therefore the upwind engine is critical. You might naively think that the opposite would apply if the drift downwind is limiting. But, with wings level and tracking straight, I can't see why either should affect the lateral force, and I therefore can't see how Flaperon can justify "To cut a long story short,the downwind engine IS the critical engine in case of EO".

[ClippedCub]

If you're in the air, it's easy -- you bank upwind to provide an equal and opposite force, and you have a sideslip

You mean crab relative to heading, not side slip, but you get the point.

[bookworm]

You mean crab relative to heading, not side slip, but you get the point.

No, I mean side slip -- if you want both the heading and the ground track to remain aligned with the runway. But that constraint is only necessary on the ground. In the air, you could, of course, simply eliminate the lateral force by yawing into wind, but you can also oppose the lateral force by banking into wind.

[ClippedCub]

Must be an airline operations procedure. If you're in a pickle, once in the air, 'yawing' into the crosswind, keeping the ball centered, to maintain runway heading, imposes less drag on the airframe than banking into the wind keeping runway heading. Increases rate of climb after takeoff. Increases (T-D)/W.

[linksys]

Love to do a 25kt crosswind during a V1-cut, base-check.

[Flaperon777]

Bookworm,
My dear,I shall repeat my earlier post that during takeoff with a V1 cut, in the air the downwind engine IS the critical engine.But on the ground the opposite holds true.But how often do u stay on ground after V1.Therefore,to answer the question that Mac posted,AND for the purpose of this post only(we could go on forever with other variables),for a V1 cut,it is the downwind engine which is 'critical'.That is IF there is such a thing as 'critical engine' on jets anyways.
My 2 cents worth...

[bookworm]

ClippedCub

I'm sorry, I wasn't very clear. The idea of banking into the wind was merely to illustrate the presence of the lateral force that needs to be opposed when the aircraft has a slip angle. I wasn't suggesting that it's a good operational procedure. That said, in an aircraft that develops significant lift before rotation speed, banking into wind by lifting the downwind wheel may be the only way of staying on the runway, when the weight on the wheels is insufficient to oppose the slip force. It's just the reverse of a wing-down crosswind landing. There are (at least, light) aircraft that have sufficient rudder authority to remain straight, but are limited by their ability to avoid drifting off the runway before rotation.

Flaperon

I'm afraid I still don't follow your logic. Once in the air, the constraint to align heading with track is removed. The aircraft can simply yaw into wind, and there is no longer an "upwind" or "downwind" engine.