What happened
A private pilot and a certified flight instructor (CFI) were conducting a local flight with a third occupant aboard. The primary objective of the flight was to provide dual instruction to the private pilot in preparation for obtaining a multi-engine rating. At 18:18, one of the pilots contacted Houston TRACON to request radar advisories and stated their intention to perform some air work in the area. The last transmission from the aircraft occurred at 18:47, when the controller advised that they were clear of other traffic. Approximately four minutes later, radar contact with the aircraft was lost.
The aircraft was located where it had impacted the ground in a relatively flat area, appearing to be in a wings-level attitude. Investigation revealed evidence that the aircraft was spinning left at the time of impact. Radar data indicated that the pilots had executed a series of stalls just before the aircraft entered a rapid descent from approximately 7,800 feet, which corresponded to a density altitude of roughly 10,250 feet.
The investigation
For the specific weight and configuration of the aircraft, the stall speed (Vs) was determined to be 81 knots. The published minimum control speed under sea level conditions was also 81 knots, while the intentional one-engine inoperative speed (Vsse) was 86 knots.
Findings
The pilots were operating at a density altitude significantly higher than the pressure altitude, which degraded aircraft performance. The execution of a series of stalls in a multi-engine aircraft at high density altitude likely led to an aerodynamic stall and subsequent spin from which recovery was not possible given the low altitude.