The restriction is FL250 for the 737. During recurrent training we had just that scenario, one pack inop, and it was stressed to us that FL250 is just a MEL requirement not something you have to obey once in flight. About the same time the Swedish accident investigation board released a report of a 737 that had a decompression. They had initially a bleed trip off but continued climb to FL410. Then a second bleed trip off as a result of one air cond pack not being able to pressurize. Due to the fact that these things continue to happen led them to recommend EASA and FAA to change the QRH to reflect the same limitation as the MMEL does (FL250).
Conclusions The assessment made by the crew when the climb had to be aborted was in line with the company's procedures. The flight altitude could be reduced without disruptions or delays and stabilised at Flight Level 100. The oxygen masks in the cabin were activated manually. The commander carried out a debriefing with the crew and passengers after landing. The persons that had experienced discomfort in connection with the flight received help in dealing with their impressions and experiences.
Since the introduction of this aircraft model in the late nineties, the Type Certificate Holder, Boeing, has carried out a number of controlled introductions of improved components, but the result of this work has not extended the operating time before a failure has occurred.
The operator's maintenance measures have changed following similar incidents in the past; a checking procedure has been introduced. In the event that one of the Bleed Air Systems is non-operational, checks are made to ensure that the remaining system has the capacity to maintain the cabin pressure during flight up to Flight Level 250. The occurrence discussed in this investigation has re- sulted in implementation of functional tests on mentioned components on B- Check.
The FAA's airworthiness directive to modify the detection and presentation (ATA 31) of malfunctions in the pressurisation system is a sign that the design still has a high fault incidence and requires improvement.
SHK believes that the limitation introduced for application when flying with one Air Conditioning System inoperative, in accordance with MMEL 21-01, in which the operating altitude is limited to Flight Level 250, also should be taken into consideration when one system becomes non-operational during a flight in progress.
The incident was caused by the remaining cabin pressurisation system not be- ing able to pressurise the cabin at the altitude where the airplane operated.
Recommendations EASA and the FAA are recommended to:
Act to change the Boeing B737 QRH – NNC “Bleed Trip Off” so that a limita- tion of the flight altitude should be taken into consideration in the event of fail- ure of one pressurisation system during flight in the same way as when this is identified before dispatch (Cf. MMEL point 21-01).
RL 2013:03 R1
Full report:
http://www.havkom.se/virtupload/repo...202013_03e.pdf