fuel pump failure

Hi William,

I love to test and research things and this just happened to be one of them. My test was strictly that, a research test and not for any operations. Just something I wanted to know.  Inquiring minds wanted to know. LOL

This test was done with a Flight Design CTSW high wing.

I did a research test years ago about this very thing. Of course Rotax says 2.2 psi, but I wanted t know how low a pressure would really work. This only works with a high wings because of gravity. Low wings must have an electric pump back up and even high wings should. 

So I pulled the inlet hose off the pump and the outlet hose. I took a metal tube and connected the two hoses. So now the fuel was only gravity fed. I ran the engine up to full throttle and it never coughed or sputtered. Then  I thought this wasn't very realistic. So I replaced the hoses on the pump and removed the pump and just let it hang there basically totally dead. Then I put a blind over the open hole on the gearbox so it wouldn't lose pressure or oil. So when a pump fails it fails open. I ran the engine again and I could get up around 5K rpm, but any more than that the engine did sputter from lack of fuel. This was only gravity fed pressure.

So to answer your question will the engine run if the pump fails,,, yes, but it would be extraordinarily rare that such an event would ever happen. Pump diaphragms can leak if it's getting too worn or old, but totally failure probably isn't in your future.

MFG's put limit specs on many things to keep whatever someone is using in a safe zone and so people won't push the limits and get in trouble or get hurt. Liability for MFG's is in play too.

So even though I might test something I still stay within a MFG's published limits when using their equipment. It's for the product longevity and safety that we should stay within any MFG's specs during usage.

Just in case you are interested in the FAA requirements, they are in 14 CFR 23.955 and the following parts apply to your question: (a) General. The ability of the fuel system to provide fuel at the rates specified in this section and at a pressure sufficient for proper engine operation must be shown in the attitude that is most critical with respect to fuel feed and quantity of unusable fuel. . . . . .  In addition— (1) The quantity of fuel in the tank may not exceed the amount established as the unusable fuel supply for that tank under §23.959(a) plus that quantity necessary to show compliance with this section.  (b) Gravity systems. The fuel flow rate for gravity systems (main and reserve supply) must be 150 percent of the takeoff fuel consumption of the engine.

So, in your particular A/C case and assuming you are qualified to open your fuel system, with the fuel system test requirements described above, remove the fuel line coming off of the fuel pump and, at the level of the carb entry points, test the fuel flow rate by timing how long it takes to fill a clean, gallon jug.  The math the will answer your question for your plane for the worst possible case of losing your fuel pump as that is when it becomes a gravity fed system.

All:

If you are getting dragged into the concept that your engine will run on gravity feed only you must consider this.  The carburetors are designed with the knowledge that gravity feed will require more flow (less resistance) in the system.  This is commonly misunderstood.  For example.  We are well aware of motorcycles, with the fuel supply directly over the engine,, flow without a fuel pump.  This works in that the resistance is reduced to get the correct volume needed into the float bowls.  in the case of a system designed for a fuel pump there are some parts inside that are sized differently to allow this to work and take into consideration the variable configuration issues such as low wing aircraft or long fuel line runs.  

The main difference is the inlet needle orifice size of the seat.  Gravity feed, motorcycle type, have larger inlet seat bores.  These can be 3 or 3.3 mm in some cases.  When we have a fuel pressure designed system (Rotax calls out at least 2.2 PSI to get full volume at WOT) then the inlet needle seat will be much smaller, I believe it is 2mm.  So you will see power is possible with a gravity feed but when you get to the high volume flow rate needed for max power it will sag and you run out of fuel in the bowl.  

Cheers

You shouldn't fly with pressures below the 2.2 psi lower limit. My test was just that a test to see what happens and how the engine would respond to lower pressures.

Just because you can some some things doesn't mean you should. Stay within the operating limits.