What happened
On the morning of 19 October 1999, an HS-125-400A, registered as ZS-OIF, was undergoing pre-flight preparations at Lanseria. The aircraft was initially parked inside a hangar where the pilot began his inspections around 0500Z. During this period, an offer of assistance from a maintenance staff member to assist with hydraulic system pressurization was declined by the pilot.
Following the initial inspection, the aircraft was towed to the international departures apron for refueling. As the crew continued their checks, the co-plane arrived to assist. At approximately 0540Z, after the external inspections were finished, the pilot entered the cockpit to perform internal checks. While the parking brake was engaged, the co-pilot proceeded to remove the wheel chocks. Immediately upon their removal, the aircraft began rolling forward. Despite an attempt by the co-pilot to replace the chocks, the aircraft continued its momentum, eventually striking an adjacent hangar. The collision resulted in extensive damage to the aircraft's wing skin and under-belly.
The investigation
Investigators examined the state of the aircraft's systems following the collision. Upon arrival at the scene, it was noted that the accumulator gauges both indicated approximately 1000 PSI. Because this specific pressure level is maintained by nitrogen gas within the accumulators, this reading indicated that there was zero hydraulic system pressure available to support the braking system.
Findings
- The hydraulic system had not been pressurized during the pre-flight inspection process.
- The removal of the chocks occurred while the aircraft lacked the necessary hydraulic pressure to ensure the parking brake remained effective.