Can commercial airliners theoretically taxi backwards using reverse thrust?
If this is possible, why isn't it common?
I can already imagine some safety reasons...
This is called Powerback, most aircraft can do it, but it is not done very often.
In a jet aircraft, the three main problems are:
So a tug is both cheaper and safer.
In propeller aircraft, reverse is more efficient and does not throw up as much debris, so it is sometimes used. But a tug is still preferred because transport aircraft don't have any rear visibility, so the pilots can't see where they are taxiing. With tug the driver can see behind the aircraft, and a ground marshaller walks along side each wingtip with an intercom connected to the plane.
Lastly, since aircraft are able to turn almost on the spot using differential braking and thrust and 180-degree nosewheel steering, at airports with few facilities there is generally enough space around the aircraft to permit the pilot to reverse out easily.
It can be done, in fact the DC-9 and MD-80 aircraft are approved for backing up using reverse thrust. It is called "powerback".
It is rarely used since it is quite fuel consuming, noisy and increases the risk of sucking up debris near the gate area causing damage to the engines.
Here's a video of an MD-80 backing up.