*** Updated 23 Nov 2020 ***
Digital Cabin Pressure Control System (DCPCS)
The aircraft is pressurised by bleed air supplied to the packs and controlled by outflow valves.
The auto system will fail if either:
Cabin Pressure Control System (CPCS) installed in a 737-200C. Notice the extra SMOKE CLEARANCE controls.
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. The normal (AUTO) scheduled rate of climb is either 600 or 750 slfpm and either 350 or 500 or 750 slfpm for descent depending upon the customer specification.
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.
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.
Photo - Frode Lund
The pressurisation system of all series of 737 ensures that the cabin altitude does not climb above approx 8,000ft in normal operation. However in 2005 the BBJ was certified to operate with a reduced cabin altitude of 6,500ft at 41,000ft (ΔP of 8.99psid above 37,000ft) to increase 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.
Is a customer option for operations into airfield elevations of up to 10,000ft (all NG), or 13,500/14,500ft (-6/700 series only). Changes include: Addition of high altitude landing selector switch to transfer to the high altitude mode, cabin altitude at warning horn activation is increased, an extra hour of emergency oxygen (bottles stored in aft cargo compartment), 10 minute take off thrust if engine inop; winglets, carbon brakes and fwd Cof G options are recommended. For the 737-700 the takeoff envelope is increased to 19,000 ft & 0.50Mn. The option is usually taken with increased MLW, carbon brakes and RNP
Photo showing the high ground idle N1 whilst taxying at 6,900ft elevation, Kunming China (Photo Sukhwinder Phuller)
Max differential pressure:
Max differential pressure for takeoff & landing: 0.125 psi
Max negative differential pressure: -0.1 psi