Date: 201902
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Falcon 50
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Flight phase: Parked
Aircraft component: Parking Brake
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Distraction
Anomaly: Aircraft Equipment Problem Less Severe; Ground Event / Encounter Other / Unknown; Ground Excursion Ramp
Detector: Person Flight Crew
When detected: Taxi
Contributing factors: Human Factors; Airport; Aircraft
Primary problem: Aircraft

Narrative: Our aircraft was taxied to a busy and confined ramp area occupied by three large jets either taxiing in or out at approximately the same time. We were temporarily parked to make room and all engines shutdown. Parking brake was set awaiting our aircraft to be towed to another parking space within a few minutes. The APU was running and another jet aircraft was starting engines in the adjacent spot so noise was a factor. One pilot was walking outside our aircraft door while the other remained inside the cockpit or cabin. At no point did the pilot inside exit our aircraft or leave the cockpit or cabin unattended. Ground equipment and personnel were nearby and arranging to come straight over to tow our aircraft to a different spot. There were several notable distractions outside events on the ramp including an unattended aircraft that came loose under tow and was rolling backward on a collision course with ours. Luckily the other aircraft was stopped by a pilot running out from a hanger and opening the door to jump inside. The pilot inside our aircraft was discussing a minor cabin equipment issue with operations by phone while going between the cockpit and cabin when this event occurred. The mutual assumption was that our aircraft would be connected for towing and at some point had been chocked. The parking brake had already been set for several minutes and was holding our aircraft in position on the same ramp as it had done many times before. The outside pilot then went into the terminal to check on passengers. The pilot remaining inside our aircraft felt a movement consistent with a tug being connecting to the nose gear and went forward to the cockpit expecting to see one in place and then release the parking brake. No tug was seen and the aircraft was felt to start rolling backward despite the parking brake still being set. The pilot quickly occupied the left seat and seeing that the parking brake was still set; applied both brake pedals fully and turned on the standby hydraulic pump attempting to stop. The aircraft rolled straight back until it was either stopped by this braking action or by both main wheels going just a foot or two off the ramp area into soft ground. No damage occurred but the main wheels were stuck and the aircraft later towed back onto the ramp with proper recovery equipment.




Synopsis: Falcon 50 pilot reported while waiting for a tow the aircraft rolled backwards off the ramp area; even with the parking brake set.