Originally Posted by
davidviolin
I still smell jet fuel when the 737 starts the engines after pushback.
During pushback and engine start the air conditioning is coming from the APU which is in the tail of the airplane. Anything you smell is coming into the system from the APU air intake just forward of the tail. Depending on which way the wind is blowing, the odors could be coming from your airplane's engine or other nearby airplanes, fuel trucks, etc.
On the 737 we normally start the No. 2 engine first. During that engine start the a/c system is off, with only the recirc fans providing airflow, because all of the APU's bleed air is needed to start the engine. Immediately after the engine is started (takes about 50 seconds) the pnuematics are reconfigured so that the No. 2 engine bleed air is powering the right pack to supply fresh air to the cabin. If the No. 1 engine is going to be started right away the left pack remains off until both engines are running. If the taxi will be on a single-engine then the APU bleed runs the left pack until it is time to start the No. 1 engine.
During start, you are never breathing air from the engine that is being started.
Air still passes through the engine, where engine oil contamination is a possibility.
You have to look at what is inside "the engine". Air flows in only one direction through the engine, front-to-back. It is not like a a reciprocating engine where there is a lot of movement in various directions. All of the engine oil and hydraulics are down-stream of the compressor section where the pneumatic air originates. If there was a leak that was spraying oil or hydraulic fluid into the compressor section it would become apparent very quickly.
Even on the video they show the bleed air being tapped from in front of the combustion section. Combustion gasses and fluids can not flow "upstream" against the very strong forward-to-aft airflow through the engine.
Originally Posted by
FlyWorld
but if engine is started at idle or plane goes into reverse for push back from taxi, that is the moment when engine exhaust can be sucked in. No?
No. The engines don't go into reverse for pushback--they aren't even started yet. Often, the first engine (usually No. 2 on the 737) is started during pushback. As I explained above, both packs are off during the first engine start and the engine being started can not provide bleed air during start (as it isn't yet producing any).
The old DC9 and 727 with their tail-mounted engines were capable of power-back operations but I don't know of any airlines that still do powerbacks in their DC9 descendants (MD80/90/B717). Even when they did, the packs would automatically shut off when the reversers were deployed to prevent compressor stalls in the older engines.