UKAAIB and BASI
The same incident:
|BASI Occurrence Brief 199805392|
|Location:||100 km SE London|
|Date:||Saturday 28 November 1998|
|Time/Zone:||1300 hours UTC|
|Highest Injury Level:||None|
|Aircraft Manufacturer:||Boeing Co|
|Type of Operation:||Air Transport, High Capacity, International, Passenger, Scheduled|
|Damage to Aircraft:||Nil|
|Departure Point:||London United Kingdom|
AAIB Bulletin No: 6/99 EW/C98/11/7 Category: 1.1
Approximately 45 minutes after departing London Heathrow Airport (LHR), reportedly when the aircraft was still in the climb, an 'equipment cooling' amber message was displayed on the Engine Indication and Crew Alerting System (EICAS) screen, together with the amber flashing attention light. After a brief discussion with the aircraft's maintenance organisation, the crew decided to return to LHR where the aircraft landed without incident. It was subsequently established from the aircraft's Central Maintenance Computer that there was an apparent fault associated with the Electronic Equipment (E/E) bay cooling system ground exhaust valve. A related circuit breaker was also found to have tripped. This valve is located on the lower centreline of the aircraft near Station 540 in an open bay below the floor of the forward cargo compartment, and vents the cooling airflow from the equipment cooling system in the E/E bay overboard when the aircraft is on the ground. In flight, the valve closes and the cooling airflow is re-circulated within the system. This exhaust valve also operates as a smoke clearance valve by opening in flight if smoke is detected in the electronic equipment cooling system (ECS).
Insulation blanket materials
The airframe thermal and acoustic insulation blankets on Boeing aircraft, in common with almost all large aircraft, are fabricated by encapsulating insulating material (typically fire retardant expanded foam or glass fibre materials) within a thin reinforced plastic bag, tailored to fit the appropriate local structure. One of the functions of the bag is to seal the insulating medium against the ingress of water, oil and grime etc to preserve the fire resistance of the blanket and to avoid an unacceptable increase in weight. When examined approximately one week after the incident it was apparent that the damaged insulation bag had not been re-sealed, and evidence of the burnt bag material remained.
All materials used for such blankets are tested by the manufacturer to the Boeing Material Specification documents, in this case BMS 8-300 type I, grade 0.3, for the insulating foam and BMS 8-142 type 11, class 2, for the insulation blanket bag. The blanket in question had recently been changed for a 'lightweight' item fabricated by the operator from approved materials. These were polyimide foam, which had not burnt, and a polyester scrim reinforced polyester film, which had burnt (Figure 2). Certification testing of these materials includes a requirement to pass 'vertical' flammability tests, as specified in Federal Aviation Regulation (FAR) 25.853, Appendix F and in which the material test sample is presented vertically, alongside the heat source. However, a recent report issued by the FAA indicated that the primary response during thermal degradation of such blanket film materials is for the film to rapidly 'shrink away' from the heat source and the report therefore questioned the validity of this current vertical flammability certification test in simulating realistic combustion conditions. This report described an alternative flaming 'cotton swab' test method which it considered more rigorous for certification testing. Boeing has recently incorporated this latter test method into its own material specifications requirements. Two other blanket bag materials, which remain in service but which are no longer manufactured, were also reported upon in this FAA report, ie metallised PET film which was considered 'flammable and which possibly could propagate a fire in a realistic situation', and the much more effective 'Kapton' polyimide film bag material (originally installed at manufacture on all Lockheed L10-11 Tristar aircraft). This latter material is currently being re-evaluated for future widespread use as insulation blanket bag material on public transport aircraft due to its excellent flammability resistance.
|Click HERE for a similar Aug 2002 incident involving a BA 747 just ex-Sydney|