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My flight training manual (FTM) says that at low altitudes we throttle back our trainers in order to not exceed recommended power settings.

But why is that? Is the air close to sea level just too dense for the standard Cessna 172 engine? And at 8000 feet it's just thin enough for us to cruise at full throttle?

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    $\begingroup$ What is FTM? The question could be improved by explaining. Flight Training Manual? If so, which one? Some are better than others. Also there were at least a couple of grammatical errors in this question, some of which have now been corrected. Also please explain in more detail what you mean by "the normal Cessna engine". Also consider modifying your title so that it actually asks a question, the same question that is filled out in more detail in the body of the question. $\endgroup$ – quiet flyer Jul 23 at 17:19
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    $\begingroup$ It would be helpful to know what engine we are talking about. I once flew a C172 that had a Porsche engine. $\endgroup$ – Aaron Holmes Jul 23 at 21:28
  • $\begingroup$ Aaron, the engine is Lycoming O-320. $\endgroup$ – leha007 Jul 23 at 22:30
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    $\begingroup$ @leha007 you should put any relevant information in the question, not in comment. $\endgroup$ – Manu H Jul 24 at 6:25
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These engines are not designed to run at maximum horsepower output all the time with a lean mixture. They are designed to cruise at 50-75% of their rated max power, and deal with that level of internal heat and friction over the long term. Even this is fairly hard compared to a car that runs at perhaps 20% of rated power when cruising at 60 mph.

Running them wide open means maximum heat, maximum wear, for not much benefit considering the power required to go faster is nearly the cube of the speed increase. You must run with the mixture full rich to keep the engine cool at all, and fuel burn is a lot higher because you are dumping unburned fuel used for cooling at full power out the exhaust. It's not worth it in other words, unless you are racing, so the concept of cruising at not more than 3/4 of rated max power is a very old convention in piston engines, and mixture leaning shouldn't really be done above that.

You can run one of these engines at full throttle with a rich mixture with the RPM near red line all the time if you want, but only if you don't mind paying for the 20 thousand dollar premature overhaul from flogging the poor engine like a mule (the exhaust valves will hate you for all eternity).

It's pretty much the same thing as hooking up a big heavy trailer to your car, that required foot on the floor all the time just to stay at highway speed. How many miles do you think that car's engine would last?(Although the Lycoming being run that hard will still probably outlast the car engine in the same shoes.)

However, when you go up, by the time you get to 8000ft, 75% of maximum output is all that is available with wide open throttle (WOT) in the thin air, so 75% cruise, normal operation, requires WOT and no harm is done. And as you go up further, you find you can't even get 50% with WOT and pretty soon you're at your service ceiling.

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    $\begingroup$ Yes. 75% requires, depending on the RPM, somewhere around 22-23" of manifold pressure (you don't normally get a MP gauge with fixed pitch props). WOT gives you just under ambient atmospheric pressure in the intake manifold due to the residual restriction of the carb venturi and throttle plate etc. If the barometer says 30" of mercury at sea level, WOT will give somewhere around 28-29" in the intake passage. At 8000 ft the ambient is down to around 22-23" so the best you can get is maybe 21" at WOT, just enough to make 75% at the RPM you will see at that altitude at WOT (just below redline). $\endgroup$ – John K Jul 23 at 23:39
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    $\begingroup$ the Lycoming being run that hard will still probably outlast the car engine in the same shoes - really? I consider the car engine an absolute marvel of reliability considering the abuse they get - owners forgetting to service cars for tens of thousands of miles past their dates, flogging them through dirty atmospheres with all sorts of contaminants splashing all over them, precious little steady state running.. Aircraft engine would probably quickly die in a car, and maybe fortunate that Honda don't make aircraft engines; they'd put everyone in the service industry out of work! :) $\endgroup$ – Caius Jard Jul 24 at 9:41
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    $\begingroup$ @CaiusJard I've seen videos of idiots revving lamborginis in traffic til they catch fire. Same idea. $\endgroup$ – Maaark Jul 24 at 10:20
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    $\begingroup$ Not really. If you've ever had a Lyc apart the difference structurally is quite striking. And the car engine spends its life at 20% of rated power or less, with brief spurts higher, and almost never at WOT. You have to be racing it to get it to an equivalent load. There are aircraft conversions of Honda Civic engines out there, and they do ok although they need a reduction drive, but they wear out at 1000 hrs and won't run 2000-3000 hours like a Lyc will. A/c engines are more like industrial or tractor engines. $\endgroup$ – John K Jul 24 at 12:45
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    $\begingroup$ If you are well past best L/D, the power goes up with almost the cube of speed. Underscores your point that it isn't worth it. $\endgroup$ – Peter Kämpf Jul 24 at 17:20
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Most Cessna 172s have fixed-pitch propellers. Designing a fixed-pitch prop always requires making some compromise between climb performance and cruise performance. Typically this means that at low altitude in horizontal flight, at full throttle the engine would exceed redline RPM, so you have to throttle back. At higher altitude the engine produces less power at a given RPM, so it isn't a problem there. It's possible to use a "cruise prop" that has a coarser pitch and allows full power (or closer to full power) in low altitude cruise, but this comes at the expense of not making full redline RPM at the start of the takeoff roll, and therefore lower acceleration and longer takeoff runs.

Variable-pitch propellers (of which "constant-speed" propellers are a subset) avoid this dilemma, though they have their own downsides such as increased complexity and a bunch of extra weight right on the nose where you don't want it.

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I agree with the above answers in terms of the limitations related to engine. However I wanted to also add that it could be possible that going full throttle at a low altitude would cause the aircraft to go out of its structural flight envelope as well. So as much as being an engine related issue, it could also be due to structural considerations of the airplane.

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    $\begingroup$ Not in level flight on a 172. $\endgroup$ – pericynthion Jul 24 at 18:50

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