jessjess (asker) May 7, 2015:

The client went for "freight position", since I didn't have more context I figured it was general enough and they couldn't explain it more in detail anyway. Thank you all very much for your help.

Daryo Apr 16, 2015:

a bit more context would be useful although the general idea is quite clear.

When loading an airplane there is a character called a "loadmaster" whose job is crucial: distribute the load inside the cargo airplane in an optimal and safe way.

That would include deciding which crate goes where inside the cargo airplane, and how it must be orientated.
So most likely

Effectuer une rotation de 90° des pièces avant chargement en position avion par le grand côté du conditionnement.

means these crates must be loaded on the truck in the same position/orientation they will have to have in the airplane, so when the truck stops next to airplane the crates could be pushed straight into the plane (without messing around to orientate them in the right way)

you could also look at

http://www.btb.termiumplus.gc.ca/tpv2alpha/alpha-fra.html?la...

but it doesn't fit here

all this assuming that these crates are going to be loaded in a cargo plane ...