Jet Aircraft Critical Engine

NJGov

NJGov

New Member
In turbine-jet aircraft, which engine is the critical engine with respect to Vmca and Vmcg?

From my understanding (please correct if wrong), the critical engine is the upwind, outboard engine - on either a 2 or 4 engine aircraft while on the ground.

A turbine-jet aircraft has no critical engine in the air.

Additionally, would that differ for an aircraft like the 727 (3 engine, tail config)?
 
You are correct, for the purposes of V1 cuts, the upwind (outboard) engine is the "critical" engine. The same would be true for a 727, whichever "external" motor is upwind.

As for systems and such, I don't really know of any jet aircraft that have a critical system, ie landing gear, electrical, etc on one engine so the thinking of the critical engine as it applies to systems doesn't really, well, apply.
 
For the KC-135 there was a defined "critical engine failure speed" and a "non-critical engine failure speed" but there wasn't any defined "critical engine". CEFS was the speed at which a decision to continue the takeoff would take exactly the same distance as the decision to stop. Non-critical engine failure speed was just a poor name for something that was really just an adjusted refusal speed.

Of course for VMCg the worst case scenario was if you lost a downwind outboard engine (4 engine airplane), so that was the situation that was assumed for any of the engine failure stuff-- the engine lost was the downwind outboard engine. I think it was the same for VMCa, the downwind engine was the bad one to lose, not the upwind.

The reason for the downwind engine being the bad one was because the numbers assumed that you'd be able to maintain directional control on the runway and deviate a maximum distance from centerline... I haven't flown that airplane in 2 years, but I think the distance was 50 feet. So if you have a wind from the left pushing you towards the right side of the runway and then you lose a right engine you'll drift FURTHER right. If you lost a Left engine, it would counteract the wind.

I think that the long story short on this one is sometimes the manufacturer of the aircraft may use some non-standard terminology, so when in doubt reference your own aircraft's Operating regulations (whatever they are, a POH, an owner's manual, or a Dash 1).
 
Fish, I agree with the statement you gave, but, from what I understand (without quoting any specific reference):

a critical engine is one that most adversely affect performance if rendered inoperative.

During a normal takeoff, with two or four engines, a turbojet aircraft would still need to use right rudder to maintain centerline (to counteract the weathervane effect of a left crosswind)

I may be mistaken, but would like to know the correct answer if anyone else wants to chime in :)
 
Disclaimer...this is my own thoughts...other material introduced into the discussion may prove me wrong.

"Critical Engine" is an FAA certification parameter used to evaluate single engine performance on the ground and in the air. Nowhere, is wind introduced into the certification discussion concerning engine out flight. Therefore, I would not consider wind when determining which engine is "critical". i.e. in a twin engine jet...neither or both or critical.

The crosswind component will make the maneuver more or less difficult, no doubt. But I don't think it makes a difference in regards to certification.

In the simulator...the instructors conductor V1 cuts in both upwind and downwind scenarios. The downwind manuever is more difficult.

However, in a checking environment, either engine may fail...either upwind or downwind. The evaluator is required to check the V1 cut with the critical engine failed. So this lends me to believe that the FAA does not consider wind in determining the critical engine.

Also, lateral deviation during certification must be 30' or less from centerline.
 
While this may or may not affect which engine is termed "critical," it is generally relavent to what y'all are discussing. In a crosswind jet engine takeoff there are two opposing directional effects going on.

1. A weathervaning effect due to the rudder and surface area aft of the CG.

2. An anti-weathervaning or downwind steering effect due to engine intake momentum drag. Imagine the aircraft when it is sitting still on the runway in a crosswind. The air enters the engine moving left or right (the direction of the wind) and exits the engine heading straight back. The air has changed directions as the result of a force exerted on it by the engine intake. The reaction of that force causes the aircraft to yaw downwind since the intakes are generally forward of the CG.

This explains why loss of the downwind engine can be worse than the upwind, or vice versa.
 
Realms09 said:
The air enters the engine moving left or right (the direction of the wind) and exits the engine heading straight back. The air has changed directions as the result of a force exerted on it by the engine intake. The reaction of that force causes the aircraft to yaw downwind since the intakes are generally forward of the CG.
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I must say this is one of the strangest explanations I have heard.

The critical engine in a jet is the last one running. Weathervaning is cause by the crosswind on the vertical stab (much more surface area than the side if an inlet). One engine failure will enhance the weathervaning, the failure of the other will (partially) offset the weathervaning tendency. Either of which can be reversed if an engine "overspeeds" before it shuts down.
 
What you've listed mathces weathervaning, the first effect I mentioned. However, the effects of "weathervaning" and "intake momentum drag" are completely separate. Weathervaning will vary with wind velocity and surface area distribution, while "intake momentum drag" will vary with wind velocity and thrust setting. At high thrust settings the engines will be processing a lot of air, all of which is accelerated from moving somewhat sideways to moving purely axially. The resulting force on the intake is downwind, so on for wing mounted engines the yawing moment is away from the wind, opposite the weathervaning tendency. This explains why B767 found the downwind cut more difficult. However, on the business aircraft you likely fly the engines are fuselage mounted with the inlets aft of the CG. The "intake momentum drag" would reinforce the weathervaning tendency and it would be harder to differentiate the effect.
 
Realms09 said:
The resulting force on the intake is downwind, so on for wing mounted engines the yawing moment is away from the wind, opposite the weathervaning tendency. This explains why B767 found the downwind cut more difficult.
Click to expand...

It would be helpful for you to note your background so that we can understand your point of view.

The main reason a wing mounted jet reacts differently is the location of the engine along the centerline/CG , the distance the engines are from the centerline/CG, and that the engine are located below the CG. I'm not saying that there isn't any intake drag or effect, but I don't believe that it can outweigh design/placement of the engine or outweigh the surface area effect from the vertical stab.
 
Background: Aerospace engineering

You're right that it may not dominate, but it is a real effect that enters into the equation and is something to be aware of. In some situations it may be the driving force, depending on the aircraft design (high thrust, small vertical stab, low airspeed). I invite B767 and other wing-mounters to elaborate more on their RTO training experiences.
 
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