History of the Flight
On 20 January 1994, at 1341 central standard time, a Calm Air International Hawker Siddeley HS 748, operating as Canadian Partner flight CAV 1557, departed Winnipeg International Airport, Manitoba, for a scheduled domestic flight to Thompson with a planned stop at Flin Flon. The crew performed a pre-flight check, including hydraulic system fluid quantity.
Approximately one hour after departure, about 150 nautical miles north of Winnipeg, the right hydraulic pump low-flow warning light flickered, followed by the left pump light and a drop in main hydraulic pressure below the normal 2,000-2,500 psi to 1,800 psi. Both lights remained on. The crew contacted the company duty manager and decided to divert directly to Thompson, bypassing Flin Flon. Passengers and cabin crew were informed.
About 25 minutes before arrival, main hydraulic pressure dropped to zero. The crew performed the hydraulic emergency drill, turned off nosewheel steering, and noted brake system pressure remained at 2,000 psi, indicating stored back-up pressure. They briefed flight attendants for possible evacuation and requested Thompson Flight Service Station to have emergency response equipment on standby.
Upon arrival, the crew received weather and runway condition updates and were instructed to land on runway 23. They lowered the landing gear manually using the auxiliary system, circling several times to obtain a down-and-locked indication. After securing the cabin, they landed. Following a normal touchdown, the crew increased propeller pitch to slow the aircraft while using rudder for directional control.
As the aircraft decelerated, the pilot applied wheel brakes but felt no braking action. The first officer observed the brake pressure gauge fall rapidly to zero. With reduced rudder effectiveness, the aircraft veered right. The crew issued a brace command and feathered the propellers as the aircraft departed the right side of the runway. It slowed in deep snow, traveling about 40 feet before stopping. Passengers deplaned through the aft main cabin door without injury; evacuation was smooth. The accident occurred at Thompson Airport (latitude 55°48′N, longitude 097°52′W) at approximately 1615 CST, during daylight, at 729 feet asl.
Injuries to Persons
There were no injuries. All 4 crew members and 41 passengers were uninjured.
Damage to Aircraft
The right propeller sustained minor blade deformation from snow contact. The propeller was sent for repair, and the right engine was removed for inspection.
Other Damage
No other damage occurred.
Personnel Information
The pilot-in-command, aged 42, held an Airline Transport Pilot Licence (ATPL) with 12,891 total flying hours, including 6,041 on type. The co-pilot, aged 38, also held an ATPL with 8,500 total hours, 2,900 on type. Both had similar recent flying experience.
Aircraft Information
The aircraft was a British Aerospace Hawker Siddeley HS 748-2A, serial number 1745, manufactured in 1976, with a valid Certificate of Airworthiness. Total airframe time was 24,788 hours. It was powered by two Rolls Royce Dart 534-2 engines and equipped with Dowty Rotol propellers. Maximum takeoff weight was 46,500 pounds. Fuel used was Jet A. The aircraft had a history of hydraulic issues in cold weather; recent snags included replacement of the No. 2 maxaret unit and pump replacements.
Meteorological Information
At Thompson, Environment Canada reported cloud layers at 1,200 feet broken and 21,000 feet broken, visibility 10 miles in light snow and blowing snow, temperature −17°C, winds from 270° at 17 knots. Tower reported surface winds from 270-300° at 10 knots gusting to 15.
Aerodrome Information
Thompson Airport has runway 05/23, asphalt, 5,800 feet long by 150 feet wide. The aircraft departed the right side just beyond taxiway Charlie, about 2,450 feet from the runway 23 threshold. A runway condition report indicated 90% frost cover with 10% compacted snow patches; the James Brake Index was 0.420, which could increase stopping distance by 45%.
Hydraulic System and Brakes
The main hydraulic system uses two engine-driven pumps, accumulators, and supplies landing gear, brakes, nosewheel steering, and propeller brake. The auxiliary system uses a hand pump for gear lowering. Wheel brakes have two independent systems with maxaret anti-skid units that return fluid to the reservoir. Non-return valves isolate brake accumulators; a full charge allows about nine moderate brake applications.
Post-Occurrence Examination
The aircraft was brought to a warm hangar. Fluid added to the main reservoir revealed a leak in the right main landing gear maxaret return line. The line was removed and sent for laboratory analysis. It was an Aeroquip type 666-5 medium-pressure hose with Teflon inner tube and stainless steel braid. A split encompassing one-third of the tube circumference was found at the leak. The split orientation suggested overdeflection from external loading rather than normal flexing. The line was slightly stiffer than a newer sample, but this was not the primary failure cause. The line had been installed on 18 March 1988 and had no specified service limit.
A brake system check was performed but not detailed in the source.