During ground operation the aircraft will be depressurized until the GRD/FLT switch is selected to FLT after engine start. The cabin will then automatically drive to 200 ft. below ambient pressure (.1 psi). After takeoff the pressurization controller will climb the cabin proportional to the aircraft climb rate on schedule up to a max of 500 fpm. When within about 1000 ft. of the set cruise altitude (FLT ALT) the controller will then trip a relay and enter a cruise mode and will maintain a constant cabin altitude with either a differential of 7.45 (below FL 280) psi or 7.80 psi (FL 280 and above). During cruise mode the controller will allow minor fluctuations in the differential up to 7.90 psi to maintain a constant cabin altitude. When descent is begun to about 1000 ft. below cruise altitude a descent relay trips and the controller will program cabin descent down to selected LAND ALT at max rate of no more than 350 fpm. The controller will actually descend the cabin to 200 ft. below the set LAND ALT to allow the aircraft to land slightly pressurized, just as in takeoff. During taxi in the FLT/GRD switch will be positioned to GRD which will then allow the aircraft to be fully depressurized.
An amber AUTO FAIL light is caused by:
> Loss of AC power
> Excessive cabin rate (+/- 1800 fpm)
> High cabin altitude (14,000 ft.)
AUTO FAIL will cause the controller to automatically go to STBY mode. The selector will still be in AUTO however. When the selector is positioned to STBY the AUTO FAIL light will go out.
STANDBY Mode-
During normal operation the destination airport elevation minus 200 ft. is set in the standby CAB ALT selector. If the AUTO mode fails during flight the controller will then begin to descend to the planned landing altitude if no other action is taken. QRH procedures will direct to set for cruise altitude as needed.
If standby mode is planned then the controller will simply follow whatever is set in the standby CAB ALT selector. Set FLT/GRD switch as in normal operation. The pilot will set 200 ft. below departure airport elevation before takeoff. After takeoff the pilot will select the proper cabin altitude in the CAB ALT selector based on a small chart on the pressurization panel itself. The pilot may adjust climb and descent rates as needed when in standby. When ready to descend dial in the destination airport elevation minus 200 ft. in the CAB ALT selector. Aircraft will be slightly pressurized on landing. Select GRD on FLT/GRD switch to depressurize after landing.
Manual mode-
Manual mode allows the pilot to directly control the outflow valve if both AUTO and STBY modes have failed. The same valve is controlled by both AC and DC motors. The AC motor is faster than the DC motor. The pilot will select appropriate manual mode and the green MANUAL light will come on. The pilot can then hold the manual switch to CLOSE or OPEN as needed and follow the progress on the valve indicator.
FLT/GRD switch set to FLT (flight) allows the aircraft to be pressurized on the ground to .1 psi (equals 200’ below airport elevation). This allows a smoother transition to pressurized flight during takeoff. Set to GRD allows the aircraft to fully depressurize on the ground. The pressurization system knows from the air/ground safety switch whether the aircraft is inflight and the system will not depressurize if the aircraft is inflight and the switch is set to GRD. The FLT/GRD switch is only “active” when on the ground.
If a descent is begun before getting to within about 1000 ft. of cruise altitude then pressurization will begin off schedule descent to return to departure field elevation. An amber OFF SCHED DESCENT light will illuminate. If this is not intended due to ATC not allowing planned final cruise altitude, etc. simply dial new cruise altitude into pressurization controller. Pressing the FLT ALT knob will cancel the OFF SCHED descent.
In normal operation the pressurization FLT-GRD switch will not prevent aircraft from pressurizing but will allow aircraft to have slight pressurization on ground when set to FLT to prevent takeoff pressure “bump”. On ground if set to GRD the outflow valve will fully open.
Equipment Cooling Light is looking at airflow. Lack of airflow will cause light to come on. Selecting Alternate will activate the alternate fan. The IRU’s are qualified to run for at least 180 mins. with no cooling air (do NOT turn off IRU’s during cooling failure simply for lack of cooling). TRU’s are rated at 50 amps without cooling (from normal 65 amps).
The forward cargo compartment is warmed inflight when more than 2.5 psi differential exists by air flow from the E&E compartment flowing up around it. The recirc fan normally provides this warming flow but the forward outflow valve can also be used if the recirc fan is off (except when forward outflow valve closed to maintain pressurization).
