Definition

This article deals specifically with issues pertaining to manual control of an aircraft's pressurisation system, which is typically required by a non-normal checklist (NNC) for failure of the system's automatic control. For information on other types of pressurisation problems, such as rapid decompression, please see Pressurisation Problems: Guidance for Flight Crews. Nothing herein supersedes aircraft-specific guidance in an aircraft flight manual (AFM).

System Overview

A typical pressurisation system consists of an automatic cabin pressure controller, an outflow valve, a safety valve (or positive pressure relief valve), and a negative pressure relief valve. In normal operation, the pressure controller regulates the outflow valve to maintain the desired cabin altitude. The safety valve relieves any overpressure. The negative pressure relief valve equalizes pressure when outside pressure becomes greater than inside pressure. This can happen in cases of rapid descent. 

Description

Manual pressurisation control involves direct manipulation of an aircraft's outflow valve. If automatic pressurisation fails, the NNC or Quick Reference Handbook (QRH) will normally direct pilots to to take manual control, usually by moving a pressurisation master switch to the MANUAL position. Another switch on the pressurization panel, either a toggle switch or a rotary knob, will allow control of the outflow valve by selecting either INCREASE or DECREASE (or similarly labeled positions). To increase the cabin differential pressure and lower the cabin altitude, the switch moves the outflow valve toward the closed position. To lower the differential pressure and raise the cabin altitude, the switch moves the outflow valve toward the open position.

In modern glass cockpit aircraft, the multifunction display (MFD) usually allows selection of a synoptic page for the pressurisation system. This page normally includes a graphic for outflow valve position. This helps the pilot visualise what is happening with the system as he or she operates the manual control switch.

It is important to remember that GREATER cabin differential pressure means LOWER cabin altitude. LESS cabin differential pressure means higher cabin altitude. Crews have been known to confuse this relationship when task saturated and under stress. Errors stemming from misunderstanding the system can result in high cabin altitude that drops the passenger oxygen masks, or overpressurisation, which can become painful for passengers and crew. One example is the 20 February 2023 accident aboard a Fokker 70 in Papua New Guinea that caused serious injury to four occupants and minor injuries to 18 others.

Differential pressure gauge from a DC-4E, 1939. SKYbrary photo by Thomas Young, taken at the U.S. National Air and Space Museum's Steven F. Udvar-Hazy Center.

Technique

When using manual pressurisation, the QRH or other emergency procedures will usually include a table that lists the recommended differential pressure to maintain a desired cabin altitude at a given aircraft altitude. To avoid overcontrolling, a good technique is to "beep" the control switch, or to hold it in the desired direction only momentarily. Then, watch the trend by monitoring cabin pressure, rate of change, and outflow valve position. If necessary, "beep" the switch again. Small movements of the outflow valve can make surprisingly large changes in cabin pressure.

Initially, little movement of the outflow valve may be required, especially if the automatic controller fails in cruise flight. The outflow valve's last commanded position might maintain desired pressure until the aircraft begins descent. Then, the crew may need to make minor and brief adjustments of the valve's position during descent.

IMPORTANT NOTE

Automatic pressurisation systems fully open the outflow valve on landing, relieving all residual pressure inside the aircraft. When using manual pressurisation, this usually DOES NOT happen. That means the crew must remember to open the valve manually. QRH procedures often direct the crew to fully open the valve and depressurise the aircraft before landing, at a fairly low altitude, such as 5,000ft MSL.

If the crew forgets this step, residual pressure could make it difficult or impossible to open plug-type doors and hatches. Worse, if a door is forced open under these conditions, the pressure differential could cause crew or passengers to be ejected from the aircraft. A situation like this caused the death of a U.S. Navy SEAL in 1996 when a Boeing C-17 Globemaster landed with residual pressurisation.