On October 19, 2002, about 2000 eastern daylight time (EDT) [or 0000 coordinated universal time (UTC)], a Boeing 757-200, TF-FII, operating as Icelandair flight 662, experienced a stall while climbing from flight level (FL) 330 (i.e., 33,000 feet) to FL 370. The flight lost about 7,000 feet during the recovery and then diverted to Baltimore-Washington International Airport (BWI), Baltimore, Maryland. There were no injuries to the 191 passengers or 7 crewmembers and no damage to the airplane.
ANALYSIS/CONCLUSIONS
During the takeoff roll as the captain was about to call "eighty" knots, the first officer called "hundred." The captain noted that the standby airspeed indicator agreed with the first officer's and decided to continue the takeoff and address the anomaly of his airspeed indicator after takeoff. The pilots indicated that EICAS messages appeared and disappeared several times after takeoff and during the climb, including the messages MACH/SPD TRIM and RUDDER RATIO. Checklists for MACH/SPD TRIM and RUDDER RATIO messages did not mention an unreliable airspeed as a possible condition. The modifications associated with Boeing Alert Service Bulletin 757- 34A0222 (and mandated by FAA Airworthiness Directive 2004-10-15 after the incident), which had not been incorporated on the incident airplane, would have provided a more direct indication of the airspeed anomaly. According to information in the Icelandair Operations Manual, these EICAS messages (in conjunction with disagreements between the captain and first officer airspeed indicators) may indicate an unreliable airspeed. Overspeed indications and simultaneous overspeed and stall warnings (both of which occurred during the airplane's climb from FL330 to FL370) are also cited as further indications of a possible unreliable airspeed. The crew did take actions in an attempt to isolate the anomalies (such as switching from the center autopilot to the right autopilot at one point during the flight). However, this did not affect the flight management computer's use of data from the left (captain's) air data system, and the erroneous high airspeeds subsequently contributed to airplane-nose-up autopilot commands during and after the airplane's climb to FL370. During the climb the captain's indicated airspeed began to increase, and the overspeed warning occurred. The first officer indicated that at this time his airspeed indication and the standby airspeed indication both decreased to about 220 knots and his pitch attitude felt high. Despite agreement between the first officer and standby airspeed indications and the pilots' belief that the captain's airspeed indicator was inaccurate, control was transferred from the first officer to the captain. Pitch attitude continued to climb and airspeed continued to decay after the captain assumed control. The airplane's pitch attitude became excessively high until the airplane's stick shaker activated and the airplane stalled. Although stall recovery was eventually effected and the airplane was leveled at FL300, the lack of appropriate thrust and control column inputs following the stall delayed the recovery. Evidence from the investigation indicates that 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.
Source:
http://www.rnf.is/media/skyrslur/2002/M-08602_AIG-34,_flugatvik_TF-FII_vid_Baltimore_skyrsla_NTSB.pdf