Uncommanded Pitch-Up Event Reported On Approach

The FAA has issued a Special Airworthiness Information Bulletin (SAIB) concerning de-icing procedures of Boeing 737-600/-700/-700C/-800/-900/-900ER airplanes.

The SAIB was prompted a report from an operator of a Model 737-800 airplane of an uncommanded pitch-up event after intercepting the glide slope during a normal approach. The elevator failed to properly respond to autopilot commands, and the flight crew reported that the elevator responded only after higher column forces were applied.

It was determined that de-icing fluid can enter the tail cone during airplane de-icing with the horizontal stabilizer trimmed to the full leading-edge-up position. This fluid can then freeze around the elevator power control unit (PCU) input levers, restricting their movement. Boeing testing has shown that positioning the horizontal stabilizer in the green band range during de-icing will reduce the amount of fluid that can enter the tail cone through the elevator linkage side cutout.

Procedural changes have been made to the 737 Flight Crew Operations Manual (FCOM) D6-27370-TBC, dated September 25, 2014, and the Boeing 737-600/700/800/900 Aircraft Maintenance Manual (AMM), Task 12-33-01 “Cold Weather Maintenance Procedure,” dated October 15, 2014, to reduce the amount of fluid that can enter the tail cone through the elevator linkage side cutout. The revised FCOM procedure instructs the flight crew to position the horizontal stabilizer at the calculated take-off setting within the green band range for de-icing. The revised AMM procedure instructs maintenance personnel to position the horizontal stabilizer to five units of trim. Additional guidance for these procedures can be found in Boeing Fleet Team Digest 737NG-FTD-27-13005.

The FAA recommends that flight crews and maintenance technicians of the referenced airplanes follow the procedures outlined in the above service information.

(Image from file. Not incident airplane)

FMI: SAIB