Narrative:

After pushback, all 3 engines were started normally. The captain began taxi toward the ramp exit, and I moved the flap lever to the 15 degree detent. Before exiting the ramp we attempted a check of the speed brake/flap warning system, which sounds a horn if speed brakes and flaps are extended simultaneously. The horn did not sound and I then checked the flap gauges which read: outboard flaps, 15 degrees; inboard flaps, up, leading edge flap green light, on. Captain stopped the aircraft and we asked ramp tower to look us over, and called company maintenance. Also, the second officer stepped back into the cabin to visually inspect the flaps, and returned with the news that the outboards were extended and the inboards were, indeed, up. Maintenance confirmed this further upon their arrival. Also, during this time, we advised the #3 throttle and received a takeoff warning horn, warning of an incorrect condition for takeoff. Captain elected (with full concurrence of crew) to return to the gate for corrective action. Maintenance determined the problem to be a faulty inboard flap gauge, replaced same and flight continued as scheduled. Human factors considerations: the flap gauges in the large transport are arranged and calibrated such that the needles are at 9 O'clock in the up position and at 3 O'clock in the 15 degree position. This results in parallel needles if one set of flaps (inboard or outboard) do nt extend for takeoff. Since a quick scan of gauges, checking for parallel needles, is a common habit pattern, there is nothing to immediately call attention to this conditions--a potentially deadly flap setting. The warning horn test was what called our attention to the problem. Also, the takeoff warning horn would have further warned us, except that it was not supposed to work in this situation. The takeoff warning horn is wired to the outboard flaps and they were in the 15 degree takeoff position. Therefore, the takeoff horn should not have sounded. The speed brake/flap warning horn, on the other hand, is wired to the inboard flaps, and it functioned normally in this case, because it sensed the flaps were up, therefore no horn. Bottom line: we potentially could have taken off with inboard flaps up, outboards extended, with unknown results. Manufacturer claims this cannot happen, and further that the flap gauge arrangement is 'not a problem.' unfortunately, they are wrong! Recommendations: redesign large transport flap gauges so that the up position indication is at 12 O'clock. Then, a split flap indication such as we had will be much more noticeable and we won't have to rely on a warning horn system which may or may not work. I consider this to be an extremely important issue, which has been ignored too long and must be addressed before more people die.

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Original NASA ASRS Text

Title: ACR LGT FLAP GAUGE PROBLEM DETECTED BEFORE TKOF. RETURNED TO RAMP FOR MAINTENANCE ACTIVITY.

Narrative: AFTER PUSHBACK, ALL 3 ENGS WERE STARTED NORMALLY. THE CAPT BEGAN TAXI TOWARD THE RAMP EXIT, AND I MOVED THE FLAP LEVER TO THE 15 DEG DETENT. BEFORE EXITING THE RAMP WE ATTEMPTED A CHK OF THE SPD BRAKE/FLAP WARNING SYS, WHICH SOUNDS A HORN IF SPD BRAKES AND FLAPS ARE EXTENDED SIMULTANEOUSLY. THE HORN DID NOT SOUND AND I THEN CHKED THE FLAP GAUGES WHICH READ: OUTBOARD FLAPS, 15 DEGS; INBOARD FLAPS, UP, LEADING EDGE FLAP GREEN LIGHT, ON. CAPT STOPPED THE ACFT AND WE ASKED RAMP TWR TO LOOK US OVER, AND CALLED COMPANY MAINT. ALSO, THE S/O STEPPED BACK INTO THE CABIN TO VISUALLY INSPECT THE FLAPS, AND RETURNED WITH THE NEWS THAT THE OUTBOARDS WERE EXTENDED AND THE INBOARDS WERE, INDEED, UP. MAINT CONFIRMED THIS FURTHER UPON THEIR ARR. ALSO, DURING THIS TIME, WE ADVISED THE #3 THROTTLE AND RECEIVED A TKOF WARNING HORN, WARNING OF AN INCORRECT CONDITION FOR TKOF. CAPT ELECTED (WITH FULL CONCURRENCE OF CREW) TO RETURN TO THE GATE FOR CORRECTIVE ACTION. MAINT DETERMINED THE PROB TO BE A FAULTY INBOARD FLAP GAUGE, REPLACED SAME AND FLT CONTINUED AS SCHEDULED. HUMAN FACTORS CONSIDERATIONS: THE FLAP GAUGES IN THE LGT ARE ARRANGED AND CALIBRATED SUCH THAT THE NEEDLES ARE AT 9 O'CLOCK IN THE UP POS AND AT 3 O'CLOCK IN THE 15 DEG POS. THIS RESULTS IN PARALLEL NEEDLES IF ONE SET OF FLAPS (INBOARD OR OUTBOARD) DO NT EXTEND FOR TKOF. SINCE A QUICK SCAN OF GAUGES, CHKING FOR PARALLEL NEEDLES, IS A COMMON HABIT PATTERN, THERE IS NOTHING TO IMMEDIATELY CALL ATTN TO THIS CONDITIONS--A POTENTIALLY DEADLY FLAP SETTING. THE WARNING HORN TEST WAS WHAT CALLED OUR ATTN TO THE PROB. ALSO, THE TKOF WARNING HORN WOULD HAVE FURTHER WARNED US, EXCEPT THAT IT WAS NOT SUPPOSED TO WORK IN THIS SITUATION. THE TKOF WARNING HORN IS WIRED TO THE OUTBOARD FLAPS AND THEY WERE IN THE 15 DEG TKOF POS. THEREFORE, THE TKOF HORN SHOULD NOT HAVE SOUNDED. THE SPD BRAKE/FLAP WARNING HORN, ON THE OTHER HAND, IS WIRED TO THE INBOARD FLAPS, AND IT FUNCTIONED NORMALLY IN THIS CASE, BECAUSE IT SENSED THE FLAPS WERE UP, THEREFORE NO HORN. BOTTOM LINE: WE POTENTIALLY COULD HAVE TAKEN OFF WITH INBOARD FLAPS UP, OUTBOARDS EXTENDED, WITH UNKNOWN RESULTS. MANUFACTURER CLAIMS THIS CANNOT HAPPEN, AND FURTHER THAT THE FLAP GAUGE ARRANGEMENT IS 'NOT A PROB.' UNFORTUNATELY, THEY ARE WRONG! RECOMMENDATIONS: REDESIGN LGT FLAP GAUGES SO THAT THE UP POS INDICATION IS AT 12 O'CLOCK. THEN, A SPLIT FLAP INDICATION SUCH AS WE HAD WILL BE MUCH MORE NOTICEABLE AND WE WON'T HAVE TO RELY ON A WARNING HORN SYS WHICH MAY OR MAY NOT WORK. I CONSIDER THIS TO BE AN EXTREMELY IMPORTANT ISSUE, WHICH HAS BEEN IGNORED TOO LONG AND MUST BE ADDRESSED BEFORE MORE PEOPLE DIE.

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.