CEN11FA144

The pilot’s decision to conduct an instrument approach in icing conditions without the anti-ice system activated, contrary to the airplane flight manual guidance, which resulted in an inadvertent aerodynamic stall due to an in-flight accumulation of airframe icing.

The flight encountered light rime icing during an instrument approach to the destination airport. The copilot was the pilot flying at the time of the accident. He reported that the airframe anti-icing system was turned off upon intercepting the instrument approach glide slope, which was shortly before the airplane descended below the cloud layer. He recalled observing light frost on the outboard wing and tip tank during the approach. The stick shaker activated on short final, and the airplane impacted left of the runway centerline before it ultimately departed the right side of the runway pavement and crossed a slight rise before coming to rest in the grass. The cockpit voice recorder transcript indicated that the pilots were operating in icing conditions without the wing anti-ice system activated for about 4 1/2 minutes prior to activation of the stick shaker. A postaccident examination of the airplane did not reveal any anomalies consistent with a preimpact failure of the flight control system or a loss of anti-ice system functionality. A performance study determined that the airplane’s airspeed during the final 30 seconds of the flight was about 114 knots and that the angle of attack ultimately met the stick shaker threshold. The expected stall speed for the airplane was about 93 knots. The airplane flight manual stated that anti-ice systems should be turned on prior to operation in icing conditions during normal operations. The manual warned that even small accumulations of ice on the wing leading edge can cause an aerodynamic stall prior to activation of the stick shaker and/or stick pusher.

HISTORY OF FLIGHT On January 6, 2011, at 1101 central standard time, a Learjet 35A, N800GP, was substantially damaged during a hard landing and runway excursion on runway 22 (8,001 feet by 150 feet, concrete) at the Abraham Lincoln Capitol Airport (SPI), Springfield, Illinois. The flight was being conducted under 14 Code of Federal Regulations Part 135 on an instrument flight rules (IFR) flight plan. Instrument meteorological conditions were encountered during the instrument landing system (ILS) approach. Visual meteorological conditions prevailed at SPI at the time of the accident. The pilot and one passenger sustained minor injuries. The copilot and remaining three passengers on-board were not injured. The flight departed from the Chicago Midway International Airport (MDW), Chicago, Illinois, at 1034, with an intended destination of SPI. The pilot reported that the airplane was de-iced before takeoff. The departure from MDW and the descent into SPI were normal. He recalled that the airplane was in clear air during the cruise portion of the flight. They re-entered the clouds about 5,000 feet mean sea level (msl) on descent into SPI and remained in the clouds until they descended below the cloud layer on the approach about 5 miles from the runway. They were in visual conditions for the remainder of the flight. He noted that the airplane accumulated a trace of rime ice on the wing tip tank and windshield during the descent and approach. The pilot stated that the master warning and stick shaker activated when the airplane was on short final. The airplane impacted left of the runway centerline before it ultimately departed the right side of the runway pavement. The airplane crossed a slight rise before coming to rest in the grass. He directed the copilot to assist the passengers in evacuating. The copilot noted that he was the pilot flying at the time of the accident and he was positioned in the left pilot seat during the flight. The copilot stated that the airplane was above the clouds during cruise and it re-entered the clouds on descent into SPI. He reported that he used the autopilot for the initial portion of the approach and then hand flew the airplane after descending below the cloud layer. The copilot reported that the airplane anti-icing was on during the entire cruise portion of the flight and no icing was observed on the airframe during the initial portion of the approach. The airframe anti-icing system was turned off upon intercepting the ILS glide slope, which was shortly before descending below the cloud layer. He recalled observing light frost on the outboard wing and tip tank during the approach. The copilot noted that the reference speed for the accident approach was 119 knots and his target airspeed was 124 knots. The copilot recalled that he slowed the airplane earlier than usual in order to extend the wing flaps and landing gear. This allowed him to operate at a higher engine power setting to increase the effectiveness of the anti-icing system. He noted that with the wing/stabilizer and the engine anti-ice systems on, an engine speed of approximately 65-percent is required in order to maintain cabin pressurization. Also, the airplane needed 60 to 65-percent engine power to hold a standard 3-degree glide slope with full flaps and landing gear extended. He stated that the airplane performed normally without any indication of a malfunction or abnormal condition prior to the accident. The copilot stated that during final approach, the captain noted "ref plus five." About that time, the stick shaker went off. He noted that the airplane was in the vicinity of the airport fence line or runway threshold at the time. He was not sure of the airspeed at the time the stick shaker activated. The copilot reportedly applied full power and at the same time the captain stated "full power." The left wing dropped causing the airplane to veer to the left. He attempted to correct to the right. He was not sure exactly when or where the airplane touched down, but he thought that it was while he was trying to correct to the right. After the airplane came to a stop, the captain directed him to evacuate the passengers. One of the passengers reported that the flight seemed routine until immediately before the accident. He felt some lateral movement in rear of the airplane, which was somewhat more than he had experienced on previous flights. He recalled that the airplane began to sink; the left wing dropped and the airplane seemed to “flip-flop” left to right. Engine power increased, but the airplane bounced unusually hard. He thought that the initial impact was on the airplane landing gear, but that the second and third impacts were on the belly of the airplane. His perception was that the airplane was skidding across the ground and that they were not on the runway or a tarmac at that time. He noted that the subsequent evacuation went smoothly and that the fire crews were on-scene very quickly. A second passenger reported that the left pilot’s windshield appeared to be obscured except for a clear area immediately in front of the pilot. He recalled that the airplane veered to the left at the beginning of the accident sequence. The pilots seemed to be trying to stabilize the airplane and execute a go-around. The left wing initially dropped 45 to 60 degrees and then the right wing dropped a similar amount. He recalled hearing a “popping” sound as the airplane impacted the ground. After the airplane came to rest, the occupants exited and moved to a safe distance away from the airplane. A review of the cockpit voice recorder (CVR) transcript revealed that the flight was cleared for the ILS runway 22 approach at 1055:51 (hhmm:ss). The controller informed the pilots that the flight was located about 9 miles from the approach fix LICOL at that time. Seventeen seconds later, the pilot informed the controller that they were descending into the cloud tops at 4,100 feet msl. At 1056:30, the copilot noted that the anti-ice system was off. Eighteen seconds later, the pilot noted the accumulation of rime ice on the airframe and that they were descending through 3,500 feet msl. At 1058:49, the copilot noted that the wing anti-ice was off and that the engine nacelle anti-ice was on. At 1101:01, the CVR recorded a gasp followed by the pilot commanding “full power.” The sound of the stick shaker activating was recorded at 1101:05, with a sound similar to impact beginning at 1101:06 and lasting for about 10 seconds. PERSONNEL INFORMATION The pilot, age 44, held an airline transport pilot certificate with single and multi-engine land airplane class ratings. The single engine airplane rating was limited to commercial pilot privileges. The certificate also included LR-Jet (Learjet) and CL-600 (Bombardier Challenger) type ratings. The CL-600 type rating was limited to second-in-command privileges only. The pilot was issued a first-class airman medical certificate on September 21, 2010, without any restrictions or limitations. According to documentation provided by the operator, the pilot had accumulated about 5,932 hours total flight time, with 827 hours in Learjet model 35 airplanes. He had accumulated about 65 hours in Learjet model 35 airplanes within 90 days of the accident. The pilot’s most recent regulatory checkride was completed on October 28, 2010. The copilot, age 25, held an airline transport pilot certificate with single and multi-engine land airplane class ratings. The single engine airplane rating was limited to private pilot privileges. The certificate also included a CE-510S (Cessna Mustang single-pilot) type rating. The copilot was issued a first-class airman medical certificate on March 2, 2010, without any restrictions or limitations. According to documentation provided by the operator, the copilot had accumulated about 1,657 hours total flight time, with 802 hours in Learjet model 35 airplanes. The copilot