“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Sunday, April 04, 2010


That's the model number of my Piper Saratoga, a 1981 model. Now, I know what you're thinking (at least if you haven't followed my blog for long and maybe even if you have). What's a guy who writes about energy savings, fuel efficiency, and peak oil doing in a private airplane?

The answer is that I've wanted to fly since being a small child and  thus I acquired my Airman Certificate ("pilot's license") in 1981. In 2001, well before I began to concern myself with energy related matters, I acquired the airplane. I've been all over the country in it, and have flown it for around 500 hours.

As transportation security measures have increased, the attractiveness of driving to the hangar, performing the preflight inspection, taxiing out and flying to the airport nearest my destination (as opposed to an air carrier airport) becomes more and more attractive. This increases the range I'm willing to fly myself. There are drawbacks as well: I don't have the weather capabilities of the airlines; outside of about 650 (nautical or 748 statute) miles I'll need to stop for fuel; adverse winds have a huge effect on ground speed; and it's quite costly. For very long distances, the airlines are significantly faster though with the hub and spoke system, one hour early arrivals, waiting for baggage, and other delays, the distance at which airlines become faster door to door is longer than one might think. Also, for business meetings, having my transportation ready when I am means that I don't have to leave early to catch my flight.

Parsing the model number, PA stands for Piper Aircraft, it's the 32 series (no significance to that), R indicates that its landing gear is retractable (chicks don't dig fixed gear), 301 is the maximum continuous horsepower of the Lycoming TIO540-S1AD engine (actually, it's one more for some obscure reason), and T indicates turbocharging. As to the engine, T is turbocharged, I is injected, O is (horizontally) opposed (cylinders), 540 cubic inches is the displacement. It's a "dash S1AD" variant, the nomenclature has no obvious significance.

So I have a turbocharged aircraft capable of developing about 300 horsepower. This power is only used on takeoff and initial climb, typical cruise power is about 70% of this number or about 210 horsepower. Just for fun, if I assume that the engine efficiency at this power setting is 25% and that there are about 125*10^6 joules in a gallon of avgas, my airplane should burn 18.04 gallons/hour. I actually run at a flow of about 18 gallons/hour as shown on my fuel flow meter (pilots, obviously continuously concerned with fuel, think in terms of pounds or gallons per hour). As I've repeatedly stated, I just love it when calculations and data or two methods of calculation agree.

These 18 gallons at 13,000 feet will typically take me about 168 nautical miles or 193 statue miles (the "knot" is one nautical mile or 6080 feet per hour). So, in terrestrial terms, I get 193/18 or 10.7 m.p.g. Not really so good. The Saratoga is known as a bit of a sky-borne SUV, it has six seats and three baggage areas. It's a little draggy and a bit heavy in comparison to piston single engine airplanes.

For a given amount of shaft power, the airspeed achieved (at any particular "density altitude") is controlled by propeller efficiency and total drag. I can increase my range and consequently my "specific range" (miles per gallon) by flying at slower airspeeds. At about 140 knots (a bit over 160 m.p.h) I can reduce the fuel flow to around 10 gallons/hour. This works out to 16 m.p.g., more along the lines of what you might expect for a fuel thirsty road vehicle.

But one of the main reasons people buy airplanes is to go fast, and so I rarely use this method. It can, however, result in a quicker trip if the extended range results in the elimination of a fuel stop. Such a stop typically adds about 45 minutes to a trip. But really, by the time I've been in the plane over five hours, I'm ready for a stop.

Finally, if I put passengers in three of the five seats remaining (more than that results in the need to fly with partial fuel loads, thus reducing range) I can achieve a reasonable figure for "passenger miles per gallon" of 42.8. To be candid, that rarely happens though. No amount of mental gymnastics can make N8409Y a fuel efficient way to travel.


Anonymous said...

The fact that you can often go in pretty much a straight line from A to B counts for a lot though. A good rule of thumb is that the distance by air in kilometres is the distance by road in miles because of the way that the roads wiggle around.

E.g., it's a gliding silver distance (50 km) from my local airfield (Booker, High Wycombe, Buckinghamshire, England) to Lasham (Hampshire) but 50 miles from one to the other by road.

Except in cases where there happens to be a direct motorway between the two points this works pretty well. I guess that in the US mid-west or other areas with roads laid out on a grid this effect would be smaller, varying from a factor of 1 when going north-south or east-west to 1.414 (root 2) when going diagonally.

Still, on average, you can multiply aeroplane m.p.g by 1.6 to get the value to compare with cars.

King of the Road said...

A very interesting point. To briefly investigate, I took my most common lengthy trip, from Southern California to my brother's house in Colorado Springs. Door to door in my car, the trip is 1159 statute miles and, on average, I'd use about 55.2 gallons of auto gas.

For the airplane, I drive from my house to the hangar and my brother drives me from the airport to his house. This is about 52 miles total and about 2.3 gallons of auto gas.

The airplane, using low altitude airways (I fly IFR and direct routings are difficult in the west due to restricted airspace and military operations areas) the flight is 741 nautical miles (853 statue miles) and will use about 67 gallons of avgas plus about 3 gallons to start, warm up, taxi out and taxi in, run up, etc.. So the total for the trip is 72.3 gallons.

Since it's really door to door that I'm concerned about, I'll use that distance and the airplane trip achieves an equivalent of (1159/72.3) or 16.0 miles per gallon. As you stated, that's not too far off of 1.6 times 10.