Ten Gallons

Ten gallons — about thirty bucks worth of avgas — what’ll it get you?

I had the happy opportunity to fly the Diamond Aircraft DA40-180 DiamondStar recently evaluating it as the primary trainer for a program I’m working with. Don Rollins, Diamond salesman at Central Flying Service in Little Rock, Arkansas went up with me for a little over an hour of takeoffs, landings, steep turns and stalls, and an ILS. I flew a 2002 Cessna 172S (over from Chattanooga for the evaluation. But enough unsolicited product placement… what’s this all about?

The 180 horsepower Skyhawk “trued out” at about 125 knots on the trip over — burning just under 10 gallons per hour. Essentially the same engine in the DA40 pulled the DiamondStar at 140 KTAS in high-speed cruise … at the same fuel burn as the Cessna. It got me to thinking about what 10 gallons — including tax, about thirty dollars worth of avgas at most places in the U.S. — buys in the way of airspeed and passenger-moving.

I owned a 1946 Cessna 120 for about seven years. If you’re not familiar with the 120, think Cessna 152 without the back window, equipped with a fabric-covered wing, a tailwheel, and a Continental 85 horsepower engine. That C-85 would lift two hefty seat-fillers (about 600 pounds useful load with full fuel … if I remember correctly) and cruise around 95 knots, burning five gallons per hour. In the C120, 10 gallons equals 190 nautical miles (in still air), or 380 admittedly bare bones, VFR passenger seat-miles (one seat moved one mile). Put another way, the C120 got about 19 miles to the gallon.

For a while, I flew a Citabria with a 150-hp engine, burning around eight gallons per hour and “truing out” near 120 knots, flat-out. Ten gallons, or about one and a quarter hours, meant 150 nautical miles in still air, or 300 passenger seat-miles for roughly $30 in avgas. That’s roughly 15 miles to the gallon. I’m sure there are IFR-equipped and certified Citabrias out there, but most of these fine ships are likely VFR cruisers.

How about that C172S? 10 gallons equals about an hour’s flight, or 125 nautical miles/12.5 miles per gallon. The Skyhawk’s weight and balance reports it has a useful load of 845 pounds — so put 53 gallons of fuel (318 pounds) and there’s 527 pounds left over for passengers … about three average people. So “full fuel” 10 gallons in a C172S gets you 375 passenger seat-miles (almost exactly the same as the Cessna 120!), although in this case with top-notch IFR avionics including moving map GPS and autopilot. Takeoff off with partially filled fuel tanks and you can get 500 seat-miles for $30 in fuel in a C172S — even better than Southwest Airlines — provided you can fill the seats.

The DiamondStar’s 10-gallon range is around 140 nautical miles, or 560 passenger seat-miles in still air, at 14 miles per gallon with the seats filled. It’s very well equipped for IFR cross-country flight. Max fuel is 40 gallons (240 pounds), so in the airframe flown (875 pounds useful load — almost exactly the same as the C172S I flew) it’s still a three-adult ship with a generous baggage allowance — netting 420 passenger seat-miles in calm skies.

Step further up to something like a 300-horsepower Beech Bonanza and you’ll burn about 15 gallons per hour at high cruise, at an optimal lean-of-peak (LOP) setting. True airspeed will run around 160 knots in most airframes. Ten gallons, then, nets about 107 nautical miles and 428 passenger seat-miles (four adults on board) … about the same as the DiamondStar. But it’s also a lower 10.7 miles per gallon. Lean more traditionally on the rich side of peak (ROP), and the same Bonanza will fly at about 170 knots at 18 gallons per hour. This gives you 94 miles (9.4 miles to the gallon) and (with four on board) 376 passenger seat-miles for your $30 fuel investment.

Take off in a twin-engine Baron and you can see 170 knots (LOP) or as much as 190 knots true airspeed on the rich side of peak exhaust gas temperature. LOP, then, results in 57 miles range or 228 seat-miles; fly ROP and you may get 51 miles down range, or 204 passenger seat-miles. Readings like 5.7 miles per gallon (LOP) or 5.1 miles per gallon (ROP) were pretty normal when I was flying a Baron.

Just for grins let’s look at the specs for a King Air and a Cessna jet. A King Air C90B seats six, and Beech claims a maximum cruise speed of 246 knots true. The twin PT-6 turboprops will pull the King Air about 31 miles with 10 gallons of fuel (3.1 miles per gallon which, being jet fuel, costs a little less than avgas), or 186 passenger seat-miles if the takeoff fuel load is low enough to permit filling all the seat.

A Citation CJ1 will cruise (at optimum weight and altitude) at about 120 gallons per hour (turbine fuel burns are usually expressed in pounds per hour, but we need to do the conversion to make the comparison to piston airplanes). It’ll zip at about 380 knots true airspeed, carrying six passengers (although weight and balance limitations apply). That translates to about five miles or 30 passenger seat-miles for 10 gallons, again usually costing a little less than avgas. That’s half a mile a gallon. Of course, the idea here is speed, and if time is money, well, it seems this aircraft caters to those who have more money than time. Oh well, “if you have to ask…

The point is this: there are a lot of different ways to calculate the efficiency of an airplane. True airspeed and payload capability aren’t everything for everybody, or every mission. Especially if most of your trips are relatively short and only carry you, the bigger, faster airplanes with more powerful engines aren’t always worth the extra cost.

BOTTOM LINE: As fuel becomes a more significant part of airplane costs, and other factors (like insurance and maintenance) take a bigger chunk of our flying budgets, you might want to be more creative in investigating which airplane meets your mission needs for the least amount of money.