fuel pumps:For up or down engines?

[PPRuNeUser0179]

hi everyone
just a question about fuel pumps.
ive noticed recently when watching aviation clips on youtube (normally start ups IE:bell 206 start up) that they activate the pumps.ive also noticed that these heli's have their tubine above the fuel tanks.so it makes sense that they would need to use the pumps.Other choppers IE:R22,R44,R66 have their engines just a fraction below their tanks,and ive noticed that they dont use fuel pumps.

so what im asking is,do all helicopters that have their turbines above their tanks all need fuel pumps or is this just a coincidence?

Ive also seen planes use pumps to only PRIME their engines before start up.IE:C172.
But yet Ive also noticed that the kingair 200 or 350 (cant remember which one) used fuel pumps through-out the entire start up.The pilot only turned them off once the plane was ready to taxi.

1:)Is the use of fuel pumps for technical reasons or is it simply just because the turbine is higher than the tank and it just needs to be boosted through the pipes?
2:)If it is just because the turbine is higher than the fuel tanks,why would the kingair need it through-out the start up if the turbine is level with the tanks.

I know that this is not the simplist way to ask this question but i dont know what else to say.

anyway,thanks in advance

Daniele

[3 Point]

Hi Daniele,

First thing is to say that you may find some responses here saying these are silly questions - they are not! There are no silly questions! If you don't understand something you should ask. It shows that you are thinking about things and have an enquiring mind - both very good signs at such an early stage of your career in aviation.

Now, to the answer; not such a simple question!

In general you have got it right, with low engines and high wings gravity can do the work of sending fuel to the engines, hence no pump in a Robinson or a C172. With a low fuel tank and a high engine then obviously a pump will be required, in fact a pump and a back up because with just one pump a single faiure would stop the engine - not a good thing!

Once you start to look in more detail there are many many factors which could come into play sch as the routing of the pipes, are there any high points in the pipe runs or any possible air/vapour locks? What is the specific engine's requirement for fuel pressure both during start and during normal running? Is there a need at certain stages of flight (take off and landing perhaps) to anticipate a failure of the single pump and protect against engine stoppage by running the second pump?

Many many questions and as a pilot it's unlikely that you will ever be aware of all the subtle factors which come in to play. The only people who are (or should be!) are the designers, engineers and test pilots who built the thing and wrote the manual so - the only safe answer for you (and all other pilots) is to use the procedres in the manuafacurer's operating manual.

Happy landings

3 Point

[PPRuNeUser0179]

hey 3 point

i thought about it before I posted it,but ive been told and ive seen post where people say "THERE ARE NO SILLY QUESTIONS."
And i truly believe it :)

this question has been haunting me for days on end (dont know why i didnt ask my instructor first) but I thought it was such an obvious anwser but then my mind started telling me that it was more complicated than that :ugh:

Im sure that there must be more factors contributing to the use of fuel pumps but atleast I know one of the reasons:)

thanks again 3 point


Daniele