Narrative:

When reviewing flight papers we noted a maintenance item where the action taken did not seem to match the problem. One of the two air conditioning systems (packs) had displayed warnings of no airflow when selected to operate during ground operations with the auxiliary power unit (APU) as the pneumatic source. In response to this problem maintenance prohibited use of the automatic cabin temperature controller in favor of manual temperature control. While some failures in the automatic system can cause airflow to be automatically disabled (pack trip); the chronic write-ups didn't suggest any such failures had been detected. Neither did the maintenance history reflect any diagnostic activity suggesting pack trips. Aircraft are able to operate quite normally with an inoperative automatic temperature controller while other pack restrictions are more problematic.upon arrival at the aircraft we observed the right pack warning of no airflow in spite of compliance with the deferral. Pneumatic duct pressure was abnormally low. A pack valve will not open without sufficient air pressure from the pneumatic source. Inadequate pressure suggests either a pneumatic leak or insufficient output from the source; the APU in this case. Weak pneumatic output from an APU is not a threatening condition. The same cannot be said of pneumatic leaks. Pneumatic air temperatures regularly exceed 400 degrees. The aircraft utilizes pneumatic air for air conditioning; pressurization; wing anti-ice; landing gear operation; electronic equipment cooling; and other lesser functions. A cursory check revealed the right pack operated normally if air was not simultaneously ducted to the left pack. But routing air to the left pack would cause the right pack to fail for lack of airflow. This simple check verified adequate pressure from the APU and implicated the left pneumatic system downstream of the left pack valve as having a leak of sufficient magnitude to starve the right pack.when presented with the indications and my opinion the mechanic resisted further diagnosis. He urged me to accept the aircraft based on the maintenance deferral and evidently believed the deferral provided latitude to ignore the cockpit warning that compliance with the deferral failed to extinguish. He indicated that the aircraft had been operating for at least four days in the current condition and therefore should be safe to continue. When I didn't budge he offered an engine run to diagnose whether that configuration would provide sufficient airflow to operate the right pack; still evidently believing the pack warning in the current configuration could be ignored. Using engine bleed air; the left and right pneumatic systems are isolated rather than joined. So that configuration would mask the leak I suspected. I accordingly disagreed with using an engine run to determine whether the aircraft should be accepted. Eventually maintenance checked for pneumatic leaks on the right side of the pneumatic system. Then; after additional persistence on my part; ultimately checked the left side where a 'substantial' leak was found downstream from the pack valve. The aircraft was removed from service.

Related reports:

Google
 

Original NASA ASRS Text

Title: B767 Captain becomes suspicious of pack MEL for low air flow from the APU and suggests to Maintenance that a bleed air leak is the more likely culprit. Maintenance is not convinced but is eventually persuaded to check the bleed air ducting and does find a substantial leak. Aircraft is taken out of service.

Narrative: When reviewing flight papers we noted a Maintenance item where the action taken did not seem to match the problem. One of the two air conditioning systems (packs) had displayed warnings of no airflow when selected to operate during ground operations with the auxiliary power unit (APU) as the pneumatic source. In response to this problem Maintenance prohibited use of the automatic cabin temperature controller in favor of manual temperature control. While some failures in the automatic system can cause airflow to be automatically disabled (pack trip); the chronic write-ups didn't suggest any such failures had been detected. Neither did the Maintenance history reflect any diagnostic activity suggesting pack trips. Aircraft are able to operate quite normally with an inoperative automatic temperature controller while other pack restrictions are more problematic.Upon arrival at the aircraft we observed the right pack warning of no airflow in spite of compliance with the deferral. Pneumatic duct pressure was abnormally low. A pack valve will not open without sufficient air pressure from the pneumatic source. Inadequate pressure suggests either a pneumatic leak or insufficient output from the source; the APU in this case. Weak pneumatic output from an APU is not a threatening condition. The same cannot be said of pneumatic leaks. Pneumatic air temperatures regularly exceed 400 degrees. The aircraft utilizes pneumatic air for air conditioning; pressurization; wing anti-ice; landing gear operation; electronic equipment cooling; and other lesser functions. A cursory check revealed the right pack operated normally if air was not simultaneously ducted to the left pack. But routing air to the left pack would cause the right pack to fail for lack of airflow. This simple check verified adequate pressure from the APU and implicated the left pneumatic system downstream of the left pack valve as having a leak of sufficient magnitude to starve the right pack.When presented with the indications and my opinion the Mechanic resisted further diagnosis. He urged me to accept the aircraft based on the Maintenance deferral and evidently believed the deferral provided latitude to ignore the cockpit warning that compliance with the deferral failed to extinguish. He indicated that the aircraft had been operating for at least four days in the current condition and therefore should be safe to continue. When I didn't budge he offered an engine run to diagnose whether that configuration would provide sufficient airflow to operate the right pack; still evidently believing the pack warning in the current configuration could be ignored. Using engine bleed air; the left and right pneumatic systems are isolated rather than joined. So that configuration would mask the leak I suspected. I accordingly disagreed with using an engine run to determine whether the aircraft should be accepted. Eventually Maintenance checked for pneumatic leaks on the right side of the pneumatic system. Then; after additional persistence on my part; ultimately checked the left side where a 'substantial' leak was found downstream from the pack valve. The aircraft was removed from service.

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.