B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 170
Confusion; Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem
Human Factors
After departure from ZZZ; enroute to ZZZ1; we were passing about FL360; when we got a left bleed off light. We applied the QRH procedure; and the light reset momentarily. Before the light reset; I noticed; as the Pilot Flying; the cabin altitude started to rapidly increase also. The bleed tripped again; and this time the cabin altitude was increasing toward 10;000 feet.  As it was approaching 10;000 feet I called for oxygen masks and started a descent as the Cabin altitude warning light/horn activated. It was then the Pilot Monitoring (PM); the First Officer realized the right bleed switch was off.   He turned it on; and the cabin altitude quickly recovered. We reset the trip on the left side as well; and all indication were normal. This all occurred very quickly; from FL360; descending to FL320. All ops were then normal; and we continued the flight normally.   We discussed this for quite a while during the flight; and both agree the switches were in the correct position at takeoff. The taxi out at ZZZ was not rushed; and we were stopped at the end of the runway when we completed the before takeoff checklist; and we both agreed the error was not there. On all previous flights of this trip; we routinely used the packs in high after takeoff for cooling.   We think possibly; in this instance; the PM may have inadvertently turned off the right bleed valve mistakenly while attempt to return the right pack to auto.
The Captain reported that just prior to leveling off the cabin altitude began to climb. After completing the appropriate troubleshooting checklist it was discovered that a bleed switch was in the wrong position. The bleed switch was repositioned to the 'on' position and the cabin pressure returned to normal.