Critical Engine

[disco87]

Hi,

First off, I don't fly a jet, but I was reading that in an aircraft powered by turbofan engines there is not a critical engine. My understanding of the critical engine is that it is the engine, that if it were to fail, would give the most adverse handling qualities.

My question is that if you were on the takeoff run with a crosswind from the right and the right engine failed, this would cause more asymmetric problems than if the left engine failed. In this scenario I would have though that the right hand engine is the critical engine.

Is my thinking wrong in this?

Thanks

[FE Hoppy]

In practical terms you are absolutely correct. However in certification of the aircraft there is no requirement to include a crosswind and therefore there is no defined critical engine.

[john_tullamarine]

Further to FE Hoppy's post, the older CAA Standard required a 7kt crosswind for Vmcg.

The practical real world consideration of strong crosswinds is somewhat ill-considered in the Industry. Taking off with an upwind failure can push the Vmcg up significantly. Typical figures vary from 0.5kt/kt upwards, depending on Type.

Periodic discussion points in PPRuNe relate to the consideration of

(a) crosswind on Vmcg and

(b) the effect of bank angle on Vmca.

A useful report to peruse for which these were material considerations relates to the training accident to the B707-400 at Prestwick in 1977 (http://www.aaib.gov.uk/cms_resources.cfm?file=/6-1978%20G-APFK.pdf).

Sobering reading and, in my view, essential for all multi-engine pilots.

[FE Hoppy]

Further to JT's post.
The RAF Tristar fleet was a little perturbed a few years ago when a civilian contractor was able to lift more than them from a rather restrictive airfield in a dusty place due to the fact that Boscombe Down had used the older UK standards for RAF ops while the contractor had the same type but with an FAA AFM which gave them a 12 kt lower vmcg. Same jet different rules different max take-off :confused:

[john_tullamarine]

.. and, I would hope, the Contractor's pilots would have been mindful of that significance on occasions when the takeoff were to be performed in strong crosswind conditions.

The nature of the yawing moment as the real Vmcg is approached is that the gradient steepens VERY rapidly in the closest few knots.

Get caught out with this one and the situation might become irrecoverable VERY rapidly.

Then, why don't we see aircraft running off the side of runways every day of the week ?

(a) most takeoffs are with a speed schedule well above Vmc limiting speeds

(b) the conjunction of limiting takeoff with failure and strong crosswind is rare

(c) engine reliability is such that the probability of a failure in such circumstances is rare.

Just don't get caught out in circumstances where the speed selection was optional - ferry flight, for instance.

[FE Hoppy]

I think oblivious would be a more accurate description of their position ;-)

[john_tullamarine]

I suspect that we both agree that it would be funny were it not so serious a problem generally with less than ideally trained and knowledgeable pilots ?

[Capn Bloggs]

IIRC, the 146 has a critical engine stated in the AFM; outboard engine on the upwind side.

[FE Hoppy]

IIRC, the 146 has a critical engine stated in the AFM; outboard engine on the upwind side.

The AVRO RJ AFM I have makes no mention of a critical engine. It does however state that VMCG applies at zero crosswind and increases by 5 kt per 10 kt crosswind component.

The original 146 my well have been certified before the regs changed.