> I see it as a waste, an aircraft arrives in the gate, and the time
>it takes to get ground power connected an engine is left running...
The only number I recall was "8 minutes" for B757. It was cheaper to keep one engine running for up to 8 minutes waiting for ground air/electric than to start APU and shutdown both engines during those 8 minutes. This economic model took into account the APU cycles and wear/tear on APU as well as fuel burn.
OTOH; once one has the MD90 up/running on internal power, there is no way I'm going to shift electrical power to any outside source... NO EXCEPTIONS! That means the APU (if it is working) is going to be on everytime the aircraft parks. One electrical "hic-up" and the plane could be down for 20-40 minutes trying to get everything working properly again (if not longer).
>then it always seems as soon as the crew deplanes the next crew
>comes on and the 1st thing they do is light off the apu.
Inbound crew is leaving and may not know when the plane is departing (or they may not care) so shutting down may be appropriate (checklist item?). Outbound crew will have two major concerns; a) reliable electrical power and air source for starting and, b) comfortable cabin air temperature.
>Also when do you normaly light the apu off?
The "ideal" situation is to have the APU up and running no more than 1 minute prior to parking at the gate. Reality means we usually start it immediately after clearing the runway (I prefer one-engine taxi).
>Also ive seen aircraft doing single engine taxis after powerback or only
>lighting one engine after pushback. I know this is a very common
>procedure but what are y'alls opinons of it...
Very common. No big deal.
>This makes me think of NW 255 where they pulled the aural warning
>system ciricuit breaker since they did single engine taxies so tehy
>wouldnt have to deal with being told the slats werent set when
>they pushed the throttle forward to taxi and ended up taking off
>without without the aircraft configured for departure.
Never pulled the C/B on the MD80 for single engine taxi. One just lives with the warning(s) since it is so rare anyway. OTOH, the MD80 trained folks driving the MD90 always use too much power... all the time!