Under Pressure: Lessons in Piston Cabin Pressurization

Under Pressure: Lessons in Piston Cabin Pressurization

Under Pressure: Lessons in Piston Cabin Pressurization

The COVID-19 pandemic has had a paradoxical effect on aviation: flying hours are way down while maintenance shops are way busier. I think this is largely due to the reduction in flight hours producing downtime that owners then use to have their airplanes worked on. This has certainly been the case with my own Cessna 340. 

Earlier this year, when all the teleconferencing started to replace face-to-face meetings, our customers stopped using the airplane and flying hours dropped. So, I decided it would be an opportune time to fix all the various squawks my airplane accumulated over the past year – one of the biggest being a failure to maintain rated cabin pressure at altitude. Other pilots who fly the airplane dealt with this problem by simply staying below 15,000 feet (providing a 10,000-foot cabin). But with the higher flight level requirements of the approaching winter weather, this became one of the first issues that needed to be fixed. It turns out there has been no easy, quick fix.

Cabin pressurization in piston-powered airplanes is a completely different animal than it is in turbines. The biggest difference is the very limited supply of high-pressure air. Turbine engines have large fans that are huge air pumps, with excess pressurized air available. By comparison, available air in piston engines comes from the engine’s turbochargers. They have very small fans driven by exhaust gas, which drives another surprisingly little fan that pressurizes the air. Any change in the amount of exhaust gas, such as a power reduction, will lessen the amount of pressurized air available from the turbochargers. 

As altitude increases and the air becomes less dense, the work required by the turbochargers to deliver the same number of oxygen molecules also increases, leaving less available for the folks in the cabin. Generally, even if the cabin is completely sealed, once the airplane’s altitude is in the mid-20 flight levels, the amount of air available from the turbochargers is less than required to keep the cabin below 12,000 feet. For this reason they are rarely flown above FL280. Then there is the issue that the air available from the turbochargers drops considerably as power is reduced. If one engine fails on a piston twin in the flight levels, the cabin will quickly go above 12,000 feet simply because of insufficient air input. 

Another problem is that most of these piston airframes are decades old, with nearly all of them exhibiting leaks that were not present when they left the factory. In addition, they were designed with a lot of attention to keeping the weight down, making their structures relatively light and more susceptible to pressurization leaks. After thousands of flight hours and many compression cycles (each requiring some slight flexing and expansion of the fuselage), all kinds of small air leaks develop that are very difficult to track down. 

To further complicate things, most of these airframes have since had various electronic and other upgrades completed, each requiring additional holes made in the pressure vessel. For example, many piston twins now have JPI engine monitoring systems. This requires a large bundle of wires to be run from the engines through a hole in the pressurized portion of the fuselage near the wing root then up to the instrument panel. The wire penetration near the wing root is usually sealed off with something like ProSeal. This sticky black material that can eventually dry out and lose its flexibility, resulting in small leaks.

After crawling around the insides of the 340 for the past couple of weeks, I have also come to realize that the design of the pressurization system in these airplanes is really a half-baked affair compared to that in turbines. Some features leave you wondering, “What were they thinking?” For example, on the supply side, the pressurized air comes hot from the turbochargers, runs through an intercooler, then into the cabin. The hoses used to connect the parts and move the air are made up of a flexible material that appears to me like a sewer drain hose in an RV, with hose clamps and all…pretty flimsy stuff. 

The square panel in the forward pressure bulkhead (visible in the wheel well) that left me wondering ”What were they thinking?” The black sealant is ProSeal.

On further poking around you discover other things that seem wacky. The forward bulkhead for the cabin pressure vessel is just in front of the instrument panel, or immediately aft of the nose baggage compartment and nosewheel well. The bulkhead itself is a flat sheet of aluminum when intuition tells you it should be curved like the end of a propane bottle. But you rationalize that maybe a flat sheet is good enough given the requirement. Then you see there is a large square, flat inspection panel with 90-degree corners attached to the outside of the bulkhead wall, rather than the inside. This means that as pressure increases, the panel is pushed away from its attachments as opposed to into them. To compensate for this design, there are small bolts and nuts every inch or so all the way around it, held together with a substantial amount of ProSeal placed there by the mechanics chasing down leaks. Square corners focus stress on small areas, which is why all the windows and doors on pressurized airplanes have rounded corners. It almost leaves you wondering whether maybe the junior aeronautical engineers were assigned to designing the forward bulkhead while the senior ones worked on more visible designs such as doors and windows. 

Soapy water was sprayed on wiring bundles as the cabin was pressurized via a vacuum cleaner. The bubbles demonstrate leaks not adequately stopped with sealant.

Design issues aside, just plain age causes another odd problem that can occur after a significant leak is fixed – and it can drive the mechanics nuts with frustration. What happens is, once they fix the cabin to pressurize up to a higher differential than it could before, inevitably, the new higher pressure differential blows out other leaks somewhere in hard to see or access areas. This requires more expensive chasing by the mechanics (most of whom know about this possibility and warn the pilot before any test flight). 

In their pursuit of chasing down leaks, one solution the mechanics use to make the leaks more obvious is to “bomb” the airplane. This involves hooking up the outlet side of a vacuum cleaner to the inbound pressure source for the cabin and setting off a special smoke bomb. With all the doors and windows closed, they look for places smoke might be leaking out. Vacuum cleaners actually don’t put out that much air, so the cabin pressure usually does not exceed much above 2.0 with this method, and often is not as revealing as was hoped. 

In my airplane the problem started last year when the cabin failed to pressurize to more than a differential of 2.8 (whereas it should be about 4.0). But knowing it was a time-consuming problem to fix, we did not worry about it as long as it didn’t need to fly very high. Besides, the mechanics said they would take care of it during the annual in June. They did indeed find a significant leak and fixed it. Once we crossed 4,500 feet during the first flight after, however, a loud sound of leaking air came from somewhere in front of the pilot’s seat, and the cabin failed to pressurize much above 3.0. This required a return to the shop, where it was discovered that the boots surrounding the gear extension system were also dried out. Apparently, all it took for them to crack was a cabin to pressurize to 3.0. The boots were fixed and another test flight was made. This time the cabin pressure reached 3.2, but again a loud sound developed, appearing to come from forward of the cockpit. This turned out to be a leak in the heating system hoses. 

While all of this work was going on, the airplane was essentially grounded except for test flights. Although I am routinely told flying piston twins without full pressurization is somewhat common in the industry (provided the altitude is kept below 12,000 feet), it still makes me feel a little ill at ease. Maybe this is because in the jets I fly (Learjets in particular), any failure of the cabin pressurization system is a grounding event. Of course, in pressurized piston twins, the airplane not only climbs much slower but also rarely gets to an altitude where a sudden depressurization event would be immediately catastrophic. For example, at FL220, an altitude common for pressurized piston twins, the useful period of consciousness should the pressurization system fail is about 10 minutes. Relatively speaking, there is a lot of time to do something about it. Whereas in a Lear at FL450, you lose consciousness in 10 to 15 seconds if the pressurization goes out. Maybe it is just conservatism that comes with increasing age, but I think despite this knowledge, my own routine use of the 340 will be limited until the failure to pressurize fully is fixed. 

The mechanics are a skilled bunch, working assiduously and under pressure to get this airplane problem fixed by the time COVID-19 goes away. Hopefully, both will happen soon. 

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