what_next wrote:
Passengers don’t like unpressurised aircraft (me neither).
I see that as a very important point. I don’t plan to go to grass runways and I have the feeling that pressurized cabin beats shorter runway requirements. An open question is how well the pressurized cabin works after 30+ years and how much more expensive the maintenance gets.
Vladimir wrote:
An open question is how well the pressurized cabin works after 30+ years and how much more expensive the maintenance gets.
That’s kind of a circular reference: It the maintenance over the last 30+ years was good, the pressurisation system will require no extra (expensive) maintenance.
There is not too much that can go wrong. The pressurisation controller is a pneumatic-mechanical device that was installed in every pressurised Cessna from the 1960ies until the Citation V. It is a very proven component with which I never had the least bit of trouble (other than with the later electronic controller). Then there is the door seal which, AFAIK, has to be replaced in regular intervals. An extra cost factor of course. The outflow valve is the third component which can fail, but again, unlike the newer electronic ones, it is very reliable. Unless the cabin is always full of heavy smokers.
Like many other system of these older aircraft, the pressurisation works best when it is excercised regularly. Letting the airplane rot in the corner of a hanger and pulling it out for a short flight every eight weeks will result in a very “bumpy” pressurisation. But usually after a few cycles everything returns to normal on it’s own.
@what_next: Thanks for all replies, very helpful! If you had to choose, would you go for a C340 or C303?
Vladimir wrote:
If you had to choose, would you go for a C340 or C303?
Unless I have to fly out of 600m grass strips (which I don’t) always the 340.
what_next wrote:
That’s kind of a circular reference: It the maintenance over the last 30+ years was good, the pressurisation system will require no extra (expensive) maintenance.
Doesn’t the fact that the aircraft is pressurised increase maintenance cost on other parts of the aircraft, compared to a non-pressurised aircraft? The cabin has to be kept airtight and is subject to additional fatigue from expansion and contraction.
Airborne_Again wrote:
The cabin has to be kept airtight and is subject to additional fatigue from expansion and contraction.
The only critical area regarding airtightness is the door seal. And this is taken care of by maintenance schemes. For the rest, almost every aircraft is mostly airtight, even the non-pressurised ones (fabric covererd apart). Otherwise you couldn’t leave it outside in the rain. And there is enough supply of pressurisation air for the small leaks around control cables and other cables and lines that go into the pressurised hull and which are difficult to seal completely.
Regarding fatigue: That’s an issue for airliners where quite a few get 50,000 to 100,000 pressurisation cycles on their structure during their service life. But if you look in the adverts for pressurised piston twins, especially in Europe, you will find that most have less than 5,000 flying hours. At 2 hours per average flight, this means they only have 2,500 cycles on their hull which is next to nothing.
This is a useful article on the 340.
I would not regard the 340 as a weather topping pressurised aircraft. The 340A has a cabin altitude of 7,500 at FL200 when new, thirty or forty years on my understanding is that to get the same cabin altitude you are probably at FL160-180. The 340 is even lower. Chasing down leaks is not at the top of the maintenance budget so older airframes are unlikely to achieve design psi.
I would still posit that your average T303 is in better shape than your average 340.
JPT have a 340A for sale which gives you some idea of values if avionics and interior are not kept up to standard.
As said above, the huge difference is pressurization. If you really want it (frequent hard IFR flights, with non-oxygen compatible passengers) the C340 is your aircraft, because that one critical feature outweighs the drawbacks (age, complexity, etc.).
If you only occasionally need it and/or are fine with oxygen, the 303 is better. A pressurized Crusader would be a complete slam-dunk decision, but that does not exist.
C.
RobertL18C wrote:
Chasing down leaks is not at the top of the maintenance budget so older airframes are unlikely to achieve design psi.
I am pretty sure that if you look at a decent aeroplane like this one here ( http://www.aircraft24.com/multiprop/cessna/340a—xi122090.htm ) pressurisation issues will not be a factor at all. It will easily achieve book figures. And the avionics is good to go for quite some time.
Once you go pressurized it’s impossible to go back! 
Had zero pressurization problems on my Aerostar. Very simple and reliable system. Sure, it requires the turbos to be working well, and they last about 1000hrs between overhauls, but other than that the controller, outflow valve and seals are pretty simple and relatively cheap to maintain. On the Aerostars the air pumps (vacuum pumps if you wish) inflated the door seal. If it had a pinhole leak, they had to keep inflating them constantly, which could make your pumps last shorter. But if you have a good seal, the load on the pumps is very low. There was also an upgrade available for an electric door seal pump that took the load of the air pumps completely, and many Aerostars had this upgrade.
