Hacker News Reader

Comment by yunohn | Hacker News Reader

> 1,800 hours time-before-overhaul

That’s surprising! Do you mean the entire life of the engine, or just one run? If the former, doesn’t that get expensive? I imagine that’s not very sustainable.

View on HN

Entire life of the engine. After 1800 hours total operation, the engine should be overhauled: taken apart, cleaned, each part examined and replaced if necessary. Note that per FAA regulations in the USA, only actual flight time counts toward this limit, not time spent taxiing on the ground.

It'll take a normal private pilot a few years to reach 1800h of operation, so it's more economical if you're not flying the plane continuously for weeks at a time.

But yeah, it's expensive. Owning a plane is expensive. It depends on if it's worth it to you to operate the plane. Small single-engine planes like the Cessna 172 are commonly used for recreation or training. So if you use it often, such frequent overhauls will pay for themselves in terms of safety and reliability.

Maybe when all-electric recreational aircraft become common they won't have to be maintained quite so much...

Interesting, thanks for the context. TIL!

Piston engines like you get in a Cessna 172 are also about the worst-case for overhaul times, since there's so much mechanical complexity and because they're often run at/near maximum rated power for long periods.

Turboprop engines (effectively these are jet engines with propellers attached) like the ubiquitous P&W PT6A have TBOs up to 4,500 or even 6,000 hours.

Interesting, I thought everything in Cessna 172 is simple and reasonably forgiving.

It is. It only gets more complex and expensive from there.

Most private pilots will never get to 1800 hours. Even 100 hours per year is a lot.

Electric airplanes are already used in training in Europe. They are less expensive to operate on an hourly basis just because of fuel/energy cost. But they are probably also cheaper to overhaul; the Emrax motors in Pipistrels are relatively simple, and it would probably cost less to buy two and swap one out (sending the other one back to Emrax for a new bearings) than to do one thermal engine overhaul. And almost no downtime!

(I work on another electric airplane which will take flight in 2023.)

Is there anything you can share regarding the electric plane? I had a look at your profile and couldn't see any links beyond your keys.

Pie Aéronefs, s.a. of Switzerland

I've flown an electric plane. Granted it was only about 60 cm long from nose to tail (and I flew it from the ground) but I can speak to its simplicity, low cost, and ready availability of replacement parts compared to ICE-based planes of similar size. :)

In a car running at 100 kph, that would be 180000 km (roughly 110 miles). Most cars would need at least some parts replaced by that time as well..

I think you forgot the K on your 110K miles

Yet only 54000 km at the more typical average speed of 30 km/h. Are aircraft engines typically under high load at cruising speeds? It would seem they are generally overbuilt to overcome the high demands of takeoff.

I think they are under more of a load a cruising speed than you'd expect. At speed both the car and plane are primarily overcoming wind resistance, which is more about speed and elevation than anything. If I'm understanding right the plane would be going more than highway speed within a few thousand feet of the ground for most of it's (flying) life so it's probably putting more stress on the engine than a car on the highway would.

More importantly: if your car engine dies you gradually slow down and pull over. If your plane engine dies calmly pulling over isn't an option. It's much easier to justify spending the money ensuring a plane's engine is reliable.

They're generally outputting up to around 75% of their rated horsepower at cruise.

I'd say that's closer to going 110km/h or 65mph in 2nd or 3rd gear for most cars.

For example, a typical performance cruise power setting in a Cessna 172SP might be 70% rated power (180 BHP at sea level), which is way, way, way more than you'd ever use for a sustained period in your car.

Highway cruising might take 20-40 HP depending on your vehicle, which is probably more like 10% of rated power. 70% of rated power in a typical American car is probably like 120 MPH.

Also worth mentioning is that they didn't spend all their time on cruise, but had to dip down to refuel from a truck twice a day, putting more work on the engine.

To put the power-over-overhaul-run into a more comparable perspective, I think it's helpful to compare the gallons of fuel consumed [as a proxy for the amount of energy/work output]. A small aircraft engine like in a 172 might be 10 gallons per hour, or 18K gallons of flight time over an engine run.

18K gallons will take a typical road car around 500K miles, so an airplane engine getting "only 1800 hours" of flight time likely outperforms the typical auto engine.

Overhauling an aircraft engine is a very serious undertaking involving removing the engine from the plane, shipping it away for complete disassembly and replacement of many parts.

Reply on news.ycombinator.com