737 Pressurisation

The aircraft is pressurised by bleed air supplied to the packs and controlled by outflow valves.

The auto system will fail if either:

Cabin altitude exceeds 13,875ft CPCS; 15,800ft DCPCS.
Cabin rate of climb or descent exceeds 1890 sea level fpm CPCS; 2000 sea level fpm DCPCS.
Loss of AC power (transfer bus 1) to auto computer for more than 3 secs CPCS. Loss of DC power (DC bus 1/2) to auto computer DCPCS.
Differential pressure exceeds 8.3 psi CPCS, 8.75 psi DCPCS.
Other fault in pressurisation controller.Digital pressurisation controllers have two automatic systems (AUTO & ALTN) instead of a standby system, these alternate every flight. If the auto system fails, the standby / alternate system will automatically take over. The AUTO FAIL light will remain illuminated until the mode selector is moved to STBY / ALTN (tidy but not necessary). On CPCS panels, the cabin rate selector, for use in standby mode, adjusts cabin rate of change of altitude between 50 and 2000fpm, the index is approx 300fpm.
If you have to return to your departure airfield, do not adjust the pressurisation panel. You will get the OFF SCHD DESC light, but the controller will program the cabin to land at the take-off field elevation. If the flight alt selector is pressed, this facility will be lost.

In manual mode, you drive the outflow valve directly. The sense of the spring-loaded switch can be remembered by:

“Moving the switch towards the centre of the aircraft keeps the air inside."

The 737NG pressurization schedule is designed to meet FAR requirements as well as maximize cabin structure service life. The pressurization system uses a variable cabin pressure differential schedule based on airplane cruise altitude to meet these design requirements. At cruise altitudes at or below FL 280, the max differential is 7.45 PSI. which will result in a cabin altitude of 8000’ at FL280. At cruise altitudes above FL280 but below FL370, the max differential is 7.80 PSI. which will result in a cabin altitude of 8000’ at FL370. At cruise altitudes above FL 370, the max differential is 8.35 PSI. which will result in a cabin altitude of 8000’ at FL410. This functionality is different from other Boeing models which generally use a fixed max differential schedule thus can maintain lower cabin altitudes at cruise altitudes below the maximum certified altitude.

In all 737's the pressurisation system ensures that the cabin altitude does not climb above approx 8,000ft in normal operation. However in 2005 the BBJ will be certified to a reduced cabin altitude of 6,500ft at 41,000ft thereby increasing passenger comfort. The payback for this is a 20% reduction in airframe life cycles, ie from the standard 75,000 down to 60,000 cycles. This is not a problem for a low utilisation business jet but would be unacceptable in airline operation where some aircraft are operating 10 sectors a day.

Cabin Altitude Warning
The cabin altitude warning horn will sound when the cabin altitude exceeds 10,000ft. It is an intermittent horn which sounds like the take-off config warning horn. It can be inhibited by pressing the ALT HORN CUTOUT button. Note the pax oxygen masks will not drop until 14,000ft cabin altitude although they can be dropped manually at any time.

Following the Helios accident where the crew did not correctly identify the cabin altitude warning horn, new red "CABIN ALTITUDE" and "TAKEOFF CONFIG" warning lights were fitted to the P1 & P3 panels to supplement the existing aural warning system.

High Altitude Landing System
This is a customer option for operations into airfields with elevation of up to 14,500ft (12,000ft on some versions). There are also enhancements to the DCPS, an extra hour of emergency oxygen and the cabin altitude warning horn is inhibited.

Limitations
Max differential pressure:

Series Max Diff
1/200's 7.5psi
200Adv's 7.8psi
Classics 8.65psi
NG's 9.1psi
Max differential pressure for takeoff & landing: 0.125 psi

Max negative differential pressure: -0.1 psi