What happened
On 20 January 1994, a Calm Air International Hawker Siddeley HS 748 was performing a scheduled domestic flight from Winnipeg to Thompson, Manitoba, with an intended stop in Flin Flon. Approximately one hour into the flight, the crew noticed flickering low-flow warning lights for both the left and right hydraulic pumps, followed by a significant drop in main hydraulic system pressure.
After consulting with company management, the crew decided to divert directly to Thompson. During the approach, the main hydraulic pressure dropped to zero. To conserve remaining pressure, the crew deactivated nosewheel steering and manually lowered the landing gear using the auxiliary system.
Upon landing on runway 23, the pilot attempted to use the wheel brakes to decelerate. However, the crew observed the brake pressure gauge drop rapidly to zero, resulting in a total loss of braking action. As the aircraft slowed, the rudder became ineffective, and the aircraft began to veer toward the right. The aircraft eventually exited the right side of the runway and came to a stop in deep snow. All 45 passengers and crew were evacuated through the aft cabin door without injury.
The investigation
The investigation focused on the source of the hydraulic fluid loss and the sudden disappearance of brake pressure. Technicians examined the right main landing gear maxaret return line and discovered a circumferential split in the inner tube core. This failure was attributed to overdeflection from an unidentified external source rather than standard service wear.
Investigators also looked into why the brake system failed during the landing roll. While the aircraft's history showed previous hydraulic issues during extreme cold, no mechanical faults were found in the brake system during post-incident testing in a warm hangar. The investigation also considered the impact of the runway surface, which was heavily covered in frost and snow, and the presence of a right cross-wind that contributed to the aircraft's departure from the runway.
Findings
- The primary cause of the main hydraulic pressure loss was a leak in the right maxerant (anti-skid) return line caused by overdeflection from an undetermined source.
- The loss of directional control during the landing roll was caused by the loss of brake pressure, the specific cause of which remained undetermined.
- The aircraft departed the runway because it weathercocked into the prevailing cross-wind as the rudder lost effectiveness at low speeds.
- Effective coordination between the flight crew and cabin crew facilitated a safe and orderly evacuation.