Max altitude

[172510]

I'm wondering what can legally limit max altitude.
My 172's documents says that the ceiling is 14000 and max operating altitude is 17500.
I understand that climing above 14000 might take a long time or might not be feasabile at all, and that climing above 17500 is forbidden.
Why is it forbidden to fly above 17500? What could be the limiting factor?

My question is rather for the DA42 the ceiling of which and max operating altitude are both 18000ft. What certification critierion is not met if the plane is flown above 18000ft?

[GipsyMagpie]

Could be for a whole bunch of reasons. All you got to do is respect it. Could be what they tested it to, the absolute ceiling (rate of climb at zero), could be a handling issue. Why would you want to go up that high anyway.

[Genghis the Engineer]

Interesting question.

Both are part 23 aeroplanes. Taking CS23, it has no requirement there to set a maximum operating ceiling - although there's nothing I know of that would stop a manufacturer declaring one if they see good reason.

Part 23 does refer to an absolute ceiling defined by zero rate of climb, and a service ceiling defined by 100fpm RoC.


I have a C172M manual to hand, and can't see anything about a ceiling in it at-all.

Diamond post a DA42 manual online at http://www.diamondaircraft.com/libra...ev.%207%29.pdf (warning - takes a while to download, or did me anyhow). As you rightly say, it states:-

2.11 OPERATING ALTITUDE
The maximum operating altitude is 18,000 ft (5,486 m) pressure altitude.

With no explanation. The perf data in the same manual seems to show adequate climb performance at FL180 with both engines at MCP, so it's hard to see any good reason why this should not be permitted.

The main thing I can think of, and it's wild conjecture, is that somebody has decided that as it's unpressurised then FL180 is a safe ceiling in supplementary oxygen. That, arguably is rather pointless and over-conservative if it is the case.

The other possibility I suppose is some kind of engine limitation? Again however, I can't think of anything about the IO360 that would create a particular issue? On the other hand, there are high altitude ignition system mods available for the 360 which I don't think that the Diamond has. Maybe its playing safe for that reason?

G

[darkroomsource]

Have you ever flown a small plane at it's max altitude?
I took a Cessna 150 up to it's max altitude, and it got real squirelly.
I didn't stay long, I was at full throttle, going along at a nice clip, and the stall warning horn was going on and off the whole 5 minutes I was there.
This may or may not have anything to do with it, but personally, I don't have the desire to do it in any other aeroplane to find out.

[keith williams]

The fact that the stall warning kept sounding suggests that the max altitude is at or close to the aerodynamic ceiling (where the low-speed buffet boundary and the high-speed buffet boundary converge).

[darkroomsource]

Yes Keith, that's exactly it.

Another way of saying it: "The higher you go, the closer the stall speed is to the maximum speed of the aircraft."

EDIT:
I should not have said stall speed, what I meant was that the stall angle of attack changes due to the air density or lack there-of, such that the speed required to maintain flight without stalling gets closer and closer to the maximum speed the aircraft is capable of.

[Machdiamond]

The high speed buffet boundary of a Cessna 150 is certainly something you want to approach very carefully

[keith williams]

The high speed buffet boundary of a Cessna 150 is certainly something you want to approach very carefully

If by that you mean that the aircraft doesn't have enough power to reach its high speed buffet boundary then you may well be correct.

But as altitude increases, the two boundaries will converge with maximum speed somewhere between the two. And as darkroomsource has said

The higher you go, the closer the stall speed is to the maximum speed of the aircraft.

This certainly sounds like being the case where full throttle straight and level flight results in the stall warning sounding.

[darkroomsource]

Yes, it's not that the speed of the aircraft (C150) was at the high speed buffet boundary, it could have been at 1/2 that value or even 1/3 (although my guess would put it above 1/2).

But the speed required to maintain an angle of attack that would not stall the aircraft gets faster and faster as you climb higher (the wing can't generate as much lift because the density of the air is reduced - hopefully my stating this is obvious to the ones currently in the conversation, but this is more for the OP).

So even though the speed of the aircraft is safely below the high speed buffet boundary (as we're calling it), the low speed buffet boundary (as we're calling it) gets gets faster and faster such that it reaches the maximum speed the aircraft is capable of attaining with that engine at that altitude at that temperature.

Now, since the high speed buffet is also decreasing (as far as I understand), there could also be a situation where the speed of the aircraft is faster than the high speed buffet boundary. However in something like a C150, I don't think that's very likely. I know it happens in other aircraft, because I've heard stories of Lear jets getting too high too soon (before they've lost enough weight due to fuel burn off), or of pilots trying to hand-fly a jet at high altitude in similar circumstances. Any aircraft flying faster than the high speed buffet boundary has issues, that's why it was so difficult to "break" the sound barrier. My recollection is that one test pilot discovered that the specific aircraft he was in required reversing the elevator controls at the edge of the buffet.

[JOE-FBS]

I hesitate to disagree with G but this is in CS-23 amendment 3:

CS 23.1527 Maximum operating altitude
(a) The maximum altitude up to which operation is allowed, as limited by flight, structural, powerplant, functional, or equipment characteristics, must be established.
(b) A maximum operating altitude limitation of not more than 7620 m (25 000 ft) must be established for pressurised aeroplanes, unless compliance with CS 23.775 (e) is shown.

[BOAC]

The high speed buffet boundary of a Cessna 150 is certainly something you want to approach very carefully

well, I liked it....

[Genghis the Engineer]

Originally Posted by JOE-FBS

I hesitate to disagree with G but this is in CS-23 amendment 3:

CS 23.1527 Maximum operating altitude
(a) The maximum altitude up to which operation is allowed, as limited by flight, structural, powerplant, functional, or equipment characteristics, must be established.
(b) A maximum operating altitude limitation of not more than 7620 m (25 000 ft) must be established for pressurised aeroplanes, unless compliance with CS 23.775 (e) is shown.

bu66er, how did I miss that.

I shall do some more homework!

G


Postscript: yep, you're right - really don't know how I missed that before. Not that is says much, cross-checking through the standard, all that's really said is that the aeroplane has to behave acceptably up to that ceiling. So, just a bit of commonsense really - largely I think ignored as most part 23 aeroplanes don't declare a ceiling in my experience.

[9 lives]

I think that the altitude limitation for the DA 42 is associated with the diesel engine restart ability. We flew a Avgas powered DA 42 to 19,500 during hot fuel testing - no problem.

I've had a normally aspirated, carburetted C 185 to 20,800 feet during mogas testing. I got it through 21,000, but it would not stay up there. At that altitude, full power was 12" MP, and I was flying at 72 MPH, with the stall horn screaming. The mogas worked fine though!

For pressurized aircraft, the 25,000 limitation may be associated with the passenger oxygen system flow requirements, which change at 25,000 feet. I can tell you that with the thinner air there, hand flying can be more difficult. I hand flew the Piper Cheyenne up there a few time, and it was no fun whatever!

[pineappledaz]

Good to see I'm not the only one who has tried this in a 152

[172510]

I think that the altitude limitation for the DA 42 is associated with the diesel engine restart ability.

Is it a certification requirement that an engine can be restarted at high altitude?
Why don't they say it in the AFM?

[9 lives]

Is it a certification requirement that an engine can be restarted at hight altitude?
Why don't they say it in the AFM?

The requirement is here:

FAR 23.903: ......

(d) Starting and stopping (piston engine).
(1) The design of the installation must be such that risk of fire or mechanical damage to the engine or airplane, as a result of starting the engine in any conditions in which starting is to be permitted, is reduced to a minimum. Any techniques and associated limitations for engine starting must be established and included in the Airplane Flight Manual, approved manual material, or applicable operating placards. Means must be provided for--
(i) Restarting any engine of a multiengine airplane in flight, .... my bold

There are many examples of a design requirement being complied with by flight manual limitation, without the underlying reason for that limitation being explained in the flight manual. For this reason, unquestioning compliance with flight manual limitations is wise, as there may be an angle you've failed to consider, as to why exceeding a limitation is unwise.

That all said, it is speculation on my part about the starting limitations of the diesels in the DA-42, All my engine testing experience in the DA-42 was the Lycomings. My diesel experience in the DA-42, was to take it so the mechanic could buy some Mercedes engine parts to get it running properly again!

[172510]

(d) Starting and stopping (piston engine).
(1) The design of the installation must be such that risk of fire or mechanical damage to the engine or airplane, as a result of starting the engine in any conditions in which starting is to be permitted, is reduced to a minimum. Any techniques and associated limitations for engine starting must be established and included in the Airplane Flight Manual, approved manual material, or applicable operating placards. Means must be provided for--
(i) Restarting any engine of a multiengine airplane in flight
My understanding is that the maximum altitude at which an engine can be started may be below the maximum operating altitude, provided it's documented in the AFM.

[9 lives]

My understanding is that the maximum altitude at which an engine can be started may be below the maximum operating altitude, provided it's documented in the AFM.

That sounds reasonable, and I've flown turbine twins like that.

However, I would not approve a single engined aircraft to operate at an altitude higher than I could prove engine restart ability - what reason would there be to do that? I see no reason to cool the engine of a single engined aircraft, during a glide down to the maximum starting altitude, to then really have to work at getting it running again. Just tell the pilot not to fly that high, it's simpler.

As I mentioned, the manufacturer does not always state in the flight manual why a limitation is placed on an aircraft.

[172510]

The DA42 is a twin, the diesel engines are liquid cooled.
The plane can sustain 18000ft on a single engine.
I think that the issue of temperature loss before attempting to restart is less critical than it would be with air cooled engines

[9 lives]

It was explained to me by the certification authority that diesel engines have had difficulty demonstrating cold restart due to the glow ignition system being inadequate. I do no have first hand experience with this, as my DA-42 diesel flying was not for the engine testing, and my DA-42 Lycoming 360 flying had no problems with cold restart, I did dozens.

We did briefly fly the Lycoming powered DA-42 at 19000 feet on one engine (as the other one had quit due to fuel exhaustion of the hot fuel being tested). But, we did not actually try to maintain altitude up there, as we were headed down anyway. A DA-42 can maintain single engined flight at 18000 feet?

That said, I would expect that manufacturers, who are not marketing their aircraft for high altitude use, would rather just limit them by altitude, rather that going to the additional effort to demonstrate compliance way up there. I often negotiate with clients what they want to show compliance to, so as to not expend extra test effort for no perceived gain.