Troy, AL, USA
N1DC
Learjet LJ-60
According to witnesses, the airplane collided with two deer shortly after touchdown. Following the collision, the airplane continued down the runway with the tires smoking, veered off the right side of the runway near the end, crossed a taxiway, impacted into a ditch and burst into flames. After the accident, the captain and first officer both reported that the thrust reversers failed to operate after they were deployed during the landing. Examination of the landing gear found all three gear collapsed. The right and left main tires had areas of rubber that were worn completely through. The flaps were found extended, and both thrust reversers were found in the stowed position. Examination of the cockpit found the throttles in the idle position, and the thrust reverser levers in the stowed position. Aircraft performance calculations indicate that the airplane traveled 1,500 feet down the runway after touchdown, in 4.2 seconds, before striking the deer. The calculations also indicate that the airplane landed with a ground speed of 124 knots. At 124 knots and maximum braking applied, the airplane should have come to a complete stop in about 850 feet. However, investigation of the accident site and surrounding area revealed heavy black skid marks beginning at the first taxiway turnoff about 1,500 feet down the 5,010 foot runway. The skid marks continued for about 2,500 feet, departed the right side of the runway and proceeded an additional 500 feet over grass and dirt. The investigation revealed that deer fur was found lodged in the squat switch on the left main landing gear, likely rendering the squat switch inoperative after the impact with the deer, and prior to the airplane’s loss of control on the runway. Since a valid signal from the squat switch is required for thrust reverser deployment, the loss of this signal forced the thrust reversers to stow. At this point, the electronic engine control (EEC) likely switched to the forward thrust schedule and engine power increased to near takeoff power, which led to the airplane to continue down the runway, and off of it. Following the accident, the manufacturer issued an Airplane Flight Manual revision that changed the name of the “Inadvertent Stow of Thrust Reverser During Landing Rollout” abnormal procedure to “Inadvertent Stow of Thrust Reverser After a Crew-Commanded Deployment” and moved it into the emergency procedures section. [ This Brief of Accident was modified on April 5, 2010, based on information obtained during NTSB Case No. DCA08MA098. ]
[ This factual report was modified on April 5, 2010, based on information obtained during NTSB Case No. DCA08MA098. ] HISTORY OF FLIGHT On January 14, 2001, at 1345 central standard time (cst), a Learjet LJ-60, N1DC, collided with two deer during landing and ran off the end of runway 7 at the Troy Municipal Airport, Troy, Alabama. The airplane was registered to and operated by Ark-Air Flight Inc. The business flight was operated under the provisions of Title 14 CFR Part 91. Visual meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan was filed. The airline transport pilot (ATP) and the ATP-rated first officer received serious injuries. The airplane was destroyed and there was a post-impact fire. The flight originated from Love Field in Dallas, Texas, at 1030 cst on the day of the accident.. According to witnesses, the airplane collided with the deer shortly after touchdown. After the collision, the airplane was observed to continue down the runway with the tires smoking, veered off the right side of the runway near the end, crossed a taxiway, impacted into a ditch and burst into flames. Local rescuers were able to extricate the crew before the fire engulfed the cockpit. According to the two pilots on board, the thrust reversers failed to operate when engaged. PERSONNEL INFORMATION The captain held an ATP certificate with privileges for airplane multiengine land, with type ratings in the Learjet LJ-60. He also held commercial pilot privileges for airplane single engine land, and was a certified flight instructor (CFI) for airplane single and multiengine instrument airplanes. The pilot reported 20,750 total civilian hours on his most recent second class medical certificate, dated June 5, 2000, with no waivers or limitations. The pilot's most recent biennial flight review was in May of 2000, in the Learjet LJ-35. The first officer held an ATP certificate with privileges for airplane multiengine land. He also held commercial pilot privileges for airplane single engine land, and was a CFI for airplane single and multiengine instrument airplanes. The first officer reported 3,500 hours total flight time in all aircraft, including 750 hours in the LJ-60. The first officer’s most recent first class medical certificate was dated February 7, 2000, with no waivers or limitations. The first officer’s most recent biennial flight review was dated May of 2000, in the LJ-60. AIRCRAFT INFORMATION The accident airplane, a Learjet model 60, serial number was manufactured in 1994 by Learjet. FAA records show the airplane was registered to Ark-Air Inc., on June 12, 1997. The maintenance records revealed the airplane was on a manufacturer's inspection program and was last inspected on August 8, 2000. The airplane had accumulated a total time of 2,088 hours at the time of the accident, including 241 hours since its last inspection. METEOROLOGICAL INFORMATION Visual flight rule (VFR) conditions prevailed at the time of the accident, with visibility at 10 statute miles, ceiling at 8,000 feet above ground level, temperature 16 degrees Celsius, dew point 8 degrees Celsius, wind 170 degrees at 8 knots and altimeter 30.19 inches of mercury. WRECKAGE EXAMINATION Investigation of the accident site and surrounding area revealed heavy black skid marks beginning at the first taxiway turnoff about 1,500 feet down the 5,010 foot runway. Two fragmented deer carcasses were found several hundred feet beyond the initial impact.. The skid marks continued for about 2,500 feet and departed the right side of the runway near the Instrument Landing System (ILS) shack at the end of the runway and proceeded an additional 500 feet over grass and dirt. The airplane was found resting on its left side down an embankment. The cockpit section of the fuselage was crushed upward into the crew seats. The fuselage aft of the cockpit was consumed by the post crash-fire. Examination of the landing gear found all three gear collapsed, and deer fur was found lodged in the squat switch on the left main landing gear. The right and left main tires had areas of rubber that were worn completely through. The flaps were found extended, and both thrust reversers were found in the stowed position. Examination of the cockpit found the throttles in idle, and the thrust reverser levers in the stowed position. MEDICAL AND PATHOLOGICAL INFORMATION Both pilots were hospitalized with serious injuries. There was no toxicology testing conducted. ADDITIONAL INFORMATION Aircraft Performance Calculations Specialists from Bombardier Aerospace Learjet performed basic aircraft performance calculations for the accident landing. The calculations took into consideration the following assumptions: Empty weight of 15,800 pounds; estimated fuel load of 1,100 pounds; and the aforementioned reported weather conditions at Troy. The calculations indicated that the airplane traveled 1,500 feet down the runway after touchdown, in 4.2 seconds, before striking the deer. The calculations also indicated that the airplane landed with a ground speed of 124 knots. At 124 knots and maximum braking applied, the airplane should have come to a complete stop in about 850 feet. Cockpit Voice Recorder Sound Spectrum Analysis A sound spectrum study was conducted using the Cockpit Voice Recorder (CVR) as the source of the sound. The study was conducted by the National Transportation Safety Board's Vehicle Recorders Division in Washington, DC. The study revealed that the engines’ fan speed increased from 8,727.5 (82.3 percent) rpm to 9,590 (90.4 percent) rpm between 11 seconds after touchdown and 18.2 seconds after touchdown. This increase in fan speed does not achieve the calculated takeoff N1, however, this engine speed is higher than achievable on a reverse thrust schedule. According to the engine manufacturer (Pratt and Whitney of Canada), N1 is governed during reverse operations. The governing limit varies, and is based on ambient conditions and the airspeed. At 100 knots or greater, the maximum governing limit for N1 should be about 85.2 percent (according to Bombardier Aerospace) instead of the 90 percent found on the engines 20.4 seconds after landing. Thrust Reverser Operation Normal operating procedures call for deploying the thrust reversers within 4 to 6 seconds after landing. Pilots are to deploy the thrust reversers by raising piggyback levers located in the cockpit with the throttles. With the loss of the squat switch on the left main landing gear, the thrust reverser relay box would deenergize the deploy solenoid, and the thrust reverser would move to the stowed position. As the thrust reversers complete the stow cycle, the EEC's switch to the forward thrust schedule , and if the piggybacks remain at the max reverse position, the engines rpm begins to increase to near takeoff power. Post-Accident Actions Following the accident, Learjet manufacturer issued an Airplane Flight Manual revision that changed the name of the “Inadvertent Stow of Thrust Reverser During Landing Rollout” abnormal procedure to “Inadvertent Stow of Thrust Reverser After a Crew-Commanded Deployment” and moved it into the emergency procedures section. Wreckage Release The airplane was released to the owner’s representative USAIG, 2635 Century Parkway Suite 120, Atlanta, Georgia.
On ground collision with deer during landing roll, and the inadvertent thrust reverser stowage caused by the damage to the landing gear squat switch by the collision, and subsequent application of forward thrust during rollout.
Source: NTSB Aviation Accident Database
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