Cessna 172 fuel pump switch and mixture control on start

[737Andi 6]

Hi,

I have seen in several Cessna 172 startup youtube videos that the fuel pump switch is shortly turned on and that the mixture control lever is then pushed in.

Shortly after that the fuel pump is however switched off before the engine start is performed.

Can somebody please explain to me in more detail why the fuel pump switch needs only to be activated once before start and is then not used any more.

Thanks!

Best regards

Andi 

[skelsey 766]

The electric fuel pump is used essentially as a primer to draw some fuel in to the cylinders prior to start.

After starting the engine-driven fuel pump is sufficent in most circumstances. The electric fuel pump may be used in certain circumstances (e.g. to alleviate vapour lock at high outside air temperatures, or as part of typical 'trouble checks' in the event of engine problems) but per Cessna is not otherwise generally required.

That said, many pilots/schools/clubs etc may well have different approaches/procedures/habits.

[vbazillio 2,463]

Another background story around this kind of startup procedure with pump ON then off (mixture cut-off...) like in the A2A C172 Trainer is that some says : "it's strange I saw another start-up procedure on Youtube where the mixture is full rich, where there's a primer".

Recent 172 (SP, G1000...) have fuel injected engine. Older one (N, M...) have older engine with carburator 😉 and "primer" (manual pump). The startup procedure is different.

Edited September 26, 2019 by vbazillio