PACK / PACK TRIP OFF

P6-4A,

Temp at ACM Compressor Oulet > 199 degrees C / 390 degrees F
Temp at ACM Turbine Inlet > 99 degrees C / 210 degrees F
Temp in Supply Duct > 121 degrees C / 250 degrees F
Failure of Primary and Back-Up Pack Cont dual fault
Failure of Primary or Back-Up Pack Cont single fault (on recall)

OR

PACK LT ILLUMINATED

Temperature selector (all) ............................. Select warmer temperature

Both air mix valves are driven to full cold and temperature selectors are disabled.
Selecting a warmer temperature will therefore NOT move the air mix valves.

The temperature has to decrease by itself. This step has only been added in view of preventing the same problem from occurring again after the TRIP reset. The aim of selecting a warmer temperature is to decrease the workload on the pack.

(600-700-BBJ) With temperature selector(s) in MANUAL, a warmer temperature has to be selected after the TRIP Reset. With the selector(s) in AUTO, verify the air mix valves are moving to full hot ! If not, select MANUAL !

If one PACK light remains illuminated (QRH)
A single PACK light stays illuminated (QRH Phase 2)

Notice the NNC [PACK TRIP OFF] does not have this condition (QRH) - is completed with no further actions (QRH Phase 2).

This is because the pack valve is always automatically closed when the PACK TRIP OFF amber light illuminates, while the Pack Valve may still be open when the PACK amber light illuminates to indicate a single or dual controller fault. The Pack Valve must then be switched OFF - closed - manually. Hence the difference in caption PACK vs. PACK TRIP OFF.

On GND
(800-900-BB2) Refer to MEL 21-32 [Pack Temperature Control Systems] for dual failure of both Pack Temp / Control Systems.

(800-900-BB2) PACK on recall indicates single failure of one Pack Temperature Control System. No restrictions. The fault will often disappear once airborne and flaps are UP ...

MEL 21-01 [Air conditioning Packs]

Aircraft can be dispatched at FL 250 (one Pack) or FL 100 (unpressurized)

• verify fuel for flight at FL250
• Verify enroute weather at lower altitude
• Advise ATC / Dispatch (flight plan)
• Takeoff perf credit for Packs OFF not authorized ; accomplish takeoff perf computation with A/C ON - AUTO, however Vmcg must be determined based on AC Packs OFF
• differential thrust on takeoff is no factor. No adjustments are required for N1
• After engine start : - affected Pack Switch OFF, - Isolation Valve CLOSE

! With the Pack Valve blocked open, the Pack Valve must be manually closed to permit engine start. Refer also to MEL 21-03 [Pack Trip Warning System]

In FLT
A single pack should be capable of maintaining the cabin pressure altitude at or below 8,000 feet SLE with acceptable temperatures up to the airplane maximum certified ceiling. However, due to seal leaks, not every aircraft is able to maintain pressurization at cruising altitude with a single pack.

Continue flight at the actual or planned flight level. If cabin pressure cannot be maintained, descend to a lower flight level and verify fuel range.

The remaining pack will automatically regulate to HIGH (when flaps are UP). (800-900-BB2) Turning the related Temperature Selector(s) to OFF will usually eliminate the pack from tripping over and over again.

When the flight is planned with a single pack, cruising altitude is restricted to FL250 as per MEL 21-01. However, if the pack fails in flight, the NNC allows you to climb or maintain planned cruising altitude up to aircraft certified ceiling. The logic behind this difference is dispatch redundancy on ground versus operational fuel range in flight.

The temperature probe in the Left Pack is different from the probe in the Right Pack and is more sensitive. The temperature limit measured at compressor outlet of the ACM is a protection against overload caused by a failing pack valve, while the temperature limit at turbine inlet duct protects the ACM against overspeed in case of insufficient ram air flow.

(800-900-BB2) Each Electronic Pack Controller consists of a primary and a standby Pack Control. If a primary Pack Control fails (PACK on recall), the affected PACK is controlled by the standby Pack Control of the opposite Electronic Pack Controller.

DURING SUMMER

The Pack trip is probably caused by an overload of the Air Cycle machine due to high cooling demand. Selecting a warmer temperature will offload the ACM. Selecting the opposite pack to ON may help the cooling process.

DURING WINTER

When a warm temperature is demanded, the Pack trip is most probably caused by an excessive temperature in the supply duct. If the DUCT OVERHEAT / ZONE amber light is illuminated at the same time, the air mix valves may be blocked in hot position due to a mechanical failure or because of electric power loss.
If the DUCT OVERHEAT / ZONE amber lights are not illuminated, failure of the Duct Overheat Sensor may be suspected. (600-700) With the temperature selectors in AUTO, either an additional failure of the Max. Topping / Duct Temperature Limit Bridge may be suspected, or a loss of AC Transfer Bus power. Check the...

The NNC calls for warmer temperature, however it is suggested to select a cooler temperature. Verify that the air mix valve(s) are actually moving.

Passengers seated at rows 7-8 (*) will be the first to complain about the cabin temperature being too low. This is because the diffuser outlets near the Sidewall Riser Ducts tend to have a higher airflow.

Conditioned air from the Main Distribution Manifold flows through two or more Sidewall Riser Ducts. These riser ducts supply the overhead distribution ducts.

The riser ducts follow the airplane contour along the left and right fuselage :
Left : 2 riser ducts (800-900-BB2-BB3)
1 riser duct (600-700-BBJ)

Right : 2 riser duct (BBJ)
1 riser duct (600-700-800-900-BB2-BB3)

(*) The exact row depends on the 737-variant and seating configuration. The riser ducts are located in the sidewall where the windows are missing forward of the overwing emergency exit(s).
In winter time when the fuselage is covered with frost, you can notice the complete riser duct from the outside as warm air passing through the duct causes the frost to disappear.

SUBSEQUENT FAILURE(S)

WING ANTI-ICE VALVE OPEN or WING-BODY OVERHEAT opposite side
The NNC calls for switching off the affected engine bleed air. However, since this will result in a loss of both packs and thus a loss of pressurization, consider to :

• Retard thrust on the respective engine
• Advise ATC for immediate descent
• Descend to 10,000 feet (or minimum safe altitude if higher)
• Perform the applicable NNC [WING-BODY OVERHEAT], [WING ANTI-ICE VALVE OPEN]
• Continue unpressurized to destination or diversion field (trip fuel at 10,000 feet is approximately 10 x distance)

With a low actual cabin altitude (in climb or at an intermediate cruising level), you can switch off the affected engine bleed air. Cabin pressure altitude will climb at 1,000 to 2,000 fpm, and it should not reach 10,000 feet before the airplane is at FL100.

ENGINE FAILURE or BLEED TRIP OFF opposite side
Pressurization is lost !
Position the failed Pack Switch to OFF, causing the Isolation Valve to open.
Use the remaining pack with the opposite engine to pressurize.

PACK TRIP OFF / PACK opposite side

• Pressurization is lost
• Main Outflow Valve will drive to full close
• Cabin pressure altitude will climb at 1,000 to 2,000 fpm
• Advise ATC for immediate descent
• Descend to 10,000 feet ( or minimum safe altitude if higher)
• Continue unpressurized to destination or diversion field
  (trip fuel at 10,000 feet is approximately 10 x distance)