High Altitude Operations - Pressurization

Air is pumped into the sealed cabin — bleed air from a turbine engine, or a turbocharger/compressor on a piston aircraft — and the exit is regulated by an outflow valve. By regulating the air exit, the outflow valve allows a constant inflow of air to the pressurized area (PHAK ch. 7). Controlling how fast air leaves is what controls cabin altitude.

A typical system maintains a cabin pressure altitude of about 8,000 ft at the aircraft's maximum designed cruising altitude, which prevents rapid cabin altitude changes that could be uncomfortable or cause injury, and permits a reasonably fast exchange of air to remove odors and stale air (PHAK ch. 7).

Differential pressure is the difference between the cabin's inside pressure and the outside pressure — the system holds that difference, not manufacturing air.

Why it limits the system: the fuselage can withstand only a certain maximum cabin differential pressure, set by the structural strength of the cabin and, often, by the relationship of cabin size to probable rupture areas such as windows and doors. Differential control prevents that maximum from being exceeded; once the difference reaches the design maximum, a further increase in aircraft altitude produces a corresponding increase in cabin altitude (PHAK ch. 7).

Teach it with the pressure table: standard pressure is 14.7 psi at sea level, 10.9 psi at 8,000 ft, and 4.8 psi at 28,000 ft (PHAK ch. 7).

• Outflow valve — regulates the air exit and therefore cabin pressure (PHAK ch. 7)
• Cabin air pressure safety valve — a combination pressure relief, vacuum relief, and dump valve:
  • Pressure relief prevents cabin pressure from exceeding a predetermined differential above ambient
  • Vacuum relief prevents ambient pressure from exceeding cabin pressure by letting outside air in when ambient exceeds cabin
  • Dump valve — actuated by a flight deck control switch; positioned to ram, a solenoid opens the valve and dumps cabin air to the atmosphere

(PHAK ch. 7.)

• Cabin differential pressure gauge — indicates the difference between inside and outside pressure; monitor it to ensure the cabin does not exceed maximum allowable differential
• Cabin altimeter — a check on system performance
• Cabin rate-of-climb (or descent) indicator

The first two are sometimes combined into one instrument (PHAK ch. 7). All pressurized aircraft include automatic visual and aural warning systems (PHAK ch. 7).

Decompression is the inability of the aircraft's pressurization system to maintain its designed pressure differential, caused by a system malfunction or by structural damage (PHAK ch. 7).

• Explosive decompression — a change in cabin pressure faster than the lungs can decompress, possibly resulting in lung damage. Unrestricted lung release normally takes 0.2 seconds, so most authorities consider any decompression occurring in less than 0.5 seconds to be explosive and potentially dangerous.
• Rapid decompression — a change in cabin pressure in which the lungs decompress faster than the cabin.

During an explosive decompression, occupants experience:

• Noise and a momentary dazed feeling
• Fog, dust, or flying debris filling the cabin — the fog forms from the rapid temperature drop and change in relative humidity
• Ears clearing automatically
• Air rushing from the mouth and nose as air escapes the lungs

(PHAK ch. 7.)

Teach the fog specifically: pilots who have not been briefed on it mistake it for smoke and start troubleshooting a fire while hypoxic.

Rapid decompression decreases the period of useful consciousness because oxygen in the lungs is exhaled rapidly, reducing pressure on the body, decreasing the partial pressure of oxygen in the blood, and cutting effective performance time to one-third to one-fourth its normal value (PHAK ch. 7).

So the TUC table from Task II.N is optimistic in exactly the scenario where you need it. For this reason an oxygen mask should be worn when flying at very high altitudes — 35,000 ft or higher — and crewmembers should select the 100 percent setting on a demand or pressure-demand regulator at high altitude (PHAK ch. 7).

• Hypoxia is the primary danger — quick, proper use of oxygen equipment is necessary to avoid unconsciousness (PHAK ch. 7)
• Evolved gas decompression sickness — when body pressure drops sufficiently, nitrogen comes out of solution and forms bubbles inside the person, with adverse effects on some tissues
• Being tossed or blown out of the aircraft if decompression is caused by structural damage and occupants are near openings — which is why occupants near openings should wear safety harnesses or seatbelts at all times when the aircraft is pressurized and they are seated
• Wind blast and extremely cold temperatures from structural damage

Rapid descent from altitude is necessary to minimize these problems (PHAK ch. 7).

Sequence it so the student has one memorized action, then a plan:

1. Oxygen mask on, 100 percent, verify flow — you are on the TUC clock, cut to one-third or one-fourth by the decompression (PHAK ch. 7)
2. Emergency descent to a safe altitude — rapid descent is required to minimize hypoxia and decompression sickness (PHAK ch. 7)
3. Passengers on oxygen, occupants secured
4. Then troubleshoot, declare, and divert

Fly the AFM/POH procedure for the specific airplane — the manufacturer's guidance and procedures take precedence over general handbook recommendations (AFH ch. 13).

• A simulated failure that becomes real. If you demonstrate a malfunction by manipulating the actual system, brief the restoration procedure first and set an altitude and cabin-altitude limit for the exercise.
• Both pilots impaired simultaneously. Unlike an engine failure, a pressurization failure degrades the instructor's judgment along with the student's. Masks first, discussion later — establish this as a hard rule before takeoff.
• The airplane outruns the student. Pressurized airplanes are usually faster, higher, and more automated than what the student has flown. AI.II.E.R4 names instructing in unfamiliar aircraft or with unfamiliar avionics as a risk in its own right; know the system yourself before you teach it.
• Complacency about oxygen. A working pressurization system means nobody has touched a mask in months. Include a mask check in the preflight and make the student do it.