Loss of Control in Flight · NTSB CEN21FA459
ROCKWELL INTERNATIONAL 690B — Hiles, WI
| Date | September 28, 2021 |
| Location | Hiles, WI |
| Aircraft | ROCKWELL INTERNATIONAL 690B |
| Purpose of flight | Aerial Observation |
| Conditions | Day · Visual Meteorological Cond |
| Phase / occurrence | Enroute-cruise Loss of control in flight |
| Pilot age | 27 |
| Pilot total time | 1,019 hrs · Experienced |
| Time in type | 300 hrs |
| Fatalities | 3 |
Probable cause
NTSB findings
- Aircraft-Aircraft oper/perf/capability-Performance/control parameters-Airspeed-Not attained/maintained
- Aircraft-Aircraft oper/perf/capability-Performance/control parameters-Angle of attack-Capability exceeded
- Personnel issues-Task performance-Use of equip/info-Aircraft control-Pilot
- Not determined-Not determined-(general)-(general)-Unknown/Not determined
What happened
The company pilot and two employees had departed on an aerial imagery survey flight of forest vegetation. The airplane began to level off at an altitude of about 16,100 ft mean sea level (msl) and accelerated to a maximum recorded groundspeed of 209 knots. Less than 2 minutes later, the groundspeed decreased to about 93 knots, and the airplane descended about 500 ft while on a steady heading. The airplane subsequently entered a rapid descent and a right turn, and “mayday, mayday, mayday” and “we’re in a spin” transmissions were broadcast to air traffic control (ATC). A witness, who was located near the accident site, noticed the airplane nose down at high rate of speed and then saw the airplane spinning rapidly about its longitudinal axis. The airplane wreckage was located in remote wetlands and wooded terrain.
Postaccident examination revealed that the airplane impacted the ground in a nose low vertical attitude and at high speed. All major components of the airplane were located at the accident site. Examination of the airframe, engines, and propellers revealed no preimpact mechanical malfunctions or failures that would have precluded normal operation.
According to the aircraft performance study for this accident, when the airplane pitched down, the normal load factor decreased rapidly from about 1.6 to less than 1 G. A rapid decrease in normal load factor is consistent with a stall when the wing exceeds its critical angle of attack. At that point, the air flow becomes separated at the wing, and the wing can no longer generate the necessary lift. If the airplane is in uncoordinated flight at the stall, a spin can result. Thus, the pilot likely did not maintain adequate airspeed, causing the airplane to exceed its critical angle of attack and enter a stall and spin.
An important but unknown factor before and during the initial stall was the behavior of the pilot regarding his flight control inputs, including his possible attempt to recover. The airplane’s Pilot Operating Handbook states that spins are not authorized and does not include a procedure for inadvertent spin recovery.