B737-400
Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Confusion; Training / Qualification
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance
Flight Crew Returned To Departure Airport; Flight Crew FLC complied w / Automation / Advisory; General Declared Emergency; General Maintenance Action
Aircraft; Procedure
Conducting initial operator experience as Check Airman; First Officer [was] pilot flying. Repairs supposedly made from previous day. After departure: Autothrottles locked in THR HOLD. TCAS failed. Mode Control Panel (MCP) would not allow movement of airspeed bug; engagement of VNAV or LNAV. Cabin began an uncontrollable climb. Manual pressurization made rate of climb worse. Stopped climb at FL180. [We] declared an emergency. Allowed the First Officer to make an expedited decent to 10;000 FT after ATC clearance received. ACARS again would not allow diversion message to be sent. Informed flight attendants; company and passengers we were returning to the departure airport. First Officer landed after level 1 automation visual approach. Landing was uneventful. Cabin altitude never exceeded 7;000 FT. Crew/passenger O2 not used.
The Reporter stated that he flew this aircraft the day before when only pressurization and autothrottles failed. Maintenance did not pick up on any failure except the pressurization write-up. Therefore; the Pressurization Controller was replaced and the aircraft signed off. The fact that only a few systems reverted to the ground mode the day before was somewhat confusing. During this second event; the following day; all systems affected reverted to the ground mode which made diagnosis of the root problem clearer. The nose gear ground air switch was the failed component and it was replaced which apparently corrected the problem.
A B737-400 nose gear ground/air sense squat switch failed to the ground mode preventing aircraft pressurization and other systems from functioning so an emergency was declared and the flight returned to the departure airport.