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Dear rhurt,
I happen to have in my possession an old “Air Facts” magazine from September 1972 (equivalent to today’s “Flying Magazine”) that has a review of the 1973 T337G that was written by Leighton Collins (father of Richard Collins). In this extensive review, Leighton answers your question specifically. I will quote it verbatim. Before I do however, a humble opinion for your thought. Anytime an airplane is operated outside it’s certified parameters, the insurance company would most likely cry foul and the FAA would certainly cry foul. Ya all be careful out there. Nuff said!
Here is the answer to your question according to Leighton Collins for the 1973 T337G:
Why 20,000 feet?
“This 20,000 foot operational limitation is quite interesting. Limitation is not a good word because this suggests that maybe there is some weakness in the pressure vessel which determines the 20,000 foot figure. That isn’t the case at all. If you went above 20,000 feet the cabin would not overpressurize because the regular and back up valves would take care of that just like they do at the lower altitudes, maintaining and guarding the 3.35 psi differential. But the cabin altitude would go up and at an indicated altitude of about 12,000 feet cabin altitude a red light would come on indicating the need for supplemental oxygen. So the 20,000 foot limitation, as far as the pressurization system is concerned safety-wise really had nothing to do with pressurization structure. But when a manufacturer puts on a placard take him at his word, because if you don’t something is going to happen that you don’t like. In this case it is probably that the engines would overheat.
They settled on the 20,000 foot figure at Cessna for two reasons. The first is economic and the other will surprise you.
Turbocharged piston engines will usually deliver 75% power up to around 25,000 feet and in most of the pressurized piston engine aircraft up to now that was settled on as the limiting altitude mainly because it meant maximum cruise speed and above that altitude would start slowing down. But Cessna was shooting for pressurization at a price. They could have selected 25,000 for the 337G like they have for their larger pressurized airplanes, but to get a 10,000 foot cabin at that altitude would have meant a thicker skin cabin and heavier windshield and windows and more weight. Additionally engine cooling would have raised the weight and cost because even though it is sometimes unbelievably cold thin air just doesn’t carry away the heat from the cylinder head fins. All in all, if the 337G’s operational altitude were 25,000 feet, which it could have been as well as not, the price might easily have been another $25,000.
A Policy Decision
The other consideration in settling on the 20,000 feet was a very laudable one and rather unique. At Cessna they reasoned that this airplane would introduce high altitude operations to an entirely new group of pilots, one that might not be as aware of the operational facts of life at 25,000 feet as they might need to be. For instance, at 25,000 feet if cabin pressure is lost there is barely time to get an oxygen mask. Additionally, getting a clean airplane from 25,000 to 15,000 quickly requires some doing. As they figure it, anyone inadvertently depressurizing the cabin at 20,000 can make it to 15,000 in time. Or at least have gotten on the masks by then. They also gave a thought to when the airplane would get old and the door seals might not be maintained properly. In short, there’s got to be something between the bottom and the top rung of a ladder. We think the decision was a wise one.”
The End
Hope this helps. The article is 21 pages. This was one portion of it that relates directly to your question.
Ed Asmus
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