You’re right JasonC, got my wires crossed.
So the conclusion seems to be that on a pressurised twin such as the 340, the “practical” ceiling is more a function of maintaining reasonable climb rate, than the pressurisation capabilities.
Presumably the climb rate is both performance and cooling limited. Even if you had the higher power RAM engines, for example, you might be power setting limited by engine temps.
What do the PA46 (piston) pilots on here consider to be a practical ceiling, in terms of reasonable time to top of climb? I can’t see that a 340 would do worse than this, unless very heavy.
ortac I agree completely, I wonder if this was a design constraint to start with? The cruising altitude with a cabin altitude at 7,500 or 8,000, take your pick and arguably a marketing number, seems to coincide roughly with a 500 fpm climb.
On maintenance the pressurised hull, not necessarily system components, adds complexity. Recall on older airframes wiring, fuel and control lines are nicely aged into Japanese ultra premium whiskey terms! Any remedial work is much more due to routing stuff through the hull. A good test is the eau d’avgas aftershave you are experiencing in the cockpit.
I have come across the 8k feet cabin altitude before as the maximum allowable in airliners. If it is real, could it be derived from the max allowable diff p at max altitude of FL410? I suppose such subjects would be covered by the HPA or ATPL theory 
I have come across the 8k feet cabin altitude before as the maximum allowable in airliners.
That’s a part 25 certification requirement (no. 841 if you want google to find it more quickly). This kind of stuff is not part of the normal ATPL syllabus. A C340 is certified under part 23 so it does not apply.
I think the 8000ft cabin altitude also comes from some other stuff, to do with medical limits on the carriage of passengers in “less than perfect health”.
For example the portable oxygen units in widespread use (visit your local hospital outpatient waiting room, anytime) – this is one of many vendors – tend to be rated to 8000ft only.
We did this in several threads in the distant past – example. Most of those units are accordingly useless for our aviation use.
Indeed. 10,000ft is probably marginal for smokers and other unhealthy people out there, of which the numbers are increasing unfortunately. But I wouldn’t hesitate to climb to 10k cabin altitude if I needed to as do that unpressurised already.
The 340 maintains a 8,000ft cabin to 20,100ft and 4.2psi thereafter so 23,500ft ceiling for 10,000ft cabin.
https://www.twincessna.org/twin-cessna320.htm
The Skyknight was a top dog in its day with 220 KTAS – I still remember the marketing brochure with owner fondling a Purdie, black lab looking up loyally, wife/girlfriend in a cocktail dress in the background…marketing straight out of the Hefner world.
Great looking alpha twin and one keeps being advertised in Pilot.
ortac wrote:
What do the PA46 (piston) pilots on here consider to be a practical ceiling, in terms of reasonable time to top of climb? I can’t see that a 340 would do worse than this, unless very heavy.
For the piston I flew mine at 240/250 all the time but many people tend to only fly them in the high teens low 20s. Time to climb was around 30 mins for 250 on a normal day.
I fly my PA46 piston at MTOW almost all the time I do a serious trip and it climbs less well than Jason’s did, takes me about 38min to get to FL250. I prefer to stay around FL180-FL200 usually, I feel like I’m being kind on my turbos by doing that, but if weather or ATC favours a higher (or lower) route I’ll have no hesitation.
