PA-30 Take off Fuel Flow

[T210DRVR]

My gauge on the PA-30 shows a redline fuel flow of 16 GPH/Side. However, I'm only seeing 12-13 GPH/Side for full throttle take off at sea level on a cool day. I have limited EGT indication, but from best I can tell EGT's aren't abnormally high. Both left and right fuel flow match up very close at full throttle.

What are you seeing?

[N3322G]

My 39 digital gauge sees 16 per side at sea level at max mp and rpm at standard temp. Used to race that way a lot.

Is your take-off roll as expected? If it is then I'd suggest the old - check the gauge by topping the tanks, flying at power settings you think you should burn 16 gph per side for a certain time increment and then top the tanks again and see how much you burned.

If the take-off roll isn't per POH, then there's a different issue and you need someone with greater mechanical knowledge than me to help.

[Zach Grant L1011jock]

Normal full power fuel flows are specified to be 13.8 -15.5 gph for an IO-320 as per the Lycoming technical docs. This is of course std day, sea level 2700 rpm. It depends on how your servo was set up, and as you can see the tolerances aren't very tight. Hope this helps. BTW, the pressure type flow gauges are notoriously inaccurate...it doesn't matter what type airplane it is installed in. Invest in a flow transducer type FF and live happily ever after. The best money you can spend on your new turbo bird would be a GEM with FF. You will save the money you spend easily in maint. you don't have to do.
-Zach

[Jay]

My pressure type fuel flow indicates about 12 GPH at full power on a sea level take off. My JPI (transducer based) fuel flow indicates 15 to 16.5, depending on DA. Likewise at cruise according to the pressure fuel flow I'm doing 7 GPH when the JPI (and the amount of fuel I put in the tank) says I'm doing 9.5 GPH.

For all practical purposes I use the JPI fuel flow as primary. I regard the pressure type gauge as an aid to troubleshooting, but other than that it just takes up space in the panel while keeping the airplane legal.

Best,

Jay

[SLIMDREDGER]

I had a similar 12 - 13 GPH take off reading on the right engine after both engines were overhauled the last time. I had always seen 16 GPH +/- on both engines for take off and the left one with the fresh overhaul still read 16 GPH.

The right engine seemed to be developing full power on take off and the temperatures and pressures were normal ...... but I would not accept the low reading.

We checked all the plumbing for both engines and found no defects. I finally switched the lines at the gages and the problem moved to the other gage.

After all of the above, I sent the right Bendix unit back for checking.
It was found that an improper venturi had been installed during the overhaul process.

The corrected unit shows 16 GPH on takeoff now.

A. H. Powers ICS 2978

[HUESTERSHOOTER]

I have low fuel flow (4-5gph) on the right side when attempting to get takeoff power, confirmed by engine performance and JPI. The problem has been isolated to aft of the firewall by running a clear plastic line from left fuel selector to right engine, so it is not fuel servo, valves, etc., and the right fuel selctor has just been overhauled by Webco. Could it be a restriction in the boost pump, a collapsed fuel line, or something else? Problem exists with boost pump on or off. Any and all suggestions appreciated, we are baffled.

[N3322G]

Have you tried using the crossfeed function to further isolate? To run from left main, left selector on main and right selector on crossfeed. Be sure to drain well first if you haven't been draining the crossfeed in the past. Then try the same to run the left engine cross feeding to the right main.

There are some more ideas in the trouble shooting section of the Service Maunal, end of Chapter IX. Free under the Tech Tab on the home Page of this site. Assume your mechanic has this at their side.

Given this is your first post, the Twin may be new to you. No offense intended but below is the fuel hygiene we use and has worked well.

N3322G FUEL HYGIENE

This is the process we use to sump fuel on our Twin Comanche. It has evolved from the methods we were taught. The results are that the fuel sump screens are remarkably clean at each 50 hour AD inspection – so much so that shops have commented upon their cleanliness. Sumped fuel has been entirely water and debris free for almost two years or more of flying. I should mention that all bladders were replaced in 2009/2010, fuel door seals are fresh, engine fuel lines are happy and I do my best to keep the fuel caps in good condition. When home, the aircraft is hangared in Texas where the heat eats rubber for lunch.

1. Before the first flight, place 30 qt sterlite bin bottom under sump drain lines (or something that will catch fuel)
2. Start with selectors on main tanks
3. Sump right main by holding up rear sump drain
4. While holding sump open, change to right aux (notice movement of mechanism in sump area)
5. While holding sump open, change right selector to crossfeed
6. Release sump drain
7. Return selector to right main
8. Sump left main by holding up forward sump drain
9. While holding sump open, change to left aux (notice movement of mechanism in sump area)
10. While holding sump open, change left selector to crossfeed
11. Release sump drain
12. Return selector to left main
a. If fuel in tips - with selector on right aux, turn on master and switch to right tip
b. Sump right tip by holding up rear sump drain
c. Return switch to aux
d. Return selector to right main
e. If fuel in tips - with selector on left aux, turn on master and switch to left tip
f. Sump left tip by holding up forward sump drain
g. Return switch to aux, master off
h. Return selector to left main
13. Check fuel in bin. If debris, individually sump tanks until sumped fuel is clean
a. If fuel in tips, sump right tip external sump on tip
b. If fuel in tips, sump left tip external sump on tip
c. If nacelle tanks have fuel sump at external sump valves both sides
d. If Miller wet wings have fuel, sump at external sump valve on both sides
14. Fly
15. Fuel main and aux tanks (and any other desired tanks)
16. After last flight of day, sump main and aux tanks again.

Hope this helps.

[N3322G]

Might also check using aux tanks to further isolate.

When did the fuel flow problem start? If it wasn't there before the O/H, that might be a staring place.

[HUESTERSHOOTER]

Have tried all of your suggestions early on in the troubleshooting. Cross feed and switching tanks makes no difference. When we ran a clear plastic tube from the right fuel selector to the left engine the problem followed to the left engine. The right fuel selector has been overhauled by webco and we still have the problem.