What happened
During an instrument landing system (ILS) approach conducted in snow showers with visibility limited to 0.5 miles, the aircraft experienced a critical loss of airspeed. The flight was following a procedure utilizing distance measuring equipment (D/DME) to identify localizer fixes. As the aircraft progressed through the approach, it maintained altitudes consistent with the previous navigation configuration, which relied on a VORTAC located 4.7 nautical miles beyond the ILS DME.
Upon reaching the outer marker, the aircraft was at an altitude of 900 feet. It successfully intercepted the glide slope from a position approximately 1.8 nautical miles from the runway threshold, maintaining an altitude 500 feet above the decision height and 700 feet above the touchdown zone. However, during the 28 seconds the aircraft remained on the glide slope, its airspeed decreased until it reached stall speed. This resulted in the aircraft dropping below the glide slope and impacting the ground 1.3 nautical miles before reaching the threshold.
Findings
Investigation into the cockpit configuration revealed that the pilot's FLT DIR DME-1/DME-2 switch, which manages the DME display on the horizontal situation indicator (HSI), was positioned to the DME-2 setting. Additionally, the NAV-2 radio was tuned to the VORTAC frequency. The investigation noted that the decay of airspeed to stall speed was the primary factor leading to the descent below the glide slope, noting that recovery from such a stall can require altitudes up to 800 feet.