# NTSB CEN12IA502: Boeing 737-924ER, N37420, Denver, CO, July 31, 2012 - Type: NTSB incident investigation - Classification: Incident · No injuries · Incident investigation · Final report - Date: July 31, 2012, 09:09 local (15:09 UTC) - Place: Denver, CO - Coordinates: 39.861667, -104.673057 - Nearby events: https://openairdocs.org/map.md?lat=39.8617&lon=-104.6731&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Defining event: [Enroute-descent: Birdstrike](https://openairdocs.org/ntsb-events/901) - Highest injury: No injuries - Injuries: 157 uninjured - Status: Completed, report March 24, 2014 - Report: Approved by Office Director - NTSB office: Central Region - Parties: United Airlines, FAA - United Airlines CMO - NTSB case key (mKey): 84541 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN12IA502 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/84541/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=84541 - Page: https://openairdocs.org/ntsb/CEN12IA502 ## 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. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Object/animal/substance: animal(s)/bird(s)](https://openairdocs.org/ntsb-findings/03022045): Environmental issues › Physical environment · Effect on operation (probable cause); cases with it: [Boeing 737-924ER, 1 case](https://openairdocs.org/ntsb-findings/03022045/cases.md?model=13848A3), [All aircraft, 358 cases](https://openairdocs.org/ntsb-findings/03022045/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Enroute-descent: Birdstrike](https://openairdocs.org/ntsb-events/901) (defining event); cases with it: [Boeing 737-924ER, 2 cases](https://openairdocs.org/ntsb-events/901/sequence.md?model=13848A3), [All aircraft, 253 cases](https://openairdocs.org/ntsb-events/901/sequence.md) ## 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. ## Factual narrative 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 whiledescending 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. ## Aircraft - Registration: [N37420](https://openairdocs.org/aircraft/N37420), registered now, S/N 33457, built 2008 (OpenAirDocs) - Reported make and model: BOEING 737-924ER - Registry model: [Boeing 737-924ER](https://openairdocs.org/model/boeing-737-924er) (registry code 13848A3) (OpenAirDocs) - Registry engine: [CFM International CFM56-7B26 series](https://openairdocs.org/engine/cfm-international-cfm56-7b26-series) (registry code 13045, turbo-fan) (OpenAirDocs) - Category: Airplane - Serial number: 33457 - Engines: CFM INTL CFM56-7B26 26300 lbs thrust turbo fan; CFM INTL CFM56-7B26 26300 lbs thrust turbo fan - Damage: Minor damage - Flight: Part 121 - Operator: United Air Lines Inc - Registered owner: Continental Airlines Inc - Airworthiness certificate: Standard, Transport - Departed: DFW, Dallas, TX - Destination: Denver, CO - Flight plan: IFR - Airport: DEN, Denver International Airport, runway 26, 12,000 × 150 ft, concrete ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 51; injuries: none; airline transport, flight engineer; airplane ratings: multi-engine land; instrument ratings: airplane; medical: class 1, without waivers/limitations; seat: left; flight hours: 20,000 total, 10,000 as pilot in command, 220 in the last 90 days, 80 in the last 30 days, 8 in the last 24 hours - Co-pilot; age 47; injuries: none; flight engineer, military, commercial, airline transport; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane, helicopter; medical: class 1, without waivers/limitations; last flight review June 1, 2012; seat: right; flight hours: 9,000 total, 3,000 as pilot in command, 70 in the last 90 days, 70 in the last 30 days, 8 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 220° at 19 knots, gusting 26 - Visibility: 10 statute miles - Lowest ceiling: 20,000 ft broken - Temperature: 73 °F, dew point 52 °F - Altimeter: 30.17 inHg - Observed at: DEN, 45 nm away ## Docket (9 files) The files NTSB released with the investigation on January 31, 2014, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=84541. 1.2 MB in all. - Cockpit Voice Recorder 12 - Specialist's Factual Report (item 1, .PDF, 93 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40376247&FileExtension=.PDF&FileName=Cockpit%20Voice%20Recorder%2012%20-%20Specialist%27s%20Factual%20Report-Master.PDF - First Officer's Statement (item 2, .PDF, 36 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404475&FileExtension=.PDF&FileName=First%20Officer%27s%20Statement-Master.PDF - Captain's Statement (item 3, .PDF, 39 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404476&FileExtension=.PDF&FileName=Captain%27s%20Statement-Master.PDF - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 4, .PDF, 264 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404478&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Redacted.PDF - Bird Strike Report - 1 (item 5, .PDF, 120 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404485&FileExtension=.PDF&FileName=Bird%20Strike%20Report%20-%201-Redacted.PDF - Photo of damage to radome (item 6, .PDF, 188 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404481&FileExtension=.PDF&FileName=Photo%20of%20damage%20to%20radome-Master.PDF - Photo of left pitot tube (item 7, .PDF, 160 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404482&FileExtension=.PDF&FileName=Photo%20of%20left%20pitot%20tube-Master.PDF - Statement of Party Representatives to NTSB Investigation (Boeing) (item 8, .PDF, 229 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404487&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation%20(Boeing)-Redacted.PDF - Statement of Party Representatives to NTSB Investigation (United Airlines) (item 9, .PDF, 107 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40404488&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation%20(United%20Airlines)-Redacted.PDF From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.