National Transportation Safety Board
Aviation Incident Final Report

 

 

Location:

Anchorage, AK

Incident Number:

ANC03IA001

Date & Time:

10/09/2002, 1740 AKD

Registration:

N661US

Aircraft:

Boeing 747-400

Aircraft Damage:

None

Defining Event:

 

Injuries:

404 None

Flight Conducted Under:

Part 121: Air Carrier - Scheduled

Analysis

The instrument flight rules scheduled international flight was in cruise flight at FL350 with the autopilot engaged, when it abruptly rolled into a 30 to 40 degree left bank.  There were indications that the lower rudder moved to the left blowdown limit, and remained there.  Emergency procedures failed to correct the problem, and the flight diverted to the nearest airport for an emergency landing.  As the airspeed decreased during the approach, the lower rudder deflected further to the left.  The flightcrew used asymmetric engine thrust to maintain heading.  After landing, the lower rudder remained deflected fully to the left, and could not be repositioned until the hydraulic pressure was relieved.   An inspection of the forged aluminum manifold of the lower rudder power control module (PCM) revealed the end portion which houses the yaw damper actuator had fractured off from the main portion of the manifold.   The lower rudder PCM and the flight data recorder (FDR) were sent to the NTSB laboratory.  The data retrieved from the FDR showed an initial lower rudder deflection of 17.5 degrees left, and a subsequent increase to 32 degrees (full) left.  The incident airplane has two independently supported and operated rudders, which provide yaw control.  Typically, the upper and lower rudders operate in unison.  The hydraulic actuators for the lower and upper rudders are controlled by independent power control modules.  The power control modules for both the upper and lower rudders are virtually identical.  The fractured power control module was disassembled and inspected.  The yaw damper piston was protruding from the fracture, and precluded functional testing of the module.  The individual components of the power control module were tested, and no anomalies were found.  Dimensional checks showed no discrepancies, and metallurgical testing showed the manifold material was consistent with the manufacturer's specifications.  Metallurgical examination revealed a mode of crack initiation and growth consistent with fatigue.  A non-destructive inspection process was developed, and a group of similar power control modules were inspected.  The group contained power control modules with higher and lower use cycles than the incident airplane's power control module.  No similar fractures were found.   As a result of this incident, the airplane's manufacturer has issued an Alert Service Bulletin which recommends operators perform an ultrasonic inspection on pertinent high-time lower and upper rudder power control modules.  The Federal Aviation Administration has issued a Notice of Proposed Rule Making for a airworthiness directive which would make the ultrasonic inspection mandatory for all affected airplanes.    

 

 

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this incident to be:
The fatigue fracture of the lower rudder power control module manifold, which resulted in a lower rudder hardover.

Findings

 

Occurrence #1: AIRFRAME/COMPONENT/SYSTEM FAILURE/MALFUNCTION
Phase of Operation: CRUISE - NORMAL

Findings
1. (C) FLT CONTROL SYST,RUDDER CONTROL - FATIGUE
2. (C) FLT CONTROL SYST,RUDDER CONTROL - FRACTURED
3. FLIGHT CONTROL,RUDDER - HARDOVER
4. DIRECTIONAL CONTROL - DIMINISHED

 

Factual Information

HISTORY OF FLIGHT

On October 9, 2002, about 1740 Alaska daylight time, a Boeing 747-400 airplane, N661US, experienced a lower rudder hardover during cruise flight at FL350.  The airplane was being operated as Flight 85, by Northwest Airlines Inc., as an instrument flight rules (IFR) scheduled international flight under Title 14, CFR Part 121.  The 4 flight crew members, 14 flight attendants, and the 386 passengers, were not injured.  Visual meteorological conditions prevailed, and an instrument flight plan was filed. The flight originated at the Detroit International Airport, Detroit, Michigan, about 1403 eastern daylight time, and was bound for the Narita International Airport, Tokyo, Japan.  Following the lower rudder hardover, the flight diverted to Anchorage, Alaska, where an uneventful landing was made.

During an interview with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on October 10, 2002, the captain said the airplane was at a cruise altitude of 35,000 feet with the autopilot engaged, when it abruptly rolled into a 30 to 40 degree left bank.  He said there were indications that the lower rudder initially moved left to the blowdown limit of 17 degrees deflection, and remained there.  (The blowdown limit is a function of airspeed; the lower the airspeed, the greater the allowed deflection.)  He said he declared an emergency, and diverted the airplane to the Ted Stevens International Airport, Anchorage, Alaska.  The captain said he and the first officer ran through the available emergency procedures, but none of these could correct the problem.  He said as the airspeed decreased during the approach for landing, the lower rudder deflected further to the left.  During the approach and landing, the crew used differential power to aid in directional control.  The captain said after landing, he observed that the lower rudder remained deflected fully to the left.

During an inspection of the airplane by the IIC on October 10, the lower rudder was found in the centered position.  A mechanic for the operator said during his initial inspection the lower rudder was deflected full left as witnessed by the pilot.  He said the lower rudder could not be repositioned until the hydraulic line connected to the positioning actuator was disconnected, relieving the hydraulic pressure.  An inspection of the lower rudder power control module (PCM) revealed the forged aluminum housing (manifold) of the lower rudder power control module was fractured. The end portion of the control module manifold that houses the yaw damper actuator piston, had fractured off from the main portion of the manifold.  The fractured end contained a metal end cap that was safety-wired to the manifold.  The separated portion of the manifold remained attached to the main portion by the safety wire.

The lower rudder power control module, and the flight data recorder (FDR) were removed, and sent to the NTSB laboratory in Washington, DC, for examination.

FDR INFORMATION

The data retrieved from the flight data recorder showed an initial uncommanded lower rudder deflection of 17.5 degrees to the left, and as the airplane slowed during the approach and landing, a subsequent increase to 32 degrees (full) left deflection for the remainder of the flight.

RUDDER SYSTEM INFORMATION

The Boeing 747-400 has two independently supported and operated rudders (upper and lower) which provide yaw control for the airplane.  Each rudder is positioned by a hydraulically operated power control package (PCP).  The hydraulic system operating pressure is 3000 psi, and typically the upper and lower rudders operate in unison.  The lower rudder has less surface area than the upper, and is positioned by two hydraulic actuators, whereas the upper rudder has three actuators.  The hydraulic actuators for the lower and upper rudders are controlled by independent power control modules.  The power control modules for both rudders are virtually identical and are located next to each other in the vertical stabilizer.  Each power control module contains a primary and secondary hydraulic control system, housed within a single manifold.  In the event of a failure of the primary or secondary system, the remaining system can position the rudder.  In this incident, the lower rudder power control module manifold fractured, allowing the yaw damper piston to travel beyond its normal position.  This resulted in a full left command input to the main control valve hence driving the two actuators to the full left rudder position.

TEST AND RESEARCH

The initial metallurgical examination of the fractured power control module by the NTSB laboratory revealed a mode of crack initiation and growth consistent with fatigue.  Under the supervision of the NTSB systems group chairman, the fractured power control module was returned to the manufacturer for disassembly and further inspection.  The yaw damper piston was visibly protruding from the manifold, and precluded operational testing of the manifold.  All the individual components of the power control module were tested, and no anomalies were found.  Dimensional checks of the power control module showed no discrepancies, and metallurgical testing by the manufacturer showed the manifold was made of material consistent with the manufacturer's specification.  Since a fatigue type of failure typically cannot be visually detected prior to the actual failure, a non-destructive inspection process was developed.  A group of similar power control modules that were installed on other airplanes, as well as a spare unit, were inspected.  The inspected group contained power control modules with higher and lower use cycles than the incident airplane's power control module.  No similar fractures were found.  

As a result of this incident, the airplane's manufacturer has issued Alert Service Bulletin 747-27A2397, dated July 24, 2003, which recommends operators perform an ultrasonic inspection of pertinent high-time lower and upper rudder power control modules.  The Federal Aviation Administration has issued a Notice of Proposed Rule Making (NPRM), "Airworthiness Directive; Boeing Model 747-400, -400D, and -400F Series Airplanes," published in the Federal Register on August 28, 2003, which would make this inspection mandatory on affected airplanes.    

 

 

 

 

 

 

 

Pilot Information

Certificate:

Airline Transport

Age:

54, Male

Airplane Rating(s):

Multi-engine Land; Single-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

Toxicology Performed:

No

Medical Certification:

Class 1 Valid Medical--w/ waivers/lim.

Last FAA Medical Exam:

06/12/2002

Occupational Pilot:

 

Last Flight Review or Equivalent:

04/22/2002

Flight Time:

11297 hours (Total, all aircraft), 630 hours (Total, this make and model), 43 hours (Last 90 days, all aircraft), 10 hours (Last 30 days, all aircraft), 10 hours (Last 24 hours, all aircraft)

 

 

Co-Pilot Information

Certificate:

Airline Transport; Flight Engineer

Age:

57, Male

Airplane Rating(s):

Multi-engine Land; Single-engine Land

Seat Occupied:

Right

Other Aircraft Rating(s):

None

Restraint Used:

Seatbelt, Shoulder harness

Instrument Rating(s):

Airplane

Second Pilot Present:

Yes

Instructor Rating(s):

None

Toxicology Performed:

No

Medical Certification:

Class 1 Valid Medical--no waivers/lim.

Last FAA Medical Exam:

06/13/2002

Occupational Pilot:

 

Last Flight Review or Equivalent:

01/15/2002

Flight Time:

3420 hours (Total, all aircraft), 651 hours (Total, this make and model), 129 hours (Last 90 days, all aircraft), 63 hours (Last 30 days, all aircraft), 10 hours (Last 24 hours, all aircraft)

 

 

Aircraft and Owner/Operator Information

Aircraft Make:

Boeing

Registration:

N661US

Model/Series:

747-400

Aircraft Category:

Airplane

Year of Manufacture:

 

Amateur Built:

No

Airworthiness Certificate:

Transport

Serial Number:

23719

Landing Gear Type:

Retractable - Tricycle

Seats:

429

Date/Type of Last Inspection:

08/30/2001, Condition

Certified Max Gross Wt.:

873000 lbs

Time Since Last Inspection:

5209 Hours

Engines:

4 Turbo Fan

Airframe Total Time:

50090 Hours as of last inspection

Engine Manufacturer:

Pratt & Whitney

ELT:

Not installed

Engine Model/Series:

PW4056

Registered Owner:

State Street Bank and Trust Company Trustee

Rated Power:

 

Operator:

NORTHWEST AIRLINES INC

Operating Certificate(s) Held:

Flag carrier (121)

Operator Does Business As:

Northwest Airlines Inc.

Operator Designator Code:

NWAA

 

Meteorological Information and Flight Plan

Conditions at Accident Site:

Visual Conditions

Condition of Light:

Night

Observation Facility, Elevation:

 

Distance from Accident Site:

 

Observation Time:

 

Direction from Accident Site:

 

Lowest Cloud Condition:

Clear

Visibility

 

Lowest Ceiling:

None

Visibility (RVR):

 

Wind Speed/Gusts:

/

Turbulence Type Forecast/Actual:

/

Wind Direction:

 

Turbulence Severity Forecast/Actual:

/

Altimeter Setting:

 

Temperature/Dew Point:

 

Precipitation and Obscuration:

 

Departure Point:

Detroit, MI (DTW)

Type of Flight Plan Filed:

IFR

Destination:

Narita (NRT)

Type of Clearance:

IFR

Departure Time:

1403 EDT

Type of Airspace:

Class A

 

Airport Information

Airport:

Ted Stevens International (PANC)

Runway Surface Type:

Unknown

Airport Elevation:

 

Runway Surface Condition:

Unknown

Runway Used:

 

IFR Approach:

ILS

Runway Length/Width:

 

VFR Approach/Landing:

None

 

Wreckage and Impact Information

Crew Injuries:

18 None

Aircraft Damage:

None

Passenger Injuries:

386 None

Aircraft Fire:

None

Ground Injuries:

N/A

Aircraft Explosion:

None

Total Injuries:

404 None

Latitude, Longitude:

61.174444, -149.996389

 


 


Administrative Information

Investigator In Charge (IIC):

Lawrence R Lewis

Report Date:

06/29/2004

Additional Participating Persons:

Glenn R Smith; Anchorage FSDO-03; Anchorage, AK

Publish Date:

 

Investigation Docket:

NTSB accident and incident dockets serve as permanent archival information for the NTSB’s investigations. Dockets released prior to June 1, 2009 are publicly available from the NTSB’s Record Management Division at pubinq@ntsb.gov, or at 800-877-6799. Dockets released after this date are available at http://dms.ntsb.gov/pubdms/.