What happened
On 27 February 1994, a Shorts SD3-30, registration SE-INZ, was preparing for a scheduled flight at Trollhättan/Vänersborg Airport. The aircraft had been parked for approximately 40 hours following a routine inspection. During the engine start-up sequence, the captain adjusted the propeller controls from the feather position to the taxi position. This action generated enough thrust to cause the aircraft to begin rolling forward.
As the aircraft moved, the crew discovered that the braking systems were non-functional. Attempts to use the toe-brakes, the parking brake, and the emergency brake handle failed to arrest the movement. The captain eventually steered the aircraft into a snow bank to mitigate the impact, but the aircraft ultimately collided with a terminal building after traveling roughly 25 metres. There were no injuries to the three crew members or the eight passengers on board, though the aircraft sustained substantial damage and the terminal building suffered limited damage.
The investigation
The Swedish Accident Investigation Authority (SHK) examined the aircraft's braking and emergency braking systems, specifically looking at the hydraulic architecture. The investigation focused on the interaction between the main hydraulic pressure and the emergency accumulator.
Investigators found that the aircraft had been parked with the emergency brake handle in the 'EMERGENCY' position. Over the 40-hour parking period, pressure in the emergency accumulator had naturally depleted. A critical technical detail was identified: when the emergency brake handle is moved from 'EMERGENCY' to 'NORMAL', the emergency pressure can become trapped in the system. If this trapped pressure is high enough to prevent the cut-off valve from opening, but too low to provide effective braking, the main hydraulic pressure is blocked from reaching the brake cylinders.
Findings
- The primary cause of the accident was that taxiing commenced without sufficient hydraulic pressure available for the brakes.
- A design feature of the braking system allowed for a temporary loss of braking pressure when the emergency handle was manipulated.
- The operator was unaware of the specific risk of this malfunction, as the relevant manufacturer information (Shorts Information Letter SD3-IL-120) had not been integrated into the flight manual or the company's checklists.
- Discrepancies existed between the airline's operational checklists and the manufacturer's recommended procedures, specifically regarding the verification of brake effectiveness prior to taxiing.