[ju-st]

Commercial airplanes have (at least) two engines because of the high risk of failure at take-off.

The next step for this electric engine would be to replace that second heavy, expensive engine every large airplane carries around...

Additionally, the full power of the two engines is only needed at takeoff. At cruise they only run at 30-40%. An electric motor with a relativly small battery would be enough to provide enough power at takeoff.

[phillc73]

Plenty of "commercial" aeroplanes only have one engine. The Cessna Caravan springs to mind. I run a couple of Cessna 206s for joy flights. Strictly speaking, people pay my company money for this, therefore the aircraft are "commercial."

[mikeash]

It's not just takeoff. Often airliners cruise outside of gliding range of an airport that can handle them, and a total power failure could be catastrophic there.

As long as the electric engine can provide a sufficient backup to get to an airport after a failure in cruise, though, that would certainly be an interesting possibility.