Pressurisation Systems

Why are cabins pressurised?

- Not enough oxygen pressure at the more efficient altitudes to fly at

- Safe and comfortable for pax and crew

What will the max altitude of an aircraft generally be determined by?

Max allowable pressure differential (difference between internal and ambient pressures)

How is a cabin pressurised?

Supplying more air to cabin than is required and controlling the excess air that's leaving (constant supply balanced against variable rate of outflow air)

What is the max allowable PD in general?

- Light aircraft 3-5 psi

- Large piston aircraft 5.5 psi

- Jet transport aircraft 9 psi

What happens to pressurisation of an aircraft during its life cycle?

- The higher it flies, the higher the PD

- Pressurisation, then stretching, eventually fatigued and stretched until strain hardening then potentially cracking

- Limited to a number of cycles

- Can fly lower after that (less strain)

How can an aircraft be pressurised?

- Superchargers

- Turbochargers

- Engine-driven air pumps

- Bleed air from gas turbine compressors

What's the downside to using engines for pressurisation?

Places large demands on engine so loss of power and must filter out oil or fumes

How does the supercharger provide air for pressurisation?

Air goes through intake and half the compressor (driven by the engine) goes to the engine, half to cabin for pressurisation

How does the turbocharger provide air for pressurisation?

Air goes through intake and half the compressor (driven by exhaust gases) goes to the engine, half to cabin for pressurisation

How does an engine-driven air pump provide air for pressurisation?

Roots blower on the back of a gearbox, lobes trap and move pressurised air from inlet to outlet

How does bleed air from a turbine provide air for pressurisation?

Bleed air from engine compressor (can drive a turbo-compressor) or used through a jet pump flow multiplier

What is a cabin pressure regulator?

An outflow valve, controlling the escape of excess air so the aircraft is set at a desirable altitude

What is a positive pressure relief valve?

Prevents over-pressurisation, opens automatically at a predetermined pressure. Set on the ground as the maximum allowable pressure differential

What is a negative pressure relief valve?

A vacuum, preventing cabin pressure going below the ambient air

What is a dump valve?

Dumps residual cabin pressure on landing, controlled by WOW switch and can be selected by pilots in an emergency

What is the isobaric range?

Controls cabin pressure to a selected value/constant cabin pressure altitude during various levels of flight by changing the cabin pressure differential

What is the differential range?

Limits cabin pressure to a preset differential value/limits to a maximum permissible value

How will a regulator reference changing altitude during flight?

It keeps the selected altitude, adjusting the pressure to keep it the same but if max differential pressure is exceeded, a safety valve will open to lower cabin pressure

What are the general components of a cabin pressure regulator?

- Outflow valve

- Differential control

- Isobaric control

- Dump valve

What is the rate of change of cabin pressure when ascending or descending?

Ascending = 0.26 psi

Descending = 0.16 psi

What is catching the cabin?

Cabin can't exceed aircraft altitude otherwise PD becomes negative so the negative pressure safety valve gets pushed open and outside air flows into the cabin. Cabin altitude will then descend with the aircraft

What are the malfunctions that can come with pressurisation?

Decompression and negative pressure

What happens when there is negative pressure?

An object will compress

What happens when there is explosive decompression?

Cabin pressure changes faster than lungs can decompress, generally less than 0.5 seconds, likely to have lung damage

What happens when there is rapid decompression?

Cabin pressure changes but lungs can decompress faster, not likely to get lung damage

What happens when there is slow decompression?

Automatic visual and aural warnings are installed so crew don't become incapacitated before it's detected, however is is more dangerous than rapid decompression

What will happen to the pressurisation in an emergency?

There is an emergency pressurisation mode, directing a mixture of bleed air and ram air into the cabin, when combined with fully closed outflow valves, can preserve some pressurisation of the aircraft