Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 161.15; Flight Crew Total 22693; Flight Crew Type 1605.98
Human-Machine Interface; Troubleshooting; Distraction; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed
Aircraft
After takeoff with gear up; Captains Flight Director (FD) and Flight Management Annunciations (FMA) showed different indications than what was depicted on First Officer instruments. The crew climbed to 10;000 feet per the departure SID. A steady warning horn sounded at level off. There were no Amber Caution lights; or Master Caution lights. There was no QRH checklist that addressed the steady warning horn. The crew elected to return to departure airport suspecting a possible landing gear sensing switch. There was no adverse cautions or lights except the steady warning horn. The crew landed 5;000 lbs over max landing weight at approximately 150 feet per minute vertical speed. After rollout the PSEU light illuminated (light only active on ground.) The aircraft was taken out of service by Maintenance. A Built-in Test Equipment (BITE) check of the Proximity Switch Electronic Unit (PSEU) showed the aircraft electronics had calculated that the flight never climbed above 200 feet AGL. The crew climbed to 10;000 feet indicated before assessing the cause of the steady warning horn. The BITE check determined a huge difference in actual altitude versus computed altitude. The steady warning horn was indicating erroneously an altitude deviation. The crew flew the aircraft normally to a landing. After gear down; flaps 15; the warning horn stopped sounding.
The B737 crew had different indications on their flights directors and FMAs; and a steady warning horn sounded at level off. The crew returned to the departure airport; and Maintenance discovered a faulty Proximity Switch Electronic Unit (PSEU).