Boeing 737-924ER N37420 incident near Denver, Colorado

Probable cause

An in-flight collision with a bird during descent to land, which resulted in damage to the left pitot tube and loss of airspeed information to the left and right primary flight displays (PFD). The reason for the total loss of indications on the left PFD could not be determined based on available evidence.

NTSB analysis

The flight crew reported that the airplane struck a bird while descending to land. Examination of the airplane after the event revealed a large hole in the radome and a portion of the broken radome lodged on the left pitot tube, which was also bent. The flight crew reported that following the bird strike, the captain's airspeed and altimeter were inoperative and the first officer's airspeed and altimeter indications were erratic. The captain eventually lost all instrument indications on his primary flight display (PFD). The crew referenced the standby instruments, but the readings did not make sense given the airplane altitude and power settings. As a result, the flight crew used power settings and aircraft configurations along with air traffic control (ATC) callouts of their ground speed to continue the flight to the intended destination, where they executed an uneventful landing.

The operator later reported that the standby airspeed indicator readings during the descent appeared to match the ground speed callouts provided by ATC. Data retrieved from the flight data recorder (FDR) for the left air data inertial reference unit (ADIRU), which references sensed data from the left pitot probe and static ports, confirmed a loss of airspeed information that would have resulted in an amber flag indicating "IAS DISAGREE" on the left and right primary PFD, as well as an amber flag indicating "SPD" on the left PFD. Data from the right ADIRU and the standby system were not recorded. Other air data parameters exhibited erroneous behavior that was consistent with the loss of airspeed information. The FDR continued to record altitude and attitude information throughout the remainder of the flight. Testing of the air data systems following replacement of the damaged pitot tube and cleaning of the system did not reveal any malfunctions or failures that would explain the total loss of indications on the left PFD.

Text from the NTSB. Open the NTSB docket for CEN12IA502 for the full record.