Incident Overview
During a landing descent in instrument meteorological conditions, the copilot of a Cessna 500 noticed a trace amount of rime ice accumulating on the windscreen. Neither crewmember observed ice on the wings, and the deicing boots were not activated.
Events During Approach
The approach appeared normal until the aircraft reached approximately 100 feet above ground level. At that point, the copilot described experiencing a "burble," and the airplane rolled "steeply" to the right. The pilot stated there was "no buffet and no warning." The crew attempted to recover but were unsuccessful, and the right wingtip struck the runway overrun area.
Postflight Inspection
Postflight inspection revealed light rime ice on the wings, horizontal stabilizer, and radome. The airplane was certificated for flight into known icing conditions, but no ice detector or stall warning system was installed.
Aircraft Configuration
The Citation 500 was originally certificated with full-span deicing boots on the wing leading edge, but this airplane had been modified with electrically heated anti-ice panels on the inboard wing leading edge. The deicing boots had no automatic or continuous mode.
Crew Training and Beliefs
Both pilots had taken FAA-approved systems training for the Cessna 500. The pilot stated that he had heard about "ice bridging" at the training provider and his company. The copilot stated that "unless you have 1/4 to 1/2 inch of ice" the boots should not be activated.
Flight Manual Guidance
A review of the Cessna model 560 airplane flight manual revealed that, like the Cessna 500 operating manual, it advised pilots to wait for ice to build before inflating the pneumatic de-icing boots. Both the study guide and the AFM stated that "Early activation of the boots may result in ice bridging on the wing."
Regulatory and Industry History
In 1996, the FAA conducted icing evaluations of the Cessna 560, resulting in modifications to the stall warning system but no change to de-ice boot procedures. In 1997, the Safety Board recommended that leading edge deicing boots be activated as soon as the airplane enters icing conditions because ice bridging is not a concern. A 1997 industry workshop reported no substantiation for ice bridging. In 1999, the FAA proposed airworthiness directives that would require changes to AFMs, but Cessna succeeded in convincing the FAA to withdraw them. In December 2002, the FAA issued advisory circular AC 91-74, stating there were few documented cases of ice bridging on modern boot designs. In February 2005, the Safety Board investigated an icing accident involving a Cessna 560, and the AFM still advised waiting for 1/4 to 1/2 inch of ice. In December 2006, the FAA released a report supporting activation at first detection, but no regulatory action followed. Cessna removed references to ice bridging in its AFMs but still advises waiting until ice thickness is 1/4 to 1/2 inch.
Safety Statistics
Since 1982, the Safety Board has investigated 43 icing occurrences involving turbine-powered airplanes, resulting in 201 deaths and 16 serious injuries. In the past 4 years, the Safety Board has issued 18 recommendations and one Safety Alert related to icing. As of June 2008, actions to reduce dangers remain on the Safety Board's 10 Most Wanted List, and the Board has classified the FAA response as "Unacceptable."