(19) The copilot's call of "Four hundred" is construed to mean that an altimeter indicated that the aircraft was at the MDA.
(20) The crew was unaware that the aircraft had descended through the actual MDA.
(21) The crew sighted the glow from the refinery lights during the approach, but never obtained visual contact with any part of the runway environment.
(22) The probable reason for the unrecognized descent through MDA cannot be determined; the two most likely explanations are (a) an error in the static system which caused the barometric altimeters to indicate a figure higher than the actual altitude, or (b) reliance by the crew on the radio altimeter as a primary altitude reference while executing an approach over uneven terrain.
(23) The accident might have been prevented if there had been available the nonstandard glide slope which was installed at a later date.
(b) Probable Cause
The National Transportation Safety Board determines that the probable cause of this accident was the descent below Minimum Descent Altitude during a nonprecision approach under adverse operating conditions, without visual contact with the runway environment. The Board has been unable to determine the reason for this descent, although the two most likely explanations are (a) improper use of cockpit instrumentation data, or (b) an altimetry system error.
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