Critical engine (jet)
 

I would appreciate if someone could help explain what is the critical engine on a jet in relation to direction of crosswind and whether it is better to lose the upwind or downwind engine?

Thanks in advance

On a jet there is no "critical engine" in the normal sense. This is a term used for twin prop aircraft where both props rotate in the same direction. All about the "downgoing blade effect"

I would suggest that in a situation where you have a significant crosswind, it would be better to lose the downwind engine, as the aircraft will have a tendency to turn towards the wind (weathercock) which you would need to counteract with rudder (I'm talking on a final approach of course, in level flight I don't think it makes any difference.) So losing the upwind engine will mean that you have less available rudder deflection to counteract the yawing effect of the good engine, as some of it is used up in counteracting the tendency to weather cock.

Of course I could be wrong, but that's my understanding of it.

Thanks for that answer, that is the way which i saw it but have heard other opinions suggesting the extra airflow on the keel surface of the fin and aircraft fuselage adds a restoring force, and therefore the opposite is true.

you might want to have a look at thse answers for a balanced view!!
http://www.atpforum.co.uk/archive/index.php?t-6132.html

Quote from matt_hooks:
...it would be better to lose the downwind engine, as the aircraft will have a tendency to turn towards the wind (weathercock) which you would need to counteract with rudder (I'm talking on a final approach of course, in level flight I don't think it makes any difference.)
[Unquote]

You are right that it's better to lose the downwind engine, but - generally speaking - there is no weathercocking when airborne. It is true, however, that it would help on final approach when using the side-slip crosswind-landing technique. Using the normal technique, it assists when decrabbing (kicking the drift off), immediately prior to touchdown.

In the engine failure on T/O (EFTO) case, it would help on the ground, particularly if the T/O was being continued, and during rotation. Once airborne, after allowing the aircraft to weathercock slightly to compensate for drift, it would make no difference.

Indeed Chris, I forgot to mention the ground case, but that's where it could cause the greatest problem. As for weathercocking in the air it does happen, hence why your heading and track are often two quite different numbers. However I agree that in the air it's not so important. However I still believe it has a role to play, especially with a very strong crosswind component where you require to hold a given track.

Air Navigation - Day 1
 

Sorry to bang on about this, matt, but think you may be confusing two completely different things:

WEATHERCOCKING
The tendency of an aircraft with conventional vertical tail surfaces to swing its nose into a stiff wind when it is resting on the ground (i.e., NOT airborne).

DRIFT IN FLIGHT
[Assuming no side-slip, i.e., with the aircraft flying straight IN RELATION TO THE AIR MASS.]
As you say, the difference between the HEADING (the bearing in which the aircraft is pointing) and the TRACK (the bearing of the track made good over the ground).
This results from the movement of the AIR MASS (at the altitude of the aircraft) in relation to the ground. The air mass is, after all, what is supporting the aircraft.
If the true airspeed (TAS) of the aircraft is 120kts, and there is a 90-degree crosswind of 40kts, the drift will be approximately 20 degrees.
The relationship between HDG/TAS, W/V, and TRK/GS can be mathematically represented by a triangle of velocities, which is what we old farts used to do on a Dalton Computer.

There IS a critical engine - it will be the outboard engine onto the into wind (windward) side.

Failure of the critical engine between V1 and Vr will exacerbate the tendency of the aeroplane to swing nose into wind. Failure of a downwind (leeward) side engine will be partially cancelled out by the weathercocking caused by the crosswind - so losing a downwind engine is best (but not as good as losing none!).

As far as certification goes there is NOT a critical engine.

In reality what you say is correct but it is not "the critical engine" as far as the certification goes.

Chris, I'm not disagreeing with you. On the ground it's called weathercocking (the tendency of the nose of the aircraft to swing into wind due to the aerodynamic pressure on the vertical tail surface) and in the air it manifests as drift, where the nose of the aircraft is pointing in a different direction to the direction of travel.

And the modern equivalent is the CRP5! :\

And moggie, as FE states, in the certification there is no "critical engine". There definitely IS an engine that will cause a worse situation, but to call it a critical engine might tend to mislead!

matt's progress...
 

Quotes from matt:
1) As for weathercocking in the air it does happen, hence why your heading and track are often two quite different numbers. However I agree that in the air it's not so important.
2) Chris, I'm not disagreeing with you. On the ground it's called weathercocking.... and in the air it manifests as drift.
[Unquote]

Not good enough, Hoskins... Until you have grasped the elementary - but essential - difference between the concepts of weathercocking and drift, and admitted (at least to yourself) that you were confused, I cannot allow you to proceed to Day 2 of your basic Air Navigation course !
In the meantime, kindly read my note again.

[Thinks - Is he like this with all his instructors?] :ugh:

Chris Scott - you are absolutely correct. Weather-cocking takes place on the GROUND, and drift is simply the angular difference between heading and track. It takes place in the AIR!

On the question of "critical engine" related to a jet, there is no comparison with that of a propeller driven twin. There is however an engine that we would prefer not to fail in some situations, most notably being when taking off in crosswind conditions in a LIGHTLY loaded aircraft.

In such cases (jets) the calculated V1 for a given WAT may actually compute to a speed below the "real" VMCG. The stated Flight Manual VCMG is determined without any consideration given to the effect of crosswind, so the loss of the upwind engine after V1 may, in the case of a lightly loaded aircraft departing in a crosswind, result in a loss of directional control.

The solution? If the TODA permits, increase the V1 by picking a different weight from the tables, but by no more than VR of the actual weight. V2, also should be based on the actual weight.

Worth a search of the archives as Mutt and I periodically do this one to death. Useful to recall that the effect of crosswind on real Vmcg varies from 0.5 kt/kt through to something in excess of 1.0 kt/kt .. and, if your crosswind limit is a lot of knots, and you are light and at aft CG and on a min speed schedule ... life could get very interesting if one goes quiet.

Look at it this way: when you are drifting in air, the airplane is out of contact with ground and therefore, for the constraints of movement, it is irrelevant what the underlying ground is doing.

When the airplane touches ground, anywhere, the support points affect the aircraft movement, both vertically and (unless completely freely sliding) horizontally.

I should assume that the effects of weathercocking would be rather different depending on which wheels contact ground (e. g. front wheels on ground, tailwheel off ground, vs. main wheels supported, nosewheel off ground...). Drift and sideslip should work in a more similar manner.

Critical engine etc...
 

Fact - There is no critical engine in a jet aircraft -
xxx
Many of you have established that fact here above. And obviously, the outboard engines in a 4-engine aircraft are the two "critical engines" as compared to the inboard engines. It does not matter if nº 1 or nº 4 fails, the resulting VmcG will be equal. No need to say, same applies to 2-engine aircraft, the resulting VmcG are equal with the L or R engine failed.
xxx
This is for FAA (FAR 25) certification.
However, the British CAA accounts for crosswind effect on VMCG. In the case of 747-200/300s as an example, in the minimum V1 speeds (restricted by VmcG), speeds in the FOM/QRH are different in airplanes certificated as per FAA and CAA. Those of you who operate such 747s, if you have the original Boeing FOM manuals, if they are marked at the bottom of pages by "FAA", or "CAA", next to the page revision date, it will indicate to you which certification was used (among other info) for the speed tables, and the speed tables are different. VmcG/V1 speeds are higher for CAA certification.
xxx
In the simulator, when I train pilots, I can demonstrate VmcG/V1 engine failures that are controlable at speeds as low as about 115 KIAS, or uncontrolable at speeds as high as 135 KIAS, this based on which engine power, CG position and crosswind factors...
xxx
Now again - nose wheel steering...
How do I say it again, in English, Spanish or Guarani - it is WORTHLESS -
If you think that your nose wheel steering will assist you, you are having a dream.
Read the following...
xxx
Background -
In July 2001, LV-MLP (a 747-287B) sustained damage by veering off the runway during takeoff at Buenos Aires EZE airport. The weather was partially obscured by light drizzle and fog, wind calm. The captain stated that the takeoff roll began normally with only minor corrections, with the tiller, to maintain runway centerline. Prior to receiving the "80-knots" call, the captain felt the airplane moving to the right. He applied left rudder and left nosewheel steering without any effect on the path of the airplane. Prior to departing the runway, and with right drift increasing, the captain stated that he applied full left rudder and nosewheel steering. The right wing gear damaged a few runway lights, and departed the runway concrete area. The captain then retarded the power on nº 1 engine to turn the nose of the aircraft towards the centerline, then, retarded all thrust levers to idle and the T/O autobrakes stopped the aircraft.
xxx
The probable cause of the incident was the captain's failure to reject the takeoff in a timely manner when excessive nosewheel inputs resulted in a loss of directional control of the aircraft.
xxx
We have concluded that the procedure to "guard the tiller" during the takeoff roll to 80 knots increased the likelihood for overcontrol of the nosewheel, and that we should eliminate the procedure. From then on, our procedure changed to instruct the captain not to guard the tiller, and place his hand on the wheel, after alignment for takeoff on the runway. I was instructed by the chief pilot to effect the new procedure to all types (747, 737, MD-80 and A-310s), effective immediately. There were no further problems since the procedure changed.
xxx
Further, "elevator down" to "increase nosewheel effectiveness" is worthless. It is obviously worthless at low speeds, worse, it does increase aerodynamic drag at higher speeds. Directional control should rely only on aerodynamic forces, once the aircraft starts rolling for takeoff. Elevators are kept in a "neutral/faired" position.
xxx
Boeing sent us a bulletin confirming our change of procedures without objections.
Be aware that with PanAm, the "rudder pedal-nosewheel steering" linkage in the 747 was disconnected and removed even from 747 airplanes we acquired from other airlines which were so equipped. And pilots did not "guard the tiller" during takeoff roll to "80 knots".
xxx
:)
Happy contrails

P.S. For those of you who play with "engine out ferries" qualification, like I have to do occasionally, with the 747-200s, the "VmcG-2 is 160 - VmcA-2 is 155 KIAS" with Flaps 10 and JT9D-7Q power, at 3 engine-ferry weights.

As has been previously stated there is no critical engine on a jet a/c.

However if you are flying a crosswind take off and the downwind engine fails (let keep ourselves to twin jets but the same principle applies) during the ground roll and the take off is continued the effect of the engine failure will tend to counteract the requirement for into wind rudder due to the crosswind. However as the aircraft becomes airborne the rudder will have to be re-applied to keep going straight (in the air that is - never mind the ground track at the moment although this is also important).

I remember well a B707-436 which was written off during training at Prestwick in the mid 1970s. They were departing on RW 13 and the average wind was circa 220/15 (although the speed and direction were quite variable). After V1 the training captain closed the thrust lever on the No1 engine to simulate an engine failure. The trainee was slow to apply corrective rudder and as the a/c became airborne it started to yaw rapidly at which point the training captain took over control and (if my memory serves me correctly) restored the thrust on No 1 and retarded thrust on No 4. 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. The a/c came to a halt a few yards from the fire station at PIK - all the crew got out but the a/c was destroyed (by fire I think).

One of the interesting factors in this accident which was pointed out in the investigation was that the VMCA wings level was 40 kt HIGHER than with a few degrees of bank towards the live engine. If the a/c had been banked a trifle towards the live engine they would have regained directional control and they might have got away with it!

Another factor was that the B707-436 was not fitted with a series yaw damper so all take offs and landing were made with the yaw damper disengaged which made the a/c somewhat of a challenge from a control point of view with engine out and/or crosswind.

the VMCA wings level was 40 kt HIGHER

important consideration and applies across the board ... if the bank is applied the wrong way (sillier things have happened in high workload situations), the delta goes up out of sight ... food for thought for those who like to play with Vmca demonstrations etc in flight rather than the sim ...

However, the British CAA accounts for crosswind effect on VMCG

more a point of style than reality .. 7kt versus nil wind.

(1) Weathercocking and Vmcg. (2) Sideslip.
 

Quotes from chornedsnorkack:
(1) I should assume that the effects of weathercocking would be rather different depending on which wheels contact ground (e. g. front wheels on ground, tailwheel off ground, vs. main wheels supported, nosewheel off ground...).
(2) Drift and sideslip should work in a more similar manner.
[Unquote]

(1) Yes. This is a bit off-topic, ;) , but may help to build the picture of crosswind effects on VMCG (see sooty615, john_tullamarine, BelArgUSA and fireflybob posts).

Tricycle L/G: If noseheel firmly on ground and steering is HELD central (using the pilot's tiller), weathercocking can be stopped, but this can only be used at taxiing speeds. [If the pilot takes his/her hand off the tiller, or relaxes feet on the rudder pedals, the nosewheel will be free to castor - and the aeroplane is free to weathercock.] Rudder-fine-steering, if you have it, helps at speeds up to 60 - 80 kts. Above that speed the directional control of the nosewheel(s) cannot be relied upon, and trying to use tiller-steering can be lethal. So on take-off the prime directional control is by aerodynamic rudder, mainly opposite to the crosswind. On landing you also have the option of differential brakes, which can likewise be used in taxiing.

Tailwheel L/G: Tail-draggers are much more susceptible to weathercocking, because the wings are in a flying attitude which catches the wind at low speed. Also, for a given size of aeroplane, the tail surfaces are further behind the pivot point (the main-wheels) than on a trike, giving a bigger moment. Few tailwheels are steerable. On taxiing you steer and control weathercocking with differential brakes, aerodynamic rudder, or differential power (difficult). Once the tail starts swinging, quick action is needed to avoid embarassment! However, additionally for take-off and landing, the tailwheel can often be LOCKED fore-and-aft. On take-off, weathercocking can also be anticipated by differential engine power (difficult), and on landing is invariably combatted with differential brakes.
On a crosswind take-off on the DC-3, for example, the pilot pulls the "stick" (control column) fully back initially, the propellers' slipstream forcing the locked tailwheel firmly on the ground to keep the aeroplane straight. Once the rudder becomes effective (about 40 kts IAS), the tail can be lifted quickly. Once the nose is level, the weathercocking tendency reduces, and is controlled with opposite rudder, backed up by into-wind aileron.
[For crosswind landing on the DC-3, the standard technique is to "wheel it on" well above stalling speed with the tail quite high. You then have to use more and more forward stick as you slow down. The trick is to pick the right moment to allow the tail to descend rapidly on to the ground, after which you pull the stick back to hold it down firmly. The locked tailwheel then provides directional stability, which the pilot backs up with liberal amounts of brake on the downwind side...]

(2) Not sure what you mean. Sideslip must not be confused with drift.

Let me explain SIDESLIP (already done drift to death) :rolleyes:. It is when the aircraft is travelling slightly sideways through the air (forget the ground).
For example, if you slowly put 10 degrees of right bank on, you will normally turn to the right. But IF you simultaneously feed in opposite rudder, you can stop the aeroplane turning. [I believe it works for a helicopter too.] The "ball" goes off to the right side, indicating that you are now in a sideslip. The aircraft creeps sideways in relation to the air, in the direction of the bank. DRAG is greatly increased, which can be used to get surplus height off on an approach (NOT on an airliner, please). :=

The only connection between sideslip and drift is that, on a crosswind approach, the former is sometimes used to counteract the latter, so that the nose of the aircraft can be pointing straight along the runway. [A right-banked sideslip can counteract a crosswind from the right. This technique is normally frowned upon in airliners, partly because all the loose glasses slide off the galley work-tops...]
On landing, the upwind wheel(s) touch down first. As the downwind wheel(s) come down, you already have downwind rudder to help counteract the tendency of the aeroplane to weathercock. [Rudder is backed up by aileron into wind.]

Hawker 700 and 800
 

If I remember correctly, they have different "critical" engines, when on ground, and even perhaps at lower speeds.

Due to the redesign of the lower section of the tail fin.

Perhaps a current HS bod can confirm.

widny

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?

Also, how do tandem landing gear planes handle weathercocking? Rotation is not an option, and they have a high angle of incidence.

I meant that when an airplane is completely in air and no landing gear extended, it should fly the same way whether it was retracted tricycle gear, retracted taildragger gear, retracted tandem gear or a flying boat with no landing gear at all.

When the landing gear is extended, it would start affecting drag, sideslip and yaw behaviour. But it is only when the gears touch down that they become the major control of bank and pitch attitude.

As for weathercocking: do B-2 planes, with no fin and rudder at all, weathercock in crosswinds?

For a performance engineer - am I correct in my understanding that Vmcg is predicated on failure of the 'worst' engine?

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? ;)

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.

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!

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.

On a twin engine a/c the engine that's still operating is THE critical engine! :eek:

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.




:rolleyes:

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! :D

@ 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?

@ mad (flt ) scientist,



Correct me if I´m 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.

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.

Well, OK.

I guess there´s room for that kind of doubt.



( Anyway, How hard are we supposed to look into meanings ?. Shouldn´t 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 there´s something not-written, then I can´t imagine the one who wrote it expecting me to imagine something else than what´s on the paper. I don´t think there´s any judge who can, in the end, have you busted for "not-imagining" .)


:ugh:


Just rather go by-the-book as much as possible.
Don´t wanna give others the chance to blame me for something that wasn´t even written by the one who was getting paid to be responsible for that.



Rgrds.:ok:

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!

Itswindyout

You are correct for the Hawker 700

The critical engine is the downwind or lee side engine.

This is due to negative weathercock stability on the ground.

Totally agree w/ u on that one, Sci.

Nobody happier than me that it is not lawyers to write them down. ( even though you shouldn´t doubt it´s 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) ) :yuk:



Anyway, don´t 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 one´s 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, can´t trust it much either. :


" there is NO critical gas turbine engine because the engines are positioned simetrically with opposing revolution direction "


Now I´m 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 ? )


:confused:


(Good to have this conversation.)

Rgrds.

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.

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, can´t 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.]

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.

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!

@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.