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ASN Wikibase Occurrence # 147274
Last updated: 1 December 2021
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Type:Silhouette image of generic B739 model; specific model in this crash may look slightly different
Boeing 737-924ER (WL)
Owner/operator:United Air Lines
Registration: N37420
MSN: 33457/2565
Fatalities:Fatalities: 0 / Occupants: 157
Other fatalities:0
Aircraft damage: Minor
Location:Denver -   United States of America
Phase: Approach
Nature:Passenger - Scheduled
Departure airport:Dallas, TX (DFW)
Destination airport:Denver, CO
Investigating agency: NTSB
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.

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.



Accident investigation:
Investigating agency: NTSB
Status: Investigation completed
Duration: 1 year and 7 months
Download report: Final report

Revision history:

01-Aug-2012 13:02 ryan Added
01-Aug-2012 13:06 harro Updated [Aircraft type, Cn, Source, Narrative]
21-Dec-2016 19:28 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
28-Nov-2017 20:17 ASN Update Bot Updated [Cn, Operator, Total occupants, Other fatalities, Departure airport, Destination airport, Source, Narrative]
28-Nov-2017 20:18 harro Updated [Operator, Other fatalities, Source]

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