Page:CAB Accident Report, United Air Lines Flight 823.pdf/12

 The National Bureau of Standards prepared a number of electron photomicrographs of carbon specimens taken from the free-fall window and passenger's clothing, and from various components of the aircraft found at the main wreckage site, primarily from under the cabin floor between FS-317 and FS-414. They conducted tests to determine the effects of various temperatures on paint on pieces of seat track and underfloor runners taken from the aircraft wreckage. They also examined the Janitrol fire extinguisher bottle firing strip. Aluminum paint samples exposed to heat of less than 400°F. for two minutes showed no visible effects. Color changes began after exposure for two minutes at 400°F. and blistering began in two minutes at 450°F. At 900°F. the paint darkened to dark brown or black. These results were used in conjunction with the electron photomicrographs to study various components of the aircraft for evidence of fire in flight. The examination of the Janitrol bottle firing strap was inconclusive and no determinations could be made regarding the conditions under which it fractured.

The photomicrographs of the carbon specimens were forwarded to a specialist in an effort to determine the identification of the material that produced the soot found on the wreckage and the free-fall victim's clothing. The carbon deposits taken from both the free-fall items and under floor wreckage at the crash site were identified as being the produce of incomplete combustion of saturated aliphatic hydrocarbons. Examples of this type of fuel are kerosene, gasoline, paraffin, hydraulic fluid, lighter fluid, and naphtha. Of these examples, only kerosene, hydraulic fluid, and lighter fluid were known to be aboard the aircraft. The lighter fluid was not known to be aboard in sufficient quantity to produce the amount of fire experienced. Kerosene in the form of engine fuel and hydraulic fluid were aboard the aircraft in quantity.

Additional studies of the fire damage were made by a specialist from the Massachusetts Institute of Technology and tests to determine the effect of heat on aluminum surfaces were made under his direction. These tests, made under laboratory conditions, included the exposure of numerous painted and unpainted specimens to open flames in one series and to oven heating in another series. In the open flame tests, kerosene, hydraulic fluid and turbine engine oil were used with both oxidizing and reducing atmospheres to produce soot deposits on the test specimens. In these tests it was found that soot was deposited only on those portions of specimens in the line of flame impingement. Scratches in areas of light sooting remained clean and bright, but became dark with progressively heavier deposits of soot. The soot build-up in scratches and cuts was also a function of flame impingement angle. In another series of tests, individual specimens were dipped in one of the above-mentioned fluids prior to insertion in the oven. They were retained int he oven for five minutes at temperatures ranging from 450°F., to 1000°F. Both oxidizing and reducing atmospheres were used. In these tests painted aluminum surfaces darkened progressively to 800°F. Above this temperature the dark coloring disappeared and the surfaces became silver gray. Between 700° and 800° the fluid deposits burned, leaving black deposits on all surfaces. At lower temperatures scratches and edge cuts on specimens remained bright, independent of the surface discoloration caused by the fluids and other deposits on surface finishes on the specimens. From these tests and comparison with the underfloor wreckage of N7405, he concluded that there was no positive evidence of an underfloor fire prior to impact.