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B772, London Heathrow UK, 2007

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Summary
On 26 February 2007, a Boeing 777-222 operated by United Airlines, after pushback from the stand at London Heathrow Airport, experienced internal failure of an electrical component which subsequently led to under-floor fire. The aircraft returned to a stand where was attended by the Airfield Fire Service and the passengers were evacuated.
Event Details
When February 2007
Actual or Potential
Event Type
Airworthiness, Fire Smoke and Fumes
Day/Night Day
Flight Conditions On Ground - Normal Visibility
Flight Details
Aircraft BOEING 777-200 / 777-200ER
Operator United Airlines
Domicile United States
Type of Flight Public Transport (Passenger)
Origin London Heathrow Airport
Take off Commenced No
Flight Airborne No
Flight Completed No
Flight Phase Pushback/towing
PBT
Location - Airport
Airport London Heathrow Airport
General
Tag(s) Inadequate Airworthiness Procedures
FIRE
Tag(s) Fire-Electrical origin,
Fire-Underfloor origin
EPR
Tag(s) RFFS Procedures
AW
System(s) Electrical Power,
Equipment / Furnishings,
Fire Protection
Contributor(s) OEM Design fault,
Damage Tolerance,
Component Fault in service
Outcome
Damage or injury Yes
Aircraft damage Major
Causal Factor Group(s)
Group(s) Aircraft Technical
Safety Recommendation(s)
Group(s) Aircraft Airworthiness
Investigation Type
Type Independent

Description

On 26 February 2007, a Boeing 777-222 operated by United Airlines, after pushback from the stand at London Heathrow Airport, experienced internal failure of an electrical component which subsequently led to under-floor fire. The aircraft returned to a stand where was attended by the Airfield Fire Service and the passengers were evacuated.

Synopsis

This is an extract from the aircraft accident Report (2/2009) published by the Aircraft Accident Investigation Branch (AAIB), UK:

The accident occurred during engine start after pushback from the stand. After the right generator came online an electrical failure occurred in the right main bus. The failure resulted in severe internal arcing and short circuits inside the two main power cont actors of the right main bus. The heat generated during the failure resulted in the contactor casings becoming compromised, causing molten metal droplets to fall down onto the insulation blankets below. The insulation blankets ignited and a fire spread underneath a floor panel to the opposite electrical panel (P205) (see Figure 1), causing heat and fire damage to structure, cooling ducts and wiring.

B772 MEC FIRE
Figure 1 - Location of P200 power panel and nearby panels in the Main Equipment Center (MEC). Source: AAIB 2/2009.


The flight crew responded to the bus failure and a burning smell by shutting down the right engine and taxiing to a nearby stand. The Airfield Fire Service attended the aircraft when it arrived on stand and entered the Main Equipment Centre where they discovered significant smoke but no fire. The passengers were evacuated uneventfully via steps.

The Report identifies the following causal factors:

  • An internal failure of the Right Generator Circuit Breaker or Right Bus Tie Breaker contactor on the P200 power panel inside the Main Equipment Centre resulted in severe internal arcing and short-circuits which melted the contactor casings. The root cause of contactor failure could not be determined.
  • The open base of the P200 power panel allowed molten metal droplets from the failed contactors to drop down onto the insulation blankets and ignite them.
  • The aircraft’s electrical protection system was not designed to detect and rapidly remove power from a contactor suffering from severe internal arcing and short-circuits.
  • The contactors had internal design features that probably contributed to the uncontained failures.

The accident Report provides several recommendations (See Further Reading, AAIB 2/2009, page 100) addressing design of electrical systems by manufacturer and replacement/use of specific hardware and software on Boeing 777 aircraft.

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