What happened
On 6 August 2006, a PZL-104 Wilga 35A, registration G-BWDF, was conducting aerial work involving banner towing at Dunsfold Aerodrome. Following a standard pre-flight inspection, which included the removal of the pitot tube cover, the pilot commenced the operation. The flight proceeded without incident until the banner was released.
As the pilot initiated a climbing left-hand turn at full power, the aircraft experienced a sudden thump and a noticeable stagger in flight. Despite the disturbance, the pilot maintained control and completed an abbreviated circuit, landing the aircraft safely. No injuries were reported.
The investigation
Upon landing and taxiing for refueling, a subsequent inspection in the hangar revealed that the pitot tube was missing and the right front wing spar had sustained damage. Specifically, the starboard forward link brace had parted.
Observers on the ground reported seeing the aircraft lurch and then recover during the climbing turn. Investigators examined the wing for evidence of a bird strike or impact with another object, but no such evidence was found on the wing structure. The investigation focused on the mechanics of the failure and the forces applied to the airframe during the maneuver.
Findings
- The investigation established that the pitot tube failed due to an overload caused by an upward and slightly inboard force.
- This failure of the pitot tube resulted in a secondary failure of the front wing spar under overload conditions.
- While the loss of the pitot tube would have impacted airspeed indications, the pilot was actively monitoring the instrument during the towing process and remained confident in its functionality.
- There was no definitive evidence to confirm whether the initial cause was an impact with a bird or model aircraft, structural fatigue, or an impact during ground handling.