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Monday, July 17, 2006

NTSB Issues Icing Recommendations for Saab 340s

In an incident hauntingly similar to the Halloween 1994 crash of an ATR 72, the crew of an American Eagle Saab 340 lost control owing to ice buildup, prompting the National Transportation Safety Board (NTSB) to issue recommendations calling for an upward revision in the safe speeds in icing conditions for the aircraft and the installation of a stall warning system, already in Canadian 340s. It also believes an ice detection system, an option on the 340, should also be required on US aircraft.

Turboprop icing has been a continuing NTSB concern since the ATR crash as well as a Embraer Brasilia accident in Monroe, Mich. As the result of the ATR accident, the entire turboprop fleet underwent testing to determine the impact of the same conditions that felled the American Eagle ATR.

Saab, which is still reviewing the new NTSB recommendations, indicated that the aircraft passed these icing trials and no restrictions were placed upon the aircraft for operations in icing conditions. However, in response to various icing-related incidents experienced on turboprops in the industry as a whole, in addition to the Saab incidents, the manufacturer created and introduced a winter operations program, highlighting the risks involved in winter operations and emphasizing the use of correct procedures to minimize the potential for any type of mishap. Flight operations staff made several visits to operators since then to provide additional training to pilots. Saab also indicated it was a recurring point of discussion at operator conferences. It also has issued a number of operations bulletins further highlighting the need to follow proper procedure and maintain published minimum speeds when flying in icing conditions.

The incident last January forced the left and right ailerons to simultaneously move upward, causing a 5,000-foot, 50-second dive that violently rolled the plane both right and left. The incident prompting the NTSB action was American Eagle Flight 3008 which departed San Luis County Regional Airport (SBP) en route to Los Angeles (LAX) when it encountered icing conditions during climb out. The pilots lost control at approximately 11,500 feet mean sea level (MSL). The aircraft, carrying 25 passengers and three crew who were uninjured, fell to 6,500 feet before the pilots recovered control and continued to LAX.

The impact of the icing in four incidents in the U.S. and Australia occurred at speeds five knots faster than Saab's published icing operations speeds, causing NTSB to raise questions as to whether current published speeds provide an adequate margin of safety for pilots to exit icing conditions and affect recovery of the aircraft.

The Australian Transportation Safety Board (ATSB) investigated three similar icing incidents with the Saab 340 in the past eight years, two of which resulted in an aerodynamic stall and steep dives. The incidents occurred on November 11, 1998 and June 18, 2004 in Victoria, and June 28, 2002 in New South Wales.

"A review of those reports will show that the causes of those incidents are clear," Saab told Regional Aviation News. "Failure to maintain proper speeds in icing conditions will result in a stall condition. Whether you are flying a Saab 340 or any other aircraft type, above all, you need to fly the aircraft."

The American Eagle crew reported light rime icing and moderate turbulence on the inbound leg to SBP as they were descending from 9,000 to 5,000 feet msl. The aircraft was operating with the continuous mode of boot deice system inoperable, as permitted by the company's minimum equipment list (MEL).

The aircraft departed in level two weather conditions (10 degrees Celsius or colder with visible moisture) and with the engine anti-ice on. As they approached cruise altitude, they observed rime ice on windshield wipers as well as a narrow strip of ice about a half-inch wide on the left wing leading edge. Before the captain could engage the boots, however, he felt a heavy vibration when the nose and left wing dropped and autopilot disconnected.

The DFDR showed the incident began at 130 knots indicated airspeed before the stall warning sounded. The aircraft then rolled right to a maximum of 140 degrees right wing down while the aircraft pitched to 48 degrees nose down. It rolled back to the left and back to the right. The captain leveled the wings and began pulling up on the control yoke. It was only then the first officer was able to operate the deicing boots. The FDR also reported that both the left and right ailerons simultaneously moved upward during the initial upset.

The board found that speed degraded during climb and the decay became more rapid in the final 10 seconds before the autopilot disconnect. The DFDR also revealed that about 26 seconds before the stall roll departure - at 144 KIAS - the aircraft experienced a slight rolling anomaly counter to the aileron input. Further input from the autopilot stopped the roll. However, previous investigations revealed that this action is an indication of airflow separation particularly on the wing or near one or both ailerons. The agency noted that further aileron movement in the trailing-edge-down direction can expand the airflow separation at higher airspeeds than previously thought.

The board noted concerns the aircraft could stall in icing conditions, expressed by Transport Canada during certification in 1994. The agency mandated several changes, including the addition of stall protection logic to Canadian 340s and an "ice speed" switch to be activated during flight into icing, according to the NTSB report. When activated, it provides a lower trigger in the stall warning system and a greater level of stall protection. The NTSB also said the Canadian 340s are unique among the worldwide fleet for this reason.

The ATSB indicated that had this been installed in the Australian aircraft, it would have activated nine seconds before the pre-stall buffet. It also reinforced a previous NTSB recommendation (A-98-96), resulting from the Brasilia accident, calling for installation of stall warning protection systems on all aircraft certified to operate in icing conditions. For the American Eagle flight, the system would have alerted the crew only four seconds before the start of the incident. (NTSB Safety Recommendation A-06-48 through -51.)

NTSB Icing Recommendations for the Saab 340

Marking the recommendations urgent, the NTSB called for the Federal Aviation Administration (FAA) to require:

  • Saab 340 operators to instruct pilots to maintain a minimum operating airspeed of 1.45xVs during icing conditions and before entering known or forecast icing conditions. Orders would include exiting icing conditions if performance degrades below 1.45xVs.
  • The addition of modified stall protection logic in 340s certified for flight into known icing conditions.
  • The installation of an ice detection system.
  • The suspension of autopilot use when operating in icing conditions, except during intermittent periods of high workload. The board cited the fact that all icing upsets in turboprops including the Embraer Brasilia, the ATR and the Saab occurred when the autopilot was engaged. It contends the pilots would have felt the problems sooner had they been flying manually. However, the FAA disagreed with previous recommendations to suspend autopilot operation in icing conditions.
  • The convening of a panel of aircraft design, operations and human factors specialists to determine whether the installation of a low-airspeed alert system would be feasible in all aircraft operating under Parts 135 and 121.
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