Critical engine on 747
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Critical engine on 747
I would we thankful if another forum member could answer my question if the Boeing 747 has a critical engine and the reason why.
Thanks in advance for any replies.
Thanks in advance for any replies.
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Re: Critical engine on 747
Boeing say there isn't one, but essentially it's the upwind outboard engine in a crosswind.
With rudder already being used to counter the crosswind on the takeoff roll, failure of the upwind outboard engine leaves less than full rudder to use to counter the asymmetry from the engine failure.
There has been debate here with all the history and the FAA/ARB/JAR certification issues, but a failure of THAT engine during max crosswind is pretty low. A smarter user may post a link to the earlier debate.
G'day
With rudder already being used to counter the crosswind on the takeoff roll, failure of the upwind outboard engine leaves less than full rudder to use to counter the asymmetry from the engine failure.
There has been debate here with all the history and the FAA/ARB/JAR certification issues, but a failure of THAT engine during max crosswind is pretty low. A smarter user may post a link to the earlier debate.
G'day
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Re: Critical engine on 747
Well, which first single engine failure is the worst?
Logically, as the 747 engines are chosen to have roughly equal thrust, the failure of an outboard engine generates more asymmetric yaw than inboard engine out.
Then, if there is a crosswind, logically failure of an upwind engine is worse than failure of its downwind mate.
What about the torque?
In a piston prop or a turboprop, the major rotating part that interacts with air is the propeller, so naturally a propeller creates spiral wash which moves towards the direction of rotation - and interacts with the aerodynamic surfaces, too.
But turbofans? For one thing, B747 is well out of its jet blast, right?
For another, what is the direction of the torque created by a turbofan? The air passing the fan must spiral along the direction of rotation, as the fan is imparting speed on the air. But the bypass air moves some distance inside the bypass ducts, unlike the propwash of a turboprop, which is free to spiral around the engine core... then the air entering the core is compressed by the compressor, which should make it continue spiralling. But then the core jet is slowed down by the turbine. Would it also be losing its torque in the turbine?
Logically, as the 747 engines are chosen to have roughly equal thrust, the failure of an outboard engine generates more asymmetric yaw than inboard engine out.
Then, if there is a crosswind, logically failure of an upwind engine is worse than failure of its downwind mate.
What about the torque?
In a piston prop or a turboprop, the major rotating part that interacts with air is the propeller, so naturally a propeller creates spiral wash which moves towards the direction of rotation - and interacts with the aerodynamic surfaces, too.
But turbofans? For one thing, B747 is well out of its jet blast, right?
For another, what is the direction of the torque created by a turbofan? The air passing the fan must spiral along the direction of rotation, as the fan is imparting speed on the air. But the bypass air moves some distance inside the bypass ducts, unlike the propwash of a turboprop, which is free to spiral around the engine core... then the air entering the core is compressed by the compressor, which should make it continue spiralling. But then the core jet is slowed down by the turbine. Would it also be losing its torque in the turbine?
Re: Critical engine on 747
But if 1 or 4 fail, the relevant hydraullic system will be powered by the ADPs (classic) or Aux pumps (744), so this shouldn't be a problem. And the brakes can be powered by 4, 1 or 2.
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Re: Critical engine on 747
Originally Posted by chornedsnorkack
... what is the direction of the torque created by a turbofan? The air passing the fan must spiral along the direction of rotation, as the fan is imparting speed on the air. But the bypass air moves some distance inside the bypass ducts, unlike the propwash of a turboprop, which is free to spiral around the engine core... then the air entering the core is compressed by the compressor, which should make it continue spiralling. But then the core jet is slowed down by the turbine. Would it also be losing its torque in the turbine?
There's quite a bit of swirl at various spots within the core compressor and turbine, but not much by the time the gas exits the LP turbine. However, I think some CF6 models actually have some camber in the struts of the turbine frame, to recover the last bit of static pressure from the swirl.
Last edited by barit1; 12th Jan 2006 at 01:07.
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Re: Critical engine on 747
I totally agree with Feather#3, critical engine on a 747 is the upwind one.
In a no-wind situation there is no critical engine.
G'day
In a no-wind situation there is no critical engine.
G'day
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Re: Critical engine on 747
I cannot remember seeing it the manuals, but I heard from a sim instructor that the 747 with Rollers have a critical engine, because they are triple-spool. Because of the turning moment they generate, the rudder required is not the same if number 1 or 4 is lost, hence either # 1 or 4 lost means lower Vmcg/Vmca.
Don't ask me which of 1 or 4, I have forgot.
Don't ask me which of 1 or 4, I have forgot.
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Re: Critical engine on 747
Many relate the critical engine concept to local wind conditions.
If this were true, Vx would vary with wind, V(best glide) would vary with wind, V(best range) would vary with wind... and the list continues.
Speaking not as an airliner pilot, but as a teacher of Principles of Flight, it results in the best conceptual understanding if instructor pilots and student pilots stick to textbook and certification definitions. Otherwise, very quickly, there are no simple answers, and little understanding occurs.
"Which engine is critical?" - well sir that depends on the wind.
"All right, assuming nil wind, which engine is critical?" - well sir that depends on which side of the cambered runway the aircraft is on.
"Fine. In 5kts crosswind from the right and 0.12° slope to the left, at 78KTAS (Vr being 85KTAS), which is the critical engine?" - Oh I wouldn't know sir, that's very complicated mathematics.
If your textbook relates the critical engine concept to local wind conditions, it's fair to talk about it. Otherwise, perhaps not.
Cheers,
O8
If this were true, Vx would vary with wind, V(best glide) would vary with wind, V(best range) would vary with wind... and the list continues.
Speaking not as an airliner pilot, but as a teacher of Principles of Flight, it results in the best conceptual understanding if instructor pilots and student pilots stick to textbook and certification definitions. Otherwise, very quickly, there are no simple answers, and little understanding occurs.
"Which engine is critical?" - well sir that depends on the wind.
"All right, assuming nil wind, which engine is critical?" - well sir that depends on which side of the cambered runway the aircraft is on.
"Fine. In 5kts crosswind from the right and 0.12° slope to the left, at 78KTAS (Vr being 85KTAS), which is the critical engine?" - Oh I wouldn't know sir, that's very complicated mathematics.
If your textbook relates the critical engine concept to local wind conditions, it's fair to talk about it. Otherwise, perhaps not.
Cheers,
O8
Last edited by Oktas8; 15th Jan 2006 at 03:25. Reason: Edited for smilies to work...
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Re: Critical engine on 747
LGB, we operate with Rolls Royce B2/C2/D4/D4X engines on 4 different versions of the B747. Mr Boeing states that the aircraft doesnt have a critical engine.
Interesting concept 
Mutt
If your textbook relates the critical engine concept to local wind conditions, it's fair to talk about it. Otherwise, perhaps not.
Mutt
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Re: Critical engine on 747
Studying for CX interview hey? When I went for my interview, I answered this question with the upwind engine and the interviewer was pretty happy about that. That's all, don't give a complicated answer,K.I.S.S.
Re: Critical engine on 747
Failure of the upwind engine might be more "critical" but it would be much easier to handle than failure of a downwind engine.
Imagine taking off with a 30 kt crosswind from the left. Heaps of right rudder keeping it on the centreline with aileron into wind.
Now wind the clock forward a minute to when you've already suffered a failure of the right engine and you're safely climbing away at V2 tracking the extended runway centreline with heaps of left rudder and the control wheel level.
Imagine what must have transpired for this to happen with an engine failure at V1 or during rotation! (with regard to rudder and aileron input).
Imagine taking off with a 30 kt crosswind from the left. Heaps of right rudder keeping it on the centreline with aileron into wind.
Now wind the clock forward a minute to when you've already suffered a failure of the right engine and you're safely climbing away at V2 tracking the extended runway centreline with heaps of left rudder and the control wheel level.
Imagine what must have transpired for this to happen with an engine failure at V1 or during rotation! (with regard to rudder and aileron input).
Last edited by Blip; 16th Jan 2006 at 03:57.
Out of: 'Getting to grips with aircraft performance'
''JAAR/FAR 1.1: 'Critical Engine' means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft'', i.e. an outer engine on a four engine aircraft.
''JAAR/FAR 1.1: 'Critical Engine' means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft'', i.e. an outer engine on a four engine aircraft.
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I haven't been able to find it in the books now. But an experienced (RB211) 747 Classic sim instructor told me (as I wrote previously) that for the RB211, there is a slight difference between 1 and 4 and four, because it is a triple spool and has a turning moment. But since it is not in the manual, it must be something he dreamt up himself then. I remember he mentioned it in regard to the certificated crosswind component the aircraft have to be able to remain on the runway, and that it did matter which engine it was.
Good luck with the interview in any case ...
Good luck with the interview in any case ...
Agree with Matkat...as I am sure would those peanuts that were driving the Saudi 743 back from hangar to the terminal in KL a couple of years back... with only two and three alight
No prizes for guessing where they ended up...in the ditch!!
No prizes for guessing where they ended up...in the ditch!!
