I believe the airstairs are a per operator item. I know that DL's 732's used to have them but they were removed do to weight. Aircraft could carry more without them and the stairs were not needed since most airports now have jetways. I have seen them on a 733 that is a charter aircraft.
Like a lot of other things electronic equipment,APU,engines etc.the stairs are optional too.My previous company was a charter company and those stairs were handy to escape from airport stair fees.But the current airline is a flag carrie with lots ofground equipment of its own so doesn't have it.Helps to reduce the airplane weight also.
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Considering most Airports are Equipped with Grd Stairs.It might be preffered to drop off the excess weight.
Its common in Freighters to NOT have the Airstair Installed.
Depends on Company policy.
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