Aviation Investigation Report A98H0003
1.16.3 Airflow Flight Tests (STI1-99)
Two separate flight tests were conducted to assess airflow patterns in the cockpit; the space above the cockpit ceiling, and the attic space above the forward cabin ceiling. The first test was conducted by the TSB on 27 January 2000 at Long Beach, California, using Swissair MD-11 aircraft HB-IWE and Boeing equipment and facilities. The second test was conducted on 2 December 2000 by Swissair and Boeing in Zurich, Switzerland, using Swissair aircraft HB-IWE. The Swiss AAIB attended the second test flight and collected data on behalf of the TSB.
The airflow flight tests yielded the following results:
- Smoke originating inside the flight crew compartment, forward of the cockpit rear wall, does not move aft into the cabin, but is mainly drawn downward into the avionics compartment below the cockpit floor via the left- and right-side rudder pedals.
- Smoke originating from above the ceiling liner on the right side of the cockpit or behind the avionics CB panel initially migrates to the right and goes down the ladder; when the volume of smoke is sufficient, it enters the overhead panel housing through the aft oval cut-outs and enters the cockpit through the cut-outs for the fire handles.
- Smoke originating behind the ceiling liner on the left side of the cockpit enters the cockpit in one or both of two locations: around the left window air diffusers, or through the left diffuser control slide panel opening.
- The flow of smoke originating anywhere forward of the cockpit rear wall is not affected by whether the ECON switch (cabin air recirculation fans) is selected to the ON or OFF position.
- With the ECON switch selected to the ON position, smoke originating aft of the cockpit wall, in the attic space above the forward cabin drop-ceiling, is mainly drawn aft to the recirculation fans. However, some smoke near the cockpit rear wall could be drawn forward into the cockpit attic or down the cable drop and into the cockpit. Upon entering the cockpit through the cable drop, the smoke can migrate forward to swirl in front of the left-side cockpit windows enroute to the captain's rudder pedals, or go from the closet to the right and then up to swirl in an area around the upper outboard corner of the upper main CB panel before continuing forward to the first officer's rudder pedals.
- With the ECON switch selected to the ON position, smoke released in the ceiling area above the L1 and R1 doors was only smelled in the passenger cabin in the area below these two areas.
- With the ECON switch selected to the OFF position, the air in the attic space above the forward cabin drop-ceiling would flow forward toward the cockpit. Smoke produced in the attic migrates with the airflow toward the cockpit, drawn by the air being exhausted from the cockpit via the avionics compartment cooling fan, through the outflow valve. Smoke filling the attic space also tends to drift down into the passenger cabin through any seams or openings available.
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