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As far as I know at least some (e.g. some Bombardier Dash-8 series) airplanes during takeoff may increase cabin pressure above air pressure on the ground, before starting to decrease it. In one case pressure was increased to around 760 mm Hg (1013 hPa) when pressure on ground was around 730 mm Hg (973 hPa).

Is this increase of cabin pressure desired for some reason or is it just a side effect of something? And why does it happen?

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    $\begingroup$ Would it be to ensure inflow of oxygen? How long will the oxygen inside a sealed cabin last with 100-400 people consuming it? $\endgroup$ – Mike Brass Jan 3 '18 at 10:08
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Short answer

The purpose of pre-pressurization is to avoid discomfort for passengers and crews during takeoff and landing.

By storing air in the cabin before the aircraft rotation or landing, the pressurization controller is able to smooth out:

  • The variations of bleed air supply when the engines deliver the highest thrust and need all available air.
  • The variations in both air supply and air release, due to the aircraft leaving or entering the ground effect.

However pre-pressurization involves relatively limited pressure differentials.

  • 236 ft for a B737-NG (9 hPa).
  • 140 ft for a Dash8-200-300 (5 hPa).

The case you mention seems ways higher with 40 hPa.


Pressurization system

The source of pressurization for the aircraft is commonly bleed air from the engine compressor stages. See a more complete description of the air system in a large commercial airplane (ECS: Environmental control system) in this answer.

enter image description here
(B737-800, source)

The principle is that bleed air is pumped into the cabin and escapes by a configurable exit, the outflow valve. Cabin pressure is obtained by controlling the rates the air enters and leaves the cabin. This is indeed the job of the pressurization controller. This job is more difficult at takeoff and landing:

  • Air taken from the compressor is not used for the production of thrust, which can be a problem when thrust is critically required, e.g. for takeoff roll and rotation, or go-around. For these sequences, bleed air may be not available, or may be delivered at reduced or unstable rates. Normal bleed air delivery capability is restored after takeoff when the engines need less air.

  • The ambient pressure around the aircraft is subject to variations when the aircraft rotates and leaves the benefit of the ground effect, and also when it enters this effect before the flare. So the differential pressure at the outflow valve varies as well as the bleed air supply, and both need to be compensated too.


Pre-pressurization

Unmanaged variation of bleed air supply or cabin air outflow create air bumps in the cabin. To avoid discomfort, on some aircraft the cabin is pre-pressurized at a pressure slightly higher than the ground ambient pressure.

The pre-pressurization occurs during the takeoff roll and shortly before landing. It creates a stock of air the pressurization controller can use to smooth out bleed air bumps.

Source: B737-800 FCOM:

The air/ground safety sensor signals whether the airplane is on the ground or in the air. On the ground and at lower power settings, the cabin is depressurized by driving the outflow valve to the full open position.

The cabin begins to pressurize on the ground at higher power settings. The controller modulates the outflow valve toward close, slightly pressurizing the cabin. This ground pressurization of the cabin makes the transition to pressurized flight more gradual for the passengers and crew, and also gives the system better response to ground effect pressure changes during takeoff.

This additional amount of pressure mustn't compromise the ability to quickly open the doors in case of emergency, so pressure differential is limited. On a B737-NG, the differential is 0.125 PSI (9 hPa). This is equivalent to an altitude 236 ft below the runway.

After the aircraft is airborne, or has landed, the pressurization system releases air previously stocked, to recreate a smooth altitude increase.

For the sake of completeness: The cabin will not "climb" or "descend" as fast as the aircraft, and will have its ceiling at 8,000 ft to maintain oxygen pressure at a safe level to breath without a mask.


Cabin pressure vs. atmosphere pressure

Here is a representation of the pressure differential during a flight (with the takeoff/landing highlighted. Note the cabin (dashed line) "is lower" than the aircraft.

enter image description here
(Source)


More on pressurization systems

The goal of preventing discomfort from pressure variation encompasses multiple cases, including normal takeoff/landing, with/without packs, rejected take-off/go-around, emergency return to the airport, etc. The pressurization system may not be able to compensate all of them. The crew can also manage pressure manually instead of relying on pressurization system automatic control.

See more details about all those cases in Bombardier Global 5000 Air Conditioning and Pressurization manual on Scribd, e.g. rejected takeoff management of cabin pressure:

enter image description here

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  • $\begingroup$ If that was the short answer, I'm not sure I want to read the long answer! $\endgroup$ – FreeMan Jul 13 '15 at 13:36
  • $\begingroup$ Whew! I thought you were going to add the long version! $\endgroup$ – FreeMan Jul 13 '15 at 14:32

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