Service Ceiling At Gross?

JohnSBA

JohnSBA

Pre-takeoff checklist
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JohnSBA
I was talking with an expert pilot about the service ceiling of a 172M (if I recall correctly the POH number was 13.5k'), that I found was less than eager to climb above 9kMSL. My assertion was that it would be practically impossible to reach 13k at gross in that particular plane.

Was I just too impatient? Clearly if one climbed long enough, enough fuel would have been consumed to bring the actual load well below gross. I still plan flights in the much more capable 172Q I now fly, so as to have minimum fuel during any portion of the flight where extra climb capability might be useful (mountains, etc.). Though I never did solely focus on reaching the absolute service ceiling of the 172M, my purpose wasn't to do that but rather fly near that limit with enough reserve climb capability to deal with unforeseen variables.

The discussion boiled down to two main points that I'm soliciting your comments on:

  1. Is it even possible to reach published service ceiling at gross or is that just a takeoff weight?
  2. How much margin do most pilots want when choosing their own actual service ceiling on a given flight?
 
I don't have much need (nor do I have the O2 in the plan)e to go to 20/22k. So I don't worry about it at all.
 
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I assume that the service ceiling can be reached on a day without turbulence and when the temperatures are at or below ISO, the proper Vy is used, with full throttle and the engine leaned for best power. Remember that Vy decreases with altitude and service ceiling is the altitude where the climb rate is 100 ft/minute or 1000 feet in ten minutes. Vx increases with altitude and equals Vy at the absolute altitude of the aircraft, which is above the service ceiling. You just have to use proper technique on the right day and be patient as the climb for the last 3 or 4 thousand feet will be slow.
 
JohnSBA said:
The discussion boiled down to two main points that I'm soliciting your comments on:

  1. Is it even possible to reach published service ceiling at gross or is that just a takeoff weight?
  2. How much margin do most pilots want when choosing their own actual service ceiling on a given flight?
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1 - Never tried and would need O2 to do so (PA-32, 16k svc ceiling), though the climb left when I've been at 12.5k would suggest there is no way to get to 16k.
2 - I have never come up with my own service ceiling. I'm not sure why you would. There's nothing up there to hit. OTOH, I pick my enroute altitude based on the elevation of the granite I will be traversing and how much clearance I want. FWIW when I've flown over a 11.6K pass I kept climbing until I cleared the pass. Got up to about 12.8 and then started right back down the other side.
 
I have exceeded the published service ceiling in my old Cherokee 180. I got it up to 15,500 before I decided to call it quits, but it was still climbing.
 
The Aztec supposedly has a service ceiling around 21,000. I'd only believe that on a cold winter day with just me in the plane. And it wouldn't be happy. If I'm by myself with full fuel I can do 14,000 ft pretty comfortably so long as it's reasonably chilly out, but in the summer it doesn't even want to go above 10,000. Of course, "want to" and "will" are different, and service ceilings are based on "will."

A few weeks ago on an FAA standard hot day (actually, maybe a bit hotter) with 3 people, luggage, 9 dogs, and about half fuel, it didn't want to go higher than 9,000. It would have, but it wasn't happy doing even that. I would say that a lot of aircraft probably can't reach their service ceilings, even below gross.
 
JohnSBA said:
I was talking with an expert pilot about the service ceiling of a 172M (if I recall correctly the POH number was 13.5k'), that I found was less than eager to climb above 9kMSL. My assertion was that it would be practically impossible to reach 13k at gross in that particular plane.

Was I just too impatient? Clearly if one climbed long enough, enough fuel would have been consumed to bring the actual load well below gross. I still plan flights in the much more capable 172Q I now fly, so as to have minimum fuel during any portion of the flight where extra climb capability might be useful (mountains, etc.). Though I never did solely focus on reaching the absolute service ceiling of the 172M, my purpose wasn't to do that but rather fly near that limit with enough reserve climb capability to deal with unforeseen variables.

The discussion boiled down to two main points that I'm soliciting your comments on:

  1. Is it even possible to reach published service ceiling at gross or is that just a takeoff weight?
  2. How much margin do most pilots want when choosing their own actual service ceiling on a given flight?
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A couple of things to keep in mind:

  • Service ceiling is density altitude NOT indicated altitude
  • It should be able to to reach it at gross in clean configuration while being able to maintain a 100 fpm climb.
You would need the following conditions:

  • gross
  • clean
  • pressure 29.92
  • OAT 10.9 Fahrenheit
Some other conditions:
40F, 29.92 - 12,000 ft indicated = 13,500 density service ceiling
50F, 29.92, 11,500 indicated = 13,500 density service ceiling

So if you look at the winds aloft forecast for KSBA today you'll discover that it's about 48F at 12,000 ft.

Your service ceiling today, at gross, clean configuration, strong engine, straight air-frame would be 11,500 indicated. Remember you'll only be climbing at 100 FPM at this point. It'll be a dog. 10 minutes per thousand feet.

The details matter.

(all of this calculated with my own e6b application I've developed for the iphone which hasn't been released yet)
 
jesse said:
A couple of things to keep in mind:

  • Service ceiling is density altitude NOT indicated altitude
  • It should be able to to reach it at gross in clean configuration while being able to maintain a 100 fpm climb.
Some other conditions:
40F, 29.92 - 12,000 ft indicated = 13,500 density service ceiling
50F, 29.92, 11,500 indicated = 13,500 density service ceiling


Your service ceiling today, at gross, clean configuration, strong engine, straight air-frame would be 11,500 indicated. Remember you'll only be climbing at 100 FPM at this point. It'll be a dog. 10 minutes per thousand feet.
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That explains a lot. In practice I rarely plan to go over 11,500 here in SoCal in a C172 (160 hp). I generally do <10,000 as the butter zone for most flights is 5500 to 8500. The one time I had to do 12,000 (evil MEA) the best I could get was ~75 fpm above 11,200. Near the ceiling it literally feels like you're barely hanging there.
 
jesse said:
A couple of things to keep in mind:

  • Service ceiling is density altitude NOT indicated altitude
  • It should be able to to reach it at gross in clean configuration while being able to maintain a 100 fpm climb.
Click to expand...

exactly.

Before I did an engine overhaul, I had my cherokee 140 to 7500'
indicated. Wow did it feel sluggish - turns out the DA was 9600' and
the service ceiling of my 140 is 10000'.
 
At the AOAs you'll need you need to be cognizant of true weight, density altitude, and the actual Vy for that altitude (and the correction factor between IAS, CAS and TAS). DETAILS DO MATTER.
 
This is exactly the sort of discussion I'd hoped for. I'm still not clear on how the SC is calculated. That is, if I took off at sea level in the 172M/160hp at gross on a standard day and initially managed 500fpm, how much time (fuel) would it take to reach 13kDA? If there were a way to midair refuel back up to gross, would it stay at 13kDA or start descending? What got me greatly doubting the SC in that plane was several winter days with takeoff wt. perhaps 300# below gross, and struggling to get 100fpm above 9kIA. Guess it would add a lot to the discussion if I'd accurately measured OAT and calculated DA, but since those numbers meant little to the factors involved (ATC, mountains, etc.) I just changed the flight plan and resolved to fly below 9kIA in that plane on future flights.
 
We returned from TX to WI last month in the Aztec and climbed to 13k at full gross minus the fuel it took for the 30 minute climb. I remember being suprised how happy and willing the airplane was to get there. Checking flightaware it says we were still climbing at 240fpm and although there was a line of thunderstorms across KS, MO, into IL and up in IA, we didn't fly through any type of frontal zone. I also remember climbing at 2300rpms vs a flight to FL in March when we had to climb from 12 to 14k to get over some icing and I needed to increase rpms to 2500 to establish any sort of ROC... we were again at full gross minus the climb to alt fuel, OAT that time was exactly 0C at 12k.

We've been to 14k several times and things get pretty mushy. Knowing what I know, 15k may be possible with the Aztec, but I'd really need a reason to want to do it. MP at 14k was 17.5 inches... fuel burn was down to 20gph, 10 per side.
 
Hmm. Jim is making me wonder if my plane is making full power, or alternately if I'm just less patient. :)
 
JohnSBA said:
This is exactly the sort of discussion I'd hoped for. I'm still not clear on how the SC is calculated. That is, if I took off at sea level in the 172M/160hp at gross on a standard day and initially managed 500fpm, how much time (fuel) would it take to reach 13kDA? If there were a way to midair refuel back up to gross, would it stay at 13kDA or start descending? What got me greatly doubting the SC in that plane was several winter days with takeoff wt. perhaps 300# below gross, and struggling to get 100fpm above 9kIA. Guess it would add a lot to the discussion if I'd accurately measured OAT and calculated DA, but since those numbers meant little to the factors involved (ATC, mountains, etc.) I just changed the flight plan and resolved to fly below 9kIA in that plane on future flights.
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Were you leaning for best power in the climb? The mixture will get so rich at altitude the engine loses considerable power and you won't climb.

Dan
 
Dan Thomas said:
Were you leaning for best power
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If you're talking about in the 172M yes, and above 8k'IA or so I was at Vx. I experimented with different AOA for best indicated VSI. I even experimented with level off, speed up, climb, repeat. I don't recall anything that would produce an actual averaged 100FPM climb above 9.5k'IA and this was in winter at well below gross even before counting the fuel burned to get there. Maybe I was doing something wrong, was further from standard than realized, or there was some issue with the plane. Would be nice if I'd checked out the actual conditions for this discussion, but the clear impression for me was that anything above 9k wasn't a realistic option for flight planning. Now that I'm flying the Q, we're probably going to buy a minimal O2 for 2 setup because the plane's more willing than we above 12k. One warm day soon after buying, the two of us with some baggage and half fuel went to 10.5k and were still climbing at 850FPM when we got tired of being so far from the views we most enjoy. On our recent OSH trip loaded close to gross we easily climbed above 12.5k'IA several times and that approximately matched what the terrain and GPS were indicating. As I recall, the SC on the Q is 17k and I'm more inclined to believe it - at least with our typical loads.
 
Jim Burns said:
LE is 150 SMOH
RE is 1100 SMOH

Flight log TX to WI http://flightaware.com/live/flight/N6205Y/history/20100721/1106Z/KRAS/KSTE/tracklog

Kept the nose down on the initial climb to keep the engines cool. We were definitely at gross... 192 gallons 100LL, wife, 2 teenagers, plus all their baggage. OAT if I remember right was 50F the entire time that we were at 13k.
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That sounds relatively similar weight wise to a lot of the loads I carry, although the kids I keep in back are caged. ;)

Looking at that, my guess is it may be that I'm just less patient and less tolerant of flying the plane with the IAS down as low as it will be at those altitudes. Although as far as CHTs go, up at those altitudes I'd need to have the cowl flaps open at least half and be working the mixtures to keep them where I'd like them.

Although from your avatar, your Aztec is much prettier than mine. :)
 
A few things about book service ceiling which bear consideration:
  • It is based on a "standard day" where indicated altitude and density altitude are the same.
  • It is based of use of all recommended leaning techniques for best power.
  • It is based on max gross weight.
  • It is based on a factory-new airplane with no dings, dents, or bugs, a factory-new engine pulling book rated power, and a factory-new prop with no dings.
  • It is based on correct Vy for the altitude, and Vy decreases as altitude goes up -- see the climb tables in your POH.
  • It is based on stable air with no up/downdrafts.
Thus, it is typical to see your other-than-factory-new plane on a real day run out of climb rate before it gets to book service ceiling with a full load. OTOH, it is often possible to get above book service ceiling if you're light. Whether or not it's wise to plan on flying that high depends a lot on weather and terrain. With only 100 ft/min climb rate available, it doesn't take much of a mountain wave or downdraft to push you downward despite your best efforts to maintain altitude, and if that would carry you down into rocks, airspace, or icing conditions, you would be unwise to plan to fly at that altitude.

As for time/fuel, the POH should have the data necessary to perform those computations, although in an older "owner's manual," you may have to do a lot of combining of data from different tables/charts and possibly some extrapolation.
 
Thanks.

WRT temps, I've found that I have to keep cowl flaps full open below 8000 in all but winter like OATs and at least 1/2 open above 8000. My CHT's and my oil temps refuse to play nicely if I go full closed in any configuration. I've learned from a lot of other Aztec owners that the same is true for their airplanes.

I generally lean to 5-10 degrees lean of peak at those altitudes and the Piper FF gauge shows between 13 and 14 gph per side, which is slightly higher than actual. I use 1500 EGT and 400 CHT for my max limits on any single cylinder.
 
Ron Levy said:
A few things about book service ceiling which bear consideration:
  • It is based on a "standard day" where indicated altitude and density altitude are the same.
  • It is based of use of all recommended leaning techniques for best power.
  • It is based on max gross weight.
  • It is based on a factory-new airplane with no dings, dents, or bugs, a factory-new engine pulling book rated power, and a factory-new prop with no dings.
  • It is based on correct Vy for the altitude, and Vy decreases as altitude goes up -- see the climb tables in your POH.
  • It is based on stable air with no up/downdrafts.
Thus, it is typical to see your other-than-factory-new plane on a real day run out of climb rate before it gets to book service ceiling with a full load. OTOH, it is often possible to get above book service ceiling if you're light. Whether or not it's wise to plan on flying that high depends a lot on weather and terrain. With only 100 ft/min climb rate available, it doesn't take much of a mountain wave or downdraft to push you downward despite your best efforts to maintain altitude, and if that would carry you down into rocks, airspace, or icing conditions, you would be unwise to plan to fly at that altitude.

As for time/fuel, the POH should have the data necessary to perform those computations, although in an older "owner's manual," you may have to do a lot of combining of data from different tables/charts and possibly some extrapolation.
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JOOC, where are the "standards" for establishing a service ceiling for an airplane. I couldn't find anything in part 23 about it (doesn't seem to be a requirement). I'm still wondering if there really is an FAA approved specification and your list left out the altitude of the airport of departure (sea level?). And given that the "standard atmosphere" doesn't really exist I'd think there'd have to be some means of correcting for deviations from that.
 
Ron Levy said:
A few things about book service ceiling which bear consideration:
  • It is based on a "standard day" where indicated altitude and density altitude are the same.
  • It is based of use of all recommended leaning techniques for best power.
Click to expand...

Right on. Additionally, remember that lapse rates for temperature vs alt are rarely standard. Because you are Standard on the runway does not mean you will be Standard after climbing several thousand feet. If the air is warmer than standard, you are increasing the delta between actual and density altitude as you climb.

Also, if you are from lowlands and don't have lots of experience manipulating the mixture, it's easy to mismanage it and reduce power significantly. Just a few days ago we were having another conversation with another owner who couldn't get to altitude and it turned out he wasn't leaning hard enough.
 
gismo said:
JOOC, where are the "standards" for establishing a service ceiling for an airplane. I couldn't find anything in part 23 about it (doesn't seem to be a requirement).
Click to expand...
Start with 23.69:
Sec. 23.69

[Enroute climb/descent.]

[(a) All engines operating. The steady gradient and rate of climb must be determined at each weight, altitude, and ambient temperature within the operational limits established by the applicant with--
(1) Not more than maximum continuous power on each engine;
(2) The landing gear retracted;
(3) The wing flaps retracted; and
(4) A climb speed not less than 1.3
0.420!OpenElement&FieldElemFormat=gif
.
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...and then go to 23.1587:
Sec. 23.1587

Performance information.

[Unless otherwise prescribed, performance information must be provided over the altitude and temperature ranges required by Sec. 23.45(b).
(a) For all airplanes, the following information must be furnished--
(2) The steady rate and gradient of climb with all engines operating, determined under Sec. 23.69(a);
Click to expand...
...and finally to the FAA definition of "service ceiling":
Service ceiling- The maximum density altitude where the best rate-of-climb airspeed will produce a 100-feet-per-minute climb at maximum weight while in a clean configuration with maximum continuous power.
Click to expand...

...and your list left out the altitude of the airport of departure (sea level?).
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That's not part of the issue.
And given that the "standard atmosphere" doesn't really exist I'd think there'd have to be some means of correcting for deviations from that.
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As noted, the climb performance data in the POH must allow for variations in temperature. Obviously, actual service ceiling in any situation will vary with weight and atmostpheric variations, and the data provided lets you compute actual performance on those bases. However, the book "service ceiling" in the aircraft specs section is based on max gross and STP.
 
Ron Levy said:
Start with 23.69:
...and then go to 23.1587:
...and finally to the FAA definition of "service ceiling":


That's not part of the issue.
As noted, the climb performance data in the POH must allow for variations in temperature. Obviously, actual service ceiling in any situation will vary with weight and atmostpheric variations, and the data provided lets you compute actual performance on those bases. However, the book "service ceiling" in the aircraft specs section is based on max gross and STP.
Click to expand...

Max gross at takeoff or at the SC? If the latter I wonder if they depart above MGW for that test or just alter the results for the reduction in GW due to fuel used in the climb.
 
Ron Levy said:
A few things about book service ceiling which bear consideration:
  • It is based on a "standard day" where indicated altitude and density altitude are the same.
Click to expand...
Those conditions would never exist therefore they correct for them. Also - the ISA Standard Atmosphere *will* not have density and indicated as the same altitude anywhere but at sea-level.

ISA Standard Atmosphere for 12,000 ft is:
OAT: -8.8C
Pressure: 19.4"

Calculated by my still-in-development-not-completely-tested e6b application.

A standard day *will not* give you a service ceiling where density altitude equals indicated altitude.
 
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gismo said:
Max gross at takeoff or at the SC?
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As it reads, max gross, not max gross less fuel used to get there. Either they're trying to account for inflight refueling, or they want to make sure nobody's counting on the same climb rate at 10,000 feet after taking off from Leadville versus climbing all the way up there from a sea level airport.

If the latter I wonder if they depart above MGW for that test or just alter the results for the reduction in GW due to fuel used in the climb.
Click to expand...
I'll bet there's an AC which covers this.
 
gismo said:
Max gross at takeoff or at the SC? If the latter I wonder if they depart above MGW for that test or just alter the results for the reduction in GW due to fuel used in the climb.
Click to expand...

And keep in mind, the plane they were probably using to determine the SC probably wasn't yet certified. It was a test AC so it may not have been constrained by a MGW (reality and physics would still apply though, so there is a practical MGW, but not a certified one).
 
jesse said:
Those conditions would never exist therefore they correct for them. Also - the ISA Standard Atmosphere *will* not have density and indicated as the same altitude anywhere but at sea-level.

ISA Standard Atmosphere for 12,000 ft is:
OAT: -8.8C
Pressure: 19.4"

Calculated by my still-in-development-not-completely-tested e6b application.

A standard day *will not* give you a service ceiling where density altitude equals indicated altitude.
Click to expand...
It should since the definition of DA is calibrated altitude corrected for deviation from standard conditions of temperature and pressure. Also FWIW, on the DA calculator apps I could find I get 12092 for -8C and 11973 for -9C (it only allows interger temps).
 
gismo said:
It should since the definition of DA is calibrated altitude corrected for deviation from standard conditions of temperature and pressure. Also FWIW, on the DA calculator apps I could find I get 12092 for -8C and 11973 for -9C (it only allows interger temps).
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Mine calculates:
12092 ft: -8.96C 18.96"
11,973 ft: -8.72C 19.05"

I had a bug where I was using the abs() function on a double which was screwing a few things up. fabs() must be used on double's in C.

..And actually you're right. Now that I thought about it some and verified the calculations on a 1976 atmosphere model, density altitude *would* match indicated altitude throughout the altitudes.

But my point remains. Service Ceiling is density altitude which people are very often not aware of.

(The "lance rule" is true again. When lance disagrees with me, I'm generally wrong :) )
 
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JohnSBA said:
If you're talking about in the 172M yes, and above 8k'IA or so I was at Vx.
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Remember, though, that Vx increases with altitude, so if you were at 8,000 and at published Vx you were below the actual Vx for that altitude and creating a lot of unnecessary drag which would have hampered your climb rate even further.

If you want to make your service ceiling, you'll have to carefully control your airspeed - Use a cruise climb down lower to keep your engine cool as long as possible, and when you must, slow to Vy for the particular altitude you're flying at. And be patient - That last 1000 feet is going to take a long time.

I experimented with different AOA for best indicated VSI.
Click to expand...

But did you hold each AoA long enough for airspeed and vertical speed to stabilize? Remember that the VSI has a significant lag.

I don't recall anything that would produce an actual averaged 100FPM climb above 9.5k'IA and this was in winter at well below gross even before counting the fuel burned to get there.
Click to expand...

While sea-level performance in the winter is awesome, at higher altitudes it falls off faster than during the summer. The cold air is more dense, yes, but that also means that the atmosphere isn't as thick as it is in the summer. I'm not sure where this changeover happens normally, though.
 
Lots of good points above, but I think the best way to make the service ceiling is to have engine instrumentation and do a constant-EGT climb. I have some notes on this here.

Then the ceiling - for the current conditions - will be reached naturally.

Here in the UK leaning is rarely taught in the PPL, and many pilots say stuff like "my C172 cannot get above 9k" etc. This really poor performance is usually due to bad or nonexistent leaning.

My TB20 reaches 20k in ISA conditions but in say ISA+15 it makes only 17k.

Incidentally, what ceiling would you expect for a PA28-181? Fixed pitch prop.
 
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