I can't find the BAOM reference, but here is the text from it:
IssueDate: October 16, 2013
Subject: Supplementary Procedure, Adverse Weather, De-icing/Anti-icing
Reason: To inform 737 flight crews that the stabilizer trim must be set to the calculated takeoff setting prior to de-icing the airplane.
Information in this bulletin is recommended by The Boeing Company, but may not be FAA approved at the time of writing. In the event of conflict with the FAA approved Airplane Flight Manual (AFM), the AFM shall supersede. The Boeing Company regards the information or procedures described herein as having a direct or indirect bearing on the safe operation of this model airplane.
Background Information
In the winter of 2012, a 737 customer reported an un-commanded pitch up on final approach followed by reduced elevator response rates during the upset correction. Root cause analysis determined that the reduced elevator rates were caused by ice accumulation on multiple elevator Power Control Unit (PCU) input levers. De-icing prior to takeoff caused fluid to enter the aft area that accumulated and froze on the PCU input levers during flight, which is believed to be the cause of this incident.
The current de-icing Supplementary Procedure calls for the stabilizer to be trimmed to full APL NOSE DOWN (AND), which positions the stabilizer leading edge to the full leading edge up position. Analysis and testing discovered that this position maximizes the amount of liquids that enter the tailcone near the elevator PCUs due to the opening in the tailcone sides left uncovered by stabilizer’s extreme leading edge up position. If enough fluid accumulates and freezes around the PCU input levers their movement may be restricted or jammed.
Follow-on analysis and testing of various stabilizer positions during de-icing demonstrated that setting the stabilizer trim to the calculated takeoff setting (in the green band) prior to de-icing significantly diminishes the amount of fluids entering the tailcone, reducing the likelihood of a similar event caused by ice accumulation on the PCU input levers.
Operating Instructions
Prior to de-icing the airplane, set the stabilizer trim to the calculated takeoff setting, rather than Full APL NOSE DOWN.
Administrative Information
Insert this bulletin behind the Operations Manual Bulletin Record page in Volume 1 of your Operations Manual. Amend the Operations Manual Bulletin Record to show bulletin "In Effect" (IE).
This bulletin will be cancelled in a future revision of the FCOM.
Please send all correspondence regarding Flight Crew Operations Manual Bulletin status, to the 737 Manager, Flight Technical Data, through the Service Requests Application (SR App) on the MyBoeingFleet home page.