Can a passenger jet fly upside down?

Answer 1

Commercial aircraft are definitely not designed to fly upside down and could not sustain flight.

Airplane wings have a specific shape - curved on the top and relatively flat on the bottom. Air flowing over the curved top of the wing has to travel faster than the air flowing across the relatively flat bottom of the wing because it has farther to go. This difference creates a low-pressure area (vacuum) on top of the wing, and this is where the majority of our lift is generated.

Flip the airplane over, and the lift is destroyed. While rollovers and very short times of inverted flight are possible, too long like that stresses the airframe and aircraft in ways it can't handle, and result in either structural failure, stalling, or simple complete loss of lift.

PBS Television aired a historical film of a test flight of a Boeing 707 doing a barrel roll, going inverted during the maneuver. Being momentarily inverted during a barrel roll is not technically flying upside down, however, it does support the notion that a commercial jet liner [the 707] was able to be upside down, even for an instant, and recover and land safely.
A 747, or most any airplane Can fly upside down under Specific conditions and special circumstances. IF the variables all line up just right - there's no problem. First, the engines have to be strong enough to force the plane while using flaps, rudders, elevators, etc., just right. The weight of the plane must be just right to allow the 'flip' so to speak. And the conditions inside the plane must be right for the people involved to be able to handle the plane under those conditions. A fighter jet is specifically designed to handle all these problems. Even the people on board are 'specifically' designed to handle these situations. But not on a commercial plane. So retrofitting fuel tanks (for weight) and all the interior materials would have to take place. The primary concern would be Engine Power. Without enough power to force the lower wing mechanisms to force the plane upward while up-side-down it won't work less than a "Roll." Those small flaps/elevators on the bottom of the wing are just not capable to sustain flight. Even if the Engines had the power - the flaps/elevators would have to be retrofitted to hold the energy being forced past them. SO... YES, maybe...