Learjet 60
Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 180; Flight Crew Total 6500; Flight Crew Type 3100
Ground Event / Encounter Other / Unknown; Ground Excursion Taxiway; Inflight Event / Encounter Loss Of Aircraft Control
Airport; Weather
[The flight was a] no passenger; reposition ferry flight. I was the nonflying pilot doing [operational experience] training for a captain new to the company with excellent previous experience in jets and turboprops. There had been earlier reports of light snow and freezing rain; but the weather had improved one hour before our arrival to VFR. Several aircraft had made landings at the airport so we felt confident on the safety of the mission. Landing was uneventful except for a loss of seven knots on short final which we reported to the tower along with braking action good on the runway. We landed runway 16 and used all of the runway exiting to the left at the end on taxiway Lima as instructed by ATC to taxi for the FBO. Snow and ice removal and runway/ taxiway treatments were in effect. We taxied off runway 16 onto Lima and proceeded northbound. We were taxiing at a speed of 4 knots aware of surface conditions. There was a quartering strong tailwind from our rear left. Taxiway Lima slopes down and 1500 feet into our taxi; about 1000 feet from taxiway Charlie the steering horn went off; advising us of steering problems; I called out 'stop the aircraft'. [The pilot in training] responded 'I'm trying!' At this point the airplane was in a slow skid; we were on what turned out to be black ice or clear ice. [The training pilot] had full brakes and manual steering engaged with no results. He also applied thrust reversers. We were in a slow motion sliding; twisting left turn probably caused by the winds on our tail. The airplane exited the taxiway to the left; did not impact any lights and came to a rest with the main wheels on the taxiway and the nose on the grass pointing directly south. We basically skidded and did a 180 degree turn during the skid on the taxiway. We later came to find that [the airport] treats the runways with potassium and chemicals; but that environmental rules prevent them from treating the taxiways. Taxiways are treated with sand only. The area where we slid was basically an inch deep area of water that had frozen into black ice. The speed of the event was very slow and the aircraft stopped on its own once the nose wheel was on the grass. The aircraft was towed off the grass; maintenance inspections revealed no damage and it was put back into service later that night.I recommend the airport review its policies of not treating taxiways with chemicals as the uneven terrain [provides] many places where black ice can form and; this being a 121 airport; seems contradictory to safety.
After landing safely at their destination; the flight crew of an LJ-60 found the taxiways covered in black ice and were unable to prevent the jet from sliding off the taxiway. They came to a stop with no damage or injury to the pilots; the sole occupants. Chemical dispersants are used on the runways; but not permitted on the taxiways due to environmental reasons.