2025-06 · NASA ASRS report 2255236
B737 flight crew reported a large difference in bleed duct pressures during climb. Flight returned to departure airport for maintenance.
After calling for flaps up and after takeoff checklist; the Captain noticed the duct pressure looked incorrect climbing through 5000 ft while completing the checklist. I looked up and saw right duct pressure at 40+ PSI (normal) and left duct pressure at 14 PSI. I put on the autopilot shortly after that. We verified that the bleeds/packs/iso valve were all in the correct positions. We continued to climb while the Captain looked for a checklist for the discrepancy. We requested to stop climbing early at 17000 FT assuming there may be a pressurization issue. We did continue to pressurize. We could not find a checklist but only a note in the limitations section (1.30.3 in FM (Flight Manual)) stating difference of more than 8 PSI indicates a mechanical malfunction; and that an indication of less than 18 PSI with bleeds configured for flight means we should contact maintenance. I continued to fly the plane and work the radios. We leveled at 17000 while the CA (Captain) notified Dispatch and tried to get ahold of Maintenance. We got a frequency of XXX.X but it took a while (15-20 minutes). Eventually someone came on; possibly Maintenance but they could not hear the Captain. After about 40 minutes we decided to return to ZZZ. We made the decision because we did not have a reliable checklist; could not get ahold of Maintenance; and didn't know if we could conceivably pressurize to FL350 safely. There was checklist or guidance for this issue outside of the limitations note to contact maintenance. We needed to burn some fuel to get under max landing weight so we coordinated with ATC for some holding. We notified Dispatch and ZZZ Ops we were returning. We briefed the arrival and approach and made an uneventful landing on runway XX under max landing weight. Captain notified flight attendants and made an announcement to the passengers. We returned to gate and informed Maintenance of the issue.
While running the after takeoff checklist; I noticed a large split in the bleed duct pressures. The right side appeared normal; and the left side was 15 psi or less; with the split obviously being well over 8 psi difference. I told the FO to continue flying; and I began to diagnose the problem. We decided to use the autopilot to reduce workload at this time. I did a recall and checked the lights on the pressurization panel; and I could see this was not a bleed trip off. I took a few steps to try to diagnose and reset; but I didn't want to make up a checklist or interfere with our pressurization once we were above 10;000 feet. I eventually asked to stop the climb at 17;000 feet in case we needed to use APU bleed to pressurize. The only flight manual guidance for a bleed duct pressure split directs me to contact Maintenance; so I sent a descriptive ACARS message to Dispatch and Maintenance Control. After several minutes of no response; I used the call me prompt on ACARS. I made contact with ARINC on XXX.X; and briefly with Dispatch. Almost as soon as Maintenance Control was connected; they were disconnected; and when they were reconnected; I could hear Dispatch and Maintenance Control; but they couldn't hear me. Since I had no checklist for this scenario; no direct flight manual guidance on the implications of the loss of a bleed source and our ability to pressurize in the mid 30s; no MEL to reference for the deferral of a bleed source; not enough fuel to fly to ZZZ1 at the current altitude; I decided that the safest option was to return to ZZZ. I informed Dispatch; briefed the FAs; informed the passengers; and burned fuel to get below max landing weight. We conducted an uneventful and safe air return; and Maintenance repaired the airplane.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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