Saturday, May 18, 2019

737 Pitch Trim Incidents

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.

Thursday, May 16, 2019

The Beat Goes On

On Wed, May 15, House Transportation and Infrastructure Committee, Aviation Subcommittee convened a Hearing with the NTSB and FAA regarding 737 MAX airworthiness.  Information was provided, that which was new, still confusing, important, repeated, wrong, left out, and offensive.