National Transportation Safety Board
Aviation Incident Final Report

 

 

Location:

Denver, CO

Incident Number:

CEN12IA502

Date & Time:

07/31/2012, 0909 MDT

Registration:

N37420

Aircraft:

BOEING 737-924ER

Aircraft Damage:

Minor

Defining Event:

Birdstrike

Injuries:

157 None

Flight Conducted Under:

Part 121: Air Carrier - Scheduled

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.

 

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this incident to be:
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

Environmental issues

Animal(s)/bird(s) - Effect on operation (Cause)

Factual Information

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.

 

History of Flight

Enroute-descent

Birdstrike (Defining event)

 

 

Pilot Information

Certificate:

Airline Transport; Flight Engineer

Age:

51

Airplane Rating(s):

Multi-engine Land

Seat Occupied:

Left

Other Aircraft Rating(s):

None

Restraint Used:

Seatbelt, Shoulder harness

Instrument Rating(s):

Airplane

Second Pilot Present:

Yes

Instructor Rating(s):

Airplane Single-engine; Instrument Airplane

Toxicology Performed:

No

Medical Certification:

Class 1 Without Waivers/Limitations

Last FAA Medical Exam:

06/07/2012

Occupational Pilot:

Yes

Last Flight Review or Equivalent:

 

Flight Time:

20000 hours (Total, all aircraft), 10000 hours (Pilot In Command, all aircraft), 220 hours (Last 90 days, all aircraft), 80 hours (Last 30 days, all aircraft), 8 hours (Last 24 hours, all aircraft)

 

 

Co-Pilot Information

Certificate:

Airline Transport; Commercial; Flight Engineer; Military

Age:

47

Airplane Rating(s):

Multi-engine Land; Single-engine Land

Seat Occupied:

Right

Other Aircraft Rating(s):

Helicopter

Restraint Used:

Seatbelt, Shoulder harness

Instrument Rating(s):

Airplane; Helicopter

Second Pilot Present:

Yes

Instructor Rating(s):

None

Toxicology Performed:

No

Medical Certification:

Class 1 Without Waivers/Limitations

Last FAA Medical Exam:

06/12/2012

Occupational Pilot:

Yes

Last Flight Review or Equivalent:

06/01/2012

Flight Time:

9000 hours (Total, all aircraft), 3000 hours (Pilot In Command, all aircraft), 70 hours (Last 90 days, all aircraft), 70 hours (Last 30 days, all aircraft), 8 hours (Last 24 hours, all aircraft)

 

 

Aircraft and Owner/Operator Information

Aircraft Make:

BOEING

Registration:

N37420

Model/Series:

737-924ER

Aircraft Category:

Airplane

Year of Manufacture:

 

Amateur Built:

No

Airworthiness Certificate:

Transport

Serial Number:

33457

Landing Gear Type:

Retractable - Tricycle

Seats:

191

Date/Type of Last Inspection:

 

Certified Max Gross Wt.:

 

Time Since Last Inspection:

 

Engines:

2 Turbo Fan

Airframe Total Time:

 

Engine Manufacturer:

CFM INTL

ELT:

 

Engine Model/Series:

CFM56-7B26

Registered Owner:

CONTINENTAL AIRLINES INC

Rated Power:

26300 lbs

Operator:

UNITED AIR LINES INC

Operating Certificate(s) Held:

Flag carrier (121)

Operator Does Business As:

 

Operator Designator Code:

UALA

 

Meteorological Information and Flight Plan

Conditions at Accident Site:

Visual Conditions

Condition of Light:

Day

Observation Facility, Elevation:

DEN, 5434 ft msl

Distance from Accident Site:

45 Nautical Miles

Observation Time:

0853

Direction from Accident Site:

330°

Lowest Cloud Condition:

Few / 12000 ft agl

Visibility

10 Miles

Lowest Ceiling:

Broken / 20000 ft agl

Visibility (RVR):

 

Wind Speed/Gusts:

19 knots / 26 knots

Turbulence Type Forecast/Actual:

/

Wind Direction:

220°

Turbulence Severity Forecast/Actual:

/

Altimeter Setting:

30.17 inches Hg

Temperature/Dew Point:

23°C / 11°C

Precipitation and Obscuration:

No Obscuration; No Precipitation

Departure Point:

Dallas, TX (DFW)

Type of Flight Plan Filed:

IFR

Destination:

Denver, CO

Type of Clearance:

IFR

Departure Time:

0818 CDT

Type of Airspace:

Class E

 

Airport Information

Airport:

Denver International Airport (DEN)

Runway Surface Type:

Concrete

Airport Elevation:

5434 ft

Runway Surface Condition:

Dry

Runway Used:

26

IFR Approach:

Visual

Runway Length/Width:

12000 ft / 150 ft

VFR Approach/Landing:

Straight-in

 

Wreckage and Impact Information

Crew Injuries:

6 None

Aircraft Damage:

Minor

Passenger Injuries:

151 None

Aircraft Fire:

None

Ground Injuries:

N/A

Aircraft Explosion:

None

Total Injuries:

157 None

Latitude, Longitude:

39.861667, -104.673056

 


 


Administrative Information

Investigator In Charge (IIC):

John M Brannen

Report Date:

03/24/2014

Additional Participating Persons:

Thomas Sully; United Airlines; Chicago, IL

Keith A Frable; FAA - United Airlines CMO; Chicago, IL

Publish Date:

03/24/2014

Investigation Docket:

http://dms.ntsb.gov/pubdms/search/dockList.cfm?mKey=84541