Often Crews Do Not Follow Proper Procedures During Severe Icing Incidents

The basic procedures necessary to cope with in-flight icing are not always being followed, resulting in dangerous flight conditions and sometimes fatal accidents.

A recent example is the Dec. 21, 2002, fatal crash of a Trans Asia Airways (TNA) ATR-72 twin turboprop into the Taiwan Strait. The airplane, configured to carry cargo, encountered severe icing while cruising at 18,000 feet some 30 minutes into the two and a half-hour night flight from Taipei to Macau.

The accident sequence unfolded in the space of about 18 minutes, from first indication of icing to impact. Only two minutes separated the crew's first mention of severe icing to the sound of stall warning and stick shaker. The basic aircraft certification standards do not protect against freezing precipitation. Procedures required by airworthiness directives also may be inadequate.

The TNA case is being investigated by the Aviation Safety Council (ASC) of Taiwan, with participation by ATR (Avions de Transport R�gional), the aircraft manufacturer and, as accredited representative, the Bureau Enquetes-Accidents, the French accident investigation board.

The ASC had earlier issued a safety bulletin calling on operators worldwide to reinforce proper crew responses to flight in icing conditions. In a transcript of the cockpit voice recorder, the two pilots -- Capt. Pan Teh-chung, 54, and First Officer Liu Ching-hai, 34 -- knew their speed was bleeding off and a "big chunk" of ice was observed on the ice probe located just outside the captain's port window.

After 10 months of investigation, the ASC on Oct. 28 released more than 300 pages of factual data surrounding the case. The final report is not expected to be published until late 2004. The data adds significant details to the preliminary accounts of the crash.

The information indicates the crew unknowingly flew into severe icing conditions and may have been late taking precautions. As a general rule, by the time controllability symptoms appear in icing conditions, urgent action may be necessary, such as a maximum-rate descent to get to warmer air below the icing. Such action would have the added benefit of getting to a lower density altitude, where additional thrust is available to offset increased drag from ice accumulation and thus increase the stall margin.

Weather reports reviewed by the crew before their departure from Taipei's Chiang Kai Shek International Airport on the nightly cargo run (flight GE791) to Penghu, Macau, advised of rain but made no mention of icing. Severe icing had not been observed and was not forecast.

About a half-hour into the flight, the airplane's ice detector alerted. The alert occurred about two minutes after the crew first noticed ice on the airplane. Airframe de-icing was activated for about three minutes, then turned off. A four-minute gap followed, with the airplane flying in icing conditions with the de-icing system inactive. Airframe de- icing was then turned back on, and remained on for about 11 minutes, when the flight data recorder (FDR) ceased functioning.

Until the final moments of the flight, when a cacophony of alarms sounded, the crew continued to fly on autopilot. In previous icing-related accidents, crews flying on autopilot have been lulled into a false sense of security due to the lack of apparent degradation of flying qualities, as the autopilot silently works to maintain its prescribed parameters. In this case, the crew kept the autopilot on as speed slowed below the minimum for icing conditions, and even as speed slowed below the minimum for severe icing conditions.

Following the fatal Jan. 19, 1997, crash of a Comair EMB-120 twin-turboprop in icing conditions at Monroe, Mich., the U.S. National Transportation Safety Board (NTSB) recommended that pilots should disengage the autopilot whenever anti-icing systems are activated.

In May 1999, ATR issued a number of updated changes to the airplane flight manual dealing with operation in icing conditions. The procedures and limitation were explicit, including (but not limited to): adding a 10-knot speed margin, activating anti-ice systems until the aircraft "is clear of ice," turning off the autopilot and exiting the icing conditions as soon as possible.

As of January, there were 62 ATR-72s and 17 ATR-42s in service in passenger configuration by regional operators, according to the Regional Airline Association. Many more are being operated by air cargo carriers.

The GE791 crew had added 10 knots to the airplane's target cruising speed, as called for when flying in icing conditions. It does not appear that they performed the other actions required. The pilot anxiously urged the first officer to radio a request to air traffic control for an immediate descent from 18,000 feet to 16,000 feet. Moments after the pilot indicated the aircraft was "severely iced up," it rolled beyond a 60� angle of bank and spun into the water.

It was the first fatal accident in icing conditions for the ATR-72, a stretched version of the ATR-42, since a 1994 crash of American Eagle flight 4184 at Roselawn, Ind., killing all 68 on board. Since that crash, ATR modified the flight manual to more explicitly caution crews about the airplane's limitations in icing, and ATR redesigned the wing de-icing boots to extend further back on the wing, from 7 percent of the wing chord to 12.5 percent. The TransAsia Airways ATR-72 had been outfitted with the larger wing boots.

After the GE791 accident, ATR conducted a number of flight simulations to assess the circumstances surrounding the loss. In a June 2 report to the ASC, the French manufacturer cited "non-compliance by the crew of the icing speeds led the aircraft to attitudes where, on wings polluted by severe ice, aerodynamic anomalies occur."

ATR noted that the crew was able to arrest the deteriorating speed by turning on the anti-icing system, but by then turning the anti-ice system off, the "expected nominal speed was not completely recovered." The ATR analysis concluded that it was very late in the sequence when the crew finally established the relationship between ice on the airplane and its decaying speed.

On Dec. 5, 2002, just 16 days before the crash, ATR issued a worldwide reminder to its customers of winter operations, and a number of icing events in which ATR-42 and ATR-72 crews had not followed necessary procedures. Continuous "situational awareness" and "an accurate compliance with established procedures" are necessary to prevent a recurrence of "such undesired icing reports," the ATR reminder said.

But the ATR reminder also mentioned the larger problem: aircraft are not certified to cope with severe icing, notably super-cooled drizzle drops (SCDD). At temperatures near freezing, the drops do not necessarily freeze on impact, but run back along the surface and freeze.

"Please ask your pilots to pay the most possible care in watching and detecting conditions which could exceed the certification standards and have to be escaped/avoided when inadvertently encountered," the ATR note advised.

By David Evans, Editor-In-Chief of Air Safety Week

>>The ASC reports can be accessed at: http://asc.gov.tw.<<