B74S, en-route, Anchorage AK USA, 2004
B74S, en-route, Anchorage AK USA, 2004
Summary
On 5 December 2004 a Boeing 747-100SP experienced a rapid depressurisation and subsequent minor damage after reaching cruise level near Anchorage, Alaska. The crew elected to return to Ted Stevens International Airport, Anchorage where the aircraft landed without further incident.
Flight Details
Aircraft
Operator
Type of Flight
Public Transport (Cargo)
Flight Origin
Intended Destination
Actual Destination
Take-off Commenced
Yes
Flight Airborne
Yes
Flight Completed
Yes
Phase of Flight
Cruise
Location
Approx.
137 miles west of Anchorage, Alaska
General
Tag(s)
Inadequate Airworthiness Procedures
AW
System(s)
Airframe
Contributor(s)
Damage Tolerance,
Component Fault in service
Outcome
Damage or injury
Yes
Aircraft damage
Major
Non-aircraft damage
Yes
Non-occupant Casualties
No
Occupant Injuries
None
Occupant Fatalities
None
Off Airport Landing
Yes
Ditching
Yes
Causal Factor Group(s)
Group(s)
Aircraft Technical
Safety Recommendation(s)
Group(s)
None Made
Investigation Type
Type
Independent
Description
On 5 December 2004 a Boeing 747-100SP experienced a rapid depressurisation and subsequent minor damage after reaching cruise level near Anchorage, Alaska. The crew elected to return to Ted Stevens International Airport, Anchorage where the aircraft landed without further incident.
The Investigation
The investigation of the Serious Incident was led by the National Transportation Safety Board (USA) (NTSB) which stated in the official Report:
"An examination revealed an elliptical tear in the right side of the pressure bulkhead web separating the electronics service bay from the nose wheel well. The tear extended about 12-inches along a horizontal rivet line, with both ends of the torn skin turned downward approximately 80-90 degrees. The damaged area (STA 260-280, WL 160-170) was excised from the structure, and an examination of the fracture mechanism was conducted. The fracture examination disclosed that the web fracture was initiated by fatigue from multiple origins on the outboard web surface within the pressure vessel. Ten separate fatigue cracks propagated through the full web thickness before the onset of rapid ductile tearing. Each fatigue crack formed adjacent to a fastener hole common with the WL 170 beam, and was in line with the edge of the bonded strip doublers on the inboard surface (wheel well side). The web material met all the engineering specifications. At the time of the incident, the airplane had logged 58,185 flight hours, and 27,243 cycles/landings. Inspection criteria for the affected area are contained in a manufacturer's Service Bulletin and their Alert Revisions 1 thru 4."
Probable Cause(s) and Recommendations
The NTSB determines the probable cause of this incident as follows:
- "The fatigue and subsequent fracture of a portion of the airplane's pressure bulkhead, which resulted in a rapid decompression during cruise flight."
Following the examination of the incident airplane, Alert Revision 4 was amended to specify more frequent inspections of the affected area.
- "Alert Revision 4 adds the requirement for repeated inspections of areas 1 and 2, at 500 flight-cycle intervals once the airplane has reached 20,000 flight cycles. It also decreased the inspection intervals of area 3 from 6,000 flight-cycles to 1,500 flight-cycles."
Related Articles
- Ageing Aircraft - Structural Failure
- Airworthiness - The System
- Defect Assessment
- Damage Tolerance
- Cooperative Development of Operational Safety and Continuing Airworthiness Programme
