Not the most important subject I know, but it 'bugs' me not a little :)In general, when do pilots of commercial Jets start their engines? I've been to EGLL a few times both as a spectator and a passenger. Sometimes engines were started at the Terminal, more often after pushback. I have a theory about this, but I'd like to know factors actually govern engine start up.KenSomerset, England

Good question. Generally, you are cleared to start the engines by the crew on the pushback truck or ground staff at the parking position/gate. Ultimately, the PIC decides when to start the engines, but only after he was cleared to do so.Most of the time, one engine is started during pushback and the second engine after the pushback is complete and the truck has disconnected.It depends on the airfield, the parking position and the traffic. Theoretically, you can start the engines at any time, if you just pay attention that the exhaust does not endanger people, traffic or supplies on the tarmac.Hope this helps.

Hi Patrick,That about covers it all, I think :) I mentioned my theory, it does include exhaust blast and it's possible effects. I've noticed at Heathrow, in particular, at some terminals aircraft park left and right (sort of back to back); those aircraft invariably start engines after pushback. Your answer covers those instances well.My interest in this subject is because my VA counts Flight Time as blocks off/blocks on - time can be 'wasted' by pushing back and then starting engines, instead of starting at the Gate. Now I can make the decision to start engines taking all those factors in your reply into account.Very helpful indeed, thank you.KenSomerset, England