The Joys Of Pressurization

Airplanes are different animals. I may wonder about such things as Cessna's full-time Electronic Stability Protection (a kind of full-time attitude protection system) on the TTx, but I don't argue with the obvious benefits of pressurization. It's such a good idea, it's a shame we no longer have folks such as Burt Rutan or the late Roy LoPresti to come up with innovative and economical methods of pressurizing an airplane.

Unlike automobiles, where the aforementioned options are all related to convenience and comfort, pressurization in an aircraft is directly connected to safety and performance. Pressurization allows an airplane to fly higher and faster, above the terrain
and much of the weather, meanwhile pampering its crew and passengers with air-conditioning. It's perhaps aviation's ultimate creature comfort.

I've been fortunate to fly a wide variety of pressurized aircraft, from the Lancair IV-P to the Swearingen SJ30 corporate jet, and current generation aircraft fly behind pressurization differentials as high as 9.0 psi.

Contrary to what you might imagine, there's nothing very exotic about pressurization. At sea level, we have 14.7 psi of atmosphere pressing down on us; at 50,000 feet, most of the atmosphere is far below, so the pressure is only 1.69 psi. If you're flying at 50,000 feet with a 9.0 psi differential, your total pressure will be 9.00 plus 1.69, or 10.69, roughly equivalent to a 7,500-foot cabin altitude. You can't use a straight-line function for middle altitudes because the pressure is obviously heaviest at the bottom of the atmosphere where the weight of the air above pushes down the hardest. There are charts on the Internet to provide exact atmospheric equivalent, if you're interested.

Operationally, pressurization couldn't be much simpler. You simply set the controller to 1,000-1,500 feet above the elevation of either your departure or destination airport, whichever is higher, turn the system on and go fly.

The controller will allow the cabin altitude to rise as the airplane climbs, usually at about half of the aircraft's climb rate. When the cabin altitude reaches the preset limit, the controller will automatically level the cabin regardless of the pilot's selected cruise altitude.

If the pilot opts for a cruise height that's higher than the pressurization differential can support, the cabin altitude will begin to rise as the airplane climbs. For example, a 5.0 psi differential will support a cabin altitude of 9,000 feet at an airplane altitude of 25,000 feet. If the airplane climbs to 26,000 feet, the cabin will rise accordingly to 10,000 feet.

Pressurization has some peripheral benefits that might not seem immediately apparent. Night flying presents special vision challenges for pilots, as the lack of oxygen reduces visual acuity. If you have pressurization, you can depart and set the differential to ground level or below once the aircraft is in flight. With a 5.0 psi differential, you could climb to 12,000 feet, and the cabin would never climb above sea level. Your eyes would be tricked into thinking you were still at sea level.

It's perhaps only appropriate that pilots who most need pressurization are those who have arrived financially, often older aviators. Pressurization is a totally transparent system when it's working properly. Most of the time, you simply set it and forget it. If you do it right, you won't even know it's there.

I have a good friend who owns an early Piper Malibu and says he uses the pressurization for an extra charge of oxygen when he's feeling tired. He flies his airplane up and down the West Coast for business several times a week and says he'll sometimes deliberately set the pressure controller to the absolute bottom setting---about -2,000 feet---and enjoy refreshing, high oxygen levels on short flights around coastal California while cruising comfortably at 8,000 feet or more.

Of course, the question everyone wants answered is how much does pressurization cost. That can be tough, as it's not typically something you can buy as an option, but there have been a few models over the years that have been built in both pressurized and unpressurized configurations.

Back in 1980, Cessna produced a model 335 twin that was basically the popular 340 without pressurization. The 335 incorporated the features that had made the pressurized version popular---turbocharged, Continental engines, airstair door, wing and nose lockers---but excluded all the pumps, controllers and plumbing of the original twin. The result was an airplane that sold for $271,275, rather than the model 340's $330,855. In other words, you could save about $60,000 if you were willing to forego pressurization. You could also benefit from nearly 200 pounds more payload.

Unfortunately, the timing was all wrong, and sales suffered accordingly. Sale of twins was already winding down after the boom of the '70s, and Cessna knew the upcoming model T303 Crusader would further erode 335 sales. Cessna shut down the 335 program after only 65 airplanes had been built in the first and only year of production.

Piper tried a similar project in 2008 with the Matrix, an unpressurized version of the Mirage, and was met with considerably better success. The Vero Beach, Fla., company removed all of the Mirage's pressurized bits and pieces, and realized an empty weight reduction of about 180 pounds, roughly one extra passenger's worth.

That was a welcome improvement as the standard, six-place Mirage was essentially a four-seat-plus-full-fuel airplane. Perhaps more significantly, the company was able to reduce the price from $1,141,500 (on the Mirage) to $785,500 (on the Matrix)---a savings of more than $350,000. Again, performance was the same. Matrix pilots could achieve Mirage performance by strapping on an oxygen mask. To date, some 200 Piper customers have opted to buy their PA-46-350P without pressurization.

An inflatable cabin makes everything easier. There's no mask to worry about, though you'll be expected to keep the emergency oxygen system charged and ready in case the cabin loses pressurization. You and your passengers can usually plan to arrive at your destination more comfortable and relaxed. Perhaps best of all, pressurization provides options you can't appreciate without it. You may be able to fly higher, more direct and over, rather than around, the weather.

Now, if I could only figure out a way to pay for it.