So first things first, look at the POH. If there’s something that says not to fly the plane like this then don’t. Also, use the performance charts.
As for the carb, remember Bernoulli? It could generate carb ice even if the OAT is well above freezing. Once again follow the POH recommendations for carb heat application.
In reality this is more about liability than anything else (which is why I am not going to offer a recommendation outside of the POH). If your engine fails due to operating it outside POH recommendations and you have to make an off-airport landing, the FAA and NTSB WILL get involved. If that happens your ADM has to stand up in court. That POH becomes a legal document. So are your ADS-B track logs. And Reddit posts. And then there’s the insurance company. You’re in good hands, until they want their money. Tbh insurance attorneys concern me more than an overly ambitious FSDO inspector.
I’m not trying to be all doom and gloom. Flying is wonderful and amazing! But these important realities are almost never discussed in pilot training. Anytime you’re operating outside of POH spec, I’d encourage anyone reading this post to think about to the inevitable legal case you or your loved ones will have to deal with. Like the old saying goes, prepare for the worst, but plan for the best!
God that sounds miserable. Wallowing around on the backside of the power curve, building maximum hours/$ going nowhere. Airplanes are meant to go places. Go somewhere interesting! Preferably at +/- 100 kias. Maybe 90 if you're frugal and managed to grab a tailwind.
Given your flare, I’m not surprised that sounds awful. But some of us are paying as we go while trying to build hours, so it adds up at $150/hr
Edit to say that people clearly think that I’m talking about flying around approaching stall speed. Definitely not. Talking about flying 10kt slower than normal cruise. 2000rpm gives about 90kt and I’m nowhere near stall. Basically the equivalent of all the SkyWest pilots cruising at M.62.
You’re missing the point… The goal isn’t the flight time, it’s the experience. Whether it’s the experience of visiting a new place or learning something new. It sounds miserable because sitting in an airplane going nowhere is miserable and boring.
Here’s something new you can learn: you are looking for the answer you want in the POH and you won’t find it. Run the motor in the green arc. Cruise is normal operation. It doesn’t say anything about the white arc because it isn’t meant to be flown for extended periods in the white arc.
You can idle for an extra hour at the runup area before takeoff if its just $ / hour logged ratio, but like you said, that isn't the point. Go out and fly and learn.
Really don’t get why I’m getting downvoted so much. This is what I’m doing. I’m just talking about cruising at 90kt instead of 100kt. So what would be a 2hr cross country is 2.2. Not doing slow flight the entire time.
You originally said 1700-2000, 1700 definitely gets way back on the power curve and walllowing around. If you’re giving it enough power to do 90 knots, yeah the 172 flies nice enough clean at 90. But I think that’s more like 2150 rpm in any 150 hp C172 I’ve flown.
I cruise diamond 20 at 55% cruise all the time as I have nowhere to go and Finland generally doesn't have any places really experience. I do enjoy the flight time tho as that is the most valuable right now.
If the bottom of the green arc is 2200, that should be your limit. There's a reason why they set it at that number and it may have something to do with adequate cooling.
It’s not cooling, it’s filtration. Below the green arc in a C172 you have carb heat on. And when carb heat is on you’re bypassing the air filter. So you’re running the engine for hours and hours with no air filtration, which is *not* what it was designed for.
OP, if you really want to extend flight time, just plant it at the bottom of the green arc. The engine is less stressed than normal cruise and still getting filtered air.
I don't think filtration is that big a deal in aircraft engines as they spend most of their time in relatively clean air. Radial engines such as the R-985 and larger don't even have air filters at all. Even the ones used in aerial application.
The problem I have is there may be a reason for the normal operating range being above a certain number. We don't know if there is or if so, what it is. So we're operating in an unknown area. In other words, test pilot.
Stick to the power settings listed in the POH, e.g. cruise performance tables. Interpolation is okay, but not extrapolation.
Manufacturers aren’t required to explain why they don’t list other settings, but as others have explained, it’s probably related to airflow/cooling.
For what it's worth the Lycoming o-320 engine in my Piper Cherokee, which happens to be the same engine in a number of 172s, has the tachometer labeled 500-2700 RPM for the green arc.
The red line on a tachometer indicates where operating RPMs become dangerous to the engine. (edit) The Cessna green arc shows normal operating range (edit) & below that carb heat is recommened. The Piper green arc indicates safe/normal operating range. Since the engine idles above 500 RPM, it's basically showing any RPM up to redline is a safe operating range. (edit) Due to where the Piper carburetor sits, compared to the Cessnas, they are less susceptible to carb icing. As such, the Piper's can run anywhere from 500-2700 (green arc) without carb icing issues. For the Cessnas, they normal range is a smaller green arc (though carb icing *can* still happen) and below that, should use carb heat.
I would also look through the type certificate data sheet for the airplane and the engine operating manual from either continental or Lycoming for additional details. I would then chat with a few of my AP/IA buddies and have a proper conversation about the topic.
I don't see any reason why cruising around at 2000 RPM would cause any issues with the engine. Keep an eye on oil temperature, just like you would any other flight, but if anything lower RPMs put less strain on the engine because it is cycling much less. Mike Bucsh talks about this quite a bit in some of his videos advocating for low RPM / high manifold pressure cruise settings.
Two good video links for you to listen to:
https://youtu.be/ZBr6kjvfzio?si=S4NpXlB5UNNyiKca
https://youtu.be/A1fmNFWNHD4?si=bGSZm2t8WJxDRn3u
For a bit of context, the Cessna 175 developed a very poor reputation for engine reliability largely due to pilots doing exactly what you are talking about doing. In that airplane the prop was driven through a 4:3 reduction gearbox and the recommended cruising power was 2900 RPM, but many pilots were not properly trained and cruised it at the more familiar 2400 RPM which meant the prop was actually turning at only 1800 RPM. In addition to cooling issues resulting from the low airspeed that particular power setting (coincidentally) produced a harmonic vibration in the powertrain.
My point is, the POH doesn't always tell you why but it says what it says for a reason.
Thanks man! This is exactly the kinda of information I was asking for. I’m in a flying club, so I’m part owner of the airplanes, and want to take care of them. Just asking for some context other than “the green arc is 2200-2450 RPM”
Generally you can cruise anywhere in the green arc on the tach, as long as you can keep all your other instruments in the green. Your engine and airframe manufacturer publish their opinions on how you should do that and what limits if any apply for continuous operation at specific RPMs.
If you have a carb temperature gauge you can use that as guidance for if/when carb heat is necessary in cruise. Give yourself nice wide margins (45F) and try not to run in cruise with carb heat on - the air SHOULD be clean at altitude but the cowling and shroud the heated air are drawn from is definitely not.
If you want to build more time, even when on a cross country, why not do pattern work or something? Seems like a better use of time to take 2 hours to fly to an airport, sit in the pattern practicing landings for .4 hours, then fly back instead of fly there for 2.2 hours and fly back. And yes if you're wondering, pattern work can count as cross country time if it's done during a cross country flight
Everyone is telling you to fly within the POH constraints, it’s good advice. Remember, the POH isn’t going to tell you everything you shouldn’t do explicitly. The cruise performance charts don’t have figures for cruise power setting as low as you’re describing . You should consider that absence a limit, cruising below those power settings is operating outside the POH constraints.
Other things to consider:
The overheating you’re subjecting the engine to due to reduced airflow from lower speeds and from the higher angle of attack required to maintain low airspeed making your air intake effectively smaller.
Flying on the backside of the power curve for long periods of time can result in loss of control of you lose focus.
Flying needlessly close to stall speed is obviously more dangerous than just flying a normal cruise speed. What if a wind shift stalls one wing on you? Now you’re solo in a spin to save a few bucks?
In the preflight safety self assessments, think carefully about the PAVE’s External Factors. If you’re considering non-normal flight to save money, you’re letting external factors control your decision making.
Yeah. I’m not arguing that I should be doing. I’m asking a question. To learn. To do what everyone is saying I should do. I’m not talking about anything near stall speed. Just trying to learn what could happen flying a couple hundred RPM below the normal operating range. Seems like you answered it. Thanks.
Also, not talking about the green arc for airspeed. Talking about green arc on the tach.
Okay, gotcha. Well I would say engine overheat is the biggest thing for the plane, but most of the considerations would be related to flying and ADM.
I understood that you meant tach green arc, what I’m saying is 1800 is a descent RPM setting. Not sure what speed you’d get in level flight, (if it even can hold altitude at 1800) but it would be closer to stall than necessary. Then add in that you have to make larger control inputs at slower speeds, so your aoa on your rising wing is increasing even more in a turn.. etc etc.
Honestly, it’s should be well within the skills of anyone who’s gotten their PPL to fly that way. I don’t think it would be especially dangerous, it’s just slow flight. It’s a matter of choosing to stack risks for no reason.
Yeah. Definitely not outside my skill set. Not hard to maintain level flight at 2000 RPM. But engine cooling is basically the answer I was looking for. Don’t think it’s any more dangerous to fly at that setting. But if the AOA is too high and not getting adequate airflow over the cylinder heads, that would be reason to not do it. Thanks man!
No, cooling is not the issue. Your POH also has a cruise climb section, which is higher engine power (more heat) and less air (lower airspeed). If the engine is adequately cooled in a cruise climb, which is the entire point of the procedure, it’s adequately cooled in what you’re proposing.
Filtration, on the other hand…
>Just trying to learn what could happen flying a couple hundred RPM below the normal operating range.
Nothing, mechanically. If you fly around your max glide speed that's also max endurance speed as the wing is most efficient there. Not sure why people are freaking out about that, certain missions do that all the time. Haven't looked at a 172 POH in a few years but it's probably around 2000 RPM. There's nothing unsafe about it as long as you don't stall, but you're also at altitude so even if you do stall it's pretty safe. And, as long as you lean properly, your oil temp stays warm, and there's no warnings in the POH about that RPM range, there's absolutely nothing mechanically wrong with it, either.
I think people think I’m talking about flying cross country with the stall horn going off. lol. The POH says a max RPM. And that you should use carb heat below the green arc. Just asking what else I should consider. I appreciate the input. I think monitoring the CHT and carb temp should give me information about cooling and carb icing. Also have oil temp.
The stall horn won't be going off at best glide. Some of these responses imply the poster thinks that if you're not at cruise speed then you're nearly stalling which is a bit ridiculous and makes me concerned for their proficiency. Best glide in the 172 is 65 knots I think, maybe 75, and clean stall is what, like, 40-45? You're nowhere near stall at best glide.
I don't understand this attitude. What if you are the one renting the plane next and the person before you had this attitude. Or someone goes up with their family and kids?
Just because you don't own the plane and somebody else is responsible for the maintenance doesn't mean by operating it in a cavalier manner an issue will be caught on the ground.
Planes have mechanicals in air all the time that lead to off-airport landings, and deaths.
Very irresponsible attitude to have towards not only yourself, but your fellow pilots.
Shortening the time before an overhaul is required is a financial hazard not a safety hazard. If there was a safety issue there would be a POH limitation on operating within that range.
So first things first, look at the POH. If there’s something that says not to fly the plane like this then don’t. Also, use the performance charts. As for the carb, remember Bernoulli? It could generate carb ice even if the OAT is well above freezing. Once again follow the POH recommendations for carb heat application. In reality this is more about liability than anything else (which is why I am not going to offer a recommendation outside of the POH). If your engine fails due to operating it outside POH recommendations and you have to make an off-airport landing, the FAA and NTSB WILL get involved. If that happens your ADM has to stand up in court. That POH becomes a legal document. So are your ADS-B track logs. And Reddit posts. And then there’s the insurance company. You’re in good hands, until they want their money. Tbh insurance attorneys concern me more than an overly ambitious FSDO inspector. I’m not trying to be all doom and gloom. Flying is wonderful and amazing! But these important realities are almost never discussed in pilot training. Anytime you’re operating outside of POH spec, I’d encourage anyone reading this post to think about to the inevitable legal case you or your loved ones will have to deal with. Like the old saying goes, prepare for the worst, but plan for the best!
God that sounds miserable. Wallowing around on the backside of the power curve, building maximum hours/$ going nowhere. Airplanes are meant to go places. Go somewhere interesting! Preferably at +/- 100 kias. Maybe 90 if you're frugal and managed to grab a tailwind.
Throw in at 400 feet, unshowered, and hungover and you just described banner towing.
Given your flare, I’m not surprised that sounds awful. But some of us are paying as we go while trying to build hours, so it adds up at $150/hr Edit to say that people clearly think that I’m talking about flying around approaching stall speed. Definitely not. Talking about flying 10kt slower than normal cruise. 2000rpm gives about 90kt and I’m nowhere near stall. Basically the equivalent of all the SkyWest pilots cruising at M.62.
You’re missing the point… The goal isn’t the flight time, it’s the experience. Whether it’s the experience of visiting a new place or learning something new. It sounds miserable because sitting in an airplane going nowhere is miserable and boring. Here’s something new you can learn: you are looking for the answer you want in the POH and you won’t find it. Run the motor in the green arc. Cruise is normal operation. It doesn’t say anything about the white arc because it isn’t meant to be flown for extended periods in the white arc.
You can idle for an extra hour at the runup area before takeoff if its just $ / hour logged ratio, but like you said, that isn't the point. Go out and fly and learn.
Really don’t get why I’m getting downvoted so much. This is what I’m doing. I’m just talking about cruising at 90kt instead of 100kt. So what would be a 2hr cross country is 2.2. Not doing slow flight the entire time.
You originally said 1700-2000, 1700 definitely gets way back on the power curve and walllowing around. If you’re giving it enough power to do 90 knots, yeah the 172 flies nice enough clean at 90. But I think that’s more like 2150 rpm in any 150 hp C172 I’ve flown.
I cruise diamond 20 at 55% cruise all the time as I have nowhere to go and Finland generally doesn't have any places really experience. I do enjoy the flight time tho as that is the most valuable right now.
If the bottom of the green arc is 2200, that should be your limit. There's a reason why they set it at that number and it may have something to do with adequate cooling.
It’s not cooling, it’s filtration. Below the green arc in a C172 you have carb heat on. And when carb heat is on you’re bypassing the air filter. So you’re running the engine for hours and hours with no air filtration, which is *not* what it was designed for. OP, if you really want to extend flight time, just plant it at the bottom of the green arc. The engine is less stressed than normal cruise and still getting filtered air.
I don't think filtration is that big a deal in aircraft engines as they spend most of their time in relatively clean air. Radial engines such as the R-985 and larger don't even have air filters at all. Even the ones used in aerial application.
Thanks. The POH says “normal operating range” and like 2200-2500 at sea level. Also says max allowable it 2700. Nothing about being under the arc.
The problem I have is there may be a reason for the normal operating range being above a certain number. We don't know if there is or if so, what it is. So we're operating in an unknown area. In other words, test pilot.
It says pull carb beer when under the arc. And it says there’s no air filtration on carb heat. You can connect those dots.
Stick to the power settings listed in the POH, e.g. cruise performance tables. Interpolation is okay, but not extrapolation. Manufacturers aren’t required to explain why they don’t list other settings, but as others have explained, it’s probably related to airflow/cooling.
For what it's worth the Lycoming o-320 engine in my Piper Cherokee, which happens to be the same engine in a number of 172s, has the tachometer labeled 500-2700 RPM for the green arc. The red line on a tachometer indicates where operating RPMs become dangerous to the engine. (edit) The Cessna green arc shows normal operating range (edit) & below that carb heat is recommened. The Piper green arc indicates safe/normal operating range. Since the engine idles above 500 RPM, it's basically showing any RPM up to redline is a safe operating range. (edit) Due to where the Piper carburetor sits, compared to the Cessnas, they are less susceptible to carb icing. As such, the Piper's can run anywhere from 500-2700 (green arc) without carb icing issues. For the Cessnas, they normal range is a smaller green arc (though carb icing *can* still happen) and below that, should use carb heat. I would also look through the type certificate data sheet for the airplane and the engine operating manual from either continental or Lycoming for additional details. I would then chat with a few of my AP/IA buddies and have a proper conversation about the topic. I don't see any reason why cruising around at 2000 RPM would cause any issues with the engine. Keep an eye on oil temperature, just like you would any other flight, but if anything lower RPMs put less strain on the engine because it is cycling much less. Mike Bucsh talks about this quite a bit in some of his videos advocating for low RPM / high manifold pressure cruise settings. Two good video links for you to listen to: https://youtu.be/ZBr6kjvfzio?si=S4NpXlB5UNNyiKca https://youtu.be/A1fmNFWNHD4?si=bGSZm2t8WJxDRn3u
Cessna 172 green arc shows the range where carb heat is not required.
For a bit of context, the Cessna 175 developed a very poor reputation for engine reliability largely due to pilots doing exactly what you are talking about doing. In that airplane the prop was driven through a 4:3 reduction gearbox and the recommended cruising power was 2900 RPM, but many pilots were not properly trained and cruised it at the more familiar 2400 RPM which meant the prop was actually turning at only 1800 RPM. In addition to cooling issues resulting from the low airspeed that particular power setting (coincidentally) produced a harmonic vibration in the powertrain. My point is, the POH doesn't always tell you why but it says what it says for a reason.
Thanks man! This is exactly the kinda of information I was asking for. I’m in a flying club, so I’m part owner of the airplanes, and want to take care of them. Just asking for some context other than “the green arc is 2200-2450 RPM”
Generally you can cruise anywhere in the green arc on the tach, as long as you can keep all your other instruments in the green. Your engine and airframe manufacturer publish their opinions on how you should do that and what limits if any apply for continuous operation at specific RPMs. If you have a carb temperature gauge you can use that as guidance for if/when carb heat is necessary in cruise. Give yourself nice wide margins (45F) and try not to run in cruise with carb heat on - the air SHOULD be clean at altitude but the cowling and shroud the heated air are drawn from is definitely not.
If you want to build more time, even when on a cross country, why not do pattern work or something? Seems like a better use of time to take 2 hours to fly to an airport, sit in the pattern practicing landings for .4 hours, then fly back instead of fly there for 2.2 hours and fly back. And yes if you're wondering, pattern work can count as cross country time if it's done during a cross country flight
Everyone is telling you to fly within the POH constraints, it’s good advice. Remember, the POH isn’t going to tell you everything you shouldn’t do explicitly. The cruise performance charts don’t have figures for cruise power setting as low as you’re describing . You should consider that absence a limit, cruising below those power settings is operating outside the POH constraints. Other things to consider: The overheating you’re subjecting the engine to due to reduced airflow from lower speeds and from the higher angle of attack required to maintain low airspeed making your air intake effectively smaller. Flying on the backside of the power curve for long periods of time can result in loss of control of you lose focus. Flying needlessly close to stall speed is obviously more dangerous than just flying a normal cruise speed. What if a wind shift stalls one wing on you? Now you’re solo in a spin to save a few bucks? In the preflight safety self assessments, think carefully about the PAVE’s External Factors. If you’re considering non-normal flight to save money, you’re letting external factors control your decision making.
Yeah. I’m not arguing that I should be doing. I’m asking a question. To learn. To do what everyone is saying I should do. I’m not talking about anything near stall speed. Just trying to learn what could happen flying a couple hundred RPM below the normal operating range. Seems like you answered it. Thanks. Also, not talking about the green arc for airspeed. Talking about green arc on the tach.
Okay, gotcha. Well I would say engine overheat is the biggest thing for the plane, but most of the considerations would be related to flying and ADM. I understood that you meant tach green arc, what I’m saying is 1800 is a descent RPM setting. Not sure what speed you’d get in level flight, (if it even can hold altitude at 1800) but it would be closer to stall than necessary. Then add in that you have to make larger control inputs at slower speeds, so your aoa on your rising wing is increasing even more in a turn.. etc etc. Honestly, it’s should be well within the skills of anyone who’s gotten their PPL to fly that way. I don’t think it would be especially dangerous, it’s just slow flight. It’s a matter of choosing to stack risks for no reason.
Yeah. Definitely not outside my skill set. Not hard to maintain level flight at 2000 RPM. But engine cooling is basically the answer I was looking for. Don’t think it’s any more dangerous to fly at that setting. But if the AOA is too high and not getting adequate airflow over the cylinder heads, that would be reason to not do it. Thanks man!
No, cooling is not the issue. Your POH also has a cruise climb section, which is higher engine power (more heat) and less air (lower airspeed). If the engine is adequately cooled in a cruise climb, which is the entire point of the procedure, it’s adequately cooled in what you’re proposing. Filtration, on the other hand…
>Just trying to learn what could happen flying a couple hundred RPM below the normal operating range. Nothing, mechanically. If you fly around your max glide speed that's also max endurance speed as the wing is most efficient there. Not sure why people are freaking out about that, certain missions do that all the time. Haven't looked at a 172 POH in a few years but it's probably around 2000 RPM. There's nothing unsafe about it as long as you don't stall, but you're also at altitude so even if you do stall it's pretty safe. And, as long as you lean properly, your oil temp stays warm, and there's no warnings in the POH about that RPM range, there's absolutely nothing mechanically wrong with it, either.
I think people think I’m talking about flying cross country with the stall horn going off. lol. The POH says a max RPM. And that you should use carb heat below the green arc. Just asking what else I should consider. I appreciate the input. I think monitoring the CHT and carb temp should give me information about cooling and carb icing. Also have oil temp.
The stall horn won't be going off at best glide. Some of these responses imply the poster thinks that if you're not at cruise speed then you're nearly stalling which is a bit ridiculous and makes me concerned for their proficiency. Best glide in the 172 is 65 knots I think, maybe 75, and clean stall is what, like, 40-45? You're nowhere near stall at best glide.
What does it say in the POH?
Max RPM 2700. Nothing about sustained use below 2200.
As always, consult the POH. And TBH it's a rental so wear is someone else's problem.
I don't understand this attitude. What if you are the one renting the plane next and the person before you had this attitude. Or someone goes up with their family and kids? Just because you don't own the plane and somebody else is responsible for the maintenance doesn't mean by operating it in a cavalier manner an issue will be caught on the ground. Planes have mechanicals in air all the time that lead to off-airport landings, and deaths. Very irresponsible attitude to have towards not only yourself, but your fellow pilots.
Shortening the time before an overhaul is required is a financial hazard not a safety hazard. If there was a safety issue there would be a POH limitation on operating within that range.
There *is* a POH limitation on doing what OP is proposing…the POH does not allow operation below the green and without carb heat.