A B747 Captain rejected a takeoff at about 100 KTS because he detected a difference between his and the First Officer's airspeed. An air data computer was changed upon return to the gate.
Synopsis
A B747 Captain rejected a takeoff at about 100 KTS because he detected a difference between his and the First Officer's airspeed. An air data computer was changed upon return to the gate.
Narrative
Takeoff was aborted at approximately 100 KTS due to discrepancies between my indicated airspeed and the First Officer's indicated airspeed. [We] accomplished all applicable SOPS and relevant irregular procedures. After the First Officer had applied takeoff thrust I briefly monitored the engine parameters (especially EGT) for normal range and function. Then I briefly checked my airspeed indicator and the First Officer's airspeed indicators for normal acceleration. My indicator appeared normal and showed smooth acceleration. When I glanced at the First Officer's indicator it appeared to indicate less speed and was somewhat erratic. I thought this may be due to the right crosswind conditions. At 80 KTS I looked over at the First Officer's indicator to verify. It was fluctuating and indicating something other than 80 KTS. I felt the safest thing to do was to reject the takeoff and sort out the issue on the ground rather than in the air. I did not make the 80 KTS call but announced and executed the abort maneuver. I believe my intentions and actions are in accord with the training I have received at my airline with regard to when and how to abort a takeoff.
Second reporter narrative
After a normal taxi and lineup on the runway; takeoff power was applied and was stable. The First Officer was the flying pilot. He removed his hands off of the throttles after stabilizing the throttle setting and transferred control of the throttles to the Captain; as per SOP. Passing through 80 KTS; I didn't hear the usual '80 KTS; thrust set' call. As I glanced over to the Captain's airspeed indicator and noticed the airspeed passing through 100 KTS; I then glanced over to the First Officer's airspeed indicator and observed the same. As that was happening; the Captain pulled the throttles to idle and the rejected takeoff braking was initiated. We slowed immediately and the Captain assumed control of the aircraft. The First Officer told Tower we were aborting and I got on the PA and told the passengers; 'Remain in your seats' two times. We taxied clear and kept the parking brake off as we discussed the situation as it occurred. The determination was made to return to the gate; have the tires and brakes checked per the checklist; and refuel. The ADC was changed out; although the self test performed by maintenance passed. The rest of the flight proceeded uneventfully.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.