AUTO FAIL/UNSCHEDULED-PRESSURIZATION CHANGE
AUTO FAIL or Unscheduled Pressurization Change
P6-4F
Loss of DC power on DC BUS 1/2 for > 3"
Rate of cabin pressure change > 2,00 fpm SLE
Cabin altitude above 15,800 feet
Differential pressure > 8.75 PSID
Fault in Outflow Valve Control
Fault in Pressurization Controller 1/2
AUTO FAIL + ALTN = Failure of a single controller (Automatic transfer to remaining controller)
AUTO FAIL + ALTN LT OFF = Failure of both controllers (Switch to MANUAL)
If both controllers have failed, the FLT ALT and LAND ALT show dashes.
Some AUTO FAIL faults are recoverable by momentarily placing the Mode Selector to MAN and back to AUTO.
ON GND Refer to MEL 21-14 [Cabin Pressure Control System]
controller No 1 and No 2 are located in the E&E. The controllers are identical and can be swapped for dispatch.
IN FLT
Left Pack trip off during a No Bleed Takeoff interrupts air supply from the APU and will cause the AUTO FAIL amber light to illuminate when the rate of cabin pressure change is more than 2,000 fpm SLE.
Each pressure controller receives air data from both ADIRU and engine RPM data from both SMYDCs. The PSEU provides air/gnd logic signal to the DCPCS. Failure of the Captain's EFIS Control Panel causes a fault in the pressurization system because BARO corrected altitude input is lost.
Two identical pressure controllers operate in parallel to regulate air pressure in the aircraft. The controllers act as master (Primary) and slave (Alternate).
The controller operating as master corresponds to AUTO, the slave corresponds to ALTN. Which one is master and which one is slave depends on a number of factors.
The sequence is reversed every flight.
If the master controller recognizes a fault, pressure control is automatically switched to ALTN mode. The AUTO FAIL amber light illuminates. The ALTN green light illuminates also as soon as the slave has control.
Pressure in the cabin remains (nearly) unchanged after engine start. During takeoff, the controller slightly pressurizes the cabin to an altitude of 200 feet (0.1 PSID) below departure field elevation, when takeoff thrust (*) is set. This prevents an uncomfortable pressure bump at rotation. The overpressure pushes the flight deck sliding windows and cabin doors into the airframe to avoid rattling during takeoff run.
(*) Both engines N1 RPM above 60% or N2 RPM above 89% for more than 1.5 seconds. If the engine run-up to 70% in icing conditions takes more than 1 seconds as per [Cold Weather Operation - Taxi-Out], the cabin will pressurize during taxi.
For a flight at or below 18,500 feet, cabin pressure altitude obtains and will maintain landing field elevation minus 300 feet (0.15 PSID) as soon as the airplane is airborne.
For flights above this altitude, the cabin descends to the landing field elevation minus 300 feet (0.15 PSID) when the airplane starts descent. After touchdown the controller drives the Main Outflow Valve to full open and the cabin is de-pressurized.
Rules of thumb :
correct setup of the bleed and pack switches is a requirement but still no guarantee for proper pressurization. Some faults in the pressure controller or valves may go undetected and result in incorrect pressurization.
Rule of thumb to verify proper cabin pressurization during climb (valid for cruising levels above FL250) :
At 10,000 feet these pointers should indicate approximately 1 and 4
After a no bleed takeoff, the cabin altitude is catching up with the normal climb profile and the pointers rather show 2 and 3. The same goes for takeoff from airports substantially above SL. If the cabin does not pressurize, verify bleed and pack switches, pressurization mode selector and the cargo compartment fire arm switches. (if inadvertently ARMED, a descent command is sent to the DCPCS).
Rule of thumb to verify if a typical cabin descent profile can be achieved :
Cabin altitude (hundreds of feet) = Distance required (NM)
e.g. If the cabin altitude shows 4,000 feet, the controller requires about 40 NM to descent to SL. If the remaining distance to the destination is 40 NM or more, the Auto Controller will maintain a normal rate of descent. If the remaining distance is less than 40 NM, the controller will increase the cabin pressure descent rate.
If destination elevation is substantially above SL, subtract Land ALT from Cabin ALT.
e.g. If the cabin altitude shows 4,000 feet and destination is at 1,500 feet MSL, then the controller requires about 25 NM to descend to airport elevation.