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.