GPH - Better than the book?

[Jaybird180]

I'm running some numbers from my last XC and the numbers look suspicious. Of course I don't have laboratory quality measurements for fuel used, but I derive ~5.5 GPH fuel consumption based on calculations of fuel used divided by Hobbs time (tach time was .1 higher but only would have added another variable into the calculations).

IAS was on-target based on RPM selection (115kts @ 2500RPM). The book contains performance charts for temps of 15c and -5c and 29.92". IIRC, it was about 30.3" and +5c over the 2 days. Fuel flow showed about 9 GPH during most of the enroute portions of the trip and peaked above 12 during climb. I figured that I'd get about 8.5GPH (at best) and 9.5 if I screwed things up. My enroute (unplanned) fuel stop was based on time with the conservative assumption that I screwed up. When I stopped to fill up, I was surprised by how little fuel she took to fill the tanks. I had 3 Takeoffs and climbs that should have consumed about 10-12 Gallons that is reflected in my GPH calculation as aggregate, making me scratch my head even more (I should have used more fuel).

Raw data:
Approx 28 Gallons consumed in 5.1hrs (including all taxi, run-up, takeoff, climb, approach and landing operations).

What do you guys think? 180HP Lycoming with insufficient instrumentation for safe LOP operations.

 

[poadeleted20]

You didn't properly account for the much lower consumption during taxi, run-up, approach, and landing. Remember, the Hobbs starts ticking at engine start (or maybe even master-on), and runs at a constant rate until power is removed from the meter. OTOH, except for the run-up, you're probably only burning like 1.5-2 gph from engine start until takeoff, and again from pulling the power to idle crossing the fence until engine shut-down.

Another possibility is that the tanks were truly full when you started, but not completely filled when you refueled -- those Cessna tank collars can fool folks big-time to the tune of several gallons on each side. That's burned folks in the past when they had as much as 5 gallons less in the each tank than they thought, and you really can't rely on those dinky little gauges to warn you of that. If I'd flown 5.1 at normal cruise in that 180HP 172 of yours, and the line crew said it only took 28 gallons to fill, I'd be up on the step and sticking my beak in those tanks before the fuel truck pulled away.

 

[N801BH]

You didn't properly account for the much lower consumption during taxi, run-up, approach, and landing. Remember, the Hobbs starts ticking at engine start (or maybe even master-on), and runs at a constant rate until power is removed from the meter. OTOH, except for the run-up, you're probably only burning like 1.5-2 gph from engine start until takeoff, and again from pulling the power to idle crossing the fence until engine shut-down.

Another possibility is that the tanks were truly full when you started, but not completely filled when you refueled -- those Cessna tank collars can fool folks big-time to the tune of several gallons on each side. That's burned folks in the past when they had as much as 5 gallons less in the each tank than they thought, and you really can't rely on those dinky little gauges to warn you of that. If I'd flown 5.1 at normal cruise in that 180HP 172 of yours, and the line crew said it only took 28 gallons to fill, I'd be up on the step and sticking my beak in those tanks before the fuel truck pulled away.

+1... If it's too good to be true... It usually ain't.:wink2:

 

[Jaybird180]

The tanks were a little above the tabs at the start of the trip. The bottom of the tabs measures 17.5 x 2 for 35 Gals. I purchased 26 which filled the tanks. This was at the 2.5 hour mark. I didn't have lengthy ground operations but I do lean on the ground. I continued my trip for 0.6 and landed. I flew back home which took 1.9 and the fuel was right at the nip of the bottom of the tabs upon post-flight inspection. I did not add fuel.

The airplane is now in maintenance.

 

[denverpilot]

How level was the aircraft each time and was the nose strut extended the same length?

 

[poadeleted20]

The tanks were a little above the tabs at the start of the trip. The bottom of the tabs measures 17.5 x 2 for 35 Gals. I purchased 26 which filled the tanks. This was at the 2.5 hour mark. I didn't have lengthy ground operations but I do lean on the ground. I continued my trip for 0.6 and landed. I flew back home which took 1.9 and the fuel was right at the nip of the bottom of the tabs upon post-flight inspection. I did not add fuel.

Yeah, eyeballing the fuel level versus filling the tanks. I'll bet dollars to doughnuts if you go back to the plane and pump it full, it takes another 10 gallons a side.

 

[Jaybird180]

How level was the aircraft each time and was the nose strut extended the same length?

Ironically, it's in maintenance for a suspected nose strut leak.

Yeah, eyeballing the fuel level versus filling the tanks. I'll bet dollars to doughnuts if you go back to the plane and pump it full, it takes another 10 gallons a side.

Full is 53 Gals, so you've hedged your bet at 9 gals per side to top off. Only a fool would bet against you.

 

[denverpilot]

Ironically, it's in maintenance for a suspected nose strut leak.

Suspected?

It's either standing there at the same height every day with no sticky crap all over the nosegear, or it's not.

Mine isn't suspected. It leaks.

Sloooooowly.

Mechanic is having us keep an eye on it.

Eventually the strut will need new seals, including the top one that many mechanics "forget" because it's harder to change out than the two bottom ones, and the piston will have to be re-plated to get rid of pitting from 36 years of FOD being blasted into it.

Have to have it serviced about once a year.

It got ahead of schedule this year in the cold up in Lincoln, which led to the mechanic up there giving a nice referral to a good re-plating shop in Arkansas. Nice info to have.

 

[flyingcheesehead]

What Ron said - But also, did you turn the fuel selector to one side or "off" before fueling? If not, what happens is that you fill one side, and while you do that, the fuel is slowly draining through the vent to the other tank. You put the cap on the one side, and the fuel is still flowing across (IE the first tank is no longer full, and the second tank is filling up). Then, you fuel the second tank but it's got a bunch of fuel that drained across from the first tank, so it doesn't take as much to fill.

If you let the plane sit for 10 minutes, turn the fuel selector to off, and fuel it again, it'll take several gallons more.

 

[KSCessnaDriver]

I can beat the book on what I fly, especially at the lower power setting we run. First off, the book isn't real clear on exactly how much fuel you should burn at low power, coupled with the fact that while turning 1700-1850 RPM on a Lycoming IO-360, you can basically set the mixture where you want to (as long as it'll run).

 

[Jaybird180]

What Ron said - But also, did you turn the fuel selector to one side or "off" before fueling? If not, what happens is that you fill one side, and while you do that, the fuel is slowly draining through the vent to the other tank. You put the cap on the one side, and the fuel is still flowing across (IE the first tank is no longer full, and the second tank is filling up). Then, you fuel the second tank but it's got a bunch of fuel that drained across from the first tank, so it doesn't take as much to fill.

If you let the plane sit for 10 minutes, turn the fuel selector to off, and fuel it again, it'll take several gallons more.

Didn't think of that, but I did turn the fuel off but not the selector. My selector is L-R only and there is a separate off lever.

 

[Jaybird180]

However I did wait a few mins after fueling to take samples. Fuel level was consistent after sampling with my LARGE GATS, so the fuel went back in.