![]()
The NTSB issued a recommendation to Twin Commander Aircraft after identifying an issue in an investigation involving a Twin Commander 690B plane impacting terrain after an inadvertent stall and spin. Based on the findings, the NTSB recommends the development of a supplement that documents a spin recovery technique for the inclusion of Twin Commander aircraft 690, 690A and 690B models' Pilot Operating Handbooks, as well as distributing this information to owners and operators.
On Sept. 28, 2021 a Twin Commander 690B (N690LS) took off with a company pilot and two employees on an aerial imagery survey flight of forest vegetation. The plane began to level off at 16,100 feet mean sea level and accelerated to a maximum recorded groundspeed of 209 knots. Within two minutes, the ground speed decreased to 93 knots and the plane descended about 500 ft while on a steady heading. The Twin Commander then entered into a rapid descent and right turn, transmitting "mayday, mayday, mayday" and "we're in a spin" to air traffic control. A witness near the site saw the plane spinning rapidly about its longitudinal axis. The plane impacted the ground in a nose-low vertical attitude at a high speed, killing all three on board.Photos of main wreckage
The NTSB found the probable cause to be the pilot's failure to maintain adequate airspeed, causing the plane to exceed its critical angle of attack and enter into an inadvertent stall and spin. During the investigation, the NTSB found five other accidents involving a Twin Commander in which the pilot could not recover the plane from a spin.
According to the aircraft performance study, when the plane pitched down the normal load factor rapidly decreased, consistent with a stall when the wing exceeds its critical angle of attack. When this occurs, the airflow becomes separated at the wing and it can no longer generate the necessary lift. If the plane is in uncoordinated flight at the stall, it can result in a spin. This means that the pilot likely did not maintain adequate airspeed, which caused the plane to exceed its critical angle of attack, entering a stall and spin. The NTSB notes that an important but previously unknown factor before and during the initial stall was the pilot's behavior concerning his flight control inputs and a possible attempt to recover.
There were five other related cases found during the investigation, dating from 1972 in a 690 to the 2021 crash in a 690B. On Aug. 14, 1972 in Wellsburg, West Virginia a 690 suffered a loss of control in a stall maneuver at 12,500 ft for unknown reasons, killing three. On June 24, 1987 in Hilliard, Florida a 690A climbed normally to 9,200 ft and then entered a near vertical dive and broke up, killing two. The pilot had previously mentioned rolling the plane. On June 25, 1992 in Konawa, Oklahoma a 690B was deviating around weather and climbing through to 20,500 ft msl. The plane departed a controlled flight, entered a right spiral with descent rates in excess of 16,600 fpm and suffered an in-flight breakup, killing one person. On June 20, 2013 in McClellanville, South Carolina a training flight for earwork in a 690B at an altitude block of 13,000-15,000 abruptly turned right and lost altitude, consistent with loss of plane control, killing two. On April 9, 2016 in Taylor, Texas a 690B on a recurrent training flight was clearing turns and slowing to 90 kt groundspeed at 5,000 ft when it entered a steep bank and impacted the ground in a nose-low attitude, killing two.
The NTSB notes that the current POH for the Twin Commander 690B does not include a procedure for recovering from an inadvertent spin, nor do the handbooks for the 690 and 690A. Later models, like the 690C and 690D do include a procedure. The 690B was certified in 1976 for normal category, non-acrobatic, non-scheduled passenger and non-scheduled cargo operations and under this operational category, spin recovery procedures were not required. The NTSB believes that while pilots flying in these planes would not intentionally perform spins, since an inadvertent spin could occur, spin recovery technique should be included in handbooks.
Based on the investigation and the five other related crashes, the NTSB found that the inclusion of a spin recovery procedure in the Twin Commander POH for the 690, 690A and 690B models could help pilots recover from a spin in the event they inadvertently enter a stall and spin condition.