Posts: 21
Joined: Thu Mar 16, 2000 9:08 pm

Single Engine Endurance!

Tue Jan 28, 2003 7:26 pm

A funny situation.
Let's get some guidelines.
a) For turbine engines, sfc (specific fuel consumption : kg of fuel to produce a unit of thrust or power) increases as the rating of the engine is reduced.
b) endurance mission require least fuel flow per unit of power used.
c) Typical Loiter missions need low power (30% of max power on the aircarft?)

From this point of view, is it practical (!) to use only one engine in a twin engine turboprop aircraft. Let's say 400 HP is required for the mission to be flown which may mean 30%-40% rating operation. This puts the engines in high fuel consumption operation.
On the other hand, if one engine is kept idle, and the other used at a higher rating (70% ?) to produce 400HP and improving the sfc, a significant gain can be made on the fuel flow rate. This should increase the endurance time.
Of course, rudder and ailerons need to be deflected to take care of the adverse yawing moment created due to asymmetric thrust. They will produce some drag which will reduce the fule flow rate gains to some extent.
Your comments on this type of operation are most welcome !
Posts: 1037
Joined: Fri Dec 31, 1999 4:49 pm

RE: Single Engine Endurance!

Tue Jan 28, 2003 8:25 pm

That wouldn't work. The drag caused by the rudder would negate any fuel savings you would get from running with one engine idle. Also, what if you have a failure on the engine that is running full power?

RE: Single Engine Endurance!

Wed Jan 29, 2003 12:47 am

Actually, it does work; but there are other issues that must be dealt with. Shutting down engines to increase range and endurance, while not a wide-spread practice, is commonly used on some aircraft - for example the US Navy's P-3. I'm sure you've all seen pictures of the airplane in the "patrol mode" with the two outboard engines (#1 & #4) shutdown and feathered.

I remember from back in my airline days that the specific range of the B727 was greater on 2 engines than it was on 3, in other words, it could fly further on 2 engines. Granted, there could possibly be some maintenance issues to address if you tried this at home boys and girls - having all those engine components windmilling without normal oil pressure for extended periods of time would not be a good thing.

I would assume that the same principle would hold true for most turbine powered transport category aircraft regardless of the number of engines.

I know that for the Gulfstream G100 and G200 aircraft, the loss of an engine results in no loss of range, only in true airspeed and altitude capability. This makes oceanic crossings much safer by eliminating the "wet footprint" that one had to deal with in some aircraft in the past. Now, the main endurance issue that operators of twin-engine bizjets must contend with in the event of certain failures is oxygen capacity. "In the unlikely event of the loss of cabin pressure" it's the amount of O2 that you have on board that becomes the limiting factor, not fuel.

Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Single Engine Endurance!

Wed Jan 29, 2003 2:28 am

DanAir, the now defunct British airline, at one time proposed to the UK ARB, (now CAA) to be able to shut down one engine (in cruise) of their DeHavilland Comets, in order to save fuel (the Comet being very much overpowered)...this suggestion was politely declined.

IE; not possible or allowed with civil types. End of story.
With military types...up to the service concerned.
Posts: 21
Joined: Thu Mar 16, 2000 9:08 pm

RE: Single Engine Endurance!

Mon Feb 03, 2003 8:08 pm

Thank you all for responding !
CX Flyboy
Posts: 6050
Joined: Sun Dec 26, 1999 6:10 pm

RE: Single Engine Endurance!

Tue Feb 04, 2003 10:11 pm

At low level, the 777 burns less on one engine. In an extreme fuel critical state, such as a fuel leak, it would be an idea to bring one back to idle and cruise along like that, but it really would be a last ditch effort, before...well, ditching.

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