Since the beginning of powered flight, aviators have worked to find ways to deal with the effects of higher altitude flying. The rarified air up there creates challenges for airfoils, engines, and people. The results of these efforts are applied to almost every category of flying machine, including the piston engine single.
Mooney was the first to make a serious attempt at a production pressurized piston single in the late 1960s with the M22 Mustang. There are only about eight of the original thirty-four plus units still in operation and my sister, Janice, happens to own one of them. It looks a bit like a “regular” M20 series but on steroids. The cabin is taller, wider and longer than the M20. It’s powered by a unique Lycoming TIO-541 310 horsepower engine that is only found on the M22. The wing is nearly identical to all other Mooneys, and they still put the tail on backwards. The Mustang wasn’t a bad first effort at a pressurized single but maybe just a little ahead of its time.
The P210 wasn’t the first production pressurized single engine aircraft, but it was definitely the first successful one. Cessna began with a 1978 model labeled a P210N. In truth, it was a P210M, as it shared all the systems of the nonpressurized M models, including the landing gear system. Anytime you see a P210 with main gear doors or evidence of the Uvalde door mod, you’ll know it can only be a 1978 model P210. In 1979 the P210N had all the upgrades afforded the other nonpressurized N models, including the much simplified gear system sans main gear doors.
The concept behind pressurization sounds simple enough- just pump a lot of air into a capsule and attempt to duplicate sea level pressures for maximum creature comfort. The devil is in the details of course. The air we inhale is made up of 78% nitrogen, 21% oxygen and 1% other stuff like Argon and Carbon Dioxide (rounded numbers). The problem is that our lungs use oxygen and we exhale approximately 78% nitrogen, 16% oxygen, 4% carbon dioxide and 2% other stuff. Our capsule can continue to pack in the ambient air, but the increase in carbon dioxide eventually puts everyone to sleep. The solution is to have control leakage so fresh air is continually replacing the exhaled air that flows out of the capsule. Of course, that means the compressed air source needs to be much larger to keep up with the leaks.
Very large planes have the option of an electrically driven air pump to provide the volume and pressure needed. Many jets and turboprops use bleed-air from the engines as a readily available source of compressed air. Our small single engine planes can’t afford the weight or complexity of an electrically driven air pump, so we also must look to the existing engine.
Anyone designing a piston engined plane with a pressurized cabin will most certainly also employee a turbocharged or turbo-normalized engine. These happen to have a very high volume air compressor (the turbo charger) which can be tapped for the needed cabin air supply. This usually takes the form of a venturi attached to the engine’s induction system, creating a “calibrated” leak from the engine’s intake manifold to the airplane’s cabin. There is a loss of available air to the engine, but this is accounted for by designer’s choice of turbo size. This also explains why reducing the energy (exhaust pressure) supplied to the turbo drops air into the cabin, meaning pulling the throttle too far back for descents make the cabin altitude climb.
If weight wasn’t an issue, the flying capsule could be constructed in such a manner as to maintain a sea level pressure regardless of the altitude flown. Since it is an issue, some compromise must be made between available structure and available air pressure. The P210 cabin differential pressure limit of 3.35 p.s.i. may not sound like much, but that translates into 482.4 pounds of pressure on each square foot of cabin skin surface. Modern air transport category aircraft structures are designed to operate in the 7.5 to 9.4 p.s.i. differential range. A P210 built to withstand over 1,350 pounds of pressure on each square foot of cabin structure would probably need another few hundred pounds of structure and a dedicated extra engine to run a cabin air pump.
I bring all this up because we were finally able to acquire a proper pressurization cart for P210 pressurization system troubleshooting. Previously, our troubleshooting efforts required flying the plane and operating the system in situation. No one ever wanted to sit in the baggage area to watch the outflow valves operate, go figure. We found a couple of high power leaf blowers could get the cabin up to 1.0 p.s.i. Not great but good enough for some basic testing. The addition of a true to life pressurization cart with the needed air volume capacity allows the ability to check for leaks and system operation all the way to the system red line 3.35 p.s.i. differential.
The most common problems with the P210 pressurization are related to air supply and unwanted leaks. Air supply being from the turbocharger suggests that the exhaust system and the induction system must be in top form for best performance. Leaks in the exhaust system reduce the energy that turns the compressor. Leaks in the manifold reduce the available air to the engine and cabin. The best measure of the air delivery side of the system is a part-throttle critical altitude test.
I’ve written about this test in prior articles about turbochargers, and it’s well documented in the aircraft service manual, so I won’t go into much detail. In short, the engine is set to a particular power setting for climb to altitude. No adjustment of the throttle should be needed to maintain the set manifold pressure until the part-throttle critical altitude is reached. From that point on, the manifold pressure will drop about one inch for every additional 1,000 foot increase in altitude. The altitude at which the manifold pressure begins to drop is the engine’s critical altitude.
Most of the T210 and P210 series are designed with an expected part-throttle critical altitude of about 18,000 feet. However, beyond certain serial number planes and those with a particular service bulletin installed, the critical altitude may be only 12,000 feet. Don’t assume yours is the higher. Check the service manual to be sure. Otherwise you may spend many wasted hours troubleshooting a problem that doesn’t exist. Please don’t ask how I know this. Most important, if your engine performs to the published part-throttle critical altitude, then you can be fairly certain it’s delivering sufficient air volume to the pressurization system.
The pressurization system begins at the venturi where air is tapped from the engine induction manifold. That’s where the search for air leaks must begin. A flexible hose connects from the venturi to deliver air to a heat exchanger. This hose lives in the very inhospitable environment of the engine and is very susceptible to abrasion and tears. It’s easy to inspect and not terribly expensive.
As air is compressed, it gets warmer, so a heat exchanger (radiator) is in the air delivery to cool it some before sending it to the cabin. The cooling air across the heat exchanger can be controlled to allow the warmer compressed air in the cabin for winter operations. We don’t see many problems with the heat exchanger, but its exposure to ambient elements suggest external inspections for corrosion would be the norm. From there a short hose connects the air to the cabin at the left side of the firewall.
At the air entrance to the cabin is a manually operated dump valve. If for any reason the pilot should want to stop pressured airflow into the cabin, he/she can pull a small “T” handle at the lower left of the instrument panel. This opens a valve at the firewall which “dumps” all the delivered air overboard into the engine compartment instead of in the cabin. If pulled at high cabin differential pressure, the cabin altitude will rise at an alarming rate. Be ready for ear and sinus reactions. About the time everyone recovers from the initial shock of pressure loss, it’s time to don the oxygen masks and head the plane for lower altitudes. The dump valve presents few problems but should be exercised frequently, preferably on the ground.
Now we have air to the cabin where opportunities for unwanted leaks are many. Soft targets are the most probable. By “soft” targets I mean components that are part of the pressure vessel which are fabricated with soft materials. The usual suspects are cabin door seal, emergency door seal, the accordion-like boots at the nose gear steering, and the main gear downlocks. The aft cabin floor is part of the pressure vessel, so all the access panels have seals that must do their job. Lastly, are the belly drains which allow water to drain when not pressurized but seal off as soon as cabin pressure is applied. These should last a long time but are often attacked by unknowing mechanics armed with ice picks punching away at “clogged up” drain holes.
All the flight control cables must eventually exit the pressure vessel through Chevron like seals. As the pressure increases, these seals press harder against the cables. This will create some added resistance to control movement, but higher cabin pressure usually occurs in more cruise-like configurations where control input is small. After thirty-two plus years of wear, these seals are certainly losing their ability to do their job. There are five different versions of these seals, but only two are still readily available from Textron- just another casualty of a low volume production airplane that hasn’t been built since 1986.
The fresh air inlets from the wings have one-way check valves to allow ambient air in until the cabin is pressurized. As the cabin air pressure exceeds the outside pressure, the valves close off. After many years of being ignored, these valve collect all sorts of debris and “goo” that may inhibit proper operation. These valves are buried in the overhead area and “permanently” bonded in place. Of all the components in the pressurization system, these check valves are the most difficult to access.
Last but not least are the two outflow valves mounted on the aft pressure bulkhead. They are basically the same, except one is controlled with an electric solenoid and the other with pneumatics from the altitude controller. These valves have been out of production and support for a very long time. At least one company in Florida attempts to keep the community supplied with working units. The lack of new parts means they are limited to repairs, no overhauls, and mostly with serviceable parts. These two components are the ones that concern me the most when considering the continuing viability of the pressurization system on the P210.
I’ve flown and worked on almost every mainstream piston single engine airplane produced in the United States since I was in grade school. By far, I think the P210 is my overall favorite. The cruise speeds, payload, take off and landing performance all add up to a very respectable machine. It compares favorably even when pitted against current production planes. The ability to fly long distances with friends at high altitudes and arrive relatively fresh due to the pressurization system is truly wonderful.
As with any out of production airplane, parts and technical expertise are rapidly becoming an issue with the P210. The pressurization system is actually very reliable and rarely gives trouble. But when it does, the cost can be notable. While Textron continues to increase parts’ prices at an alarming rate, there does appear to be significant interest by third party parts producers. It’s happening slowly as our very small market doesn’t offer the huge volumes and profits that others might.
I’m not one of those aftermarket parts producers. However, I am an advocate for anyone willing and able to make the effort. Just in the last two or three years, I’ve become aware of new comers in the Cessna aftermarket parts suppliers. I haven’t seen much for the pressurization system parts supply yet but will continue to be ever vigilant and will report.
Copyright © Paul New 2018. All rights reserved.
Pressurization leaks: P210N s/n 247. Can’t maintain 3.35 is it common for the A/C condenser drain to be exhausting large quanties of cabin air?
PS: The new AD spar AD does not apply to the “P” models, but the SEL does? Kinda of odd
The condenser drain should not exit any cabin air. It includes a floating ball valve to allow water to drain but no air to exit.
It is common for FAA AD to not mimic manufacture bulletins. In this case the FAA wanted to separate the earlier aircraft with no factory corrosion protection from the later models that did have standard application of epoxy primer on the spar.