Fuel Management Question

[RADIOCOMMANDER]

I have just purchased my first Cessna 337 B, and love the aircraft! I flew exactly three hours on an XC leg recently, and the the aircraft took the following fuel levels to top off when I landed and topped off:

Right Main 15.0 gallons
Right Aux 17.7 gallons

Left Main 24.2 gallons
Right Aux 10.0

My flight departed using the Main Tanks on front and back engines for a period of one hour, at which time I switched both to the Aux tanks until just prior to decent. My engines were leaned at 10 GPH each.

While the fuel burn appears to be spot on, the tank fuel use levels do not. Can anyone educate me on what this looks like to them, or is there something I have missed in my own analysis and study of the fuel system and P.O.H. ?

Many thanks !

Radiocommander

[jchronic]

The discrepancies on the individual sides do look a bit odd. Assuming that you for sure had full tanks before you took off and used the aux tanks for almost exactly one hour, and then topped everything off completely after the three-hour X-C, it seems the only explanation would be some 'delta' in the fuel being recirculated to the main tanks due to system plumbing (I couldn't explain why without delving into the maintenance manual-and then I might still not be able to explain it). Overall fuel usage looks pretty consistent.

I track my burns and fuel adds religiously on a spreadsheet (C337D) and have found that the rear engine consistently burns about 1.0-1.5gph less than the front engine (at like power settings). You appear to have had a similar slight (11.4 vs. 10.9) burn difference front-to-back, or 1.5 gal difference for the three hours, again not anything significant.

A fueling issue to be mindful of: The split main tanks on these earlier birds does mean you need to be sure the fuel has 'settled' completely before blithely assuming you're topped off - even if a visual check looks OK. This is especially so if the fuel source is a high volume pump; I had to return and land once for more fuel when this happened - luckily the fuel gauges in that airplane worked (not always the case). Some advocate doing one side, then the other, then going back again to (re)top off both sides.

One other little featurette regarding the aux tanks (I noted you pretty well drained the right aux on your flight - a bit more than I'd expect for an hour's burn): After an hour, when they're getting low, getting out of balanced flight (slipping or skidding) will quickly starve the engine. I found this out on a photo mission trying to slip the airplane for the photographer.

Anyway, welcome to the Skymaster world. They are fabulous airplanes!

Regards -
Joe

[Frank Benvin]

Radiocommander please read the article on fuel management at

http://www.skymasterus.com/

It could save your life A lot of pilots have been killed because they did not fully understand the fuel system.


Frank

[Ernie Martin]

I wrote the article Frank indicated, but I had not responded to your inquiry because I didn't have an answer. Three things come to mind -- but it's speculation and may be wrong -- and may be worth checking:

a) the fuel pump on the front engine is weak,

b) the fuel path from the left aux (I'm assuming you meant the left aux at 10.0) to the front engine is partially blocked, or

c) less likely, the return path for fuel and vapors from the front engine is partially blocked.

If you read my article, the engine normally draws roughly twice as much fuel as it needs, returning the unusued fuel always to the main. This happened on the rear engine, where the engine drew 17.7 gallons from the right aux, used roughly 9 gallons and returned the excess to the right main. However, in the front, operated for the same amount of time, only 10 gallons were drawn from the left aux. It should be more. Closer to 18. The only explanations that I can see for drawing less fuel than it should are the ones listed above.

If your full power take-off has been normal, with the gph needles on both engines at or near 18 gph and the front engine developing full power, it's probably not a weak fuel pump (point a)).

You could do a test to see if it's point b) by doing a high-speed run on a LONG runway with both engines on their auxiliary tanks (do NOT take off; reduce throttles to idle before you get to the runway mid-ponint). When you give it full power look at the gph guage, RPM and manifold pressure. If all normal, then it's probably not point b) -- and I'm stumped.

Good luck.

Ernie

[hharney]

Just some clarification, when fueling the main tanks on your B model, be very careful to check the service port after the fuel has been dispensed. Because there are two separate main tanks that are interconnected by rather small hoses, if the fuel is dispensed quickly at high volume (as Joe indicated) the main tanks will not be totally full until the fuel can reach equilibrium between both main interconnected tanks.

The inboard auxiliary tanks are single cavity and do not have the same concern.

The only other question is; I assume your B model is normal aspirated, NOT TURBO?