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The Aviation Weather Center (AWC) is conducting a research project on low level turbulence away from the influences of terrain and in the presence of stronger wind. The AWC is requesting pilots, controllers, and dispatchers make an extra effort to provide PIREPs below 12,000 feet during the nighttime hours--primarily from 0200 UTC through 1100 UTC. This includes reports of negative turbulence as well. The focus of this project runs across the central plains; from Texas to Canada, between the Rockies and the Ohio River Valley, but any nighttime PIREP is greatly appreciated. The project runs through September 1, 2012.

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Due to the computing needs of some coming operational implementations, this run likely will become less timely, and will have more missed cycles. Starting 3/20/12, the number of missed cycles will increase SIGNIFICANTLY.

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From tragedy we draw knowledge to improve the safety of us all.

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In April of 1995, both crewmernbers in an EMB-120 near Tallahassee, Florida, noticed trace icing on the outboard leading edge of the wing. The crew also observed an airspeed reduction from 180 KIAS to 140 KIAS, a pitch increase to 5° nose-up, and no apparent increase of trace icing on the leading edge of the wing. The crew activated the de-ice boots, after which the airspeed increased and pitch decreased. Information about the use of the autopilot was unavailable (This information was obtained from Aviation Safety Reporting System (ASRS) report 302910).

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On October 16, 1994, near Elko, Nevada, an EMB-120 stabilized at 160 KIAS at 13,000 feet. Both pilots checked for ice on the wings and spinner, but they did not see a significant amount. With the aircraft on autopilot, the flightcrew initiated a heading change to the right, and the aircraft began a right wing down (RWU) roll attitude. During the turn, at about 20° RWD, the stick shaker and pusher activated almost simultaneously. The aircraft rolled nearly 90° to the right and pitched over. The pilot took manual control of the airplane and recovered. Post-flight inspection of the aircraft revealed clear ice on the wing leading edge and propeller spinners. The de-ice boots were not activated during the flight because the crew did not believe the ice was of sufficient thickness to cause concern. Data from the FDR were extracted by the air carrier and forwarded to the FAA and Embraer; analysis showed a minimum airspeed of 138 KIAS before the stick shaker activated. The stick shaker activated about 10 knots above the calculated accelerated stick shaker speed. The Safety Board was not notified of this incident until after the Comair Flight 3272 accident, however, regulations do not require this type of incident to be reported to the Safety Board (This incident was described in ASRS report 286127).

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On April 29, 1993, at Pine Bluff, Arkansas, an EMB-120 was climbing on autopilot when it stalled and entered a steep descent. Three of the four propeller blades subsequently separated from the left engine. The airplane’s airspeed had decreased to 138 knots before the stick shaker activated and the autopilot disconnected. The aircraft experienced an extreme roll upset during the stall. Occasional moderate icing in clouds and precipitation were forecast for the area and for the altitude traversed by the airplane. The Safety Board concluded that an accretion of ice on the wing was the only reasonable explanation for activation of the stick shaker and loss of roll control at the higher than expected airspeeds. There was no evidence that any ice protection systems were activated before, during or after the upset and the aircrew did not recall seeing evidence of icing before the loss of control. A passenger, however, recalled seeing a “whitish” substance that appeared to be snow about 8 to 10 inches above the windshield wipers.

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On November 22, 1991, in Clermont-Ferrand, France, an EMB-120 was descending with autopilot engaged. The captain considered the descent rate too high and disconnected the autopilot manually leveling off the aircraft at 4,500 feet. As the airspeed decreased through 150 KIAS, tlie stick shaker activated. The airplane then rolled 60° to the right three times and lost 1,000 feet of altitude. During recovery, the flightcrew increased engine power and cycled the de-ice boots. The French Bureau Enquettes Accidents (BEA) obtained the FDR data and forwarded them to Embraer Avions de Transport Regional (ATR) informed the Safety Board staff of this incident duiing the Safety Board’s investigation of the October 31, 1994, ATR-72 icing accident at Roselawn, Indiana.

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In September 1991, Fort Smith, Arkansas, an unspecified aircraft type (assumed to be an EMB-120 based on systems descriptions) was in level flight at 19,000 feet with the autopilot engaged. Both pilots felt vibration through the floorboards. The pilots inspected the wings, propeller spinners, and engine inlets, which did not appear to have excessive amounts of ice. Thirty seconds after the first vibration, the stick shaker activated, the captain took manual control of the aircraft and called for all anti-ice equipment on. The aircraft did not immediately respond to rudder/elevator inputs and it entered a right bank, nose-down descent of 1,000 feet per minute. The pilots regained control at 16,000 feet (This incident was described in ASRS report 189745).

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On June 28, 1989, at Klamath Falls, Oregon, an EMB-120 was flying on autopilot at 16,000 feet in light icing and turbulence. The flight descended to 15,000 feet and the flightcrew observed light mixed rime and clear ice. The airspeed decreased rapidly, from 180 to 160 KIAS, and was followed by activation of the stick shaker. The pilot took control of the aircraft and applied maximum power as the aircraft rolled 30° to the left one time then 40° to the right two times. There was no indication that any ice protection equipment was used (This incident was described in ASRS report 115422).