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Is the fuel pump in the wrong place?

I often wondered why the fuel pumps are so far downstream.

All they can do is suck, and hope that the fuel upstream does not turn into vapour as a result.

I know that with water, the most you can suck is about a 10m height, because 10m = 1 bar which is the sea level pressure and thus would correspond to a pump at the top creating a perfect vacuum, so this is the theoretical limit for an unprimed system.

For example, how can the electric pump clear a partial blockage of the fuel filter, or clear bubbles in the pipes in or before the fuel selector, when the filter is upstream of the pump and the fuel selector. Hence I could never understand why turning on the electric pump is supposed to be done when changing tanks.

Administrator
Shoreham EGKA, United Kingdom

I often wondered why the fuel pumps are so far downstream.

In many (even most among the ones I fly) the fuel pumps are in the tanks., submerged in fuel. They don’t suck at all, but pressurise the fuel system and/or create a constant Flow of fuel in the lines thereby preventing vaporisation.

EDDS - Stuttgart

Maybe read this: Link local copy

YPJT, United Arab Emirates

In many (even most among the ones I fly) the fuel pumps are in the tanks., submerged in fuel. They don’t suck at all, but pressurise the fuel system and/or create a constant Flow of fuel in the lines thereby preventing vaporisation.

Even in a submerged pump cavitation can occur with avgas or avtur due to low atmospheric pressure vs the fluid vapour pressure…..as well as the not insignificant amount of dissolved air…(up to 15% in kerosene)…..the pumps are designed to operate in the negative NPSH (Net Positive Suction Head) regime (ie handle cavitation)

Last Edited by AnthonyQ at 03 Sep 10:19
YPJT, United Arab Emirates

It is less desirable to have pressurized fuel in longer fuel line lengths, particularly when they run through the fuselage in the cabin area. Cars changed to this philosophy a long time ago, and since then there have been many car fires resulting from pressurizes fuel bursting out of lines and catching fire. The don’t vapour lock any more, because there is no low pressure, but fire might be worse.

Everything is a compromise… Given the choice of only the two, I’d rather be gliding than on fire under power!

Home runway, in central Ontario, Canada, Canada

DAR has a point, the high pressure fuel should be kept on the opposite side of the firewall to you !

OK; I see that, but on e.g. Socatas the electric pump is under the cockpit. Same on most other types I know of. So if you are going to have it within the cockpit area, why have it after the fuel filter and not before it?

Last Edited by Peter at 04 Sep 08:23
Administrator
Shoreham EGKA, United Kingdom

To keep -contamination- bits of dust and dirt out of the pump?

Last Edited by at 04 Sep 09:09
EBZH Kiewit, Belgium

Well I think that the PA-28 fuel delivery system with it’s reliance on lift is prone to surprising variations in fuel pressure without engineers ever getting to the bottom of the reasons, and that this type of behavior varies between different aircraft of the same type. In my opinion, this is down to the marginal nature of the original design combined with in-spec variations in the performance of replacement pumps (and perhaps fuels). If anyone ever got to the bottom of it, it would be AD territory, IMHO. I wonder how many of the ‘carb icing’ accidents have alternative explanations?

With the very poor quality of typical aircraft components (compared with automotive), the idea of pressurised fuel piping in the cabin is a source of concern. But some pilots of classic types talk about fuel sloshing about in the footwell as an occupational hazard. That being said, it’s a no-brainer: engine failure beats in flight fire every time!

EGBW / KPRC, United Kingdom

If anyone ever got to the bottom of it, it would be AD territory, IMHO. I wonder how many of the ‘carb icing’ accidents have alternative explanations?

I think there is a good number of things like that in GA… I think one reason Honeywell don’t want to fix the KFC225 because of a risk of an AD grounding the entire installed base.

Administrator
Shoreham EGKA, United Kingdom
26 Posts
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