Simviation Forums

Hey

Apparently some airlines taxi on one engine till they get to the runway, then they start the other up. Then Once they'v landed they switch the other one off and go back on one!. Apprarently it saves fuel.

But could it be done at say, 35000ft? To save even more fuel, sure you'd fly slower, but the cost would decrease and tickets prices would lower, why don't they do that?

cheers
forfun

as you said, you would fly slower. People are impatient.

i spose, but.... i wouldm't really care but i guess most people just see planes as a tube with wings.

Still, it would save money  :)

I don't know much about multi-engined aircraft but I would be surprised if this is done on twin-engined types. It's handy to have at least one engine on each side running to help with steering the aircraft. Taxying on assymetric power* for any length of time would also put unnecessary strain on the the gear legs, wheel bearings, tyres & the nosewheel steering mechanism. Shutting down both inners (or both outers depending on type) is quite common on 4-engined prop aircraft while taxying so I imagine this is also done on 4-engined jet airliners. I've also seen the centre engine shut down on 3-engined types like the Trislander. This is fine for taxying but would be uneconomical for normal flying. Unless the aircraft was extremely overpowered (which I've never come across) just keeping it in the air would need more power from the engines that are still running. This would possibly use more fuel, not less.

Most aircraft use full power for take-off & are throttled back to economical cruise as soon as possible & once they reach operating altitude. This is carefully worked out for each type of aircraft & is the most economical way to operate. It also reduces engine wear.

*PS. Unless the aircraft was specifically designed for it, flying on assymetric power would be even worse & potentially dangerous.

I used to fly 20-series Lears. Mmo was .83. This airplane could  climb directly to 39,000' in about 15minutes and the power must be reduced to avoid exceeding Mmo! I've also experienced two flameouts in these aircraft. What happens when you lose one engine at this altitude? First.....Mach will decay rapidly   Second........the Angle of Attack (AOA) indicator will rapidly approach the yellow(indicating an inevitable stall), if you attempt to stay at that altitude...... As Hagar stated, power on most aircraft is reduced after takeoff and again when cruise altitude is reached but on most jets the output of each engine still remains in the 90%+ range. This Lear can achieve a 10,000fpm initial climb rate but as you approach the flight levels it falls off to less than two thousand. Usually the last two thousand feet are gained at 500-750fpm or less! That's about the same as a Skyhawk/Cherokee isn't it? Imagine losing %50 of your power in one of these airplanes(try it on the sim!).

On the occasion that I did have a flameout a "re-light profile" is in the AOM. What this means is that the "dead engine" won't even re-light until you reach a much lower altitude. So, let's say that you wanted to try your theory at one of these lower altitudes where it's a little safer...... now you have this problem.........

*PS. Unless the aircraft was specifically designed for it, flying on assymetric power would be even worse & potentially dangerous.

.......due to the above mentioned adverse yaw, the rudder has to counter the single running engine's desire to turn the aircraft. To accomplish this it must create lift on a horizontal plane but in the process, more drag is created because this large "paddle" is hanging out in the slipstream much more so than the subtle inputs required in more normal flight conditions. ......hope that helps.....

Taxying on assymetric power* for any length of time would also put unnecessary strain on the the gear legs, wheel bearings, tyres & the nosewheel steering mechanism

This is not true. The power required for taxiing is not enough for the assymentric power to affect the aircraft, Only at higher speeds does flying on one engine affect the balance and it certainly wont on the ground.

cheers
forfun

Actually it can be quite difficult in non-jet powered aircraft. Most turbofan and some turboprop powered aircraft will accelerate at idle power, which means that idle is sufficient to taxi on. This is not the case with most piston powered aircraft. For example, a high-power twin with a good load in it would be very difficult to taxi on one engine, if not impossible.

That's when you call for a push cart.

P-3 does long range patrol with 2 of the 4 engines feathered.

P-3 does long range patrol with 2 of the 4 engines feathered.

Not both on the same side I trust. This is common practice with long-range patrol aircraft & they're designed specifically for it.

This is not true. The power required for taxiing is not enough for the assymentric power to affect the aircraft, Only at higher speeds does flying on one engine affect the balance and it certainly wont on the ground.  

cheers
forfun

OTTOL's comments seem to confirm what I said. I was speaking from an engineer's rather than a pilot's viewpoint. I've come across plenty of pilots that regularly did things that would make any engineer cringe.

OTTOL's comments seem to confirm what I said. I was speaking from an engineer's rather than a pilot's viewpoint.

Any pilot who did it would be adding to his/her pysical workload too, particularly in a larger, older type, and even more so if a tailwheel aircraft...

I've come across plenty of pilots that regularly did things that would make any engineer cringe.

Like, erm, take off for example... , or worse, land... , if fact just getting in the aeroplane...

Charlie

just getting in the aeroplane...

Charlie

LOL I've met a few in my time who should never have been allowed within 100 yards of an aeroplane.

This topic was started about airlines flying their jets on one engine to save money, lol, i didn't say it rite. o well, its a good little disucssion .

lol

cheers
forfun

I wouldnt worry mate, most topics go off topic here very quickly. The answer to your question is in there somewhere

most topics go off topic here very quickly.

lol. tru