When is a fuel pump needed on GA aircraft? I am a flight simmer and have noticed that you can leave the fuel pump off and start/fly the aircraft. When would you use the fuel pump?
On a low wing monoplane you'll need a pump all the time. There will be a mechanical pump fitted to the engine for the purpose. On a high-wing monoplane the engine is often gravity feed.
However, during critical phases of flight (specifically take-off and landing) you wouldn't want to have that fail so a second auxiliary electric pump is fitted that the pilot switches on beforehand and can switch off once that phase is over.
The Pilot's Operating Manual will give specific recommendations for use on any given type.
Refer to the POH for the aircraft but in the PA-28-161 I fly the electric fuel pump is used on take off, landing, and while practicing maneuvers like steep turns and stalls. Most GA planes are configured with either a gravity feed fuel system (like the Cessna 172) or an engine driven fuel pump (Piper Cherokees) that can be supplemented with an electric fuel pump should it quit or become partially inoperative. The FAA has a nice document on it here (see pump feed systems) their opinion is,
The engine-driven fuel pump acts as the primary pump. The electric pump can supply fuel should the other fail. The electric pump also supplies fuel pressure while starting and is used to prevent vapor lock during flight at high altitude.
The switch you see in the cockpit controls the electric pump. As far as I know there is no way to turn off the mechanical pump (not that you would really want to).
The general startup procedure for carbureted PA-28's (low wing mono planes (fuel tank below engine)) often involves turning the fuel pump on, checking for fuel pressure, then turning the pump off. This can also help to prime the lines with fuel, if the lines are dry you will need a few extra cranks of the engine to draw fuel in with the mechanical pump which drains battery power.
Since you claim you can fly with out the pump running we can assume it is an engine pump driven plane however smaller turbine planes that still fall under the GA header may require an electric fuel pump to feed the engine.
Virtually all GA aircraft, high or low wing, use both an engine driven fuel pump and and electric auxiliary fuel pump. Typically high wing GA aircraft will use the aux pump only for priming the engine (fuel injected engines) as gravity feed from the main tanks offers a backup in case of EDP failure. The aux pump is used on low wing aircraft, both for priming the engine as well as a backup in critical phases of flight where EDP failure could be disasterous such as takeoff and landing. This is due to the low wing arrangement requiring siphon feed from the tanks to the engine.