What happened
During a flight from Hamilton, Ontario, to Burlington Airpark, a Cessna 206 (registration C-FIHV) experienced unusual handling characteristics during the initial climb. The pilot, flying solo, noted an increasing tendency for the aircraft to pitch nose-up immediately following rotation. To counteract this, the pilot applied full nose-down trim, yet heavy forward pressure on the control wheel was still required to maintain a stable pitch attitude throughout the climb.
As the aircraft accelerated, the required forward pressure increased. However, when engine power was reduced to cruise levels, the pitch attitude became easier to manage. During the final approach and landing, the need for full nose-down trim subsided, and the aircraft was landed at Hamilton Airport without further incident.
The investigation
Following the flight, maintenance engineers investigated the aircraft's trim tab actuator, which had recently been replaced due to reaching its five-year service life. The investigation established that while the actuator itself functioned correctly, the travel limits for the trim tab were incorrectly configured.
The engineer performing the installation encountered difficulties with cable routing due to limited visibility and found the maintenance manual's instructions regarding cable tension to be ambiguous. Crucially, the engineer misinterpreted the specified travel limits for the trim tab. Rather than setting the tab to move between 25° up and 5° down, the engineer set the limits from 25° down to 5° up. This error resulted in a significant loss of nose-down trim authority.
An independent inspection was also conducted by a second engineer. However, this inspection failed to identify the error because the second engineer relied on a verbal briefing from the primary engineer rather than independently verifying the rigging limits. This reliance on the first engineer's explanation undermined the purpose of the independent inspection process.
Findings
- The primary cause of the control difficulty was the misinterpretation of elevator trim tab travel limits by the maintenance engineer, which led to incorrect rigging.
- The error was compounded by the fact that the second engineer failed to detect the mistake during the independent inspection because he relied on the explanation provided by the person who performed the work.
- Previous organizational issues, including a similar misrigging incident on a different aircraft type, had created a context where engineers might misinterpret control nomenclature as referring to aircraft response rather than tab deflection.