Outflow Valve (OFV)
Outflow Valve (OFV)
Definition
The outflow valve is an essential element of an aircraft’s cabin pressurisation system. The valve regulates the outflow of air to maintain the desired cabin altitude.
Description
In modern turbine-powered aircraft, bleed air tapped from the engines’ compressor sections provides air for cabin pressurisation. After the bleed air is filtered and cooled, it enters the aircraft’s Environmental Control System (ECS), which provides heating, cooling, and pressurisation. The conditioned air enters the cabin at a faster rate than air exits the cabin. This causes the cabin pressure to rise to a level higher than the outside atmosphere. The increased air pressure brings the cabin altitude down to a comfortable, breathable level, eliminating the need for crew and passengers to breathe from supplemental oxygen.
Cabin altitude will vary with aircraft altitude. For example, an aircraft flying at 35,000 feet might have a cabin altitude of 8,000 feet. The same aircraft flying at 25,000 feet might have a cabin altitude of 5,000 feet. In any case, air must be released from the cabin at a controlled rate to maintain cabin altitude within the design limits of the aircraft. (Think of a balloon being constantly filled with air. A certain amount of air must be released for the balloon to remain the same size and not burst.) The outflow valve regulates that release of air.
Other elements of a pressurisation system include a positive pressure relief valve, otherwise known as a safety valve, and a negative pressure relief valve. If an outflow valve fails and cabin pressure exceeds desired limits, the safety valve will open to prevent overpressurisation. Similarly, if outside pressure exceeds inside pressure, as in the case of a rapid descent, the negative pressure relief valve will open to keep the pressure differential within limits.