Originally Posted by
Twin Wasp
If the electric pump fails in your car (and if it is the only pump) you're stuck on the side of the road. If all you had in an airplane was an electric pump and it failed, where would you be?
Yes, this. Need a primary and a backup for safety-critical systems in aviation. Except for a few very reliable things like wing spars and engine blocks.
High wing: Primary = gravity, backup typically electric (just like two ignition systems).
Low wing: Primary typically mechanical, backup typically electric.
Jets: Lots of fuel pumps.
Same reason some gyro instruments are vacuum, others electric in older planes.