Critical engine (jet)
Take-off segment from above taxiing speed until rudder effective - Tricycle L/G
Quote from chornedsnorkack:So... on speeds above 60...80 knots IAS, aerodynamic forces become effective. On taxi speeds, nosewheel steering can stop weathercocking.
How is a tricycle aircraft kept on a runway in strong crosswinds at speeds above taxi speeds but below the speeds where rudder becomes effective?
[Unquote]
You have quite rightly drawn attention to the gap in my narrative relating to aeroplanes with TRICYCLE L/G thanks.
[I can't help with your query on tandem L/G configurations, like the B-52; but presumably the front wheel-set is steerable? Know nothing, sadly, of the (aah) Northrop B-2.]
The awkward segment of the crosswind take-off between taxiing speed, and the speed at which aerodynamic rudder becomes effective is worthy of explanation. It's not going to be a brief one, I'm afraid...
What I might have written was something like this:
It may be pointed out that some such airliners, in the past, have had fully-castoring nosewheels with no steering capability. The D.H. Heron (4-engine 15-seater) and Dove spring to mind. Fortunately, they were propeller-driven (see below).
The key difference between the early part of the take-off run and normal taxiing is that the use of differential braking is, strictly speaking, not an option. Until the rudder becomes effective aerodynamically, nosewheel steering makes the major contribution to directional control. On a propeller-driven aircraft, propeller slipstream usually provides useful rudder control from the start (although the slipstream itself induces yaw, unless the propellers are "handed").
No such contribution is made by a jet engine. On jets, therefore, rudder effectiveness is roughly a function of indicated airspeed (IAS) squared.
On any multi-engine aeroplane (except with engines in tandem, or "piggy-back" **) it is possible, to a limited extent, to pre-empt weathercocking during the period of engine acceleration by doing a rolling take-off, and gently advancing the upwind throttle(s) slightly ahead of the other(s). However, this slow-acceleration technique will increase the length of runway used, so may not be possible on the day.
AIRCRAFT WITH TILLER STEERING ONLY
Tiller steering is suitable for steering and to resist weathercocking at taxiing speeds, and at the beginning of the take-off run. Above about 40 kts ground-speed, however, it becomes progressively more difficult not to over-control in yaw. This problem is increased on a wet runway, particularly if the runway friction is patchy. The painted centreline itself can cause intermittent loss of tyre adhesion. [Not a good point to have an engine failure, particularly on the upwind side, as discussed in previous posts.]
From about 60 kts IAS, the rudder will start to become effective on most large jets. So, for a given crosswind component, it is arguable that the worst case is when the wind is at right angles to the runway or slightly beyond, giving a tail-wind component. In these situations, there is no headwind to amplify IAS and rudder control in the early stages.
There is no doubt that, on many such older types, there is a period on the take-off when you are too fast for reliable tiller steering, but may not be fast enough for yaw control by rudder in a limiting gust. I am not sure that this was fully taken into account during certification.
AIRCRAFT WITH RUDDER-FINE STEERING
Once lined up on the runway, the pilot removes hand from tiller. On modern jets, the steering-control computer modulates the rudder-fine steering according to the ground speed (GS). Early on, the pilot cannot make full use of any available aerodynamic rudder, because the steering would veer the airplane off the runway. As the GS rises, the nosewheel angle provided by a given deflection of the rudder pedals (which dictate rudder angle) is weaned off, to avoid over-control. This allows the pilot to start to get the feel of using the rudder, but not much.
On the A320, if memory serves, the nosewheel steering is switched off completely above 72 kts GS. At this point, the pilot may well need an increase in downwind rudder to counter weather-cocking using aerodynamics alone. It's worth pointing out that, in the tailwind-component case, particularly at hot-high airfields, the IAS could still be as low as 50 kts.
Hope this helps.
** e.g., English Electric P1B Lightning?
How is a tricycle aircraft kept on a runway in strong crosswinds at speeds above taxi speeds but below the speeds where rudder becomes effective?
[Unquote]
You have quite rightly drawn attention to the gap in my narrative relating to aeroplanes with TRICYCLE L/G thanks.
[I can't help with your query on tandem L/G configurations, like the B-52; but presumably the front wheel-set is steerable? Know nothing, sadly, of the (aah) Northrop B-2.]The awkward segment of the crosswind take-off between taxiing speed, and the speed at which aerodynamic rudder becomes effective is worthy of explanation. It's not going to be a brief one, I'm afraid...
What I might have written was something like this:
It may be pointed out that some such airliners, in the past, have had fully-castoring nosewheels with no steering capability. The D.H. Heron (4-engine 15-seater) and Dove spring to mind. Fortunately, they were propeller-driven (see below).
The key difference between the early part of the take-off run and normal taxiing is that the use of differential braking is, strictly speaking, not an option. Until the rudder becomes effective aerodynamically, nosewheel steering makes the major contribution to directional control. On a propeller-driven aircraft, propeller slipstream usually provides useful rudder control from the start (although the slipstream itself induces yaw, unless the propellers are "handed").
No such contribution is made by a jet engine. On jets, therefore, rudder effectiveness is roughly a function of indicated airspeed (IAS) squared.
On any multi-engine aeroplane (except with engines in tandem, or "piggy-back" **) it is possible, to a limited extent, to pre-empt weathercocking during the period of engine acceleration by doing a rolling take-off, and gently advancing the upwind throttle(s) slightly ahead of the other(s). However, this slow-acceleration technique will increase the length of runway used, so may not be possible on the day.
AIRCRAFT WITH TILLER STEERING ONLY
Tiller steering is suitable for steering and to resist weathercocking at taxiing speeds, and at the beginning of the take-off run. Above about 40 kts ground-speed, however, it becomes progressively more difficult not to over-control in yaw. This problem is increased on a wet runway, particularly if the runway friction is patchy. The painted centreline itself can cause intermittent loss of tyre adhesion. [Not a good point to have an engine failure, particularly on the upwind side, as discussed in previous posts.]
From about 60 kts IAS, the rudder will start to become effective on most large jets. So, for a given crosswind component, it is arguable that the worst case is when the wind is at right angles to the runway or slightly beyond, giving a tail-wind component. In these situations, there is no headwind to amplify IAS and rudder control in the early stages.
There is no doubt that, on many such older types, there is a period on the take-off when you are too fast for reliable tiller steering, but may not be fast enough for yaw control by rudder in a limiting gust. I am not sure that this was fully taken into account during certification.
AIRCRAFT WITH RUDDER-FINE STEERING
Once lined up on the runway, the pilot removes hand from tiller. On modern jets, the steering-control computer modulates the rudder-fine steering according to the ground speed (GS). Early on, the pilot cannot make full use of any available aerodynamic rudder, because the steering would veer the airplane off the runway. As the GS rises, the nosewheel angle provided by a given deflection of the rudder pedals (which dictate rudder angle) is weaned off, to avoid over-control. This allows the pilot to start to get the feel of using the rudder, but not much.
On the A320, if memory serves, the nosewheel steering is switched off completely above 72 kts GS. At this point, the pilot may well need an increase in downwind rudder to counter weather-cocking using aerodynamics alone. It's worth pointing out that, in the tailwind-component case, particularly at hot-high airfields, the IAS could still be as low as 50 kts.
Hope this helps.
** e.g., English Electric P1B Lightning?
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Weathercocking / Drift
Hi all!
About weathercocking and drift... it probably depends on which "slang" are you going to use.
In the aeronautical engineering slang... weathercocking is possible once airborne as well, not only while on the ground!
Indeed, "weathercock stability" is the nickname given to tendency of the aircraft to "naturally" align its nose into the wind. Suppose you are flying straight and level with no sideslip, in perfect simmetrical airflow conditions. Any disturbance to this airflow simmetry (ie due to a small wind gust, or turbulence, or a short input on the rudder...) would induce a sideslip, and your aircraft would react putting its nose into the wind. BTW, in most cases the fuselage+wing alone is not stable, and the yawing moment mostly comes from your vertical fin. Anyway, there are many others factors which at the end sum up to give the final response, so even a (well-designed) flying wing will be stable around its vertical axis.
About weathercocking and drift... it probably depends on which "slang" are you going to use.
In the aeronautical engineering slang... weathercocking is possible once airborne as well, not only while on the ground!
Indeed, "weathercock stability" is the nickname given to tendency of the aircraft to "naturally" align its nose into the wind. Suppose you are flying straight and level with no sideslip, in perfect simmetrical airflow conditions. Any disturbance to this airflow simmetry (ie due to a small wind gust, or turbulence, or a short input on the rudder...) would induce a sideslip, and your aircraft would react putting its nose into the wind. BTW, in most cases the fuselage+wing alone is not stable, and the yawing moment mostly comes from your vertical fin. Anyway, there are many others factors which at the end sum up to give the final response, so even a (well-designed) flying wing will be stable around its vertical axis.
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PWK B707 accident 1977
"The main gear then contacted the runway again (lots of drag as much aileron in use) and folded sideways at which point a decision was made to abandon the take off."
Wonderfully understated firefly! From my comfortable P4 seat that seemed a great 'decision' at the time. As we slid on fire down RW13 (heading c.220) I called for the thrust levers to be closed and I reached over to the F/E panel to switch off the LP cocks. Somewhat futile gestures as all four engines had already deployed in various directions across PWK. Flying with wings level was very much an accepted B707 technique after engine failure on the line at that time. It wasn't until after the crash that the massive increase in VMCA with small amounts of bank the 'wrong way' was re-emphasised to Base Trainers. Rumour was that Dai Davis knew all about it but Boeing supressed it as commercially sensitive data. The crosswind that day certainly masked the initial effect of the simulated engine failure. BA subsequently recommended failures to be simulated either while the a/c was still firmly on the ground or safely airborne well after V2 achieved and to be very aware of the crosswind!
Wonderfully understated firefly! From my comfortable P4 seat that seemed a great 'decision' at the time. As we slid on fire down RW13 (heading c.220) I called for the thrust levers to be closed and I reached over to the F/E panel to switch off the LP cocks. Somewhat futile gestures as all four engines had already deployed in various directions across PWK. Flying with wings level was very much an accepted B707 technique after engine failure on the line at that time. It wasn't until after the crash that the massive increase in VMCA with small amounts of bank the 'wrong way' was re-emphasised to Base Trainers. Rumour was that Dai Davis knew all about it but Boeing supressed it as commercially sensitive data. The crosswind that day certainly masked the initial effect of the simulated engine failure. BA subsequently recommended failures to be simulated either while the a/c was still firmly on the ground or safely airborne well after V2 achieved and to be very aware of the crosswind!
Moderator
am I correct in my understanding that Vmcg is predicated on failure of the 'worst' engine?
indeed - worst case situation all round, other than the concession to crosswind. Reference the current FAR25 requirement. Further "how it's done" details are in the FTG ... but be prepared for a good read ...
Rumour was that Dai Davis knew all about it but Boeing supressed it as commercially sensitive data
That doesn't make much sense. The effect of bank is across the board and not particular to the 707 or any other bird. Figures of 30-50 kt penalty are not uncommon on larger four motor birds. If one is playing about with Vmca things, then one had best have a modicum of bank on ... worth keeping in mind that, if things start to run awry, increasing bank should/may improve the directional control problems .. at the (probably significant) expense of the desired going up/down numbers .. but, then again, it is all a case of priorities .. and the main priority is to die as late as practicable.
indeed - worst case situation all round, other than the concession to crosswind. Reference the current FAR25 requirement. Further "how it's done" details are in the FTG ... but be prepared for a good read ...
Rumour was that Dai Davis knew all about it but Boeing supressed it as commercially sensitive data
That doesn't make much sense. The effect of bank is across the board and not particular to the 707 or any other bird. Figures of 30-50 kt penalty are not uncommon on larger four motor birds. If one is playing about with Vmca things, then one had best have a modicum of bank on ... worth keeping in mind that, if things start to run awry, increasing bank should/may improve the directional control problems .. at the (probably significant) expense of the desired going up/down numbers .. but, then again, it is all a case of priorities .. and the main priority is to die as late as practicable.
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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.
adversely affect the performance or handling qualities of an aircraft, i.e. an outer engine on a four engine aircraft.
From AI GTG W/ PERFORMANCE book.
Note the i.e. at the last sentence.
( i.e. <> " this is... ". and NO other ref. to twin jets is made ).
Hope it helps.
Last edited by BESTGLIDESPEED; 27th Feb 2008 at 09:22.
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Re the PWK B707 commentary;
One of our trainers decided to fail the upwind outboard on one of ours at Avalon with a 30kt x/wind during a license renewal sequence in the '70's. The checkee [a man of some considerable experience] grabbed the thrust lever and pushed it back up with the immortal words "Don't be so bl**dy stupid!"
G'day
BTW he passed the check!
One of our trainers decided to fail the upwind outboard on one of ours at Avalon with a 30kt x/wind during a license renewal sequence in the '70's. The checkee [a man of some considerable experience] grabbed the thrust lever and pushed it back up with the immortal words "Don't be so bl**dy stupid!"
G'day
BTW he passed the check!
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@ BestGlideSpeed
Note that that definition is preceded by, at the start of the section:
In other words, the words in the FARs mean whatever you want them to mean.
On a general note, some of the FARs are very badly written, and there are inconsistencies between them even when they make sense individually; you simply can't make a logical textual analysis, such as deriving from that "i.e." that it doesn't apply to anything else.
@ Feather #3
Wonder if he'd have passed today?
Note that that definition is preceded by, at the start of the section:
As used in Subchapters A through K of this chapter, unless the context requires otherwise:
On a general note, some of the FARs are very badly written, and there are inconsistencies between them even when they make sense individually; you simply can't make a logical textual analysis, such as deriving from that "i.e." that it doesn't apply to anything else.
@ Feather #3
Wonder if he'd have passed today?
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@ mad (flt ) scientist,
Correct me if Iīm wrong but ,
I had understood that i.e. meant " ist ed ", meaning from latin : " this is " ( very aproximately ),
NOT meaning " for example " ( which, I suppose would allow us to doubt its contundency, as you say ).
Rgrds.
Correct me if Iīm wrong but ,
I had understood that i.e. meant " ist ed ", meaning from latin : " this is " ( very aproximately ),
NOT meaning " for example " ( which, I suppose would allow us to doubt its contundency, as you say ).
Rgrds.
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You are grammatically correct, that's exactly what "i.e." is supposed to mean.
But the bit at the start of the definitions list provides a universal "get out" clause, and the reality is that the FARs simply cannot be read with a strict understanding; there is so much "unwritten" in the FARs that you simply can't just read the FARs and understand the rules. I agree that's the way it should be, but it isn't the actual reality of aircraft certification.
But the bit at the start of the definitions list provides a universal "get out" clause, and the reality is that the FARs simply cannot be read with a strict understanding; there is so much "unwritten" in the FARs that you simply can't just read the FARs and understand the rules. I agree that's the way it should be, but it isn't the actual reality of aircraft certification.
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Well, OK.
I guess thereīs room for that kind of doubt.
( Anyway, How hard are we supposed to look into meanings ?. Shouldnīt we start making responsible for the putting into practice of these law-paragraphs to the same ones who write them ? What are we after all ? lawyers ? )
( If thereīs something not-written, then I canīt imagine the one who wrote it expecting me to imagine something else than whatīs on the paper. I donīt think thereīs any judge who can, in the end, have you busted for "not-imagining" .)
Just rather go by-the-book as much as possible.
Donīt wanna give others the chance to blame me for something that wasnīt even written by the one who was getting paid to be responsible for that.
Rgrds.
I guess thereīs room for that kind of doubt.
( Anyway, How hard are we supposed to look into meanings ?. Shouldnīt we start making responsible for the putting into practice of these law-paragraphs to the same ones who write them ? What are we after all ? lawyers ? )
( If thereīs something not-written, then I canīt imagine the one who wrote it expecting me to imagine something else than whatīs on the paper. I donīt think thereīs any judge who can, in the end, have you busted for "not-imagining" .)
Just rather go by-the-book as much as possible.
Donīt wanna give others the chance to blame me for something that wasnīt even written by the one who was getting paid to be responsible for that.
Rgrds.
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The stuff is written down. Just not in the regs! It's in the ACs, or Special Conditions, or Issue Papers, or "industry practice", or any of a million other places.
Part of the problem is that not only are "we" not lawyers, but neither are the people who write the regs - they're largely written by engineers for engineers, certainly on the certification side. (And by pilots for pilots I guess for operational regs). There's lots of places where a basic understanding is assumed in the regs rather than spelled out clearly and unambiguously.
Actually, in some ways I'm happy it's like that; if all it took to understand and apply the regs was basic literacy skills, I'd be likely out of a job!
Part of the problem is that not only are "we" not lawyers, but neither are the people who write the regs - they're largely written by engineers for engineers, certainly on the certification side. (And by pilots for pilots I guess for operational regs). There's lots of places where a basic understanding is assumed in the regs rather than spelled out clearly and unambiguously.
Actually, in some ways I'm happy it's like that; if all it took to understand and apply the regs was basic literacy skills, I'd be likely out of a job!
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Totally agree w/ u on that one, Sci.
Nobody happier than me that it is not lawyers to write them down. ( even though you shouldnīt doubt itīs constructor company lawyers who review most of them in order to preserve companies from unnecesary responsabilities in unwanted cases. You should see some AIRBUS abn. ops. proc. headings ! ( sound more like legal warnings) )
Anyway, donīt wanna get way too far from the subject here, let me give another ref. here, just to get back on the thread ,
even though this oneīs got me completely puzzled ( big black hole in my knowledge most likely ).
It comes from the " ace the tech. pilot int. " book that most of all of us know way too well , therefore, canīt trust it much either. :
" there is NO critical gas turbine engine because the engines are positioned simetrically with opposing revolution direction "
Now Iīm lost.... ( opposing rev. direction?! . What has this got to do w/ a fan. Do we get assimetric blade effect out of a non-lift but THRUST producer such as a fan COMPRESSOR ? )
(Good to have this conversation.)
Rgrds.
Nobody happier than me that it is not lawyers to write them down. ( even though you shouldnīt doubt itīs constructor company lawyers who review most of them in order to preserve companies from unnecesary responsabilities in unwanted cases. You should see some AIRBUS abn. ops. proc. headings ! ( sound more like legal warnings) )
Anyway, donīt wanna get way too far from the subject here, let me give another ref. here, just to get back on the thread ,
even though this oneīs got me completely puzzled ( big black hole in my knowledge most likely ).
It comes from the " ace the tech. pilot int. " book that most of all of us know way too well , therefore, canīt trust it much either. :
" there is NO critical gas turbine engine because the engines are positioned simetrically with opposing revolution direction "
Now Iīm lost.... ( opposing rev. direction?! . What has this got to do w/ a fan. Do we get assimetric blade effect out of a non-lift but THRUST producer such as a fan COMPRESSOR ? )
(Good to have this conversation.)
Rgrds.
Moderator
The stuff is written down. Just not in the regs
The difference lies in the way the regs are drafted.
If they are prescriptive, they will detail how you are to blow your nose .. a paternalistic approach to life such as we had in Australia years ago
If outcome based rather they will say that you are required to blow your nose ... suggestions as to how you might do that are found elsewhere, typically the ACs. The onus is on the end user folk to achieve the intent of the regulatory requirement.
The difference lies in the way the regs are drafted.
If they are prescriptive, they will detail how you are to blow your nose .. a paternalistic approach to life such as we had in Australia years ago
If outcome based rather they will say that you are required to blow your nose ... suggestions as to how you might do that are found elsewhere, typically the ACs. The onus is on the end user folk to achieve the intent of the regulatory requirement.
Multi-jet engines not "handed"
Quote from BESTGLIDESPEED:
It comes from the " ace the tech. pilot int. " book that most of all of us know way too well , therefore, canīt trust it much either. :
" there is NO critical gas turbine engine because the engines are positioned simetrically with opposing revolution direction "
[Unquote]
You are right; sounds rubbish to me. The engines are indeed symmetrically positioned, but to suggest they are ever "handed" (like some high-performance twin-piston-engine fighters were in WW2) is wrong. But it doesn't matter (we are told) because the slipstream from a jet engine, even a big-fan engine, does not rotate like the slipstream from a propeller.
I am not entirely convinced when considering some twin-jets, with today's high-bypass-ratio fan engines. The slipstream definitely assists rudder at low IAS on the A320, for example. [I'm talking about the CFM-56 engine, with the short fan duct. Not sure about the V2500 engine.] Whether it is rotating, I don't know. If the slipstream is rotating, it might result in some difference on the lines of a propeller-driven twin.
In the propeller (airscrew) case, the rotating slipstream impinges on the side of the fin (sorry, vertical stabiliser), giving a yawing effect. If the propellers are not "handed", the rudder is less effective in one direction than in the other. In any engine failure case, the resulting yaw has to be corrected by rudder in the direction of the live engine.
There is definitely a critical jet engine, as many have now posted, due to a crosswind effect. And I think we all agree that, if everything else is equal, the upwind engine is the critical one.
[By the way: in case anyone thinks I have lost interest in the subject of "weathercocking versus drift" he/she will be disappointed later...
Been busy ransacking the house for material in connection with Lodems's above post on his historic B707 accident.]
It comes from the " ace the tech. pilot int. " book that most of all of us know way too well , therefore, canīt trust it much either. :
" there is NO critical gas turbine engine because the engines are positioned simetrically with opposing revolution direction "
[Unquote]
You are right; sounds rubbish to me. The engines are indeed symmetrically positioned, but to suggest they are ever "handed" (like some high-performance twin-piston-engine fighters were in WW2) is wrong. But it doesn't matter (we are told) because the slipstream from a jet engine, even a big-fan engine, does not rotate like the slipstream from a propeller.
I am not entirely convinced when considering some twin-jets, with today's high-bypass-ratio fan engines. The slipstream definitely assists rudder at low IAS on the A320, for example. [I'm talking about the CFM-56 engine, with the short fan duct. Not sure about the V2500 engine.] Whether it is rotating, I don't know. If the slipstream is rotating, it might result in some difference on the lines of a propeller-driven twin.
In the propeller (airscrew) case, the rotating slipstream impinges on the side of the fin (sorry, vertical stabiliser), giving a yawing effect. If the propellers are not "handed", the rudder is less effective in one direction than in the other. In any engine failure case, the resulting yaw has to be corrected by rudder in the direction of the live engine.
There is definitely a critical jet engine, as many have now posted, due to a crosswind effect. And I think we all agree that, if everything else is equal, the upwind engine is the critical one.
[By the way: in case anyone thinks I have lost interest in the subject of "weathercocking versus drift" he/she will be disappointed later...
Been busy ransacking the house for material in connection with Lodems's above post on his historic B707 accident.]
Last edited by Chris Scott; 29th Feb 2008 at 00:44. Reason: To specify CFM-56 in the A320 case.
Wunderbra
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BGS, I think you've found another of the glaring inaccuracies in that book. Seriously, I've seen a copy of it, and if I'd paid for it I'd be demanding my money back!
As far as I know no twinjet uses contra-rotating engines. What's the point? As there's no assymetric blade effect, it really matters not a jot which way they spin. (excepting of course torque/gyrospic effects, but that's a different matter entirely!)
If I went into an interview talking about "critical jet engines" (other than "the furthest one from the fuselage") then I'd be laughed out of the room, and rightly so.
As far as I know no twinjet uses contra-rotating engines. What's the point? As there's no assymetric blade effect, it really matters not a jot which way they spin. (excepting of course torque/gyrospic effects, but that's a different matter entirely!)
If I went into an interview talking about "critical jet engines" (other than "the furthest one from the fuselage") then I'd be laughed out of the room, and rightly so.
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Chris, there is categorically NOT a "critical" jet engine. The term "critical engine" has a very narrow definition in the aviation world, meaning on a propellor driven aircraft, that engine which, due to the shifting of the thrust line because of the downgoing blade effect (note, nothing to do with slipstream on the rudder, it's all about the downgoing blade producing more thrust than the upgoing one and hence moving the thrust line away from the axis of the engine) will cause the worst assymetry (thrustline furthest from the C of G) if it fails. So it's always the engine which spins down and towards the cockpit, i.e. on an engine that spins clockwise when viewed from the front the LH engine is the critical engine, and vice versa.
There IS a jet engine that will be more dangerous to lose, but to call it a "critical engine" is misleading!
There IS a jet engine that will be more dangerous to lose, but to call it a "critical engine" is misleading!
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@JT One of the most troublesome regs for us is 25.251(b), precisely because it prescribes a specific means of compliance, which if strictly, legalistically, applied, would result in having to flight test every last rivet change on the aircraft ....
Re the contra-rotation thing; I think it's true to say no normal modern design uses contra-rotating/handed jets, but I believe there may have been examples in the past. VTOL types, I think, may have been exceptions, too, but that's more for gyroscopic reasons, and hardly relevant to an ATPL interview.
@m_h That no engine will be critical on a jet type is also dependent on symmetry of the design, and the interplay between the engines and the control systems. If, for whatever reason, the type has more control power available after one failure than another (due to an asymmetric hydraulic architecture, say) then one engine may well be critical for some handling cases, including VMC.
Re the contra-rotation thing; I think it's true to say no normal modern design uses contra-rotating/handed jets, but I believe there may have been examples in the past. VTOL types, I think, may have been exceptions, too, but that's more for gyroscopic reasons, and hardly relevant to an ATPL interview.
@m_h That no engine will be critical on a jet type is also dependent on symmetry of the design, and the interplay between the engines and the control systems. If, for whatever reason, the type has more control power available after one failure than another (due to an asymmetric hydraulic architecture, say) then one engine may well be critical for some handling cases, including VMC.