757 Packs automatice shutoff during start

[kevinh 409]

Some while ago I got corrected for a post making the assumption that the 757 packs automatically shutdown during engine start. I was wrong in that assumption. Now I have the 757, I notice that PSS seem to have made the same false assumption. The PSS 757 pack pushbuttons amber OFF lights indicate the packs go off as soon as an engine start is selected. This should not happen. Packs must be manually switched off before start.As the pneumatics isn't modelled very well it doesn't make any significant difference, except it's another EICAS message to cope with and maybe another caution to falsely sound.Hopefully PSS will add this of the things to change on any forthcoming patch.Kevin

[Guest dt1951]

Kevin,I'm not sure the packs are automitcally "shut off" during engine start. I suspect what you mean is that bleed air is not routed to the packs during engine start. And that does happen. The engines will start with the packs "on".

[kevinh 409]

No, I mean the pack OFF lights go on, indicating the pack has shutdown. There is nothing to stop air being supplied to the packs during an engine start that I am aware of. There are no valves to divert air away from the packs to the engines for starting.On some aircraft types (e.g. Airbus), selecting ground start automatically closes the pack valves. Not so on the 757. The engine bleed valve does close during a start, but the packs are being supplied by the APU or External Air, so this will make no difference.I was told in no uncertain terms on this forum by some 757 pilots & maintainers, that attempting to start the engines with packs on would lead to slow starts and possibly excessive EGT. Since PSS don't model duct pressure drop, there is no danger of this happpening in this sim.I suspect the PSS pneumatics simulation is a hang over from previous Airbus designs.Kevin