RTO's and Scans

[Groundbased]

Hi all,

Two separate questions here.

I've been reading about RTO's and the document, (NTSB study from c1990) makes frequent mention of a failure at or around v1 in the critical engine. How do you determine which is the critical engine? ( a guess on my part but is it to do with the remaining engine's thrust helping you into wind and thus increasing lift?)

Secondly I'm interested in how the instrument scan technique has developed/changed with glass cockpit aircraft as opposed to the older style, if it has atall.

Thanks in advance for any information

[A Very Civil Pilot]

1 Due to the rotation of an engines prop or fan, the thrust line is displaced to one side, rather than straight down the middle. If both engines rotate the same way, one of the thrust lines will be closer to the normal (vertical) axis of the aircraft, than the other . If the 'closest' engine fails, leaving the 'furthest' engine running, this running engine will have a greater moment arm (force x distance) producing greater yaw, and will be more difficult to control. So, the critical engine is the one, that if it fails, leaves the one with the greatest moment arm operating.

2 Glass cockpit - alot easier!

[El Desperado]

Only twin-engined prop-driven aircraft have critical engines. You could make the argument that turbo-fan/jets as well as multi-engined prop aircraft do, strictly speaking, have a critical engine, but from a pilot's perspective, the difference between engine (a) or engine (b/c/d) going is pretty much unnoticeable.

Imagine you are looking from the rear of an aircraft towards the engines. If the props rotate clockwise, the torque of the engines tries to roll the aircraft in the opposite direction - i.e. left.

If the right-hand engine fails, the left engine rotates the entire fuselage and wings anti-clockwise through the centre of the engine. Lots of power required for this.

If the left-hand engine fails, the right-hand engine rotates the aircraft anti-clockwise through the centre line of the RH engine. Much much easier - gravity is helping !

Therefore, in an aircraft with clockwise rotating props, the left engine is critical, and vice-versa.

It is a bit more complicated than this, as some aircraft have systems that are only powered by one engine or the other, and therefore 'critical' becomes more than just an aerodynamic consideration. This is generally only the case in light twins and older aircraft.

As for glass cockpit vs. standard T layout.... once you have any significant time on the former, it is hard work going back to the latter ! The big glass jets have most of the information you need to operate on two screens, be they Airbus, Boeing, Embraer etc. No scan (well, nothing close to what you need on clockwork aircraft) required really.

[knobbygb]

El Desperado, that makes sense, but why only on prop driven aircraft? Surely a similar torque force is caused by a jet engine which also has a fan (heavier???) of similar diameter rotating at a similar speed? Or when you say prop driven, did you mean piston, rather than turboprop? Thanks.

[El Desperado]

I did mean propellor-driven. The type of engine driving the propellor (turbo or piston) is irrelevant.

If you read the top bit of my post again, you'll see that I agree that, strictly speaking, a twin-engined jet (like a 757) will have a critical engine.

However... when the critical engine fails in a turbo-prop you have to work damn harder than if the other non-critical engine fails.

In a 757, for example. the controllability of the aircraft will be the same regardless of which engine fails. Also, the engines on jet aircraft tend to be fitted closer to the fuselage than prop engines for obvious reasons ! Ideally, you would like both engines as close together as possible (Lightning - one on top of the other, or the Embraer 145 - at the back of the aircraft on the thinnest section of fuselage).

I also forgot to add that in an aircraft like a King-Air or Dash-8, the props contra-rotate... i.e. the left engine's prop turns clockwise, right engine's prop turns anti-clockwise. These aircraft do not have a critical engine :)

Aircraft like a Seneca, Duchess, J31, ATP... these do have critical engines.

Hope this helps - I'm no aerodynamicist, and it's difficult to keep the answer reasonably simple without being patronising and at the same time answering your question !

[john_tullamarine]

Determining the critical engine is a little more complicated than what is suggested above .... but it doesn't matter .... the determination is a certification thing to work out some baseline data which is needed to determine AFM data for pilot use.

The only significance in routine operations is that, if a less critical engine fails, then the pilot's job may be just a little bit easier.

The point about the thrust line moving abeam has to do with the higher incidence at low speed and the effect on the disc thrust loading distribution.

I think that most of us who have flown larger four-engined propeller aircraft would disagree with the suggestion that there is no difference in which engine fails ... the variation in excitement depending on whether the outer port or starboard engine fails can be quite interesting ....

[El Desperado]

John - I stand corrected. I have flown a small(ish) four-engined turbo-prop and it didn't really matter which outboard engine failed (in the sim that is... whether there was a difference on the aircraft I, happily, never discovered). If you're talking about something like a C-130, Connie, a warbird, or one of the larger Russian transports then, I quite freely admit, this is something completely outside my area of experience. I'd be interested in learning more if you have the time......

Yep - I know crit. engine certification is a fair bit more complicated than the above, but I think it's a fair layman's assessment.

Cheers

[excrab]

My vague recollection from my multi-instructor course many moons ago is that the critical engine on propeller driven aircraft is not due to torque reaction but to asymetric propeller thrust - the down going blade having a greater angle of attack and thus producing more thrust than the upgoing. Therefore an engine failure of the left hand engine in a piston twin having american engines is more critical than a failure of the right hand engine.

As far as turboprops go the Dash 8 doesn't have contra rotating props and definitely has a more critical engine, and the rudder on the 300 series allows more movement right than left at full travel as a result. Dont know about the king air but I'll be suprised if P&W built left and right handed PT6 engines.

The only light twins I've flown with contra rotating engines were the Duchess and Navajo - hence no critical engine. The problem if I remember rightly was that not only were the engines handed but so were accessories like magnetos, starter motors etc and if a left handed one had failed it was always a right handed spare that was on the shelf.

[El Desperado]

I believed both torque and assym. prop thrust were both factors, the torque being the most important one. Haven't flown a prop in years and I'm also delighted to find I got the Dash-8 engines wrong :)

I'm pretty sure the King-Air has contra-rotating props but then I think the J-41 does too.... both PT6 powered (I hope !). If I'm wrong on one, I'm wrong on them both.

Time to stop typing after beer :)

[Fil]

On a jet aircraft with respect to a rejected take off, the engine into wind (rarely do we get a perfect headwind). This is so that when we stop we would turn so the damaged or on fire engine is downwind of the fuselage so any flames will blow away from the fuselage and hence away from the passengers.

Look back at the British Airtours 737 fire at Manchester where the flames were blown straight back towards the fuselage and note the results. I may be completely wrong but I think it was after this that RTO's started to include a low speed turn at the end.

The true aerodynamic critical engine as discussed above, is so negligible on a jet that we do not notice it nor consider it.

[rightbank]

The Kingair certainly has PT6 engines, but they are not contra-rotating.

The J41 does not have PT6s (Garrett?) and looking at photographs, they also do not appear to be C/R.

The C/R version of the Navajo is a fairly rare beast. Every Navajo/Chieftain that I have flown was not C/R.

The Seneca 1 however was C/R and therefore did not have a critical engine. Not sure about later model Senecas.

[Fil]

The Seneca II's and III's I flew under training at OATS were c/r too.