Narrative:

This report chronicles failure of an automatic system and subsequent activity to correct the failure. Both the automatic and standby modes of the pressurization controller failed while we were at cruising altitude. A reversion to the manual mode was necessary to maintain control of the cabin pressurization. The electronic pressurization is programmed by the F/east. The automatic mode is the normal mode of operation. The standby mode is the primary backup mode and requires additional programming. The manual mode is very elementary and involves positioning the cabin outflow valve manually by moving a toggle switch, the first officer becomes the controller. We had been at cruise altitude (FL350) for some time when I found it desirable to descend to FL310 because of moderate chop. When we cleared the area of chop, I requested clearance to climb to FL350. Approaching FL350 the first officer stated that he could not control the cabin altitude or rate of climb. The cabin quickly climbed to 9000' and the crew donned O2 masks. I requested descent to FL240 and the first officer tried to reverse the cabin climb in the standby mode, to no avail. The first officer selected the manual mode and toggled the outflow valve toward closed. The cabin climb was arrested at 10000'. We experienced a noticeable change in cabin pressure as the F/east assumed manual control of the outflow valve. We continued the flight to our des with a cruise altitude of FL240 with the cabin pressure control being operated in manual. Operators of automated systems are quickly programmed to rely on the system and do not supervise an automatic system with the same vigilance that they would a system that requires more active operator management. When automation fails in ways that were never envisioned by the system designers and engrs, the crew workload can increase exponentially. My crew performed their duties well and we were able to maintain control, but I can envision circumstances where the outcome might be different.

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Original NASA ASRS Text

Title: ACR LGT AUTH STANDBY PRESSURIZATION CTL FAILURE AT CRUISE ALT. SO CONTROLLED THE SYSTEM MANUALLY.

Narrative: THIS RPT CHRONICLES FAILURE OF AN AUTOMATIC SYS AND SUBSEQUENT ACTIVITY TO CORRECT THE FAILURE. BOTH THE AUTOMATIC AND STANDBY MODES OF THE PRESSURIZATION CTLR FAILED WHILE WE WERE AT CRUISING ALT. A REVERSION TO THE MANUAL MODE WAS NECESSARY TO MAINTAIN CONTROL OF THE CABIN PRESSURIZATION. THE ELECTRONIC PRESSURIZATION IS PROGRAMMED BY THE F/E. THE AUTO MODE IS THE NORMAL MODE OF OPERATION. THE STANDBY MODE IS THE PRIMARY BACKUP MODE AND REQUIRES ADDITIONAL PROGRAMMING. THE MANUAL MODE IS VERY ELEMENTARY AND INVOLVES POSITIONING THE CABIN OUTFLOW VALVE MANUALLY BY MOVING A TOGGLE SWITCH, THE F/O BECOMES THE CTLR. WE HAD BEEN AT CRUISE ALT (FL350) FOR SOME TIME WHEN I FOUND IT DESIRABLE TO DSND TO FL310 BECAUSE OF MODERATE CHOP. WHEN WE CLRED THE AREA OF CHOP, I REQUESTED CLRNC TO CLB TO FL350. APCHING FL350 THE F/O STATED THAT HE COULD NOT CONTROL THE CABIN ALT OR RATE OF CLIMB. THE CABIN QUICKLY CLBED TO 9000' AND THE CREW DONNED O2 MASKS. I REQUESTED DSNT TO FL240 AND THE F/O TRIED TO REVERSE THE CABIN CLB IN THE STANDBY MODE, TO NO AVAIL. THE F/O SELECTED THE MANUAL MODE AND TOGGLED THE OUTFLOW VALVE TOWARD CLOSED. THE CABIN CLB WAS ARRESTED AT 10000'. WE EXPERIENCED A NOTICEABLE CHANGE IN CABIN PRESSURE AS THE F/E ASSUMED MANUAL CONTROL OF THE OUTFLOW VALVE. WE CONTINUED THE FLT TO OUR DES WITH A CRUISE ALT OF FL240 WITH THE CABIN PRESSURE CONTROL BEING OPERATED IN MANUAL. OPERATORS OF AUTOMATED SYSTEMS ARE QUICKLY PROGRAMMED TO RELY ON THE SYS AND DO NOT SUPERVISE AN AUTOMATIC SYS WITH THE SAME VIGILANCE THAT THEY WOULD A SYS THAT REQUIRES MORE ACTIVE OPERATOR MGMNT. WHEN AUTOMATION FAILS IN WAYS THAT WERE NEVER ENVISIONED BY THE SYS DESIGNERS AND ENGRS, THE CREW WORKLOAD CAN INCREASE EXPONENTIALLY. MY CREW PERFORMED THEIR DUTIES WELL AND WE WERE ABLE TO MAINTAIN CTL, BUT I CAN ENVISION CIRCUMSTANCES WHERE THE OUTCOME MIGHT BE DIFFERENT.

Data retrieved from NASA's ASRS site as of July 2007 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.