ATRs and icing are a bad combination. Hang on to your hats, folks, this tale's going to be a long one. The ATR was the first aircraft to be globally certificated under the Joint Arcraft Airworthiness Standards Agreement, whereby all ICAO nations agreed to accept the airworthiness certification of an aircraft by it's originating nation at face value. No second guessing by other nations, as had been the case previously. Investigating the 1994 AmEagle crash at Roselawn IN, the NTSB determined that the unrecoverable spin was the result of a stall of the horizontal stabilizer, due to "bleed back" of ice behind the deicing boots, forming a ridge of ice on the (negative) lifting surface that detached the airflow and killed the horizontal stab's downforce. This caused the elevators to slam against the "nose down" stops, pitching the plane into a dive. When the speed increased and the stabilizer AoA decreased enough to reattach the air flow, and then stall negative, the elevators slammed nose up and drove the plane into an accelerated stall. A high G accelerated stall on a long slender wing like the ATR's is going to be super sensitive to the slightest asymmetry and highly likely to spin. With its long fuel filled wings and stretched fuselage, the ATR72 has a lot of distributed mass with a lot of rotational inertia, and it's narrow elevators are never going to get enough purchase in the roiled air of the rotation to lower the AoA and break the stall.
So what caused the bleed back icing that caused this whole event? Would you believe, droplet size? A lot of research by NTSB and FAA established that supercooled water droplets in superstable subfreezing air over the great plains of North America attained sizes seldom seen in the choppier air over Europe. These superstable droplets would remain as a flowable liquid longer after striking a fast moving airfoil, thus bleeding back farther before freezing than their European counterparts. In the case of the ATR, this was behind the boots. THE ATR HAD UNDERSIZED DEICE BOOTS!
This was news the Europeans didn't want to hear, and they insisted Roselawn was pilot error. The FAA revoked the ATR's icing certification, and the Canadians followed suit. All North American ATRs had to shift to sunnier climes, and Billy Clinton closed US airspace to foreign operated ATRs. In the end the US forced the ATR people to design and produce upgrade kits, and where feasible, to install them free of charge. This was accomplished by closing US airspace to all foreign airlines whose countries did not mandate the ATR fix. That policy was quietly maintained until the end of the Clinton administration. I don't think it's been maintained since. We may have just seen an outcome of that this week.
My ex went through AmEagle ATR72 initial training three years after Roselawn, and everybody in her class had to experience that event in the simulator. A real eye opener.
So what caused the bleed back icing that caused this whole event? Would you believe, droplet size? A lot of research by NTSB and FAA established that supercooled water droplets in superstable subfreezing air over the great plains of North America attained sizes seldom seen in the choppier air over Europe. These superstable droplets would remain as a flowable liquid longer after striking a fast moving airfoil, thus bleeding back farther before freezing than their European counterparts. In the case of the ATR, this was behind the boots. THE ATR HAD UNDERSIZED DEICE BOOTS!
This was news the Europeans didn't want to hear, and they insisted Roselawn was pilot error. The FAA revoked the ATR's icing certification, and the Canadians followed suit. All North American ATRs had to shift to sunnier climes, and Billy Clinton closed US airspace to foreign operated ATRs. In the end the US forced the ATR people to design and produce upgrade kits, and where feasible, to install them free of charge. This was accomplished by closing US airspace to all foreign airlines whose countries did not mandate the ATR fix. That policy was quietly maintained until the end of the Clinton administration. I don't think it's been maintained since. We may have just seen an outcome of that this week.
My ex went through AmEagle ATR72 initial training three years after Roselawn, and everybody in her class had to experience that event in the simulator. A real eye opener.