'73 non-turbo 337 performance

[Rick Gardner]

I may be ferrying a '73 model 337, non-turbo. Our 337 is a P337 so I would like to get some realistic performance numbers on the non-turbo '73 model for fuel planning purposes. Since much of the trip will be over water, good information is very important. Any info on reallistic performance numbers would be appreciated.

[Ernie Martin]

Rick, I fly the exact same model ('73 337E) over mostly water (last trip was Friday to MYBG in the Bahamas, next trip is 8 AM tomorrow to Nassau*). If you take the factory numbers and apply about a 5 - 10% correction factor for age (e.g., less engine HP than brand-new ones used for the factory tests, greater drag than new, wrinkle-free, super-waxed airframe)
you should come pretty close to actual performance. My typical flight is at 4500 - 7500 feet. At 500 ft I reduce to 25/25 and never touch the throttle again until near landing, adjusting RPM to 2400 at cruise. So I end up with a cruise of 19 - 21 MP (around 21 at 4500 ft and 19 at 7500 ft), with 2400 RPM, and get about 140 kts over ground (with still air) and around 20 gal/hr fuel consumption (10 per engine) at 60 deg ROP (leaning to peak EGT and then enriching about 2 1/3 divisions of the EGT gauge).

Ernie

[Rick Gardner]

Thanks Ernie, this is very helpful.

Actually, I am from Nassau, Bahamas myself so you are in my old stomping grounds. I will be over that way visiting family during the first week of February after our escorted Mayan Adventure trip in Mexico is over. Will you still be around?

The ferry flight will be from Costa Rica back to the USA. I am debating whether to stop in Honduras or Belize to take on more fuel or make the trip direct to Cozumel and refuel there for the flight up to Florida. Your numbers sound realistic. My first leg is 690 NM which would be around 5 hours with no wind plus take off fuel which would leave me close to an hour in the tanks which is where I like to be. (125 gal tanks). Of course, winds could change all of this.

[Ernie Martin]

I do the same math for long trips (I took my previous Skymaster, a '69 337D, from Miami to Boise, Idaho and back), namely 5 hours of flight and down for more fuel. That uses about 103 - 105 gal (considering take-off and climb consumption), leaving about 1 hour of reserves. It's critical, of course, that your tanks are filled to the brim (slowly, with you inspecting) and that you know from recent prior flights that there are no leaks and that fuel consumption has been as expected. On a ferry flight of an unknown aircraft, I don't know if the last two assumptions can be made, so I would err on the side of caution -- I personally would not make a 5 hour trip over water.

Notice that I don't say anything about relying on the standard fuel gauges, given their propensity with age to be unreliable. If the aircraft has a digital analyzer with fuel consumption, that takes some of the guesswork out, if you know that it's been working well.

One technique that has been employed by some to get advance notice of remaining fuel near the end of a long trip is to cross-feed one of the engines so both engines run off a single tank for 30 minutes in the middle of the flight. When you go back to normal operation, one tank will have 10 gallons less than it should and the other 10 gallons more (if this is unclear, see the Fuel Supply Management page at www.SkymasterUS.com ). So, if you are descending after the 5 hours and both engines are running, you know for a fact that the fuller tank has not 10 but 20 gallons left, because a) the emptier tank still has fuel after you "stole" 10 gallons from it, so there must still be 10 gallons of the original fuel in the fuller tank, plus b) the fuller tank has an additional 10 gallons, 5 that came from the other tank as excess fuel and 5 because no fuel was drained from that tank during the 30 minutes of cross-feeding.

One must be careful, long before you get low, to get back to a situation where there is ample fuel on both tanks, so there is no risk of starving one engine while low and in the landing pattern. This is usually done by cross-feeding the opposite engine for 20 - 30 minutes while you are still high, to equalize fuel. One good rule of thumb is that a tank which is being unused for a period of X minutes, because its engine has been cross-fed and is drawing from the other tank, will have after you return to normal (non-cross-fed) operations fuel for about X minutes of engine operation. So even if the emptier tank was about to run dry when you started the 2nd or opposite cross-feeding to equalize fuel, if you cross-feed for 20 minutes, it will have fuel to sustain the engine for 20 minutes after you end the cross-feeding.

Obviously, this is complicated and not recommended, but familiarization with these principles in the reference cited above might help you out of a bind on long-range operations. Especially with an unfamiliar aircraft.

Ernie

[Rick Gardner]

Half the route is over water, but it is the last half. I am worried about what you pointed out, no prior history in this particular aircraft.

I am already looking at La Ceiba, Honduras for a fuel stop. We go in and out of there a lot and know the airport staff pretty well so I can usually get a fairly quick turn. This only adds about 20 NM to my route.

[tropical]

My 1974 "G" model I typically fly at 5-6,000 feet at 2400 and 23"mp and true out at 160knots fairly consistent with a fuel burn of 20gph.

[Roger]

I had a 1970E and now a 1980H and find that they are both pretty similar (+/-5kts). I plan for 150kts at 2400/21-22 (at around 6000'avg) and burn 19-20gph. I have flown non stop from KART to KSGJ (900NM) in 6:00 with 30 gal to spare on more than one occasion (150gl total).

The only thing I would say about extended flight (especially over water) is that while I love my fuel computer (and would also never fly without a milti/egt/cht) I belive your regular fuel guages are really more important. On two occasions I have had fuel leaks, one in a tank seal, and one in crack near the sump drain that did not show up on the fuel computer as "fuel burned" . It's nice to know what you are burning, but it's better to know what you really have left in the tanks.