High Altitude Flame Out

[172510]

I fly a Cessna Citation.
41000ft is the maximum allowed altitude according to the limitation chapter of the AFM. The AFM says that altitudes are pressure altitudes unless otherwise stated, so 41000ft means FL410.

Although there is nothing more about that in the operation manual of the company, some experienced captains say that the limit is a density altitude. So when the forecast says that at FL410 the density altitude will be above 41000ft, they don't plan for FL410. Another limitation is ambient temperature (ISA+35 at seal level decreasing to ISA+23 above FL360), but even when far from the limit, they don't plan a level above 41000ft density altitude. They say that engines could flame out. They explain why, and of course I understand that there is an ambient density threshold below which the fuel won't burn, but I suppose that within the AFM limitations the ambient pressure remains far from that threshold.

I pointed out that neither the AFM nor the operation manual mention it, but they know better, and I dare not insist before having other opinions about that.

[rudestuff]

Experience just means you've done something, it doesn't necessarily mean you're any good at it. You'll find a lot of people put their own safety margins on top of the ones that are already there. Lots of people do things solely because someone they respected did them. 57% of all SOPs are inherited from a previous airline/aircraft/era.

[B2N2]

I used to fly a Citation with a ceiling of FL450.
Also flew with Captains that would not fly above FL380, even if it meant a fuel stop instead of non stop. Very experienced also with 20+ years in the industry.
Reasons logical to them not technical.
As long as their techniques and methods are towards the safer side then it’s not worth the arguments. They want to do FL360 with a fuel stop? If the chief pilot/ DO/ GM/owner doesn’t make a fuss then why should you.
Pick your battles for the days your coworker wants to go to FL430 ‘because the airplane can’.

[172510]

I would only insist if I'd a reason for it. For instance if we have only little extra on a long flight.

[john_tullamarine]

Looking at my copy of a 560 AFM, there is no reference to DH anywhere, but lots to PH. Certification work is based on standard conditions and then applies deltas for off standard. At standard conditions, PH and DH are the same. It would make little/no sense to presume that height references are other than to PH UNLESS the AFM specifically noted the difference - I can't recall ever having seen such a situation in respect of limitations.

Having said that, there would be no operational constraint against a crew's deciding to fly lower if the OAT is above standard - that's in the realm of operational decision making. At all times, there is only one really hard and fast rule - "don't crash" - all the rest of the rules is legalese window dressing which one is well-advised to observe.

[giggitygiggity]

Airbus guy here...

Do those aircraft not have a way to calculate a reccomended max altitude for the GW based on current conditions, eg SAT and ALT? Obviously caution would be exercised as you cross jetstreams into warmer air etc, but there isn't any specific guidance here other than to not fly above the REC MAX altitude produced by the FMS.

In reality, most of us probably wouldn't bother climbing there unless there was perhaps a 1000ft margin to the REC MAX (the true service ceiling is higher than that but the A320 FMS doesn't identify it for us).

Please appreciate that Im unfamiliar with Biz Jets, but I find it surprising that the FMS doesn't give some sensible guidance on the subject as it's simply maths and a lookup table - instead relying on some old hat knowledge that rudestuff suggested are inherited from a previous airline/aircraft/era. My airline has only ever flown the 737 and the A320 so the operational baggage in that regard is somewhat limited.

[B2N2]

that's in the realm of operational decision making

Agreed.
Any significant deviation from SAT up or down is worthy of extra caution.

[wondering]

I pointed out that neither the AFM nor the operation manual mention it, but they know better, and I dare not insist before having other opinions about that.

They know it better than the manufacturer, engineers, test pilots, certifying authority? Really?

If you want to be more restrictive than the AFM/POM that´s fine. Just don´t come up with pseudo arguments.

Does your company use a professional flight planning software/service?

[B2N2]

In my case the argument wasn’t fear of engine flame out but Time of Useless Consciousness.
A loss of cabin pressure below FL380 would give a higher chance of successful outcome then above FL410.

[Meikleour]

B2N2: That's a bit of a Freudian slip there- Time of Useless Consciousness - is that what many captains think that their copilots have as opposed to Tme of Useful Consciousness which most of us would prefer to think we have !!!!!

[Stagformation]

In my time I flew with a Skipper who never flew above F390. All sorts of justification given for the fuel plan, but the real reason was fear of exposure to cosmic radiation.

[etudiant]

In my time I flew with a Skipper who never flew above F390. All sorts of justification given for the fuel plan, but the real reason was fear of exposure to cosmic radiation.

Think that is a real issue for air crews, but not confident that staying below 40,000 feet helps a lot.
It is a topic that deserves serious medical study, but I've not seen anything large scale, although it should be easy based on flight logs.
Both the unions as well as the regulatory agencies should have ample data for a preliminary evaluation.