Date: 201210
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Aircraft: B737-400
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Squat Switch
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Workload
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: General Maintenance Action; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We took off and gear/ flaps retracted normally; but when N1 pushed it didn't engage and FMA stayed on THR HOLD. LNAV/VNAV would not engage and could not get either flight director to work except for LVL CHG; HDG SEL or VOR/LOC. We followed the magenta line of the GPS via heading select until we could get a clearance from ATC to proceed via conventional navigation. [We] got clearance from ATC to proceed on airway and transition to ILS via conventional navigation. Climbing through 10;000 FT pressurization appeared normal. Autopilot engaged and would work for ALT HOLD; HDG SEL and VOR/LOC. ATC never was able to establish radar contact; was not receiving transponder. We suspected an air/ground sensing problem. In logbook the night before there had been a write up with an air/ground sensing type problem and had been cleared with a do not duplicate. We leveled at FL310 and it was a short leg so we received clearance to descend to 10;000 FT. Descending through approximately FL250 we noticed that the cabin was climbing through about 9;000 FT and shortly reached 10;000 FT and the Cabin Alt warning horned sounded. We then accomplished the Cabin ALT Warning/Rapid Depressurization QRC which led us to the Emergency Descent checklist. We did not declare an emergency at this time. The maximum cabin altitude was approximately 12;000 FT. At 10;000 FT we removed our oxygen masks and were cleared for the ILS approach. We were in VMC conditions and with in 35 miles of the uncontrolled airport so we cancelled IFR and since wind was favoring the opposite direction we proceeded VFR on a wide long downwind and pattern maintaining a 500 FPM descent for passenger comfort. We had to consult the Performance Handbook for VREF speeds. On the downwind we decided to declare an emergency as a precautionary measure. Gear and flaps worked normally and we landed without incident and taxied to the gate with no further problems. It seems as if there was a partial malfunction of the air/ground sensing with the airplane.


Second narrative: Takeoff was normal until calling for LNAV at 500 FT-roll guidance bar didn't appear on flight director. Flew the course line manually; had Captain recycle Flight Director (F/D) switches with no effect; so engaged heading and got guidance bar to display. Noticed FMA still read THR Hold and autothrottles were not moving. At 10;000 FT pressurization was normal. Through 18;000 reset altimeters and was able to engage autopilot with level change and heading selective active in FMA. Due to THR Hold annunciation; disengaged autothrust and operated thrust levers manually. Captain was extremely busy with communications with ATC and company; but got us a conventional navigation clearance; and I concentrated on flying the airplane and navigating by VOR/DME; leveling at FL310. Received clearance to 10;000; began descent. Through about 25;000; noticed the cabin climbing through 9;000 at 500 FPM; followed by cabin warning horn at 10;000. Performed emergency descent QRH; got clearance to 7;000 and stopped the rapid descent at 10;000 FT. Weather was good VFR with a north wind; so we cancelled IFR and descended on a long left downwind; descending at 500 FPM for passenger comfort. Captain obtained landing speeds from the Performance Handbook; and the approach and landing were uneventful.Aircraft component malfunction caused the aircraft to remain at least partially in the ground mode after takeoff; causing some systems to act as though the airplane was still on the ground; and others to act as if in flight.


Synopsis: B737-400 flight crew experiences anomalies with autopilot and auto flight systems after takeoff and later pressurization problems which they attribute to a faulty ground/air sensing system. Flight lands safely at destination after experiencing a cabin altitude warning horn during descent. The aircraft had been previously written up for the same problem which Maintenance could not duplicate.