DHC-8-402Q G-JECK had been stored at Aberdeen Airport since mid-March 2020, following the previous operator (Flybe) ceasing trading. On 16 June 2020, the aircraft was to be flown empty to Weeze Airport, Germany where it was to be placed back into storage. Prior to boarding the aircraft, the pilots had checked that wheel chocks were fitted to the mainwheels and nosewheels. In the cockpit, the parking brake lever was confirmed as being set to the park position and whilst the commander reviewed the aircraft technical log, the co-pilot started the APU. The pilots checked the cockpit multi-function display (MFD) and noted that the fluid quantities of the hydraulic systems were adequate. Neither pilot noticed what the park brake hydraulic system pressure was indicating on the MFD, but this was not required to be checked until the end of the aircraft power-on checks and prior to starting the first engine. The representative of the aircraft’s owner and the dispatcher then proceeded to remove the chocks from all wheels. The pilots, onboard the aircraft, had not seen the chocks being removed. About 15 minutes after the chocks had been removed, the commander exited the aircraft to start his walkaround inspection. This included visual checks of the nosewheel tyres and the park brake accumulator pressure gauge, which indicated about 500 psi. Whilst the commander spoke with the engineers, the co-pilot then carried out a walkaround inspection. Neither pilot had authorised the removal of any wheel chocks, and neither noticed that all the wheel chocks had been removed during the walkaround. As the pilots completed the load sheet, the dispatcher boarded the aircraft via the forward left cabin door and stood near the cockpit entrance. The owner representative then noticed that the aircraft was starting to move forward and shouted to the dispatcher who alerted the pilots. Both pilots applied the toe brakes and the commander moved the park brake lever off and back to park twice, but the aircraft continued to roll forward. The commander recalled that, as the aircraft had started to move, he had noticed that the park brake hydraulic system pressure on the MFD was 0 psi. The commander also tried to steer using the tiller, but the aircraft did not respond. Having seen the aircraft start to move, the three engineers left their vehicle and ran to the adjacent right landing gear, where they tried to stop the aircraft by pushing and pulling against its main strut. As the representative ran to the vehicle to get some chocks, the dispatcher jumped from the cabin door and moved to the left gear where he pushed against its strut. However, the aircraft continued to gather speed as it crossed Taxiway D whilst heading towards the parked and empty aircraft, Embraer ERJ-145EP G-SAJS. The dispatcher then ran to the front of the aircraft, where he was joined by one of the engineers, who tried to slow the aircraft by pushing against the aircraft nose. A few second later, the engineers and dispatcher ran clear of G-JECK as it approached G-SAJS. At a ground speed of about 5 kt, G-JECK struck the underside of G-SAJS’s No 2 engine, causing its right landing gear to be lifted clear of the ground. G-JECK came to a stop with the No 2 engine of G-SAJS resting on top of its forward fuselage. There were no injuries. Conclusion: G-JECK rolled across Taxiway D from its parking position and struck G-SAJS because the nosewheel chocks had been inadvertently removed, and the hydraulic pressure in the park brake accumulator had depleted over several days to the point where it was unable to prevent the aircraft from moving on the 1ø slope.