B737-800 flight crew reported hearing an unusual sound coming from the lower forward fuselage. While conducting After Takeoff cleanup items; it was noticed the EQUIP annunciation light was illuminated. The flight crew requested priority handling and conducted an air turn back and made an overweight landing at departure airport. The forward equipment bay access door had been left open after maintenance.
Synopsis
B737-800 flight crew reported hearing an unusual sound coming from the lower forward fuselage. While conducting After Takeoff cleanup items; it was noticed the EQUIP annunciation light was illuminated. The flight crew requested priority handling and conducted an air turn back and made an overweight landing at departure airport. The forward equipment bay access door had been left open after maintenance.
Narrative
Shortly after takeoff; the CA (Captain) and I heard an unusual sound coming from the lower forward fuselage. While conducting After Takeoff cleanup items; we noticed the EQUIP annunciation light was illuminated. We ran the QRH checklist and determined the aircraft wasn't pressurizing. We called Departure; advised them of our situation; and requested to return to ZZZ RNAV XX. Approximate weight was 142;000 lbs.; so we were below maximum landing weight. Everything was uneventful; we landed and went to Gate XX shortly after landing.Upon arrival at the aircraft we noticed the #1 IDG was disconnected; MEL was properly annotated in the logbook; switches were placard. The APU would have to be online for the entire flight. Thus; we would have the DRIVE annunciation light illuminated during the flight. It's possible we missed the EQUIP light on the overhead panel thinking it's was the DRIVE light due to its close proximity. Thoroughly scan the overhead panel during the system annunciation recall.
Second reporter narrative
Shortly after takeoff on Runway XX the First Officer and myself noticed a rumbling sound coming from the lower forward fuselage. During climb out we investigated and saw the 'EQUIP' light illuminated. We referenced the QRH checklist and determined that the aircraft was not pressurizing. We made a call to Departure and requested the RNAV approach to Runway XX as we needed to return to ZZZ. The aircraft weighed approximately 142;000 lbs and was below maximum landing weight. I decided to not request priority handling as I anticipated a normal landing and taxi back to a gate. A normal approach and landing brought us back to Gate XX at ZZZ.It seems as though the First Officer and I both missed the EQUIP light illuminated on our preflight checklist. We reviewed the situation in our minds and neither of us remember seeing it. However the system was not malfunctioning therefor we had to have missed it. The largest casual factor that I can think of is due to an MEL item that was on the aircraft. One of the generators was inoperative and we were required to leave the APU generator online for the entire flight. Because of this; one of the 'DRIVE' lights on the overhead panel remained on. The DRIVE light is located a few inches away from the EQUIP light. It is possible that we mistook the EQUIP light for the DRIVE light.I believe this occurred due to a level of complacency. And possibly because a DRIVE light was illuminated nearby. From a checklist/procedure standpoint it should have been captured. A more thorough and deliberate approach to completing the checklist is the best way to prevent this from happening again. I believe I will be more vigilant in the future.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.