Takeoff and Initial Anomalies
During the takeoff roll, as the captain prepared to call "eighty" knots, the first officer called "hundred." The captain observed that the standby airspeed indicator matched the first officer's indication and decided to continue the takeoff, intending to address the discrepancy afterward.
EICAS Messages and Checklist Limitations
After takeoff and during climb, multiple EICAS messages appeared and disappeared, including MACH/SPD TRIM and RUDDER RATIO. The checklists for these messages did not reference unreliable airspeed as a possible condition. The aircraft had not incorporated modifications from Boeing Alert Service Bulletin 757-34A0222, mandated by FAA Airworthiness Directive 2004-10-15 after the incident, which would have provided a more direct indication of the airspeed anomaly. According to the Icelandair Operations Manual, those EICAS messages combined with airspeed indicator disagreements may indicate unreliable airspeed. Overspeed warnings and simultaneous overspeed and stall warnings, which occurred during the climb from FL330 to FL370, were also cited as further indications of possible unreliable airspeed.
Crew Actions and Airspeed Disagreement
The crew attempted to isolate the anomalies, such as switching from the center autopilot to the right autopilot. However, this did not alter the flight management computer's reliance on data from the left (captain's) air data system. During the climb to FL370, the captain's indicated airspeed began to increase, triggering an overspeed warning. The first officer noted that his own airspeed indication and the standby indication both decreased to about 220 knots, and he perceived a high pitch attitude. Despite agreement between the first officer and standby indications and the pilots' belief that the captain's indicator was inaccurate, control was transferred from the first officer to the captain.
Stall and Recovery
After the captain assumed control, pitch attitude continued to increase and airspeed continued to decay. The airplane's pitch attitude became excessively high until the stick shaker activated and the airplane stalled. Stall recovery was eventually achieved, and the airplane was leveled at FL300, but the recovery was delayed due to insufficient thrust and control column inputs following the stall.
Investigation Findings
Evidence indicated that the anomalies of the captain's airspeed indicator were caused by a partial and intermittent blockage of the captain's pitot tube. The reason for the blockage was not determined.