Narrative:

I am increasingly concerned about the mechanical condition of our aircraft. This is the second report I have written on this sequence referencing recurring mechanical malfunctions on our 737 fleet. I have had somewhat extensive communication with the company on this and another 737 thrust reverser issue; yet these issues are not yet resolved or fixed or addressed. Here is the verbiage I have either written to the company or posted to the union. Some text goes back to events this past early april. Text is in historical order to give a picture of events: just this week had my 5'th 737 this year taken out of service due difficulties to fly around with one bleed inoperative. Here's what happened. In my writing in the maintenance log earlier this year; I wrote up a plane with a rather large (seemed unreasonable) duct pressure split. The high pressure was 62; the low was 27. The highest split was during takeoff and climb power. [We] got to a maintenance station and plane taken out of service due FL250 restrictions; no ice etc. Considerations; since maintenance control said the prsov (pressure regulating shut off valve) needed either placarding or fixing. The ensuing questions to maintenance control (remember; we have no limits on the bleed air system; so I was confused) revealed the following maintenance limitations: 'no sustained pressure over 50 psi is allowed....no sustained pressure under 14 is allowed.......no large duct pressure splits are allowed within the previous limits if there are pressurization issues.' this is not in our books obviously; and last week had an out of service plane with a high of about 53 again; and yet again an out station took the plane out of service. Other pilots have contributed the following: at idle power; in-flight and on the ground; duct pressure should be 20ish psi minimum. Less than 19 psi indicates hi-stage problems. If is bad enough to read 10 psi or less then the hi-stage is totally inoperative. Real world issues are cooling the airplane and wing ice. 1. Descending into icing at idle power with the hi-stage inoperative would mean no wing de-ice on that side. 2. A hot day on the ground with the hi-stage inoperative means no cool air out of that side at idle power. Source: maintenance control. Another good application is if you start an idle descent (like VNAV path) from the high 30s and neither of the hi-stage bleed valves open; you can get a very insidious cabin climb and a cabin altitude warning indication around fl 290. What makes this a tough call is our memory items and procedures will have us doing an idle descent in response to the cabin altitude warning horn; however; the solution to getting the cabin pressurization back with both high stage bleed valves stuck closed is to get the power up so you have bleed air available. It is very rare that neither of the high stage bleed valves open on an idle decent but it has happened and it will happen again. By using your numbers and checking the duct pressure on an idle descent; a pilot could identify the malfunction before the cabin altitude warning sounds or the cabin altitude light illuminates and transition to a 1;000 FPM descent and save the whole mask drop scenario. Also; another good number to remember when referring to the duct pressure is 42 psi +/-8. That is the limitation on the duct pressure for each side (left and right). The duct pressure on each side must be 42 psi +/- 8 psi. I have the maintenance manual number somewhere in my kit bag at the airport if anyone is interested. The following applies to the flight that instigated this report from myself; took yet another 737 out of service. Noticed that (I missed this on taxi out; I normally catch this during the aircraft and bleeds check) at idle in flight; the left duct pressure was as low as 12 psi. Fast forward. We land; and I ask the first officer to close the isolation valve and see if the left bleed valve actually closes; by a drop to almost 0 psi. Yep; did not drop;prsov inoperative. Now; worth mentioning I have found most duct pressure issues involve; directly or indirectly; the prsov. As in i.e.; if there is a duct pressure split; try to close the bleed valve (on the ground of course). You will find quite often (I have found this at least a half dozen times this year alone) that the prsov will either not close; or will fail to close/seal completely. The above mentioned aircraft had a 'frozen' prsov. The mechanic was very appreciative of my extensive write up and knew immediately to go straight to the prsov. He said he put a wrench on it; and it would not budge. If this aircraft had experienced a wing body overheat on the left side; (remember; left side is the bad side; more structure is involved) the only option to stop the leak would be to shut down the engine. The mechanic said this plane obviously has been flying around a very long time like this. I won't mention how many bleeds off; or half bleeds off takeoffs have been accomplished in this aircraft; or above idle power applications there have been with the APU bleed open...etc.

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Original NASA ASRS Text

Title: A B737-800 Captain reported numerous aircraft have inoperative or malfunctioning HPSOV (High Pressure Shut Off Valve) which are not repaired because failure are not detected by maintenance or flight crews.

Narrative: I am increasingly concerned about the mechanical condition of our aircraft. This is the second report I have written on this sequence referencing recurring mechanical malfunctions on our 737 fleet. I have had somewhat extensive communication with the Company on this and another 737 thrust reverser issue; yet these issues are not yet resolved or fixed or addressed. Here is the verbiage I have either written to the Company or posted to the Union. Some text goes back to events this past early April. Text is in historical order to give a picture of events: Just this week had my 5'th 737 this year taken out of service due difficulties to fly around with one bleed inoperative. Here's what happened. In my writing in the maintenance log earlier this year; I wrote up a plane with a rather large (seemed unreasonable) duct pressure split. The high pressure was 62; the low was 27. The highest split was during takeoff and climb power. [We] got to a maintenance station and plane taken out of service due FL250 restrictions; no ice etc. Considerations; since maintenance control said the PRSOV (pressure regulating shut off valve) needed either placarding or fixing. The ensuing questions to Maintenance Control (remember; we have no limits on the bleed air system; so I was confused) revealed the following maintenance limitations: 'No sustained pressure over 50 PSI is allowed....No sustained pressure under 14 is allowed.......No large duct pressure splits are allowed within the previous limits if there are pressurization issues.' This is not in our books obviously; and last week had an out of service plane with a high of about 53 again; and yet again an out station took the plane out of service. Other pilots have contributed the following: At idle power; in-flight and on the ground; duct pressure should be 20ish PSI minimum. Less than 19 PSI indicates hi-stage problems. If is bad enough to read 10 PSI or less then the hi-stage is totally inoperative. Real world issues are cooling the airplane and wing ice. 1. Descending into icing at idle power with the hi-stage inoperative would mean no wing de-ice on that side. 2. A hot day on the ground with the hi-stage inoperative means no cool air out of that side at idle power. Source: Maintenance Control. Another good application is if you start an idle descent (like VNAV path) from the high 30s and neither of the hi-stage bleed valves open; you can get a very insidious cabin climb and a cabin altitude warning indication around FL 290. What makes this a tough call is our memory items and procedures will have us doing an idle descent in response to the cabin altitude warning horn; however; the solution to getting the cabin pressurization back with both high stage bleed valves stuck closed is to get the power up so you have bleed air available. It is very rare that neither of the high stage bleed valves open on an idle decent but it has happened and it will happen again. By using your numbers and checking the duct pressure on an idle descent; a pilot could identify the malfunction before the cabin altitude warning sounds or the cabin altitude light illuminates and transition to a 1;000 FPM descent and save the whole mask drop scenario. Also; another good number to remember when referring to the duct pressure is 42 PSI +/-8. That is the limitation on the duct pressure for each side (left and right). The duct pressure on each side must be 42 PSI +/- 8 PSI. I have the Maintenance Manual number somewhere in my kit bag at the airport if anyone is interested. The following applies to the flight that instigated this report from myself; took yet another 737 out of service. Noticed that (I missed this on taxi out; I normally catch this during the aircraft and bleeds check) at idle in flight; the left duct pressure was as low as 12 PSI. Fast forward. We land; and I ask the First Officer to close the isolation valve and see if the left bleed valve actually closes; by a drop to almost 0 PSI. Yep; did not drop;PRSOV inoperative. Now; worth mentioning I have found MOST duct pressure issues involve; directly or indirectly; the PRSOV. As in i.e.; if there is a duct pressure split; try to close the bleed valve (on the ground of course). You will find quite often (I have found this at least a half dozen times this year alone) that the PRSOV will either not close; or will fail to close/seal completely. The above mentioned aircraft had a 'frozen' PRSOV. The mechanic was very appreciative of my extensive write up and knew immediately to go straight to the PRSOV. He said he put a wrench on it; and it would not budge. IF THIS AIRCRAFT HAD EXPERIENCED A WING BODY OVERHEAT ON THE LEFT SIDE; (remember; left side is the bad side; more structure is involved) THE ONLY OPTION TO STOP THE LEAK WOULD BE TO SHUT DOWN THE ENGINE. The mechanic said this plane OBVIOUSLY has been flying around a very long time like this. I won't mention how many bleeds off; or half bleeds off takeoffs have been accomplished in this aircraft; or above idle power applications there have been with the APU bleed open...etc.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.