REVISED TO INCLUDE OTHER EVENTS => 23 JUNE 2019
Pitch trim is an abstract term to represent the ability to reduce column forces by moving the stabilizer and elevator. This post will dwell on stabilizer trim reported difficulties.
All airplanes provide at least two means to trim the stabilizer, mostly using two seperate actuators motors. Starting with the NG, the Boeing 737 relies on only one electric trim actuator, with manual wheel trim as the backup. There is no documented 737 accident as a result of stabilizer/pitch trim malfunction or failure (prior to JT610 and ET302).
With sparse reporting to draw from, it can be surmised that a stabilizer runaway or failure occurs about once a month, with a jam about once a year (world-wide). Just plugging this into a spreadsheet yields the failure rate for runaway, loss of function and jam. I am just assuming about 5,000 737 during the time frame for the failures accounted for, and looking ahead with a larger fleet size.
Ethiopian ET302 encountered high opposing forces due to the mistrim; it has been assumed that the ET302 actuator was not jammed, and there has been no concern raised that the ET302 clutch would have oppressively opposed trimming manually, on top of the aero loads.
