Narrative:

At FL310 with autoplt 'a' engaged, airplane began an uncommanded 600 FPM descent and deviation from FL310. We disengaged the autoplt to return to level flight. Elevator feel was restr/reduced during input to elevator control. Requested a descent to ascertain how much restr input there was with regard to the elevator. During descent, first to FL290 and then FL240, we momentarily experienced a restr elevator. It felt like autoplt was still engaged or that it was stuck in control wheel steering or manual reversion. Gave control of aircraft to first officer. Aircraft rate of descent was under control at all airspds, but input to elevator was momentarily restr at times. Planned diversion to bwi and declared emergency. Ran QRH for restr elevator control. During descent out of 15000 ft, first officer said elevator felt normal, slowed airplane and configured early on approach. During this event, electric trim operated normally. There were no warning lights illuminated for 'feel differential,' 'flight controls,' 'autoplt disengage,' or 'stabilizer out of trim.' callback conversation with reporter revealed the following information: the reporter stated the autoplt was engaged when the airplane started the 600 FPM descent. The reporter said with the autoplt disconnected the elevator feel was then discovered to be restr with reduced authority/authorized. The reporter stated that 5 pounds of force would be normal, but it required about 30 pounds of force. After landing, the reporter said that maintenance suspected elevator control cables were frozen or restricted by ice due to a possible leak in the aft lavatory. The reporter said the elevator feel computer was tested, found malfunctioning, and was replaced.

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Original NASA ASRS Text

Title: A B737-200 IN CRUISE AT FL310 DECLARED AN EMER AND DIVERTED DUE TO UNCOMMANDED 600 FPM DSCNT AND RESTR ELEVATOR FEEL CAUSED BY FAULTY ELEVATOR FEEL COMPUTER.

Narrative: AT FL310 WITH AUTOPLT 'A' ENGAGED, AIRPLANE BEGAN AN UNCOMMANDED 600 FPM DSCNT AND DEV FROM FL310. WE DISENGAGED THE AUTOPLT TO RETURN TO LEVEL FLT. ELEVATOR FEEL WAS RESTR/REDUCED DURING INPUT TO ELEVATOR CTL. REQUESTED A DSCNT TO ASCERTAIN HOW MUCH RESTR INPUT THERE WAS WITH REGARD TO THE ELEVATOR. DURING DSCNT, FIRST TO FL290 AND THEN FL240, WE MOMENTARILY EXPERIENCED A RESTR ELEVATOR. IT FELT LIKE AUTOPLT WAS STILL ENGAGED OR THAT IT WAS STUCK IN CTL WHEEL STEERING OR MANUAL REVERSION. GAVE CTL OF ACFT TO FO. ACFT RATE OF DSCNT WAS UNDER CTL AT ALL AIRSPDS, BUT INPUT TO ELEVATOR WAS MOMENTARILY RESTR AT TIMES. PLANNED DIVERSION TO BWI AND DECLARED EMER. RAN QRH FOR RESTR ELEVATOR CTL. DURING DSCNT OUT OF 15000 FT, FO SAID ELEVATOR FELT NORMAL, SLOWED AIRPLANE AND CONFIGURED EARLY ON APCH. DURING THIS EVENT, ELECTRIC TRIM OPERATED NORMALLY. THERE WERE NO WARNING LIGHTS ILLUMINATED FOR 'FEEL DIFFERENTIAL,' 'FLT CTLS,' 'AUTOPLT DISENGAGE,' OR 'STABILIZER OUT OF TRIM.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AUTOPLT WAS ENGAGED WHEN THE AIRPLANE STARTED THE 600 FPM DSCNT. THE RPTR SAID WITH THE AUTOPLT DISCONNECTED THE ELEVATOR FEEL WAS THEN DISCOVERED TO BE RESTR WITH REDUCED AUTH. THE RPTR STATED THAT 5 LBS OF FORCE WOULD BE NORMAL, BUT IT REQUIRED ABOUT 30 LBS OF FORCE. AFTER LANDING, THE RPTR SAID THAT MAINT SUSPECTED ELEVATOR CTL CABLES WERE FROZEN OR RESTRICTED BY ICE DUE TO A POSSIBLE LEAK IN THE AFT LAVATORY. THE RPTR SAID THE ELEVATOR FEEL COMPUTER WAS TESTED, FOUND MALFUNCTIONING, AND WAS REPLACED.

Data retrieved from NASA's ASRS site as of July 2007 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.