Narrative:

When the rudder trim switch on the medium large transport was turned to the 'add right rudder trim position' the switch stuck in this position until approximately 5 degree of unwanted trim was added. This is particularly disturbing since this occurred on aircraft whose rudder trim system was modified to the new standard to prevent such occurrences. I believe that this modified system is still deficient for several reasons. First the new round knob has a large flange on the bottom of this knob. This provides a very convenient spot for sticky liquids to invade and cause adhesion between the knob flange and the face plate of the panel which can cause this switch to stick. Instead this knob should be shaped like an inverted cone and thus eliminates this cause for the switch sticking. In addition there should be a light or aural warning for rudder or aileron trim in motion. Had the crew not caught this and the trim continued to be added it is very likely the autoplt would have kicked off and the aircraft would have likely produced a series of severe dutch rolls prior to being brought back under control. The second problem with this new knob is that the knob has no index marks on it. Thus you are unable to tell from a glance if the switch is stuck.

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Original NASA ASRS Text

Title: JUMP SEAT RIDER OBSERVED A RUDDER TRIM SWITCH STICKING WHEN RUDDER TRIM WAS ACTIVATED. SAYS IT WAS A NEW 'IMPROVED' KNOB BUT STILL HAS A PROBLEM.

Narrative: WHEN THE RUDDER TRIM SWITCH ON THE MLG WAS TURNED TO THE 'ADD R RUDDER TRIM POS' THE SWITCH STUCK IN THIS POS UNTIL APPROX 5 DEG OF UNWANTED TRIM WAS ADDED. THIS IS PARTICULARLY DISTURBING SINCE THIS OCCURRED ON ACFT WHOSE RUDDER TRIM SYS WAS MODIFIED TO THE NEW STANDARD TO PREVENT SUCH OCCURRENCES. I BELIEVE THAT THIS MODIFIED SYS IS STILL DEFICIENT FOR SEVERAL REASONS. FIRST THE NEW ROUND KNOB HAS A LARGE FLANGE ON THE BOTTOM OF THIS KNOB. THIS PROVIDES A VERY CONVENIENT SPOT FOR STICKY LIQUIDS TO INVADE AND CAUSE ADHESION BTWN THE KNOB FLANGE AND THE FACE PLATE OF THE PANEL WHICH CAN CAUSE THIS SWITCH TO STICK. INSTEAD THIS KNOB SHOULD BE SHAPED LIKE AN INVERTED CONE AND THUS ELIMINATES THIS CAUSE FOR THE SWITCH STICKING. IN ADDITION THERE SHOULD BE A LIGHT OR AURAL WARNING FOR RUDDER OR AILERON TRIM IN MOTION. HAD THE CREW NOT CAUGHT THIS AND THE TRIM CONTINUED TO BE ADDED IT IS VERY LIKELY THE AUTOPLT WOULD HAVE KICKED OFF AND THE ACFT WOULD HAVE LIKELY PRODUCED A SERIES OF SEVERE DUTCH ROLLS PRIOR TO BEING BROUGHT BACK UNDER CTL. THE SEC PROB WITH THIS NEW KNOB IS THAT THE KNOB HAS NO INDEX MARKS ON IT. THUS YOU ARE UNABLE TO TELL FROM A GLANCE IF THE SWITCH IS STUCK.

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.