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Jet engine wear is also measured in cycles, so starting and stopping an engine just to see if the engine runs OK would be a no go.If the PMDG figures are correct, taxiing a 744 at 20 KN GS at MTOW burns fuel at the same rate as cruising at FL400 at 500 KN TAS in the last few hours of the flight: About 4,500 lbs per engine.BTW I seriously doubt the WheelTug System would work for the 744 at MTOW: The nose wheels would simply slip due to insufficient weight, and is sufficient weight was added to achieve traction, they would break (and the aircraft CG woul be way past the forward stop).But I do wonder why they don't use a regular tug to tow out to RWY. Perhaps the tugs aren't powerful enough.Cheers,- jahman.

But I do wonder why they don't use a regular tug to tow out to RWY. Perhaps the tugs aren't powerful enough.Cheers,- jahman.

They are plenty powerful enough but to have quite a few more tugs to make that work.

They are plenty powerful enough but to have quite a few more tugs to make that work.

I meant power in the physics sense as in force times speed. While the tugs move the aircraft, they seem to be too slow at 5 KN or less, so a taxy to the RWY could take a very long time. A 3 NM taxi is not unheard of and would take 36 minutes!Cheers,- jahman.

Some engines have limitations on the starter duty cycle. For example for some B737s:First attempt: 2mins on, 20sec off.Second and subsequent attempts: 2mins on, 3mins off.

I meant power in the physics sense as in force times speed. While the tugs move the aircraft, they seem to be too slow at 5 KN or less, so a taxy to the RWY could take a very long time. A 3 NM taxi is not unheard of and would take 36 minutes!Cheers,- jahman.

We can get up to 15 mph on ours. Under tow the limit is first gear which is equivalent to a fast walking pace. Any faster than that the ride is uncomfortable for the passengers.

They do need a cool down time for restart to keep EGT in check for restart.

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.

Zach's right. Besides the starter limitations, some engines also have starting temperature limitations. If you were to start the engine above this temperature, you are most likely to get a hot start. The JT8D-219 on the MD-83 for instance, needs to be below 90 degrees Celsius (if memory serves me right) for start. Since the EGT could be above that limit after shutting down the engine, you would need the engine to cool down or you would have to motor the engine, which would imply more wear on the starter shaft, increasing maintenance costs, which in the long term, would be higher than the money saved on fuel.