NTSB performance specialists plotted radar tracks and ATC recordings in an effort to define the aircraft flight path, ground track, ground speed, rate of climb and ATC communication event-time history. They determined that the calibrated airspeed was about 130 ±10 kt. on the final approach, but subsequently decreased to about 95 to 100 kt. during the 20-sec. period prior to loss of radar contact. During the final approach and descent, but prior to the airspeed decay, the calculated flight path angle was about -3 ±1 deg., the calculated bank angle was about 0 ±10 deg. and the estimated angle of attack ranged from 4 to 6 deg.

During the airspeed decay period, the estimated AOA increased by 6 to 8 deg., ending up at 10 to 14 deg. (depending on the assumed engine power setting). According to the Mitsubishi MU-2B-60 AFM, the flaps -5 minimum control speed was 99 kt. and the wings-level power-off stall speed at the accident aircraft weight was about 91 kt.

The MU-2B-60 AFM Approach Checklist calls for flaps lowered to 5 deg. as speed drops below 175 kt., at which time the gear can be lowered. (The AFM also recommends that the operator set the flap switch to 5 deg. and wait for an indication before going to the 20-deg. position.)

The Before Landing Checklist states that the flaps are set to 20 deg. (below 155 KCAS) or 40 deg. (below 120 KCAS). The AFM contains performance charts for “Landing Approach Speed — Flaps 20 Deg.” and “Landing Approach Speed — Flaps 40 Deg.,” but it does offer a chart for a landing approach speed using 5 deg. of flaps.

The AFM also states, “Use of 40-deg. flaps for landing considerably restricts the go-around capability should an engine failure occur in the approach or landing phase. During landing, do not select 40-deg. flaps when operating in icing conditions. The FAA has determined that ice accumulations on the tail plane of many aircraft may result in a reduced down force on the horizontal stabilizer when full flaps are used. This reduced down force may result in the aircraft pitching nose down.”

The general instructions for the MU-2 approach procedures state: The minimum airspeed when using 5 deg. of flaps is 140 kt. (25-30% torque). Check gear down, flaps 20 deg. when approaching glideslope intercept. The minimum airspeed with 20 deg. of flaps is 120 kt. Perform landing check (approximately 25% torque). When landing is assured, check flaps 20 deg. (or 40 deg. of flaps below 120 kt.). At the threshold, fly Vref airspeed (20% torque). At touchdown, retard the power levers to flight idle stop.

Just why the pilot flew the approach with flaps set at 5 deg. — intentionally or unintentionally — is not known. Ultimately, the NTSB determined the probable cause of the accident was simply “The pilot's failure to maintain adequate airspeed during the instrument approach, which resulted in an aerodynamic stall and impact with terrain.”

Unfortunately, there is little unique about this accident. Single-pilot minimums approaches always require maximum situational awareness on the part of the pilot flying. While it is unknown whether the rated-pilot passenger was participating in any way in this phase of the flight, there have been many cases in which distractions developed when a single pilot was being “assisted” or even offering training to a passenger in the right seat.

Single-pilot minimums approaches are just about the most-demanding non-emergency maneuvers required of high-performance aircraft pilots. These operations require planning, use of practiced routine procedures and broad situational awareness. As primary flight instructors are fond of remarking to their students, “You've got to stay on your game and fly the airplane all the way to the chocks.”