Incident Overview
An Air Canada Canadair Regional Jet, operating as flight ACA646, landed on runway 15 at Fredericton, New Brunswick, at 2229 Atlantic daylight saving time. During the after-landing roll at low speed, the aircraft yawed left and exited the runway approximately 5,500 feet beyond the threshold. The aircraft came to a stop when its nose gear sunk into the soft ground adjacent to the runway. There were no injuries to the crew or passengers, and no damage to the aircraft.
Flight Crew and Aircraft
The captain held an airline transport pilot licence with 12,200 total flight hours, including 535 hours as pilot-in-command on the aircraft type. The first officer held an airline transport pilot licence with 8,500 total flight hours and about five hours on the aircraft type. The aircraft weight and centre of gravity were within limits. No aircraft system deficiencies were reported by the flight crew or identified during the post-incident maintenance inspection.
Approach and Weather
The aircraft was cleared by Moncton Area Control Centre for a straight-in instrument landing system approach to runway 15. Runway 15 is 6,000 feet long and 200 feet wide. The crew contacted the Fredericton Flight Service Station when 100 miles west of the airport and requested the runway surface condition. The FSS specialist advised that there was no James Brake Index report, that sweepers would be on the runway until about five minutes before arrival, and that an updated RSC would be given prior to landing. About six minutes before touchdown, the flight crew received the following RSC: runway 15/33 100-foot centre line, 60% bare and wet, 20% light slush, and 20% light snow; outside the centre line, one inch of slush and snow mixed.
Environmental Canada weather at 2200 ADT (29 minutes before landing) reported: precipitation ceiling 800 feet obscured, visibility 3/4 mile in light snow, temperature 0°C, dew point -1°C, wind 030° true at 10 knots. At 2248 ADT (19 minutes after landing), conditions were unchanged. About four minutes before touchdown, runway visual range was reported as 6,000 feet with light setting strength five. Two minutes before touchdown, wind was reported as 060° magnetic at 10 knots.
Loss of Directional Control
During the after-landing roll, as indicated airspeed decreased through 40 knots, the aircraft yawed left. To counteract the yaw, the pilot flying reduced reverse thrust and then stowed the reversers on both engines, while braking and maintaining full right rudder. However, the reversers were unintentionally stowed before the engines had spooled down to idle reverse, causing the aircraft to transition to forward thrust with a higher than idle setting. The left engine reverser was redeployed, then the right, with the left engine thrust increasing above idle. The aircraft continued left and exited the runway.
The aircraft stopped when the nosewheel tire sank into soft ground. After stopping, left engine forward thrust was selected in an unsuccessful attempt to return to the runway. The aircraft was then shut down, and passengers were transported to the terminal.
Investigation Findings
Flight data recorder analysis showed that approach and touchdown speeds were normal, ground spoilers deployed at touchdown, and thrust reverse operated for 24 seconds, maintained at 84% N1 for 17 seconds. Aileron and rudder inputs were applied to counteract the left crosswind. The runway surface condition reported did not exceed recommended limitations for the aircraft type. A JBI reading was not produced because the runway was contaminated with water and slush. The flight crew reported the runway was 100% snow-covered at landing.
The maximum demonstrated crosswind component for landing or takeoff is 24 knots (dry runway). The operator's manual advises considering another runway when crosswind on a wet or slippery runway exceeds 15 knots; the crosswind was 10 knots. Although manufacturer recommends reverse thrust below 30% N1 at speeds below 60 knots to reduce foreign object damage risk, the flight crew maintained 84% N1 until 35 knots.
The Air Canada CL-65 Aircraft Operating Manual cautions: when changing thrust from reverse to forward idle, pause at idle reverse to allow engines to unspool. If reversers are stowed while engines are still spooled up, there will be a noticeable decrease in deceleration or a forward surge. It also advises reducing reverse thrust symmetrically if control difficulties are experienced, and releasing the brakes if directional control difficulties occur.
Simulator landings approximating the conditions could not duplicate the left veer. With crosswind near 20 knots, a left yaw and slide to the right (downwind side) occurred, as identified in training manuals. At about 20 knots crosswind, the aircraft exited the runway off the right side.
Runway contamination at touchdown was likely greater than reported due to ongoing snow. The RVR of 6,000 feet allowed seeing departure end runway lighting at touchdown. Braking effectiveness was reduced by slippery conditions. The combination of left crosswind, slippery runway, and high reverse thrust at low speed resulted in the aircraft weathercocking left into the wind. Since engines had not spooled down before reversers were stowed, the resultant forward thrust arrested deceleration and led to the excursion.
When reverse thrust was reselected, idle reverse on the left engine contributed to increased rate of left turn. Maintaining braking after the veer minimized tire cornering forces. Had brakes been released, tire cornering forces could have helped counteract the veer. The rudder was ineffective at low forward speed.
Causes and Contributing Factors
The flight crew was unable to maintain directional control of the aircraft on landing. Contributing to the loss of control was the crosswind, the slippery runway surface, and the forward thrust that resulted when the reversers were stowed before the engines reached idle power.