BLEED TRIP OFF

P6-4AB
ENG. Bleed Air Temperature > 254 degree C / 490 degree F Or
ENG. Bleed Air Pressure > 220 PSI

On GND. Refer to MEL 36-05 [Engine Bleed Air Shutoff Valve], MEL 36-09 [High Stage Valve], MEL 36-08 [Engine Bleed Trip Off Lights]

In FLT. An engine bleed trip off during initial climb is most likely caused by an overpressure, often after a No Bleed takeoff. Selcting climb thrust [N1] prior to placing the bleed switches back to ON reduces the probability to get such an overpressure.
Duct pressure can be further reduced by selecting [CLB-2] on the FMC N1 Limit page, by selecting 1,000 ft/min [V/S] on the MCP, or by momentarily applying anti-ice on the associated engine, provided TAT is below 38 degree C / 100 degrees F, the aircraft is above 1,500 feet AGL and obstacles have been cleared.

PACK switch (affected side) OFF
The NNC does not instruct switching the Bleed Air switch to OFF because the bleed air valve is already closed (tripped). The pack is switched off to invoke high flow on the opposite pack and to avoid two packs operating from one engine.

One pack is capable of maintaining adequate cabin pressurization and air conditioning up to aircraft certified ceiling However due to seal leaks, especially on lder aircraft, pressurization should be closely monitored. If cabin pressure cannot be maintained, descend to a lower altitude and verify fuel range.

Use of wing anti-ice above 35,000 feet may cause bleed trip off
This note applies to normal operaitons with both engines supplying bleed air, each to one wing. The need for wing anti-ice at high altitudes is remote because any moisture is usually frozen and ice particles do not adhere to the wing.

Avoid icing conditions
A single bleed air system cannnot deliver sufficient volume for pressurization and engine anti-ice and wing anti-ice to both wings at the same time. If icing conditions cannot be avoided and wing anti-ice is required, consider mometarily switching-off the remaining pack below 10,000 feet and applying wing anti-ice for one or two minutes. The cabin pressure altitude climbs at 1,000 to 2,000 fpm until pressurization is restored. Do not use APU bleed air for wing anti-ice to one or both wings. If the APU is used as an alternate air source below 17,000 feet, the APU bleed air valve must be closed prior to applying wing anti-ice.

SUBSEQUENT FAILURE(S)

PACK TRIP OFF / PACK opposite side
Pressurization is lost
Position the failed Pack Switch to OFF, causing the Isolation Valve to open. Use the remainingack with the opposite engine.

ENGINE FAILURE or BLEED TRIP OFF opposite side

• Pressurization is lost
• Main Outflow Valve will drive to fully closed position
• Cabin pressure altitude will climb at 1,000 to 2,000 fpm
• Advise ATC for immediate descent
• Descend in the airway to single engine service altitude or 17,000 feet where the APU may be used as an alternate air source
• Select Engine No 1 Bleed Air to OFF (to extinguish DUAL BLEED amber light)

WING-BODY OVERHEAT opposite side
The cheklist calls to switch off the affected engine bleed air. However, since this will result in a loss of both packs and thus loss of pressurization, consider to :
- Retard thrust on the respective engine
- Advise ATC for immediate descent
- Accomplish the NNC [WING-BODY OVERHEAT]

- Left WING-BODY OVERHEAT :
   - 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)
- Right WING-BODY OVERHEAT :
    - Descend to FL170 in airway
    - Use the APU as an alternate air source

If this event occurs at low altitude, you can indeed switch-off the affected engine bleed air immediately. The cabin pressure altitude wll climb at 1,000 to 2,000 fpm, and it should not reach 10,000 feet before the airplane is at FL100.

WING ANTI-ICE VALVE OPEN opposite side
Checklist calls for switching off the affected engine bleed air and pack. However, since this will result in 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 NNC [WING ANTI-ICE VALVE OPEN]
- Continue unpressurized to destination or diversion field
(Trip fuel at 10,000 feet is approximately 10 x distance)

If this event occurs a low altitude, you can indeed switch-off the affected engine bleed air immediately. The cabin pressure altitude will climb at 1,000 to 2,000 fpm, and it should not reach 10,000 before the airplane is at FL100.