Incident Overview
Air Labrador Flight 1471 (LAL1471), a De Havilland Dash-8, departed St. John's, Newfoundland, for Deer Lake at 0725 Newfoundland daylight time (NDT). Immediately after take-off, the first officer, who was the pilot flying, noted excessive back pressure on the control column and an unusually high pitch-up attitude. The first officer applied elevator trim to counteract the back pressure, but even with the trim control wheel moved to the full nose-down position, the problem persisted. The first officer informed the captain, who then took control. The captain found the elevator trim control wheel jammed in the full nose-down position and, after leveling at 5,000 feet, applied a heavy force to free it. When the trim freed, the aircraft made a sudden nose-down pitching movement, and control column force returned to normal. The flight crew advised the Gander area control centre (ACC) of the situation and received priority clearance to return to St. John's. The aircraft landed at 0739 without further incident.
Pre-Flight and De-icing
The aircraft had been hangared overnight and was brought to the terminal ramp at approximately 0515. The initial inspection found the aircraft contaminated with a mixture of snow and ice. After boarding passengers, the crew taxied to the de-icing bay, shut down engines, and had the aircraft de-iced. During the six-to-seven-minute taxi from the de-icing bay to the departure position, the only precipitation observed was light rain with some light snow mixed in. The pre-take-off control check revealed no anomalies, and a visual inspection of the spoilers and leading edge of the wings showed no contamination before the take-off roll. The take-off roll was normal until rotation, when abnormal back pressure was felt, increasing as the aircraft accelerated.
Inflight Events and Communication
Several transmissions occurred between the aircraft and Gander ACC before the captain requested return clearance. The ACC controller asked if the captain wanted priority, which he then requested and received. The captain explained that he had not asked for priority earlier because he wanted to ensure the aircraft was under control and to determine the cause of the problem. Another aircraft was on approach ahead of LAL1471, and the captain felt it would have been too rushed to land in front of it.
Post-Flight Inspection
Approximately 45 minutes after landing, the aircraft was placed in a hangar. Inspection of the horizontal stabilizer and elevators revealed ice and slush contamination. However, the crew reported flying through freezing rain on the return, and the aircraft was exposed to freezing rain on the ramp before being hangared. A thorough examination of the complete elevator control system found no deficiencies. The aircraft was returned to service without recurrence of the problem. Weather at St. John's at the time was reported as indefinite ceiling 100 feet obscured, visibility one-half mile in light freezing rain and fog.
Investigation Findings
The de-icing organization was approved, and the crew operating the de-icing equipment were trained and qualified per Canadian Aviation Regulations. The de-icing truck was positioned with the boom and bucket about two feet above the horizontal stabilizer, allowing the operator to see the entire surface. The truck applied 670 liters of Type I fluid at correct temperature. According to the Hold Over Table, for the outside air temperature and precipitation type, the hold-over time was two to five minutes; if expired, a pre-take-off contamination inspection was required. The company's operations manual specified inspection of the spoilers and wing leading edge only.
When the first officer applied full nose-down elevator trim, the trim mechanism jammed. This condition had been addressed in De Havilland Aircraft Safety of Flight Supplement Number 1 (dated 30 October 1992). The captain's application of high breakout force to free the trim followed the supplement's procedure. Investigators replicated the jamming by applying full nose-down actuation while the aircraft was hangared; the trim could be freed using the supplement's procedure. The elevator trim tab actuators (pre-modification #8/0509) were removed and replaced with post-modification units; both sets jammed and freed similarly. The removed actuators were tested and operated normally down to -20°C. The trim tab system has no mechanical stop; only the extremes of travel of the screw jack act as a stop.
Engineering Analysis
The Dash 8 uses a spring-tab mechanism to assist elevator control. As the pilot moves the elevator, the spring tab moves in the opposite direction to reduce control forces. If the spring tab is immovable, back pressure on the control column increases with airspeed. The analysis focused on ice effects, as no mechanical discrepancies were found. The most probable scenario was that spring tab movement was obstructed by ice. Two possible explanations: contamination from freezing precipitation after de-icing, or residual ice/snow not removed during de-icing because the trailing edge of the elevator points downward when stationary, making it difficult for the de-icer to see between the elevator and spring tabs. Post-de-icing contamination alone was unlikely to cause obstruction but could have exacerbated residual contamination.
Cause Determination
The Transportation Safety Board's engineering report (LP 60/98) concluded that the elevator spring tab movement was restricted after take-off, probably due to ice or frozen snow accumulation in the gap between the spring tab and the elevator. When full nose-down trim was applied, it jammed. The captain applied high breakout force per the Safety of Flight Supplement. The captain did not request priority until prompted by ATC, after elevator control had returned to normal. The probable cause, as stated in the report's causes and contributing factors, was that the elevator spring tab movement was obstructed, probably by ice contamination, resulting in restricted control column movement after take-off.