History of Flight
On May 13, 2013, about 1245 Pacific daylight time, a Gates Learjet 35A, registration N22MS, overran the runway during landing at McMinnville Municipal Airport, McMinnville, Oregon. The airplane, registered to Evergreen Equity, Inc. and operated by Evergreen International Aviation, Inc., was on a post-maintenance positioning flight under 14 CFR Part 91. The airline transport pilot, commercial rated copilot, and a company mechanic/passenger were not injured. The airplane sustained substantial damage. The flight departed Grand Junction Regional Airport, Colorado, about 1145 mountain daylight time, en route to McMinnville. Visual meteorological conditions prevailed at McMinnville, and an instrument flight rules flight plan had been filed.
The pilot reported that he had been briefed by the mechanic prior to departure that all preflight inspections were completed. The crew performed an extensive pre-start checklist due to the airplane's maintenance status. All systems, including brakes and nose wheel steering, were normal.
The flight and landing approach were uneventful. After touchdown about 500 feet beyond the approach end of runway 22, the pilot deployed spoilers and pulled the thrust levers to the idle/deploy detent. The DEPLOY lights did not illuminate. Recycling the levers did not deploy the reversers. With about 2,000 feet remaining at a speed of about 25 knots, the pilot applied foot pedal brakes but felt no response, though the brake pressure gauge indicated normal pressure. The copilot also attempted braking, reporting loose pedals and no deceleration.
Near the runway end, the copilot pulled the thrust levers past the deploy detent, increasing engine speed and causing acceleration. The pilot moved the levers back to idle/deploy. Engaging the STEER LOCK switch did not produce a response. Just before reaching the stopway end, the pilot activated the emergency braking lever, but the airplane rolled off the stopway, through instrument landing system antennas, and down an embankment.
The pilot stated this was his first use of the emergency brake system in a Lear 35. He did not feel the need to use it earlier because the airplane appeared to be stopping on its own, and he feared locking the brakes with inoperative steering.
Personnel Information
The pilot held an airline transport pilot certificate with ratings for airplane multiengine land and commercial privileges for single-engine land. He had type ratings for the B-727, B-747, DC-8, G-IV, and LR-JET, and a flight instructor certificate. He reported 17,563.2 total flight hours, with 996.9 in the accident make and model. In the previous 90 days he flew 17.2 hours, and 2.2 hours in the last 30 days. He held a first-class medical certificate with corrective lens limitation.
The copilot held an airline transport pilot certificate with multiengine land ratings and private privileges for single-engine land. He had a DHC-8 type rating (SIC privileges). He reported 2,553 total flight hours, with 94 in the accident make and model. In the previous 90 days he flew 17 hours, and 4 hours in the last 30 days. He held a first-class medical certificate with no waivers or limitations.
Meteorological Information
An automated surface weather observation at 1153 at McMinnville reported wind from 220 degrees at 7 knots, visibility 10 statute miles, scattered clouds at 2,600 feet, broken at 3,100 feet, overcast at 5,500 feet, temperature 17°C, dew point 12°C, and an altimeter setting of 30.08 inches of mercury.
Wreckage and Impact Information
The airplane came to rest in a bramble patch down an embankment beyond the ILS antennas at the departure end of runway 22. The fuselage remained intact. The nose landing gear folded aft, and the belly skin just aft of the nose gear door was punctured, with damage to belly stringers. Both wing leading edges exhibited multiple indentations, crushing back to the forward spar. Rivet dislocation around the cabin indicated deformation of primary structure. Tire marks were present in soft mud beyond the runway, but no corresponding skid marks were on the runway surface.
Flight Recorders
The airplane was equipped with a Universal CVR-30 cockpit voice recorder. The unit recorded the last 30 minutes on two channels (intercom and cockpit area microphone). The recording supported the crew's statements. It began with the airplane at flight level 280, with the pilot flying. Over 27 minutes, the crew communicated with ATC, conducted an uneventful descent, and completed checklists. The landing gear was heard extending. Shortly after, three clicks and a touchdown sound were recorded. Four seconds later, the pilot said the thrust reversers had not deployed. The copilot asked if the control was all the way back; a click and engine noise decrease followed. Engine noise then increased, and the pilot said they were not working. Thirty seconds after touchdown, engine noise increased again. The pilot then said "emergency" and three snaps were heard followed by engine noise increase. The pilot said "emergency brake...okay" and engine noise decreased. Over 8 seconds, the crew expressed concern about stopping before the runway end. A thump sound (impact) was heard. For the next 33 seconds, engine shutdown and crew egress sounds were recorded. The total time from gear contact to impact was 63 seconds.
Tests and Research
Squat Switches: The airplane had redundant squat switches on main landing gear strut torque arms, designed to activate when on ground. Post-accident examination showed that both left and right squat switch assemblies were partially detached from their upper scissor arm fork pads. On both sides, all three switch attachment screws were loose, with the switches rising between 1/8 and 1/4 inches from the pads. The switch roller bearing surfaces were not contacting their cams, deactivating the switches. This would cause the airplane systems to interpret the airplane as in air-mode while on ground. The bolt threads were sharp, with no flattening. The mating surfaces were lightly coated in dirt, grime, and paint debris, with no fretting damage.
Thrust Reversers: The airplane had Garrett TFE731 engines with an Aeronca electro-pneumatic thrust reverser system. The flight manual states that after touchdown, the thrust reverser levers must be pulled back to Reverse Idle/Deploy; the UNLOCK light illuminates during translation, and DEPLOY light when fully deployed. A throttle solenoid then allows further retraction for reverse thrust. Correlation between lever positions and engine PLA matched manufacturer's nominal values. According to the maintenance manual, a throttle control interlock prevents reverse thrust above about 45% N1 until both reversers are deployed. At 45% N1, static thrust per engine is about 531 pounds.
Emergency Braking System: The airplane had an emergency air brake system: a control valve connected to a high-pressure air bottle, which feeds hydraulic brake lines via shuttle valves. The handle is on the pedestal; lowering it admits high-pressure air. The emergency air pressure indicator range is 1,800 to 3,000 PSI. The flight manual says the handle must be pushed down about 2 inches before braking begins, recommending smooth application to prevent skidding. Post-accident examination showed the pressure indicator needle one needle-width below fully charged (3,000 PSI). A test: with the lever depressed 2 inches, pads began contacting disks. Cycling the lever to full down (4.5 inches) five times produced resistance, pad movement, and hissing air. Pressure dropped to about 2,500 PSI. The pilot stated he felt minimal resistance and could have pushed it further; he should have applied it earlier.
Maintenance: The airplane had undergone an FMS upgrade, pitot static/transponder check, and RVSM certification at West Star Aviation in Grand Junction, Colorado. The accident flight was the first after maintenance, a positioning flight to the operator's base. The mechanic performing the FMS upgrade stated he had performed hundreds of such upgrades and did not need to configure the airplane for flight-mode. The quality assurance manager stated standard practice required an orange ribbon to flag disabled systems; both he and the mechanics stated the squat switch was not disabled during maintenance. The last work order item included a return-to-service statement referencing 14 CFR 91.409(f)(3). According to Bombardier/Learjet, the equivalent inspection program is the Learjet Phase A5, which includes inspecting squat switches for condition. The pre-delivery checklist did not specifically denote a squat switch check but listed nearby items. The airplane's exterior preflight checklist also did not call for a squat switch check.
The maintenance facility's quality assurance manager reported that a post-maintenance test flight was typically performed, but Evergreen declined and instead sent a mechanic to perform a check. The Evergreen mechanic assigned stated he reviewed work orders, found minor discrepancies that were resolved, and performed a ramp check. He had no prior experience maintaining a Learjet 35A; his primary involvement was heavy maintenance. The company's usual Learjet mechanic was unavailable.
The airplane's logbooks showed the last procedure requiring air-mode configuration was replacement of the right-side stall warning system switch on November 29, 2012 (60 flight hours and 27 landings prior) at the Evergreen facility. The mechanic who performed that work stated he did not disable the squat switches.
Performance: The pilot reported the airplane weight at 12,000 pounds. Using the Learjet 35 series Crew Checklist and Quick Reference Handbook, the landing distance for dry field conditions at the airport elevation and 16°C would have been about 2,600 feet.