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Comment by jonplackett | Hacker News Reader

Question: A private jet is obviously bad for the environment, but how does a propeller plane compare carbon-wise to driving a car the same distance?

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Not good. A Cessna 172 cruises at 140 mph and burns 8.5 gph of 100LL fuel, the equivalent of 16.5 mpg, and this is at cruise speed, not accounting for taxi, run-up, and flying the pattern during a normal flight.

Not to mention we're still burning lead in most propeller aircraft as well.

Fortunately this isn't a very popular way of getting around.

It’s interesting that it’s not insanely bad. I assumed it would be maybe 2-10X worse but it’s in a similar ballpark.

Would a full on private jet be way worse?

Would these planes carry a fair few people than a car? If so what’s it like taking passenger numbers into account?

I guess super-rich folk probably would be happy to fly just themselves in a big private jet and also be travelling waaaay further than they ever would in a car so it’s probably a silly comparison anyway

There's actually a huge range in private planes. This M600 plane does about 7 mpg, 250 knots at 45 gph. A typical light jet might do 400 knots at 125 gph (3 mpg). But a light jet cannot not fly nonstop to a different continent. For these trips, a large business jet might do 450 knots at 300 gph (1.5 mpg), but only the very super rich can actually afford these. Even most celebrities will be flying the airlines for long international trips.

Thanks for the calculations! It’s weird how a more advance technology like a jet can be so successful yet an order of magnitude less economical. Usually it’s the reverse. But when you’re rich it’s time that’s money

The short answer is that higher speeds come at a steep price. The relationship between velocity and the power required to overcome drag at that velocity is cubic, so that large business jet requires on the order of 50x more power to fly at 450 kts than it would flying at 120 kts, like the little Cessna might.

Piston engines are more efficient than turboprops which are more efficient than turbofan/turbojet engines. Newer piston engines are by far the most efficient option but are not used frequently as the piston fleet is very old.

To compare to a car, just using vehicle efficiency (not per passenger) a typical cessna is in the realm of a full size SUV in terms of carbon footprint. Both are about 10-15 mpg. The aircraft in the article is far worse but only because it's rather large and turbine powered. Optimized designs like the Long-EZ can easily hit in excess of 30mpg cruising at 160mph.

But planes fly in straighter lines and will use less distance to get to their destination compared to cars. So you can't really use mpg. Best will be to measure gallons used per hour.

Depends on the route. Flying IFR you can get routed all over the place. Also, head/tailwinds exist. It's more just to show ballpark relative efficiency numbers.

A typical single engine piston plane might fly at 130kts burning 10 gallons of aviation fuel (100LL) per hour. Or, roughly 15 statute miles per gallon. More efficient options are available on modern planes. Most of the fleet is old though.

Maybe the best case scenario is a small modern plane flying a Rotax 912, making about 24 mpg on standard auto fuel.

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