Ask Pete!

by Pete Schoeninger

Q: Why is it that I have 40-gallon fuel tanks in each wing of my Cessna 182 for a total of 80 gallons of fuel, but the manufacturer says that only 75 gallons are useable? Is it that the remaining 5 gallons are in the fuel lines and carburetor, or does altitude and pressure have anything to do with it?

A: The fuel lines and carburetor hold very little fuel. The majority of unuseable fuel in your airplane is in the bottom of the tanks, where not all fuel can flow into the engine. The reason is that sometimes the pickup point is a bit off the “floor” of the gas tank, or the location of the pickup point(s) may prohibit all fuel from flowing out of the tank, especially with a nose down or nose up attitude.

Some airplanes have peculiar fuel plumbing and it is very important when flying a new make/model to make yourself aware of them. Here are a couple of examples I have experienced: Old Bonanzas draw more fuel into the engine than they burn, and return the excess into the left main tank. So you almost always take off on the left main tank. Cherokee Sixes have four (4) fuel tanks, and take off is authorized using any of them. But the two outer tanks are only 17 gallons. At full take off power, you are burning 32 gph (in the 300 hp version), and you can run a tip tank from full to empty in just over half an hour at high power! Some old Cessna 172s required switching to the left or right single tank (from the commonly used “both” fuel selector position) when flying above 5,000 feet. In some twin-engine airplanes, not all tanks can be accessed by either engine. So many pilots will burn fuel out of the tank that cannot be used by the opposite engine early in a flight. That leaves fuel available in the most unlikely event you end up running on one engine, and need fuel from fuel tanks on both sides of the cabin. Cessna 150s and many others have the best fuel selectors, either on, or off. More should.

Q: I have a 65 hp J-3 Cub. I see about 70 mph at cruise. A friend has an 85 hp J-3, which is only 5-10 mph faster than mine. (Yes, I know all Cubs are slow!) But his rate of climb is way, way better than mine. Why is his rate of climb so much better?

A: Aeronautical engineering very simplified. Many airplanes require at least 50% power to sustain level flight. Available power above that can be used for climb.

Let’s assume both of you have props that allow full power at climb airspeed. And let’s assume both Cubs thus need about 32 hp for level flight (half the 65 hp you have) at sea level. So at sea level you have 33 hp (65 – 32 = 33) available for climb. Your pal with 85 hp has 53 hp (85 – 32 = 53) available for climb, which is 60% more power to climb with than you have – a very substantial difference. If you want a real thrill, get in an old (light) 150 hp Super Cub. I owned one that could climb at over 1,000 fpm when solo on a hot summer day, and would approach 1,500 fpm in the winter.

EDITOR’S NOTE: Pete Schoeninger is an aviation consultant and aircraft appraiser who lives in Wisconsin. He is an experienced fixed base operator, aircraft salesman and airport manager. Email your questions about all things aviation to: Pete.Harriet@gmail.com. For assistance with aircraft appraisals or fixed base operator and airport management consultation, call 262-533-3056. Any answers provided in this column are the opinion of the author and not necessarily this publication, or its editor, publisher, owners and affiliates.

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