I took a trip in my plane on Tuesday that was faster, more fuel efficient, and higher than ever before. Usually, I fly at about 6,000 feet and get ground speeds of about 130 knots (nautical miles per hour) which is the equivalent of about 150 MPH (statute miles per hour). The plane burns 9.5 gallons per hour of gas so my mileage is about 13.7 nautical miles per gallon or 15.8 statute miles per gallon. (Airplanes use nautical miles; cars use statute miles.) Not bad.
Tuesday was special, though. The weather was particularly fine; the winds were strongly out of the west; and I needed to fly east, from St. Charles, MO to Gaithersburg, MD. Airplanes generally get more efficient as they fly higher. The air is thinner so there is less air friction. Normally aspirated (non-turbocharged and non-supercharged) piston planes like mine are generally at their best between 6,000 and 11,000 feet. Above 6,000 feet, the engine no longer gets enough air to generate “full” power but the reduced drag makes up for it. The result is that the plane gets the same speed more efficiently since lower power means reduced fuel consumption.
On this particular day, the winds got stronger as altitude increased. I planned the flight for 13,000 feet and took my oxygen bottle along (strongly recommended for flights above 10,000 feet and legally required above 12,500 feet). After a couple of hours, air traffic control adivised me that I would either need to deviate to the south or climb to 15,000. The conversation went something like this.
Me: Well, I’ve never been up to 15,000 feet. Let’s see if the plane will go that high.
ATC: No, I don’t want you to go up to where you can’t breathe or anything!
Me: (laughing) Oh, it’s OK. I have oxygen. The book says that this plane should go that high but I have never tried it.
ATC: That’s good. N7430J, climb and maintain one five thousand.
Me: N7430J, up to one five thousand.
I trimmed the plane for climb and took about eight minutes to climb those last couple of thousand feet, slow and steady. Once up there, pushed by even stronger tailwinds, I was seeing ground speeds of 174 knots (200 MPH) and burning less than 7 gallons per hour.
I calculated the density altitude (the altitude corrected for temperature and weather) at 16,200 feet so the plane had readily climbed, and was nicely flying, well above its 15,000 service ceiling. I actually found this a bit comforting since the plane was acting a little bit squirrely. If I nudged the yolk or hit a bit of turbulence, the plane would oscillate and need a gentle touch or two to make it settle down again. Normally, this plane is rock steady and flies pretty much hands-off. I had not seen any of this behavior down at 13,000 feet (density altitude 14,200 feet). Since it was daylight and smooth flying, I opted to stay at 15,000 rather than descend but I would not plan another flight above the service ceiling.
The view was spectacular and it was a blast to be “up there” where I had never been before.
When all was said and done, I was on the ground in Maryland in just 4.1 hours on 29.4 gallons of gas. In aviation terms, that works out to 7.2 gallons per hour overall; pretty darned good for a 180 HP Lycoming engine. In automotive terms, that works out to 28.8 miles per gallon for what would have been an 848 mile road trip.
