The crew boarded G-JOTR while it was parked on a taxiway at London-Biggin Hill Airport, but airfield restrictions required it to be towed to a designated area on the apron for engine start. The towing route followed the edge of the apron, which was damp and downhill, and involved a 90ø left turn to line up for start. A combination of unfamiliarity with a relatively new operating base, towing in reverse and difficulty interpreting “indistinct” taxiway markings meant the tug driver entered the left turn later than intended. The late entry made the turn excessively tight, and the tug lost traction on the damp surface and jack-knifed. The resulting combination of a sharp towing angle and the aircraft’s downhill momentum caused the towbar shear pin to break. The commander applied the brakes immediately on hearing “the sound of snapping metal” but was unable to prevent the aircraft rolling forward into the tug. The operator’s internal investigation report made several safety recommendations, including a review of towing procedures, enhanced driver training, the use of radios by towing teams and improvements to taxiway markings. The company also reviewed its apron risk assessment and towing was added to their audit plan. On 10 February 2021 the aircraft was ferried to Cranfield for repairs. On 1 July 2021 the aircraft was flown back to London-Biggin Hill Airport.