Date: 201209
Local time of day: 0601-1200
Location: ZZZZ.Airport, FO
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Aircraft component: APU Pneumatic System & Ducting
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Human factors: Troubleshooting; Confusion; Situational Awareness; Communication Breakdown
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural MEL / CDL
Detector: Person Flight Crew
When detected: In-flight
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Company Policy; Human Factors; Procedure; Logbook Entry; MEL
Primary problem: Human Factors

Narrative: Runway 36; planned for a bleeds off; flaps-5; takeoff 26K maximum power. Engine bleeds off; APU bleed 'On' and Isolation Valve 'Closed' per checklist. Duct pressures normal. At cleanup altitude; after flaps up; First Officer begins to reconfigure bleeds to normal operations. Before he moved a switch; I noticed right duct pressure at 5 PSI and left duct pressure at normal (40 PSI). After [we returned the] bleeds to Normal; both [pneumatic] ducts at 35-right and 40-left; NORMAL. We mulled this over for a few minutes; QRH; checklist; Volume 1 had no information for our situation. I used FMC to contact Dispatch. Passed on info from above and asked to have this relayed to Maintenance Control Technical for an answer. Maintenance Control came back with a Precooler Condition Valve possibly being bad. [I had] never heard of this valve. Looked in MEL and this was addressed; but did not apply once airborne. I asked Maintenance Control; other than this rare instance; how am I supposed to know if this valve failed. He replied that except for this rare instance; I would have no idea. Continued flight [was] uneventful. Dispatch asked for me to write it up as it happened; and she would have Maintenance meet aircraft upon arrival. We arrived; no Maintenance; departed aircraft and went home. Placard Pressure Indicator. I noticed that this aircraft flew [a turn] before late departure. No information about a valve being bad or being placarded. The MEL for this valve (if this was the problem); was not addressed. The MEL said maximum FL250 and no icing. All I can see that was done was placarding the Duct Pressure Gauge. This gauge was working fine. So; was the problem addressed or not. This aircraft was also supposed to fly to [another destination] after a turn. Did Maintenance just push this B737-800 aircraft along without advising the flight crew? This had me a little concerned. As Maintenance Control said; I would not know that there was a problem; especially since they placarded the Duct Pressure Gauge. Provide the Captain with the proper information about the status of aircraft. Read logbook online; [trying] to confirm aircraft was repaired or deferred correctly.

Callback: Reporter stated they had the B737-800 in the bleed configuration he describes due to the aircraft's gross weight; high altitude international airport they were departing from and a cool morning.



Synopsis: Pilot reports that Maintenance Control may have applied an incorrect MEL to their B737-800 aircraft after he noted that right duct pressure was only 5-PSI and left duct at 40-PSI; with engine bleeds off; APU bleed on and Isolation Valve closed.