The flight deck of this Bristol Britannia has a navigator and an engineer as well as two pilots. The engineer is for fixing stuff when it goes wrong during the flight.

When did they stop doing that?

The flight engineer is mainly their to manage the engines, if revs and prop speed aren’t synchronised the aircraft flies inefficiently and you can get severe vibrations. Flight engineers went the way of navigators when automatic systems became smart and reliable enough.

Some planes the engineer would crawl into the wings mid flight to top up the oil on the engines http://en.wikipedia.org/wiki/Convair_B-36_Peacemaker