By GENE COOK EAA 382511

114 Imperial Ave. Friendswood, TX 77546

I have noticed in two recent articles on hooking up fuel systems the authors have suggested that if a

mechanical fuel pump is used an electric backup pump should be added in case the mechanical pump fails. The suggested piping in both of these articles is shown in Figure 1.

carburetor

The problem that I see with this arrangement is that if the mechanical pump fails due to a failed diaphragm, the electrical backup pump is only going to feed the leak of the failed mechanical pump. Feeding a leak in the failed mechanical pump will only serve to increase the possibility of a fire rather than getting fuel to the carburetor. If the mechanical pump has failed due to an internal blockage, the electrical pump would be precluded from getting fuel to the carburetor since it

mechanical fuel pump

would have to flow through the failed mechanical pump. I think that a better arrangement is shown in Figure 2. The pumps in this arrangement are each piped in series with a check valve that allows flow in the direction of the carburetor, but not in the other direction. These check valves would prevent backflow through the other pump (if the other pump fails). If the two fuel pumps are installed in this fashion, they will provide beter protection for a failed fuel pump.

electrical fuel pump

fuel tank

Figure 1

mechanical fuel pump

carburetor

electrical fuel pump Figure 2

Readers are invited to submit entries to EAA's Hints For Homebuilders, Att: Golda Cox, P.O. Box 3086, Oshkosh, Wl 549033086. Entries will be reviewed by a panel of EAA judges. Readers whose hints are published in any EAA magazine will be awarded a 3/8" Drive Socket Wrench Set from Snapon Tools. Aircraft Spruce & Specialty will award a $25 gift certificate plus a current catalog, and American Saw & Mfg. Co. will award a Lenox 4012 Hacksaw Frame. Members are also invited to submit hints of an electrical nature. Any hint used will receive a Fluke Model 23-2 Multimeter with Holster from the John Fluke Mfg. Co., Inc. The contest will run from August through July of each year with a Grand Prize being presented by Snap-on Tools (KR657 Roll Cab and KR637 Top Chest), Aircraft Spruce & Specialty ($250 gift certificate), and American Saw & Mfg. Co. (Lenox VBKMA-6 Vari-Bit Kit). A Grand Prize will also be awarded by the John Fluke Mfg. Co. These awards will be presented during the EAA Convention. Our thanks go to our sponsors for these awards. SPORT AVIATION 91