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; heading sel or VOR/localizer. 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 altitude hold; heading sel and VOR/localizer. 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 altitude 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.

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Original NASA ASRS Text

Title: 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.

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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.