Aircraft Fuel Booster Pump - Centrifugal & Immersed in Fuel

NPSH_A consists of atmospheric pressure plus (elevation above pump x specific gravity of fluid) minus fluid vapor pressure and pipe friction pressure loss in the suction piping. In low altitude cases, atmospheric pressure is 14.68 psi, which is commonly used as a default value. In your case of high altitude, this value does not apply; and thus greater caution is called for. (As I think you already understand.)

I would imagine this issue has been addressed before, but you may need to search carefully for fully correct literature.

One way to tackle it (as in refrigeration vessels and boiler feedwater tanks) is to mount the pump below the tank or vessel, thereby gaining some liquid head.

I am not closely familiar with this special situation, so please take these notes as preliminary only. But also be on guard against any old pump Joe.

There is no correct way to put limit on flying conditions. This would all have been worked out prior to certification of the aircraft operational envelope.

I guess. I'm not an A&P mechanic, so I can't say for sure.

I think your off just a little in your understanding of fuel boost pumps used in aircraft. I will speak to general aviation planes as I have not work on transport units. The boost pump most commonly used is a piston or diaphragm style unit. The tanks are vented to avoid a vacuum form being created and air-locking the fuel from the pumps.

Low winged aircraft such as Pipers, Beach's etc, will use fuel pumps to produce the need pressure due to the tanks being mounted lower then the engine. On some of the larger piston engined units that use fuel injection they may also use a boost pump on take off, start up etc. High wing aircraft such as Cessna's and a like do not use fuel pumps as gravity takes care of fuel pressure. But, like the large engined units in Piper and Beach they also will use fuel pumps with fuel injected style engines. Only to ensure fuel at critical times in flight. Fuel inject pumps generally will handle all other forms of flight.

There are several good publications out there on engines and fuel systems. EAA has a great resource of books on these subject. One I would recommend is Fire Wall Forward by "Tony Begilis" (I think I spelled his last name correctly)

Limitations as expressed in the flight manual for a particular aircraft are virtually always of the nature: "Fuel boost pumps must be on for takeoff and landing." "Fuel boost pumps must be on for all normal operations." etc. etc. The determination depends upon the operating envelope of the aircraft, the functional requirements for the boost pumps, etc. But the limitations are usually simple, at least have been in my limited experience. I have not seen a flight manual limitation such as "Fuel boost pumps must be on for operation at over 20,000 feet ASL." But then there are plenty of flight manuals I have not read.

The same physical boost pump can have different operating limitations when installed in different aircraft models.

From the boost pump manufacturer's perspective, the limitations would need be more completely specified, because the audience is not the pilot, but the aircraft engineer who has to decide if the pump will work in his application.

CR4 is a great place to find answers to a lot of things, but, it is not the place to come for advice in designing or modifying aircraft fuel boost pumps. Being that you posted as Anonymous, we have no idea where your located and what government agency dictates regulations for fuel boost pump certification. Here in the USA, we have to deal with our FAA and under title 14CFR parts 21, 43 and 91. You can not deviate from the manufacturer's original design without prior approval from, either the manufacture or the Administrator(FAA) on form 337. The fuel pumps, both main and boost pumps, must supply positive fuel flow to the engine(s) at any altitude or attitude that the airframe is certified to, normal, utility, acrobatic, commuter, or transport category.

My question to you, Anonymous, Are you qualified to make the design or modifications to the fuel system(s)?

DJ A&P Mechanic, QA Inspector,Commercial Aircraft

Did an Anonymous Post disappear? Your post does not reply to any specific other post.

Great article. Thanks for the info, this is really a helpful post. BTW, if anyone needs to fill out FAA form 337, I found a blank form in this link http://goo.gl/4WjnSr. This site PDFfiller also has several related forms that you might find useful.

Hi Paul!

Thanks for sharing this site PDFFiller which I find very useful. I just found the form I need. Here is a link to the blank FAA form 337 that I was able to fill out http://goo.gl/2nQbTt