Incident Overview
On July 31, 2012, at approximately 0909 mountain daylight time, a Boeing 737-924ER (registration N37420) operating as United Airlines flight 1475 struck a large bird while approaching Denver International Airport (DEN), Colorado. The aircraft was registered to Continental Airlines, Inc., and operated by United Airlines under 14 CFR Part 121 as a domestic passenger flight originating from Dallas/Fort Worth International Airport (DFW) at 0818. Visual meteorological conditions prevailed, and the flight operated on an instrument flight rules flight plan.
Crew Actions
The flight crew reported hearing a loud bang from the front of the airplane during descent, believed at the time to be a bird strike. Following the strike, the captain's airspeed and altimeter became inoperative, while the first officer's airspeed and altitude indications were erratic. The crew referenced standby instruments, but the readings did not align with the aircraft's altitude and power settings. The captain initially lost airspeed and altitude indication and eventually lost all instrument indications. The crew used power settings, aircraft configurations, and air traffic control (ATC) callouts of ground speed to continue the flight to DEN, where an uneventful landing was made. The operator later reported that the captain stated the standby airspeed indicator readings during descent seemed to match the ground speed callouts provided by ATC.
Aircraft Damage
Examination of the airplane after the incident revealed a large hole in the radome on the nose, measuring about 12 by 24 inches. A portion of the broken radome lodged on the left pitot tube, which was bent. Bird remains were found within the left engine and on the tail. Subsequent examination identified the bird species as a White Faced Ibis. No injuries were reported.
Air Data System Details
The airplane was equipped with three separate and independent air data systems: left, right, and standby. Each used independent sensors and displays, though some comparative display messages relied on data from both left and right systems. The flight data recorder (FDR) computed airspeed and altitude from the left ADIRU, based on sensed data from the left pitot probe and static ports. The right ADIRU and standby system data were not recorded by the FDR.
FDR data showed a rapid loss of airspeed while descending through approximately 11,500 feet pressure altitude at a computed airspeed of about 290 knots. Many air data parameters exhibited erroneous behavior: airspeed dropped to 45 knots, wind data decreased to zero, and pressure altitude increased. The left angle of attack measurement diverged from the right by about 0.5 degrees. The rapid drop to the "no computed data" (NCD) value of 45 knots was consistent with a complete loss of pitot pressure on the left air data system. According to the airplane manufacturer, this would have been consistent with an amber "SPD" flag on the left primary flight display.
Post-Incident Testing
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.