Cessna 172N - Performance Questions

[CC268]

Hey guys,

I am setting up a DUATs Aircraft Profile for the plane that I fly (I will also input this into ForeFlight for future use). Few questions:

1. So...POH says at Sea Level that Climb Rate is 770 FPM...however I fly out of KSDL which is at 1500' which on the chart is about 700 FPM. So for the profile - should I be basing everything off of the "best possible performance at sea level" or input the performance specs that based on my home field elevation.

2. Also...trying to figure out what the fuel burn per hour during climb is...chart in the POH is leading me to believe that fuel burn is roughly 10GPH during climb (full power). Any help with this?

3. POH also says true airspeed at 8000ft and 75% power is 122kts - this seems kind of fast? Is this what I should put for the Cruise True Airspeed?

4. Descent Performance - Not sure what to put for Descent True Airspeed. Yea on Approach I aim for 65kts, but I am obviously much faster than that when just descending down. Descent Fuel Per Hour? Obviously less than the Cruise 7GPH, but not specified. I put 500FPM for Descent Rate...

I know that is a lot to answer, but any help is greatly appreciated.

Thanks

 

[dmspilot]

The calculations DUATS and most flight planners do are are not all that sophisticated. As far as I know, it doesn't account for changing performance at different altitudes. Just put in average numbers. If it were me, I might use something like that following, based off the 172N POH and personal experience:

Climb: 600fpm - 80 KTAS - 11gph.

Cruise: 2400RPM - 110 KTAS - 8gph. (The 172N POH says 75% at 8,000 feet is 2650RPM. That's a lot of noise.)

Descent: 500fpm - 110 KTAS - 6gph.

 

[Pilawt]

Good questions, but the answers are not simple.

The numbers in that POH can and should be taken with not just a grain of salt, but ...

First, those numbers were derived from test flights by professional test pilots in brand-new, squeaky-clean, perfectly-rigged airplanes with strong, new engines. Often they didn't even have a full complement of navigation and communication antennas to dirty up the aerodynamics. And what we often forget today, those tests were done in a very competitive environment. A one or two knot difference in cruise speed on the performance chart might have caused a prospective 1977 buyer to decide against a 172N and buy a Piper Warrior or Grumman American Cheetah instead. You think the marketing department didn't have its thumb on the scale when those books were written?

Second, light airplanes are by their nature individually hand-built, and not identically mass-produced like automobiles or refrigerators. So there may be subtle differences from one airframe to another that would affect performance. It might not be unusual for serial number 'x' to be a couple of knots (or more) faster or slower than serial number 'x+1', even when new and identically equipped. And they're no longer new. The newest 172N on the planet is now almost 37 years old, and over the years they've all had varying degrees of wear and tear, addition or deletion of equipment, etc., etc., that affect the performance. It's safe to say that you won't find two identical 172Ns, or any two that perform exactly alike.

Bottom line, you need to fly a particular airplane for a while to get accustomed to its quirks before you can plot performance with any kind of precision. For your purposes, do you need that much precision?

Taking your questions in order:

1. You fly out of Scottsdale? Gotta factor in the temperature. Your density altitude on the surface is rarely less than 3,000', and often can be much higher. You might get a loaded 172N to climb at 500 or 600 fpm for a short time, but more likely your average climb to cruising altitude would be more along the order of 400 fpm, or maybe 300 if you're going higher. Look at the "Time Fuel And Distance to Climb" chart on page 5-15 of the 172N POH. For purpose of flight planning aids like DUATS, I prefer to be pessimistic. Call it 350 fpm.

2. For a shortcut, refer again to page 5-15, and again factor in elevation and temperature. But to understand the concept, consider this. The book says fuel burn at 75% power is 8.5 gph. Well, 8.5 / 75% = 11.3 gph at 100% power, for a really rough estimate. But that cruise power figure is based on lean mixture. You'll be climbing with a richer mixture. So I call it 12 gph while climbing. Can never hurt anything by over-estimating fuel consumption for flight planning purposes.

3. Yeah, 122 KTAS at 8,000' in a stock 172N does sound fast, for the reasons discussed above. Does your airplane have wheel and brake fairings? The book numbers assume the fairings are all on. Those fairings can make a difference of around 5 knots. In earlier years, when the fairings were "optional at extra cost" equipment, Cessna manuals had a footnote on the performance charts about deducting 'x' knots if fairings not installed; I don't see such a footnote in the 172N manual. Assuming an "average" 172N, I'd guess about 115 KTAS with full fairings and 110 without. YMMV.

4. I interpret "descent performance" to refer to the descent from cruising altitude down to traffic pattern altitude. I'd just add 5 knots to your cruising speed and deduct 1-2 gph from your cruise fuel flow, and that should be close enough for government work. Again, pessimism can't hurt in flight planning. Optimists land in cornfields (or worse) with dry tanks. And here's the one instance where you do have almost full control over a performance parameter in a 172 -- rate of descent. A 500 fpm descent is reasonable, and easy on passengers' ears.

 

[MAKG1]

122 KTAS at 8000 is about 105 KCAS. That's about right for a 172N.

Book numbers are generally close or the plane is modified or sick. But you do have to calculate TAS -- and density altitude -- correctly.

 

[CC268]

Thanks a lot guys! Very nice of you and dms to write that out!

 

[CC268]

Ohh another quick question if you guys don't mind.

So when you do your planned TAS (true airspeed) for the CLIMB I put in 72 kts for CAS (best rate of climb) and then the outside air temperature...but what do you put for the Pressure Altitude for a climb? If I am climbing from 1500 to 8500 do you just put in an average of say 4500?

Obviously for the Cruise portion of flight I use 8500 and cruise CAS of about 110

 

[coloradobluesky]

Just use the average. Figure 75% power fuel burn in the climb.

 

[CC268]

Thanks!

 

[MAKG1]

If you're doing a real long climb, like up to 12500 in a 172, you will spend almost all your time at the high end. The POH has a time and fuel to altitude chart. Use this. If it takes 30 minutes to get to 12000, your climb rate is 400 FPM. IRL, you're quite vulnerable to sink and any errors will arrest your climb. And you can only climb at Vy when performance limits are a factor; any other speed will not climb. Some POHs (Piper) get this wrong.

Vy goes down with altitude, as well, in IAS. It's worth more than 5 knots at 10,000 in a 172, and the difference in climb rates up there is huge.

 

[musket]

The calculations DUATS and most flight planners do are are not all that sophisticated. As far as I know, it doesn't account for changing performance at different altitudes. Just put in average numbers. If it were me, I might use something like that following, based off the 172N POH and personal experience:

Climb: 600fpm - 80 KTAS - 11gph.

Cruise: 2400RPM - 110 KTAS - 8gph. (The 172N POH says 75% at 8,000 feet is 2650RPM. That's a lot of noise.)

Descent: 500fpm - 110 KTAS - 6gph.

I appreciated these numbers...found them very useful for online aircraft profiles. Also, I'm new to the forums.

 

[Alex Christoff]

The calculations DUATS and most flight planners do are are not all that sophisticated. As far as I know, it doesn't account for changing performance at different altitudes. Just put in average numbers. If it were me, I might use something like that following, based off the 172N POH and personal experience:

Climb: 600fpm - 80 KTAS - 11gph.

Cruise: 2400RPM - 110 KTAS - 8gph. (The 172N POH says 75% at 8,000 feet is 2650RPM. That's a lot of noise.)

Descent: 500fpm - 110 KTAS - 6gph.

DMS I know this post is a late one but thank you for that basic but very useful information.

 

[TooTall]

I track my fuel burn per hour based on tac time, no hobbs or foreflight. tac is more conservative. After a few trips, you'll get some very accurate average fuel burn numbers to plug in. If I went by hobbs or foreflight, I'd run out of fuel. My 0300 gets about 7 gph hobbs and 9 tac. I use 10 gpm.

 

[RyanB]

I track my fuel burn per hour based on tac time, no hobbs or foreflight.

Gauging fuel consumption by the tachometer doesn’t seem like the most consistent or conservative method, when compared to Hobbs or just regular clock time. Note the time you start and the amount of time you’ve been in the air and adjust accordingly for your hourly fuel consumption.

 

[Palmpilot]

Gauging fuel consumption by the tachometer doesn’t seem like the most consistent or conservative method, when compared to Hobbs or just regular clock time. Note the time you start and the amount of time you’ve been in the air and adjust accordingly for your hourly fuel consumption.

If he calculated gallons-per-hour as fuel consumed divided by Hobbs hours, that would give him an over-optimistic GPH. I think that's what he was getting at.

 

[TooTall]

Yes, what Palmpilot said. Hobbs time increases as the clock moves forward. Tach time, on the other hand, moves faster or slower depending on engine RPM. So for example, while taxiing, tach time is only about 40% of Hobbs time. For me, tac time on a given flight is always less than hobbs so using it to divide into fuel used is safer? Yall agree?

 

[RyanB]

Hobbs time increases as the clock moves forward.

No it doesn’t. Hobbs time is true to actual clock time.

For me, tac time on a given flight is always less than hobbs so using it to divide into fuel used is safer? Yall agree?

I partially get your rationale, but I still disagree that it’s safer. As mentioned above, Hobbs time is true to clock time, it doesn’t vary in speed like Tach time does. Because of this reason, I believe fuel consumption is best calculated off true clock time, because it’s more conservative.

After I start the engine and see the Hobbs move a tenth, I know I’ve been running for 6 minutes… two tenths, 12 minutes and so forth and then divide this into my hourly consumption. Compared to the Tach which might barely have moved a tenth of an hour in that same time. Granted, I realize I’m going to burn less on the ground while taxiing, with the engine turning slower etc., but I prefer to keep my calculation’s consistent across the board to be conservative.

 

[RyanB]

If he calculated gallons-per-hour as fuel consumed divided by Hobbs hours, that would give him an over-optimistic GPH.

It’s far better to be optimistic with fuel than the other way ‘round, which is why I disagree with his approach.

 

[nauga]

It’s far better to be optimistic with fuel than the other way ‘round, which is why I disagree with his approach.

Dividing tach time by fuel *consumed* (not remaining) will give you a higher calculated fuel burn rate than dividing Hobbs by fuel consumed. I think that's the conservativism he's referring to. I just use 10GPH for everything

Nauga,
who also flies at 120 or 180 KGS, or 150 if I must.

 

[RyanB]

Dividing tach time by fuel *consumed* (not remaining) will give you a higher calculated fuel burn than dividing Hobbs by fuel consumed. I think that's the conservativism he's referring to. I just use 10GPH for everything

Nauga,
who also flies at 120, 150, or 180 KGS.

I suppose I just prefer using a fixed metric, rather than a variable one. I record my taxi time and off time, as well as the time I’ve been in the air by standard clock time and then divide that by my hourly burn rate.

I also use 10GPH as my base figure, although in reality it ends up being ~9GPH. So, if I’ve spent 1.5 in the air (taxi time included), I’ve consumed 15gals. Then I subtract that by the amount of fuel I started with.

Different strokes I guess.

 

[nauga]

For me, tac time on a given flight is always less than hobbs so using it to divide into fuel used is safer? Yall agree?

Dividing tach time by fuel *consumed* (not remaining) will give you a higher calculated fuel burn rate...

I suppose I just prefer using a fixed metric, rather than a variable one. I record my taxi time and off time, as well as the time I’ve been in the air by standard clock time and then divide that by my hourly burn rate.

Y'all are talking about different calculations - I was talking about calculating fuel burn rate, I believe @TooTall is talking about calculating fuel burn rate, and you're talking about calculating fuel consumed. Different means to a similar end, but either means of measuring time is conservative with one calc and optimistic with the other.

Nauga,
who watches the watches

 

[RyanB]

Y'all are talking about different calculations - I was talking about calculating fuel burn rate, I believe @TooTall is talking about calculating fuel burn rate, and you're talking about calculating fuel consumed. Different means to a similar end, but either means of measuring time is conservative with one calc and optimistic with the other.

Nauga,
who watches the watches

Good observation, I misunderstood that part of the discussion.

 

[asicer]

Tach time, on the other hand, moves faster or slower depending on engine RPM.

Not all tachometers work that way.

 

[TooTall]

Well if yall are using 10 GPH then we all got to the same number, because that's what I use. Using tac, I have used as much as 9.5 GPM. Hobbs, I can get anywhere from 6.5 on up depending taxi time how hard I run etc.... I put 10 gpm in foreflight for cruise @ 2300 RPM, 13 GPM for climb and 10 gpm for decent. 2 gallons for taxi. I've never used more than that. When I say conservative, I mean; I want to land having used less fuel than I calculated in foreflight. It works well for me but I realize I'm in the minority on this.

BTW, how does your foreflight logged flight time vs hobbs compare?

 

[Palmpilot]

It’s far better to be optimistic with fuel than the other way ‘round, which is why I disagree with his approach.

What I meant by "optimistic" is that the pilot would calculate that the plane uses less fuel per hour than it actually does. That's not not better.

 

[Dan Thomas]

Fuel burn figures in the POH are also based on some fairly aggressive leaning. Some pilots don't lean nearly as much as they should, and some leave the thing at full rich even when climbing at higher altitudes. The POH will tell you to lean for smoothness at altitude in the climb. If you don't you're burning way more fuel than the charts show.







To get 122 kt true in level flight at 8000 feet, 75% power, you'll need full throttle, pretty much. The book shows that as 2650 RPM. Fixed-pitch props are normally pitched to allow the engine to just reach redline at full throttle in level flight. Not many pilots are comfortable with the throttle all the way in, or nearly so, but that's what it takes and that's what is approved in those cruise charts. You're not hurting the engine. It's not getting enough air to hurt itself.

When breaking in new engines in the 172Ms, the Lycoming service letter that outlines the break-in flight process calls for a 2-1/2 hour flight, varying between 65 and 75% power, and the last half-hour at full throttle, keeping the RPM and temps within limits. Those old Ms, without wheelpants, at 8000 DA, would do 135 MPH like that. Scoot right along. That's 117 kt. The book is not wrong. Pilots are.

 

[John Spartan]

Not sure how interchangeable this is between birds but here are the numbers I was given for the 172N I fly out of KCHD a with an O-320.

 

[idahoflier]

Some pilots don't lean nearly as much as they should, and some leave the thing at full rich even when climbing at higher altitudes. The POH will tell you to lean for smoothness at altitude in the climb. If you don't you're burning way more fuel than the charts show.

Not necessarily disagreeing with what you said, but I would offer a word caution to others. Prior to having an engine monitor installed all I had was a factory installed relative EGT guage. I would routinely lean in the climb as described. With the engine monitor installed I saw that leaning in the climb as described in the above POH would result in very high CHT's. It wasn't as surprising to me why the engine had two cylinders replaced ~ 1,000 hours...

 

[Pinecone]

There are also a different level of accuracy needed between performance for filing flight plans and doing flight planning.

In FF, you will be able to first, use data already in FF saving you a LOT of work. In fact, in your case, I would add the plane to FF and copy the data off there.

Or you can setup a custom profile and you will put in cruise speed and fuel flow for the power setting you want and for each 1000 feet (may be 2000) of altitude, using density altitude. So you would put in sea level performance info, but it will calculate based on the take off airport parameters.

For filling flight plans, they are looking for close enough to know how to have an idea of where you might be.

 

[Dan Thomas]

Not necessarily disagreeing with what you said, but I would offer a word caution to others. Prior to having an engine monitor installed all I had was a factory installed relative EGT guage. I would routinely lean in the climb as described. With the engine monitor installed I saw that leaning in the climb as described in the above POH would result in very high CHT's. It wasn't as surprising to me why the engine had two cylinders replaced ~ 1,000 hours...

When the airplane was new, the EGT probe was in a certain place on a certain exhaust riser, and the POH instructions were based on the readings from that. These things age, risers burn out, get replaced, and the mechanic drills a new EGT probe hole wherever it's most convenient or accessible, and the reading is now different from the OEM reading, and leaning might result in higher EGTs elsewhere.

Your multi-point stuff is much better, but even there the installation instructions tell the mechanic exactly where those probes are to go. Whether he does the right thing or not is another story. The exhaust coming out of the cylinder includes a flame that might strike the probe, or it might not, depending on probe location, since things are not nice and uniform with gaseous flows like that. So the monitor might show significant discrepancies between cylinders that aren't really there; OTOH they might just be due to the uneven air/fuel mix between cylinders with a carbed engine. Hard to know what's what if the installation isn't done exactly as required.

As an example, this is from a JPI manual:

 

[idahoflier]

When the airplane was new, the EGT probe was in a certain place on a certain exhaust riser, and the POH instructions were based on the readings from that. These things age, risers burn out, get replaced, and the mechanic drills a new EGT probe hole wherever it's most convenient or accessible, and the reading is now different from the OEM reading, and leaning might result in higher EGTs elsewhere.

Your multi-point stuff is much better, but even there the installation instructions tell the mechanic exactly where those probes are to go. Whether he does the right thing or not is another story. The exhaust coming out of the cylinder includes a flame that might strike the probe, or it might not, depending on probe location, since things are not nice and uniform with gaseous flows like that. So the monitor might show significant discrepancies between cylinders that aren't really there; OTOH they might just be due to the uneven air/fuel mix between cylinders with a carbed engine. Hard to know what's what if the installation isn't done exactly as required.

The point I was trying to make was that all I had was a relative EGT gauge that was on one cylinder and didn't provide any meaningful info other than indicating peak EGT on that cylinder. It provided no information on CHT and as a result cylinders were likely getting cooked by leaning in the climb. This is for my aircraft, a C172N with an O-360. I have heard of others with skyhawks that run hot in climb. Because of my experience I'm a strong advocate for engine monitors.