FAA: Icing May Affect Stall Warning On Older Aircraft

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Pilots who fly general aviation aircraft that were certified before the year 2000 should be aware that the stall-warning system may not work as expected in icing conditions, the FAA said on Monday. Pilots have reported that they felt a shudder or buffet, but attributed it to engine or propeller icing. "These reported events occurred in the cruise phase of flight, in some cases with the autopilot engaged; during landing approach; and on landing," the FAA said. Pilots in icing conditions should treat a buffet or shudder as if it is an imminent stall, the FAA said. The recommendation is part of a Special Airworthiness Information Bulletin (PDF), which provides safety information to pilots. "Do not believe the myth that 'thicker' general aviation airplane airfoils are more tolerant of ice accretion," the FAA said.

Since 2000, airplanes certified under Part 23 (normal, utility, acrobatic, and commuter categories, with maximum takeoff weights below 12,500 pounds) must comply with icing regulations that require the stall-warning system to be designed and tested with critical ice accretions along the entire span of the wing. In many new designs this results in the stall warning speed biased higher in icing conditions, the FAA said. Prior to 2000, a clear and unambiguous buffet was accepted for stall warning in icing conditions, even if the airplane was equipped with a stall-warning system and a heated stall-warning sensor. Prior to 1973, there were no requirements to test Part 23 airplanes in icing conditions. Part 23 airplanes were approved for flight in icing conditions if they were properly equipped. Many of these airplanes remain in the fleet today. The FAA bulletin (PDF) provides detailed suggestions for pilots to avoid stalls in icing conditions.