Limitation of 25,000ft when one bleed air inoperative for 737-300?
hi, everybody
I am a new guy in this forum. This forum is so professional and i wanna find some answers at here.
I am a 737-300 pilot from China. I had met a "bleed trip off" few days ago. The left "bleed trip off" light illuminated when we did take-off rolling, we continued takeoff, we asked ATC for 1500m to maintaining(in China, about 5000'), we executed the NNC "bleed trip off", but the light was still on. According to the QRH&OM, we have no altitude limit, so we continued to climb, after 3-5min, we pushed the reset switch again, light was off, we flied to the destination......
When i back to home and check with some documents, problem is coming:
1. in QRH NNC.2.4"BLEED TRIP OFF" , it says:
¡° Condition: The BLEED TRIP OFF light illuminated indicates the
related engine bleed air temperature or pressure is
excessive.
TRIP RESET SWITCH. . . . . . . . . . . . . . . . . . . . . . . . . . . . PUSH
[The BLEED TRIP OFF light extinguishes if bleed air temperature has
cooled below limits.]
If the BLEED TRIP OFF light remains illuminated:
PACK SWITCH (affected side) . . . . . . . . . . . . . . . . . . . .OFF¡±
No altitude limit.
2. in Operation Manual page 2.30.1, it says:
¡° A single pack is capable of maintaining pressurization and acceptable temperatures throughout the airplane up to the maximum certified ceiling.¡±
No altitude limit.
3. but in MEL item 36-5¡± Engine Bleed Air Valves¡±, it says:
¡°......
1. avoid icing conditions
2. left engine bleed air off:
a. when blow 17000ft, left pack use APU bleed air and right pack use right engine bleed air with isolation valve switch closed
b. turn off the APU bleed air when above 17000ft
1) limit maximum flight altitude to 25000ft,
2) isolation valve switch on and use left pack(use right engine bleed air), turn off right pack switch
......¡±
HERE, we have a maximum flight altitude limitation of 25000ft!!!
So, my question is how is the difference come. Is there any limitation of 25000ft when one bleed air inoperative for 737-300? If there is, what is 25000ft for? For structure limit? I don¡¯t think so. For one bleed air work load limit or sth. else?
Although the MEL is to be used before engine start, the instructions in it still have the value as a reference in flight. At least Boeing should add a notification¡°avoid icing conditions¡± in the QRH.