The ‘offical’ story is that the quinjet broke down.
AKA someone forgot to refuel it after their last mission.
At one point, in the 1970s, the Avengers’ get their special priority status with the US government revoked, which means, among other things, that they can’t fly the Quinjet wherever they want without filing a flight plan in advance. So, when a crisis point develops in the suburbs, and they need to get there in a hurry, they have to… take the bus.
The Avengers vol 1 #178, by Jim Shooter, David Michelinie, David Wenzel, and Pablo Marcos.
Tasertrickslove 