Critical Engine on a Jet

[av8a]

Just wondering if a jet aircraft has a critical engine and if so, how is it determined?

[GoneWest]

First ask yourself the question "How is the critical engine of a piston engine, propellor driven aircraft determined" - then see if you can answer your own question!!

[tailscrape]

If you have a strong cross wind from the right and you have an engine fire, you will probably surmise that the right engine is your critical engine. That being cos u will have to turn right in order to stop a total hull engulfment by the flames.

Make sense to you?

[TOGA_Party]

I believe the critical engine on jet aircraft is the upwind engine. Because "weather-cocking" due to x-wind combines with engine out yaw to provide the most critical controlability scenario.
Regards,
TOGA_Party.

[TOGA_Party]

av8a,
To try and explain things a little more clearly.
Imagine you are lined up on the runway,you advance the power and proceed on your take-off run. You will soon reach a speed called V1.

This speed can be equal to Vrotate (which is the speed at which we can pitch the nose of the aeroplane up,in preparation for lift-off). V1 and Vrotate MUST be equal to or greater than Vmcg. Vmcg is the "minimum control speed-on ground",which by definition means that we can keep the aeroplane undercontrol with aerodynamic devices alone,ie rudder and ailerons.

So lets say that today V1 and Vrotate are at the same speed and the wind is from the right. You pitch the nose up and the right engine fails.

A failure on the right side will cause the nose of the aircraft to swing right.Right!? And with the wind from the right striking the fin on the right,will cause the the nose of the aircraft to swing right. It's the these two effects when added together MAY overcome our ability to keep the aircraft under control with aerodynamic devices alone. Remember the nose wheel is off the ground so no steering available!

There's a bit more to it than that,but,all aircraft are put throught an exhaustive set of "certification" trials which plot aircraft performance and all V-speeds are derived from these trials.
Regards Again,
TOGA_Party

[daytrader]

Good try fellas but.....

Critical engine is the one which has centre of thrust furthest from centreline of the aircraft.

Either outboard engine with 4 engines.
Either side engine with 3 engines.
No critical engine with 2.

[Nite_Flite]

ON a prop aircraft it is the engine that would be most critical to loose. The downgoing blade gives thrust (power) if the nose is up, which would be normal flight attitude, hence the left engine thurstline is closer to the centerline of the aircraft. This is the engine you don't want to loose(!). The yaw moment if flying on righy engine only is therefore greater.

On jet aircraft it is normally not defined. The wind direction changes and so does your flightpath in the patter, which yould shift the critical engine from one side to the other. Not practical

[deltahotel]

The critical engine is the one which gives the most directional control problem if it fails.

As you cavort down the rw with a wind from the left, you have right rudder applied to stay straight (counteracting the weathercocking effect of the fin). This reduces the amount of right rudder available should the left engine (upwind) fail, so the critical engine is the upwind one.

[gingerpilot]

I always thought the critical engine was the one that worked!!!!!!!!

[M.Mouse]

So we are all agreed then?

[FlapsOne]

The critical on a HS125 for example is the downwind engine.

The 125 experiences 'negative weathercock' in that it will weathercock away from the crosswind thus any failure of the downwind engine during T/O will have the yaw enhanced by the crosswind.

On most aircraft then I guess the upwind engine(s) is the critical one.

[TOGA_Party]

FlapsOne,
I'm completely miffed! Try as I might I cannot visualise what your suggesting with regard to the HS125. I can see that with thrust lines close to the fore-aft axis there would be little yaw when an engine fails. But are you saying that in nil wind the HS125 yaws AWAY from the failed engine?
Regards,
Mystified of Perth!

[White Knight]

Daytrader; you're nearly there.

On a four engine jet, the critical engine is the UPWIND OUTBOARD engine.
a) due to the thrust line being furthest out
b) weathercocking of the aircraft into wind because of the very large tail.

On most two engine jets it will be the upwind engine, although flapsone has come up with an intriguing example of the 125 doing something a bit different.....

[daytrader]

WN,
OK failure before t/o. Point taken. Obviously for the rest of the flight there is no x wind and no critical engine on a twim jet.

[FlapsOne]

No TOGA I'm not saying that at all. I didn't even suggest it.

Of course it yaws towards the failed side in nil wind.

BUT..........a x-wind will have a significant effect on the yaw following failure. This is where the negative weathercock effect is v. important.

X-wind from the right, a/c will yaw left. Add failed left engine = more yaw.

Ergo Critical engine is DOWNWIND.

[Morse Code]

Sorry to disappoint, but there is no such thing as a critical engine on a jet. :D :D
......and tailscrape , No it don't make any sense at all and yes.....TA TA :mad:

[Capt Claret]

Morse, sorry to disappoint, from the BAe 146 AFM, page 1 of section 4.21.1, The critical engine is the outer engine on the upwind side.


av8a, if on the 146 you consider the outer left engine as being #1, and for this exercise, the wind coming from the left.

Should #1 fail, the subsequent yaw will be to the left ( one engine producing thrust LHS, two engines RHS). Until airborne, the wind from the left will strike the left side of the aircraft. With more surface, including the vertical stabiliser, aft of the CoG, the resultant yaw will also be to the left.

The critical engine is that one whos failure has the most adverse effect on controlability of the aircraft, in the scenario I've described, #1 is the critical engine.

[av8a]

Thanks for all your help guys. It is all a little clearer now!

Has anyone heard of something called the
"ASSUMED TEMPERATURE METHOD"

[Air Conditioned]

There can be more than prop rotation to be considered, e.g. some aircraft have split hydraulic systems with the Landing Gear on only one. When the engine driving that system fails the gear will be retracted only by the auxillary pump and therefore performance is predicated on gear retraction time in this worst case. Therefore that engine is declared as 'critical'.

In point of fact the only effect is probably an undeclared bonus if flying on the non-critical engine during the retraction segment. The matter of critical engines is probably only academic.

[FlapsOne]

av8a

Assumed Temperature Thrust Reduction.

A way of reducing T/O thrust to acceptable limits, within Perf A, by taking your TOW and calculating the maximum temperature that you could T/O at that weight using full T/O thrust.

A natural product of the above is, given the actual temp at T/O, is how much thrust is actually needed to T/O off at this weight.

The result is a reduced T/O thrust for a given weight and temp (and wind of course) that was calculated using the ASSUMED max temp.

A simplistic explanation but I hope it helps.

[wonderbusdriver]

The point being...

money.

You take away the peak load on the engines, thus greatly reducing wear (especially in the hot section) and actually even reducing the possibility of an eng. failure during T/O (at least statistically... NO I won´t discuss the pros and cons).

[tailscrape]

So, when i am on fire in my 757 (upwind engine) with a strong crosswind i shouldn't worry about it? Not critical?

I appreciate that what day trader says is entirley correct in a purist viewpoint, but aviation is about reality. Not theory.

So, I still maintain that in reality on a real day your critical engine is the upwind one.

I would say that is fairly critical in a twin jet.IMHO.

[erikv]

FlapsOne, could you give a more detailed explanation of the negative weathercock effect and describe what causes it?

As far as the critical engine's concerned, it's all a matter of which definition you use:
1. the critical engine is the engine that causes the greatest yawing moment when it fails (works for any multi-engine airplane).

2. the critical engine is the engine which has its centre of thrust furthest from the longitudinal axis (works for props only).

I'd vore for the first definition, as a strong crosswind can easily overcome the effect of the centre of thrust being further away from the longitudinal axis.

Erik.

[FlapsOne]

Yep.

Proportionally small fin (with very small rudder) vs proportionally long fuselage. Fuelage wins. Strong crosswind on ground has greater effect on fuselage than fin causing it to yaw away from the crosswind.

I flew 3000 hrs+ in them and guarantee it's fact.