“But the aircraft would fly on 3.”
However, on this particular day, AF1 was a 757-200 with only 2 turbofans. If one turbofan were to ingest a drone and explode and worst case drop from the wing, this would leave only one engine with thrust on the opposite wing, and then the airframe could be highly prone to flip.
If a single drone has turned up now, drone swarms are being experimented with around the world that could be placed (parked) in the ILS glide path of AF1 in an ambush on very short notice to increase probability of a strike.
STC
I was made to understand that he was using AF1 for the flight, which is a four engine 747-8 aircraft.
The one you are talking about is AF2 which is an AF C-32 used by the VP and other dignitaries. In my reading I found the C-32 was equipped with the same configured dual performance Pratt & Whitney F117-PW-100 turbofan engines. If one was taken out, it could fly with a little work from the pilots and could land. Very seldom does the president use the C-32 as the 747-8 AF1 is predominately bigger and has a greater range.
rwood