NTSB Factual Report

On July 31, 2012, about 0909 mountain daylight time, a Boeing 737-924ER, N37420, struck a large bird while approaching to land at the Denver International Airport (DEN), Denver, Colorado. There were no injuries reported. The airplane sustained damage to the radome, pilot side pitot tube, and the vertical stabilizer. The flight crew declared an emergency and continued to DEN making an uneventful landing. The aircraft was registered to Continental Airlines, Inc., and operated by United Airlines as flight 1475 under the provisions of 14 Code of Federal Regulations Part 121 as a domestic passenger flight. Visual meteorological conditions prevailed for the flight, which operated on an instrument flight rules flight plan. The flight originated from the Dallas/Fort Worth International Airport (DFW), at 0818, and was bound for DEN.

The flight crew reported that during descent for landing, they heard a loud bang from the front of the airplane that was believed at the time to be the result of a bird strike. They reported that following the bird strike, the Captain's airspeed and altimeter were inoperative and the First Officer's airspeed and altitude were erratic. The crew referenced the standby instruments but the readings did not make sense to the crew given the aircraft altitude and power settings. The captain stated that he initially lost airspeed and altitude indication and eventually lost all instrument indications. As a result the flight crew used power settings and aircraft configurations along with air traffic control (ATC) call outs of their ground speed to continue the flight to DEN where an uneventful landing was made. It was later reported by the operator that the Captain reported that the standby airspeed indicator readings during the descent seemed to match the ground speed call outs provided by ATC.

Examination of the airplane after the incident revealed a large hole in the radome located on the nose of the airplane. The hole measured about 12 inches by 24 inches. A portion of the broken radome became lodged on the left pitot tube and the left pitot tube was bent. Bird remains were found within the left engine, and on the tail of the airplane. Subsequent examination of bird remains confirmed the species of bird as a White Faced Ibis.

The airplane was equipped with three separate and independent air data systems: Left, right, and a standby system. Each air data system used independent sensors and displays except that various comparative display messages relied on data from both the left and right systems. The airspeed and altitude recorded by the Flight Data Recorder (FDR) was computed by the left ADIRU based on sensed data from the left pitot probe and static ports. The airspeed and altitude computed by the right ADIRU was based on sensed data from the right pitot probe and static ports. The airspeed and altitude from the right ADIRU and the standby system were not recorded by the FDR.

The FDR data showed a rapid loss of airspeed while descending through approximately 11,500 feet pressure altitude travelling at a computed airspeed of approximately 290 knots. Many of the air data parameters exhibited erroneous behavior consistent with a loss of airspeed, including a decrease of airspeed to 45 knots, a decrease of wind data to a value of zero, and an increase of pressure altitude. Additionally, the left angle of attack measurement split from that of the right by about 0.5 degrees. The FDR continued to record altitude and attitude information throughout the remainder of the flight and the loss of attitude reference described by the captain could not be verified through system testing following the incident. The rapid drop in computed airspeed to the "no computed data" (NCD) value of 45 knots was consistent with a complete loss of airspeed reference (pitot pressure) on the left air data system. According to the airplane manufacturer, the rapid drop in computed airspeed would have been consistent with the display of an amber "SPD" flag on the left primary flight display.

Testing of the airplane's air data systems after replacement of the left pitot tube and system cleaning did not reveal any anomalies that would explain the loss of all instrument indications described by the captain or the reported unreliability of the First Officer's airspeed and altitude indications.

NTSB 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.