Aviation Investigation Report A98H0003
1.14.9 Determination of Heat Damage
To provide a temperature reference to assess the intensity of heat damage caused by the fire, pieces of comparable materials were intentionally exposed, under controlled conditions, to heat at various temperatures for specified time durations. The materials included pieces of the aluminum air conditioning ducts, frames, and intercostals, which were typically covered with green coloured fluid-resistant (FR) primer paint. While heating these materials, it was found that the FR primer paint incrementally changed colour when exposed to increasing temperatures, thereby making it possible to determine the approximate amount of heat exposure experienced by the fire-damaged aircraft pieces. When creating the frame temperature reference exemplar coupons, (STI1-94) it was found that by elevating the temperature to a range of between 482°C to 593°C (900°F to 1 100°F) for 10 minutes, the FR primer paint would disappear from the surface of the exemplar piece, leaving bare metal. The temperatures shown in the figures within this report display either the temperature value of a particular representative coupon or the average value of a range of temperatures in instances where more than one coupon applies. These coupons were made based on a 10-minute exposure at a constant temperature. It is possible that exposure to higher temperatures, over a shorter time duration, may have created similar heat-damage patterns to those observed on the wreckage.
Pieces of non-metallic material, including wire insulation and electrical module blocks, were also exposed to heat under controlled conditions to produce exemplar coupons that enabled an assessment of temperatures reached by like materials from the aircraft wreckage.
The most severe heat damage to metal aircraft structure was identified by the presence in a few areas of resolidified aluminum metal that had once been molten or near-molten. The forces of impact on aluminum in a molten or near-molten state can create a signature, referred to as "broomstraw," at the edges of a fracture. High heat can also create a distinctive, layered, feather-like appearance at the edges of a fracture, referred to as "feathered edge."
By identifying and placing the various wreckage pieces into the reconstruction mock-up, it was possible to assess heat damage and soot patterns in an attempt to determine the origin of the fire and how it propagated. The heat and soot distribution information, together with other data such as the type, amount, and location of combustible materials, was entered into computer models so that the information could be integrated and the patterns more readily viewed and assessed.
Damage patterns indicate that the fire was concentrated in the areas above the cockpit ceiling liner and above the forward cabin drop-ceiling. Reconstruction of the wreckage disclosed significant heat damage on portions of the airframe structure and air conditioning system ducts in these areas, extending from approximately STA 338 to STA 675. Most of the heat damage in the cockpit was concentrated above the level of the bottom of the upper avionics CB panel (Z= 59) and in the area of the forward cabin drop-ceiling above Z= 61. The farthest forward deposits of significant soot in the cockpit were found on the standby compass near STA 313. The farthest aft soot deposits were found on an overhead stowage bin located near STA 1780.
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