Remember all the times someone told you to keep the apples and oranges correctly separated? There is probably some math teacher somewhere in your past who said you have to keep the apples with the apples and the oranges with the oranges. And if you didn’t, it would not work out.
Then there were the teachers who said you could only deal in apples. Or maybe they preferred oranges. In aviation, our oranges are pounds and our apples are gallons. Personally, I like oranges. Here’s why.
It is easier to keep things in oranges (pounds) than it is converting from apples (gallons) to oranges. When you are making the conversion, there is the possibility of making a serious mistake.
For the Cessna 170, at 55 percent power, the airplane burns 6.6 gph or converted to pounds, 39.6 pph. By rounding up to 40 pph, a pilot can be a little conservative with the fuel flow and make the math a little easier.
What is the advantage of using pounds over gallons? For one, it makes the weight and balance problem much easier. In my aircraft, I would load the airplane for a trip. On multiple legs, it is easier to subtract pounds along the route, instead of converting from gallons.
For example, if your takeoff weight is 2142 pounds after the personnel, baggage, and fuel is loaded, after a 1.5 hour leg it is easier to determine takeoff weight on the next leg by this math: 2142 – (40 x 1.5) = 2082. This allows the pilot to avoid extended math problem: 1.5 hrs x 6.6 gph = 9.9 gallons total. Then you have to multiply that answer by six pounds per gallon to get the weight of 60 pounds, which the pilot can easily subtract from the initial takeoff weight of 2142 to get the same new weight of 2082 pounds.
Even when the cabin loading changes on different legs, by keeping a minimal flight log as simple as the total fuel in pounds and subtracting the pounds burned on each leg, you have a much more accurate estimate of the fuel weight for the new weight and balance calculations. In other words, with full tanks in the Cessna 170 of 222 pounds (37 gallons) after a flight of 1.8 hours, you know your new fuel load is 150 pounds (222 pounds less 1.8 x 40 pph = 150 pounds).
The problem with keeping track of gallons can be complicated. Pilots and line personnel can, and have, made mistakes in converting gallons to pounds and back, sometimes with disastrous results. Later this week, we’ll take a look at the results of some of those mistakes.
Oh yeah, you really do have to keep the apples separate from the oranges.
©2011 J. Clark