As a result of its investigation of the fatal crash of a Rockwell 690B twin-turboprop on Sept. 28, 2021, the NTSB has issued a safety recommendation to Ontic, which owns the Twin Commander Aircraft product line, to add a spin-recovery procedure to the model 690 and 690 A/B pilot operating handbook (POH). The 690C/D models’ POH already has such a procedure.
The accident took place after departure from Wisconsin’s Rhinelander–Oneida County Airport. After leveling off at 16,100 feet and accelerating to 209 knots groundspeed, the 690B decelerated to about 93 knots groundspeed and descended 500 feet. According to the NTSB, “The airplane subsequently entered a rapid descent and a right turn. ‘Mayday, mayday, mayday’ and ‘we’re in a spin’ transmissions were broadcast to air traffic control. A witness, who was located about one mile from the accident site, observed the airplane in a nose-down attitude, descending at a high rate of speed, and spinning about its longitudinal axis.”
“According to the aircraft performance study for this accident, the airplane pitched down in excess of 30 degrees and descended at a rate that reached 20,000 fpm. About the time that the airplane pitched down, the estimated normal load factor decreased from about 1.6 g to less than 1 g (normal load factor is the ratio of the lift of an aircraft to its weight). A rapid decrease in normal load factor is consistent with a stall when the wing exceeds its critical angle of attack.”
The probable cause of the accident, the NTSB concluded, was “the pilot’s failure to maintain adequate airspeed, which caused the airplane to exceed its critical angle of attack and enter an inadvertent stall and spin.”
In its investigation of this accident, the NTSB noted five other Twin Commander accidents “in which the pilot did not recover the airplane from a spin.” Although the later model 690C and 690D POH does contain a spin-recovery procedure, the 690 and 690 A/B POH only cautions pilots that "acrobatic maneuvers, including spins, are unauthorized.
“The NTSB recognizes that the Twin Commander 690 airplane series was certificated in the normal category; thus, it did not undergo spin testing and, therefore, pilots shouldn’t be intentionally performing spins. However, it is possible pilots could unintentionally enter a spin following a stall, similar to this accident. Accordingly, pilots should have access to all pertinent information to promote safe flight (including spin-recovery techniques). Further, because Twin Commander has already documented this procedure in the POH for the 690C and 690D models, the NTSB believes it is prudent to include the spin-recovery procedure, appropriate to each model, in the POHs for the 690, 690A, and 690B models as well.”
According to Ontic COO Brian Sartain, “Twin Commander is reviewing the NTSB’s recommendations. The safety of Twin Commander pilots and their passengers is of paramount importance to Twin Commander and Twin Commander takes all NTSB’s recommendations seriously. Twin Commander is currently reviewing those recommendations and will take appropriate action.”
Twin Commander built five hundred and forty-one 690 and 690 A/B models and one hundred and seventy-eight 690C/D models, according to information that the company provided to the NTSB.