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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
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.
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:
| Series |
Max Diff |
| 1/200's |
7.5psi |
| 200Adv's |
7.8psi |
| Classics |
8.65psi |
| NG & Max |
9.1psi |
Max differential pressure for takeoff & landing: 0.125 psi
Max negative differential pressure: -1.0 psi
Main Outflow Valve
Controlled by the pressurisation system. Regulates the
cabin pressure by adjusting the outflow of cabin air.
Early outflow valves (shown here) opened into the fuselage. |
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Later outflow valves opened out from the fuselage. |
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From Dec 2003 onwards, the main outflow valve was given teeth to reduce
aerodynamic noise. Its frightening appearance should also help to deter
people from putting their hands in the opening. |
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Pressure (Safety) Relief Valves
These two valves, located above and below the main outflow valve,
protect the aircraft structure against overpressure if the
pressurisation control system fails. they are set at Originals 8.5psi, Classics: 8.65psi , NG & Max: 8.95psi.
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Negative Pressure Relief Valve
Prevents vacuum damage to aircraft during a rapid descent. It is a
spring loaded flapper valve that opens inwards at -1.0psid. You can
check this on a walkaround by pressing it in like a letterbox. |
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Flow Control Valve (Classic) / Overboard Exhaust Valve(NG)
Open on the ground (check this on a walkaround) to provide E&E bay cooling and also
in-flight at less than 2psi differential pressure. You can often hear this valve
opening on descent when the differential pressure passes 2psi. The OEV
also opens when the recirculation fan (R recirc fan on the 8/900) is
switched off to assist in smoke clearance. Strictly
speaking, this is an exhaust port. The actual Flow Control Valve /
Overboard Exhaust Valve is located further upstream. |
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Forward Outflow Valve - Classics only
This is is a vent for the E & E bay air after it has been circulated around the
forward cargo compartment when in-flight (The E & E bay air is exhausted
from the flow control valve when in the ground). The valve opens when the recirculation
fan (R recirc fan on the 400) is off (smoke clearance mode) or when the
main outflow valve is not completely closed (ie low diff pressure). It is located just below and aft of the fwd
passenger door. Note the NG's do not have a FOV. In-flight, equipment
air is circulated around the
forward cargo compartment and discharged from the main outflow valve | 
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Pressurisation
