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Final 3 Greens
4th Mar 2002, 14:38
I have posted this question here, rather than in the Private Flying forum, as I know that a lot of airline pilots have many hours on type: would be grateful for any insight into something that our little beauty is doing.. .. .With the electric boost pump "on", the fuel flow in the climb is a healthy 5psi - when it is selected off at 1000' (with 2500rpm set) the reading drops to under 2psi.. .. .Having tested the a/c on the ground, this effect seems to be related to the power setting - e.g. at all settings with the boost pump on, the fuel flow is solid, but with it switched off, the higher the power setting, the lower the indicated fuel flow.. .. .The POH fuel diagram shows two flows, one directly to the mechanical fuel pump and then to the carb, the other via a one way (non return) valve through a separate line (which bypasses the mechanical pump and feeds the carb direct. Presumably this arrangement provides redundancy in the event of mechanical pump failure.. .. .The fuel gauge is shown as being linked to the bypass fuel line.. .. .It would thus appear that the indicated fuel pressure drops when the electric boost pump is selected "off" due to a reduction of pressure flowing through the bypass line, as the engine pump is working strongly due to the high rpms.. .. .I guess my questions are:. .. .1) - Have I understood the way the fuel system works?. .. .2) - In the experience of pilots with time on Continental engined Pups, is this normal?. .. .3) - I am wondering what the symptoms would be if the mechanical pump started to fail (would the indicated pressure tend to rise as the mechnical pump took less flow?. .. .-and-. .. .4) - Do I need to be worried about the situation?. . . . <small>[ 04 March 2002, 09:42: Message edited by: Final 3 Greens ]</small>

ShyTorque
5th Mar 2002, 01:16
F3G,. .. .I'm not current on the Pup but I used to instruct on the RAF Bulldog and I think the fuel systems as far as the engine are probably similar, albeit the Bulldog having fuel injection as opposed to your carb. (Sounds like "trust me, I used to be a doctor!" <img border="0" title="" alt="[Smile]" src="smile.gif" /> ). .. .Firstly, fuel pressure is NOT an indication of quantity of fuel flow to your engine, in fact the fuel pump's job is only to pressurise the fuel lines to ensure a reliable supply to the carb. The carb has a float which shuts off the fuel to a level from where it is DRAWN out through the jets by engine manifold depression. (An injection system picks up fuel and re-pressurises it through the injectors). It is incorrect to think that your fuel pump/s act like a spraygun such that increased pressure means more fuel flow! (Otherwise your engine would go extremely rich when you select your electric pump on).. .. .The electric fuel pump IS there as a backup to the mechanically driven one, we used to use it near the ground (where we were unlilely to successfully bail out) and for aeros, stalls, spins, PFLs etc.. .. .The fact that you are able to see a difference in indicated fuel pressure with it selected on is a bonus, and if I remember correctly was part of the run-up checks on the Bullfrog. By observing that your normal fuel pressure increases with both pumps selected on, you then know that the electric one is working correctly, as opposed to a warning light which might only mean that the switch has been selected on.. .. .The bypass line in the system is there to ensure that one failed pump does not jeopardise the correct operation of the other.. .. .One thing I might want checked though is the normal fuel pressure capability of a fully serviceable mechanical pump. 2 psi sounds a little on the low side, I would have expected 3 psi. I suggest you just confirm this but it sounds like your system is otherwise quite normal.. .. .Hope this helps!. .. .Regards, ShyT. . . . <small>[ 04 March 2002, 23:36: Message edited by: ShyTorque ]</small>

Feather #3
5th Mar 2002, 10:33
F3G,. .. .Why don't you contact DHSL at Duxford who now have design authority for the type? They would surely be able to point you in the right direction.. .. .G'day <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />

Final 3 Greens
5th Mar 2002, 15:11
ShyTorque. .. .Your comments are very helpful. During the run up at 1800rpm the fuel pressure stabilises at circa 2.7psi which is near the book figure of 3, but there is no indication in the POH of the what the fuel pressure should be at 2500rpm (climb power.). .. .Anyway, will have it checked - it had crossed my mind that we might have a problem with the gauge or the sensor, but I'll leave that to a qualified mechanic to check out.. .. .Its very helpful to have someone elses opinion before sending her to the workshop.. .. .Many thanks. .. .Feather #3. .. .De Havilland Support are an option, I wished to see first if this was a common experience with Pups, as I don't have many hours on type and the POH is not entirely helpful.. .. .Thaks for the thought. . . . <small>[ 05 March 2002, 11:27: Message edited by: Final 3 Greens ]</small>

A and C
7th Mar 2002, 15:07
The fuel PX indicated is not an indication of fuel flow but rather an indication of lack of fuel flow !.. .. .The carb float valve controls the fuel flow by opening on demand when this happens then the fuel PX will drop and as it closes the fuel PX will rise so low RPM=low demand= high fuel PX at high RPM the float valve will be slightly open allmost all the time so the fuel PX will be lower.. .. .In short if you have positive fuel PX at max RPM . .then you are OK.. .. .Most of ST,s comments are based on the Bendix RS injection system ( and for that system are true)but this is a totaly different system.. . . . <small>[ 07 March 2002, 10:21: Message edited by: A and C ]</small>

Final 3 Greens
7th Mar 2002, 17:42
A & C. .. .Many thanks.. .. .Having spent some considerable time with the fuel system drawing recently, I understand what you are saying and am reassured.. .. .Presumably the large increase/decrease in indicated fuel pressure is also influenced by the location of the sensor (on the bypass line) and also the operation of the pumps in series, rather than the parallel arrangement on the PA28s I have most hours on.. .. .Thanks for taking the time to help my education! <img border="0" title="" alt="[Smile]" src="smile.gif" />. . . . <small>[ 07 March 2002, 14:55: Message edited by: Final 3 Greens ]</small>