Originally Posted by
romeocharlie
OP...
The amount of aircraft doing SE taxi is dynamic. This changes on a flight by flight basis dependant on a number of factors, predominantly taxi time. Once the APU is up and running and a sufficient cool-down period is completed (3 minutes for us), then the option to conduct SE taxi is there.
Here in the US it's more a practical reason in that the wait line to the off can be so long that if you have all engines burning on the taxi out you will likely not meet your min TO fuel. Indirectly I guess it's a fuel/cost saving measure rather than a more practical reason of not screwing the passengers over and returning to the gate to top up with gas.
I'm yet to see anyone take it up taxiing out and imagine that the only time it would be is when the time to take-off is significant (above 30 minutes). In this case I would also suspect both engines would be started initially, and provided it's a normal start then would proceed to shut one down (and eventually restart) after being notified that the time to take-off would be extensive. I assume the provision is there to accommodate 'unusual' situations or for airlines running out of busier airports (Heathrow?) where taxi times are consistently long.
As for taxiing in, we burn 1.2 tonne/hour at idle (B767, two-engines), so discounting the extra fuel for the APU and the higher power settings for crossing runways or getting onto the bay/during turns, I guess 2 minutes you're looking at 60kg of gas saving per time we do it.
Hope this helps.
In the USA normal practice is to taxi out on one engine until you see how long the line is. About 4-5 aircraft before your turn to take the runway then start remainder of engines with enough time to meet warm up limitations.
After landing taxi in on one after meeting cool down requirements, unless there's tight turns into the gate.