Airframe structural icing on regional aircraft is one of the top candidates on the National Transportation Safety Board's list of "Most Wanted Safety Improvements," according to John Clark, NTSB head of aviation accident investigations.

"Even with the heightened awareness in the aviation community after the recent icing related accidents, some flight crews have not operated safely in icing encounters," Clark said. "Thus, the certification standards need to be upgraded, improved detection and protection systems need to be developed and implemented, and operational procedures need to be improved."

He noted that the much-heralded 1994 ATR-72 crash in Roselawn, Ind., "prompted many of our recommendations." In that accident, freezing rain caused an ice ridge to form behind the boots. The ridge disrupted the airflow in front of the ailerons that contributed to loss of roll control and steep descent.

Last December, the crash of a TransAsia Airways ATR-72 resulted in recommendations from both the Aviation Safety Council of Taiwan and the French investigating authority that turboprop operators should review their training programs to ensure they "contain the necessary training for pilots to recognize and effectively respond to all levels of icing conditions."

France's Avions de Transport R�gional (ATR) has since issued a notice to ATR operators reiterating that training procedures provided in the ATR's flight manual need to be "strictly applied" whenever icing conditions are encountered (C/R News, Feb. 10).

Clark also noted that the crash of an Embraer EMB-120 turboprop in Monroe, Mich., resulted from a very thin, rough ice shape on the leading edge of the wing. This, in turn, "caused large aerodynamic losses and provided very few visual cues to the flight crew." The crash of a Bombardier CRJ in Fredericton, Canada, also had ice as a factor during an attempted go-around.

"Regional jets such as the CRJ and Embraer's EMB-145 have hard leading edge wings. That is, there are no leading edge slats. This wing type is a common factor in a number of major icing accidents, the most notable involving a series of DC-9-10 airplane accidents in the last 20 years," he said.

Clark said that at the last "Most Wanted" meeting held by NTSB, the EMB-120 incident near West Palm Beach, Fla., in March 2001 was highlighted. That accident involved an icing encounter, several roll excursions, a 7,500-ft. dive and severe structural structural damage to the tail. Cause was determined to involve inter-cycle ice accumulation. Testing with the National Aeronautics and Space Administration (NASA) showed that significant amounts of ice can build up between boot inflation cycles, "even though the leading edge ice protection system [was] activated." Clark added that this accident "was disturbing to us, considering the operational changes and research that have taken place in recent years."

As a result of that accident, Embraer recommended that de-ice boots be activated at a higher rate and that the autopilot be deactivated in icing conditions until a "LOW SPEED ALARM" had been installed. Embraer also released a revised training video related to flight in icing conditions and started to improve the stall-warning computer for icing operations. Additional research on intercycle ice shapes was also initiated.

Clark said that with a number of aircraft with hard leading edge wings coming into the commercial and business aircraft fleets, "training and detection must be improved." He noted that while a large amount of work has been done regarding icing, the NTSB staff is concerned "that these recommendations were issued almost six years ago, and there will be further delays before the FAA [Federal Aviation Administration] will be in a position to enact rulemaking. Until the work is done, icing will continue to be a major threat to aviation safety."

>>Contact: John Clark, clarkj@ntsb.gov<< (David Evans, editor-in-chief of Air Safety Week, a sister PBI Media publication, contributed to this report)