Narrative:

During the before takeoff checklist, the flaps were extended to 1 degree. The leading edge flaps and slats indicator lights on the forward panel failed to show the devices extended. Instead they indicated them to be in a transit state. We returned to the gate to have maintenance check the problem. The mechanics asked if the leading edge device annunciator on the overhead panel was showing the normal positions with flaps 1 degree selected. We confirmed that the indication on that panel was correct. Since the overhead annunciator panel was working, maintenance put the forward panel leading edge device lights on MEL. I had already anticipated their action and had read the MEL item in the book, and it was legal. Shortly after takeoff, just before flap retraction, we felt an unusual vibration. I immediately thought we had a leading edge device problem. We left the flaps at 1 degree and told departure we wanted to return to land. As we slowed down, the vibration decreased and we made a normal landing, except for being 2000# over maximum gross landing weight. After landing, maintenance could not find the problem. They thought it was caused by a loose trim tab on the elevator. The bad thing about simply placing the leading edge annunciator lights on MEL, instead of testing the system or fixing it, was that it caused me to believe that the vibration was caused by a leading edge device when that may have had nothing to do with it. This sets up a dangerous situation because you assume you know the cause of the problem when you do not. To correct this type of problem in the future, you cannot ever assume that you know what is wrong with an airplane until you prove it--but as long as we fly our airplanes with certain systems on MEL, it will always be hard not to question that system when a problem arises.

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Original NASA ASRS Text

Title: MLG FLT CREW EXPERIENCES VIBRATION AFTER TKOF, ASSUMES CAUSE TO BE MINIMUM EQUIPMENT LISTED ITEM LEADING EDGE DEVICE AND RETURNS FOR LNDG.

Narrative: DURING THE BEFORE TKOF CHKLIST, THE FLAPS WERE EXTENDED TO 1 DEG. THE LEADING EDGE FLAPS AND SLATS INDICATOR LIGHTS ON THE FORWARD PANEL FAILED TO SHOW THE DEVICES EXTENDED. INSTEAD THEY INDICATED THEM TO BE IN A TRANSIT STATE. WE RETURNED TO THE GATE TO HAVE MAINT CHK THE PROB. THE MECHS ASKED IF THE LEADING EDGE DEVICE ANNUNCIATOR ON THE OVERHEAD PANEL WAS SHOWING THE NORMAL POSITIONS WITH FLAPS 1 DEG SELECTED. WE CONFIRMED THAT THE INDICATION ON THAT PANEL WAS CORRECT. SINCE THE OVERHEAD ANNUNCIATOR PANEL WAS WORKING, MAINT PUT THE FORWARD PANEL LEADING EDGE DEVICE LIGHTS ON MEL. I HAD ALREADY ANTICIPATED THEIR ACTION AND HAD READ THE MEL ITEM IN THE BOOK, AND IT WAS LEGAL. SHORTLY AFTER TKOF, JUST BEFORE FLAP RETRACTION, WE FELT AN UNUSUAL VIBRATION. I IMMEDIATELY THOUGHT WE HAD A LEADING EDGE DEVICE PROB. WE LEFT THE FLAPS AT 1 DEG AND TOLD DEP WE WANTED TO RETURN TO LAND. AS WE SLOWED DOWN, THE VIBRATION DECREASED AND WE MADE A NORMAL LNDG, EXCEPT FOR BEING 2000# OVER MAX GROSS LNDG WT. AFTER LNDG, MAINT COULD NOT FIND THE PROB. THEY THOUGHT IT WAS CAUSED BY A LOOSE TRIM TAB ON THE ELEVATOR. THE BAD THING ABOUT SIMPLY PLACING THE LEADING EDGE ANNUNCIATOR LIGHTS ON MEL, INSTEAD OF TESTING THE SYS OR FIXING IT, WAS THAT IT CAUSED ME TO BELIEVE THAT THE VIBRATION WAS CAUSED BY A LEADING EDGE DEVICE WHEN THAT MAY HAVE HAD NOTHING TO DO WITH IT. THIS SETS UP A DANGEROUS SITUATION BECAUSE YOU ASSUME YOU KNOW THE CAUSE OF THE PROB WHEN YOU DO NOT. TO CORRECT THIS TYPE OF PROB IN THE FUTURE, YOU CANNOT EVER ASSUME THAT YOU KNOW WHAT IS WRONG WITH AN AIRPLANE UNTIL YOU PROVE IT--BUT AS LONG AS WE FLY OUR AIRPLANES WITH CERTAIN SYSTEMS ON MEL, IT WILL ALWAYS BE HARD NOT TO QUESTION THAT SYS WHEN A PROB ARISES.

Data retrieved from NASA's ASRS site as of August 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.