Date: 201509
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pneumatic Valve/Bleed Valve
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Distraction; Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; General Maintenance Action
Contributing factors: Aircraft; Procedure; Company Policy
Primary problem: Aircraft

Narrative: During our climb it was noted that the left duct pressure was at 10 PSI while the right was at 40 PSI. The cabin was pressurizing well and at level off when the power levers came back for cruise thrust; we got an increase 'bump' in the left duct pressure to 40 psi. There had been a previous write-up of a left bleed trip during climb several flights before so this lack of pressure during climb piqued my interest into what may be transpiring.My systems knowledge led me to believe that the low pressure bleed valve off the 5th stage bleed air may not have been open fully during the climb thereby restricting air flow to the ducts and subsequently when the power was reduced at cruise; the high pressure 9th stage bleed valve opened providing the additional duct pressure.With the previous left bleed trip in mind and since we had 4 1/2 hours of cruise; I decided to contact dispatch via cockpit phone and talk with Maintenance about what I might have and what may transpire regarding pressurization when we eventually begin descent. After describing my situation simultaneously to 2 [maintenance personnel]; they told me that all they were allowed to tell me was to follow my QRH and that was it. I told them that I had no emergency and no amber or red warnings to precipitate the use of my QRH and that I just wanted to discuss my situation with the system professionals to see if I may have an issue down the road.They told me there was a recent change from Management that does not allow maintenance personnel to confer with flight crew while airborne. They are told to instruct the flight crew to follow their QRH and that is it! I was on my own to deal with any situation that may arise. I would have liked a little more info about my aircraft situation before I began descent into a high workload environment at night. I was told to put the discrepancy in the maintenance log which I did. I followed up on the situation several days later and found that they in fact replaced the bleed valve which they found to be faulty.In my own personal opinion; I was restricted in providing a safe environment for my passengers and crew by not being allowed to discuss system intricacies with the true professionals. I understand that in an emergency/malfunction; I should use my QRH. But this was not the case. I was trying to seek out information that would possibly alleviate a situation down the road; and to take away this wonderful asset that we as pilots have while airborne; in my opinion; is detrimental to the safe operation of the airline.Please let pilots secure the knowledge of [maintenance personnel] by allowing them to discuss a situation with them while airborne.




Synopsis: A B737-800 Captain noted low left side duct pressure during climb but normal pressure at cruise and suspected a faulty bleed valve. Maintenance; as per company policy; would not discuss the issue in flight stating the QRH was the crew's only available resource.