Narrative:

Captain was receiving heads up guidance system training from check airman. Captain was in left seat for departure which has no heads up guidance system, so check airman was making the takeoff from the right seat. Prior to taking the runway, captain completed the takeoff warning horn check and received no aural warning sound. At approximately 80 KTS, the takeoff warning horn sounded, captain assumed control of the aircraft and completed the aborted takeoff procedures. After the abort, all applicable checklists were completed. The performance computer indicated a 34 min parked brake cooling time. Dispatch and maintenance were contacted and all agreed that another takeoff (after the parked cooling time) would be appropriate. Both captain and check airman agreed that the most likely reason for the takeoff warning horn sounding was the speed brake handle moving out of the 'seated' position. At least 2 takeoff warning checks were completed prior to the second takeoff attempt with no warning horn. During the second takeoff attempt, the takeoff warning horn sounded at approximately 120 KTS (V1 speed 138 KTS). Once again, captain aborted the takeoff. All applicable checklists were once again accomplished with the performance computer indicating a 74 min cooling time and warning not to get the parking brake. Maintenance met us at the gate, took temperature readings of the brakes and discovered that with the trims set at 5.3 units, a 1/2 to 3/4 turn on the trim wheel made the takeoff warning horn to sound. Maintenance stated the trim limit switch was 'out of rig' and would have to be replaced/fixed. The aerodynamics loads were activating the switch and that was why we were not receiving the horn during the takeoff warning check. An aircraft change was accomplished.

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Original NASA ASRS Text

Title: A B737 HAS TO ABORT 2 TKOFS DUE TO A TRIM LIMIT SWITCH BEING OUT TOLERANCES WHICH CAUSED THE TKOF WARNING SYS TO SOUND DURING TKOF ROLL.

Narrative: CAPT WAS RECEIVING HEADS UP GUIDANCE SYS TRAINING FROM CHK AIRMAN. CAPT WAS IN L SEAT FOR DEP WHICH HAS NO HEADS UP GUIDANCE SYS, SO CHK AIRMAN WAS MAKING THE TKOF FROM THE R SEAT. PRIOR TO TAKING THE RWY, CAPT COMPLETED THE TKOF WARNING HORN CHK AND RECEIVED NO AURAL WARNING SOUND. AT APPROX 80 KTS, THE TKOF WARNING HORN SOUNDED, CAPT ASSUMED CTL OF THE ACFT AND COMPLETED THE ABORTED TKOF PROCS. AFTER THE ABORT, ALL APPLICABLE CHKLISTS WERE COMPLETED. THE PERFORMANCE COMPUTER INDICATED A 34 MIN PARKED BRAKE COOLING TIME. DISPATCH AND MAINT WERE CONTACTED AND ALL AGREED THAT ANOTHER TKOF (AFTER THE PARKED COOLING TIME) WOULD BE APPROPRIATE. BOTH CAPT AND CHK AIRMAN AGREED THAT THE MOST LIKELY REASON FOR THE TKOF WARNING HORN SOUNDING WAS THE SPD BRAKE HANDLE MOVING OUT OF THE 'SEATED' POS. AT LEAST 2 TKOF WARNING CHKS WERE COMPLETED PRIOR TO THE SECOND TKOF ATTEMPT WITH NO WARNING HORN. DURING THE SECOND TKOF ATTEMPT, THE TKOF WARNING HORN SOUNDED AT APPROX 120 KTS (V1 SPD 138 KTS). ONCE AGAIN, CAPT ABORTED THE TKOF. ALL APPLICABLE CHKLISTS WERE ONCE AGAIN ACCOMPLISHED WITH THE PERFORMANCE COMPUTER INDICATING A 74 MIN COOLING TIME AND WARNING NOT TO GET THE PARKING BRAKE. MAINT MET US AT THE GATE, TOOK TEMP READINGS OF THE BRAKES AND DISCOVERED THAT WITH THE TRIMS SET AT 5.3 UNITS, A 1/2 TO 3/4 TURN ON THE TRIM WHEEL MADE THE TKOF WARNING HORN TO SOUND. MAINT STATED THE TRIM LIMIT SWITCH WAS 'OUT OF RIG' AND WOULD HAVE TO BE REPLACED/FIXED. THE AERODYNAMICS LOADS WERE ACTIVATING THE SWITCH AND THAT WAS WHY WE WERE NOT RECEIVING THE HORN DURING THE TKOF WARNING CHK. AN ACFT CHANGE WAS ACCOMPLISHED.

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.