JET AIRPLANE CRITICAL ENGINE
Thread Starter
Join Date: Nov 2004
Location: bkk
Posts: 285
Likes: 0
Received 0 Likes
on
0 Posts
JET AIRPLANE CRITICAL ENGINE
It is well established that due to P factor a propellor engined aircraft, given certain prop rotation relative to the airplane body has a CRITICAL ENGINE for the failure case of one powerplant.However I have been asked, as an instructor on Jets, if the same concept applies to a Jet Airplane.I do not know of any reference to support this idea (Look at SKYBRARY.AERO for example, a very very good source of aviation knowledge), and asked how this question has arised.The universal reply was that certain Interview Panel Pilots in many airlines were asking this silly question and when the applicant said there isnt a critical engine on a jet aircraft they were told the answer is a definate "yes" and they had demonstrated poor technical knowledge! The only issue I can relate to this silly idea is if you have a significant crosswind then the UPWIND ENGINE if it fails might be considered the more critical failure as you have the weathervane effect coupled with the engine swing BOTH COMBINING thus requiring a larger rudder application to keep straight etc.However every manufacturer ive talked to said THERE IS NO SUCH THING AS A CRITICAL ENGINE on a Jet airplane, so the argument is not valid from these "PANEL EXPERTS", its rubbish.Any comments, thoughts?
only situation i can think of when you might have a critical engine is during a crosswind takeoff. Crosswind from the left, using left rudder to keep aircraft aligned with centreline, Right engine fails, less rudder travel available for the asymmetric thrust
Join Date: Jan 2003
Location: By the Sea
Posts: 31
Likes: 0
Received 0 Likes
on
0 Posts
I have also been posed this question. Aerodynamically (apart from the crosswind mentioned before) there should be no difference. There is however a difference measured by what systems are supported by which engine. In some aircraft the hydraulic systems supported by each engine may result in a downgrade of the controlability if say the right hand engine should fail compared to the left hand one. This may be considered to be a critical failure as a result of some downgrading of your redundancy, or reduced controlability.
However every manufacturer ive talked to said THERE IS NO SUCH THING AS A CRITICAL ENGINE on a Jet airplane
Join Date: Sep 1998
Location: wherever
Age: 55
Posts: 1,616
Likes: 0
Received 0 Likes
on
0 Posts
You are correct.
In certification terms there is no critical engine.
However, a quick search on here will uncover lots of knowledge from the horses mouth about the effect of crosswind.
If I were posed the same question at interview I would answer in two parts. Firstly certification standards for VMC and secondly the reality of crosswind in controllability. If it were a UK operator I would mention how the old UK standard did in fact consider crosswind (7kts if memory serves) but how that is no longer the case.
In certification terms there is no critical engine.
However, a quick search on here will uncover lots of knowledge from the horses mouth about the effect of crosswind.
If I were posed the same question at interview I would answer in two parts. Firstly certification standards for VMC and secondly the reality of crosswind in controllability. If it were a UK operator I would mention how the old UK standard did in fact consider crosswind (7kts if memory serves) but how that is no longer the case.
Thread Starter
Join Date: Nov 2004
Location: bkk
Posts: 285
Likes: 0
Received 0 Likes
on
0 Posts
The context of the question regarding CRITICAL ENGINE on a Jet is with respect to a failure at V1.I dont see any way to validate the idea of a critical engine on a jet with respect to PERFORMANCE or aircraft handling (like on a prop),although im told if its say a 747 then systems loss due to engine number 1 causes problems, I dont know, I dont fly it, Peter.
I've not flown the 744 for a while so am open to correction but as I recall it performance aside a straight forward run down/ failure of number 1 ( or any one of the 4 for that matter ) shouldn't cause major problems on the 747 (400 anyway) since you shouldn't lose any major systems. I certainly wouldn't think of it as a "critical engine" in the context of this thread.
What you might have heard is that it gets a bit more interesting if for some reason you lose the #1 engine and the associated hydraulic system ( and to emphasise on a straight forward engine failure you shouldn't) since amongst other things the #1 hydraulic system is needed to raise the nose and body gear.
HTH
Last edited by wiggy; 15th Sep 2016 at 07:53.
Gender Faculty Specialist
Thread Starter
Join Date: Nov 2004
Location: bkk
Posts: 285
Likes: 0
Received 0 Likes
on
0 Posts
Just to remove any doubt about the context of the original question, it was ALWAYS asked with respect to an engine failure at or very near (just after) V1 for the continued takeoff case in conjunction with the correct meaning of both V2 and V3.Peter.
Only half a speed-brake
From AB's Getting to Grips with A/C Performance:
There are 26 instances of "critical engine" in that document, all of them being paste-copy quotes of regulations. Not a single mention that there is actually such thing, except from the very one I quoted above.
“JAR/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.
I suppose the concept applies to jets as with a non-zero angle of the thrust line to the relative airflow, you will get p-factor effects from the fans. I assume that the magnitude of those effects is small compared to everything else, especially as the intake ducts probably reduce them considerably by ‘bending’ the airflow to be more perpendicular to the fan disc. Engines on commercial jets tend to rotate in the same direction, so the potential for slight asymmetry is there but appears to be disregarded.
It seems to me that there are two common usages of “critical” when it comes to power plants: One describes the intrinsic properties of an an engine + airframe combination and the other takes the operating environment into consideration as well, e.g. crosswinds.
If you really wanted to nit-pick the question, you could argue that at V1 you are still on the ground, so in many installations the thrust line is parallel to the airflow. It’s only during rotation that p-factor effects increasingly come into play and you would have hoped that you’d be nearer V2 than V1 by the end of the manoeuvre...
It seems to me that there are two common usages of “critical” when it comes to power plants: One describes the intrinsic properties of an an engine + airframe combination and the other takes the operating environment into consideration as well, e.g. crosswinds.
If you really wanted to nit-pick the question, you could argue that at V1 you are still on the ground, so in many installations the thrust line is parallel to the airflow. It’s only during rotation that p-factor effects increasingly come into play and you would have hoped that you’d be nearer V2 than V1 by the end of the manoeuvre...
Join Date: Jun 2006
Location: Brisbane
Posts: 265
Likes: 0
Received 0 Likes
on
0 Posts
only situation i can think of when you might have a critical engine is during a crosswind takeoff. Crosswind from the left, using left rudder to keep aircraft aligned with centreline, Right engine fails, less rudder travel available for the asymmetric thrust
This question was included in the JAR ATPL POF question bank for many years and it is probably now in the EASA question bank.
In the case of airline interviews I suspect that the questioner is attempting to test whether the candidates knowledge and understanding have expanded beyond the basic propeller theory.
If the candidate only considers the term “Critical Engine” to be applicable to multi-engine propeller aircraft, then the questioner will probably conclude that he/she has not given sufficient thought to the effects of engine failure on multi-engine aircraft.
But if the candidate is aware that the generally accepted definition of the term Critical Engine is something along the lines of “The engine who’s failure would most adversely affect the performance and/or handling of the aircraft” then he/she should be able to discuss effects such as crosswinds and system layouts. A candidate who is able to do this is more likely to convince the questioner that he/she is the right one for the job.
In the case of airline interviews I suspect that the questioner is attempting to test whether the candidates knowledge and understanding have expanded beyond the basic propeller theory.
If the candidate only considers the term “Critical Engine” to be applicable to multi-engine propeller aircraft, then the questioner will probably conclude that he/she has not given sufficient thought to the effects of engine failure on multi-engine aircraft.
But if the candidate is aware that the generally accepted definition of the term Critical Engine is something along the lines of “The engine who’s failure would most adversely affect the performance and/or handling of the aircraft” then he/she should be able to discuss effects such as crosswinds and system layouts. A candidate who is able to do this is more likely to convince the questioner that he/she is the right one for the job.
