Hi all, I'm working on a fuel saving project in my college, and I'm having some problems about ground engine restart. I was proposing the following procedure:1. Start all engines after push back2. Shut down engines for 1-2 engines taxi3. Restart engine near runway threshold4. TakeoffI was being told that ground engines restart requires some time to cool down, is that true? And by taking the 747-400 as an example how long could the cool down time be? Thanks a lot =]

Hi all, I'm working on a fuel saving project in my college, and I'm having some problems about ground engine restart. I was proposing the following procedure:1. Start all engines after push back2. Shut down engines for 1-2 engines taxi3. Restart engine near runway threshold4. TakeoffI was being told that ground engines restart requires some time to cool down, is that true? And by taking the 747-400 as an example how long could the cool down time be? Thanks a lot =]

They do need a cool down time for restart to keep EGT in check for restart.Why would you start all four engines just to have to shut two down, then restart again?!

My idea is to start all of them to check if there's problem on the engines, minimizing the possibility of returning to gate when failed to start the engines on the taxiway. But I wonder if this could be done, especially in airports where taxi time is not enough to cool the engines back down.

My idea is to start all of them to check if there's problem on the engines, minimizing the possibility of returning to gate when failed to start the engines on the taxiway. But I wonder if this could be done, especially in airports where taxi time is not enough to cool the engines back down.

I've never heard of having to wait for EGTs to cool down before restarting an engine except if the shutdown was for an abnormal start, in which case you wouldn't be taxiing out anymore, as you would probably be on maintenance at that point. What you probably would have to wait for would be the starter limitations. Most air turbine starters will have rest requirements between start attempts and limitations on how many attempts can be made before a prolonged wait period is required. I'm not sure what operational advantage starting an engine you don't need yet and then shutting it down has, it just uses up a start cycle you probably would rather have kept in your pocket.

At light weights it is perfectly normal to taxi in or out on the two inboard engines. On taxi out you don't need to start the engines and shut them down again, your only limitation is you must start the engines and let them run at idle for a certain amount of time before setting take off thrust. I can't remember the exact time limitation but it is covered in the aircrafts FCOM and varies depending on engine type. No need to worry about restart limits because you never do a restart.Same thing for single engine taxi on a twin, you taxi on the engine that provides hydraulic pressure to the primary brake system and start the second engine within the time limit.EDIT - thought I should add that single engine taxi in is more common for fuel saving, it is not normally done during taxi out due to the high gross weight of the aircraft at the start of the flight. When experiencing big delays during taxi out you would obviously use common sense and shut down an engine, if that was the case you would monitor restart limits although the FADEC or ECC would be taking care of it.Regards

If this device is adopted, they won't need to start engines at all until they reach the runway!http://forum.avsim.net/topic/353342-new-replacement-for-pushback-tugs/

If this device is adopted, they won't need to start engines at all until they reach the runway!http://forum.avsim.n...-pushback-tugs/

Great thanks, sound like a very nice idea~~!!

APU uses fuel... I don't see how that's a fuel savings?

A tiny APU consumes only the fraction of fuel compared to a large jet engine. E.g. on the 767 the each engine at idle burns 6 times more fuel than the APU at 100%.

A tiny APU consumes only the fraction of fuel compared to a large jet engine. E.g. on the 767 the each engine at idle burns 6 times more fuel than the APU at 100%.

What is the flow rate of the 767 at idle?

approx 1400kg/hr

Wow that is what we burn in about a 4 hour flight.

Wow that is what we burn in about a 4 hour flight.

That is what who burns in a 4 hour flite?

Two pt6-42 engines

Yeah, on ground that big engines use a lot of fuel. But in flight and at idle she uses only half of that.