What happened
On 5 June 2011, during an aerobatic display at a festival in Foynes, Co. Limerick, a Steen Skybolt, registration EI-SAT, experienced a sudden structural failure. While performing a routine sequence, the aircraft was flying in near level flight with a slight left bank when large sections of fabric detached from the upper surface of the left wing.
The loss of fabric caused immediate and severe control difficulties. The pilot noted intense vibrations through the control stick and observed the ailerons fluttering badly. Additionally, the aluminum leading edge detached from the upper wing spar, and the pitot tube was displaced, leaving the pilot without reliable airspeed indications. Despite applying full power, the aircraft began descending at a rate of 200-300 feet per minute. Faced with limited options and low altitude, the pilot executed a controlled forced landing in a nearby field, narrowly avoiding a stone wall and sustaining no injuries.
The investigation
The AAIU investigation focused on the structural integrity of the wing and the sequence of the fabric's detachment. Technical inspections of the aircraft revealed that approximately two-thirds of the fabric on the upper surface of the left wing was missing. Investigators found that the second rib of the left upper wing had shattered into fragments.
Using ultraviolet light, investigators identified biomass residue on the wing leading edge, suggesting a possible bird strike. Further examination of the fabric showed that the failure occurred when the lacing cords pulled through the rib reinforcing tapes. The investigation also included a review of the aircraft's maintenance history, noting that the plane had recently undergone a major overhaul and held a valid Permit to Fly. An inspection of the wooden wing structure by a specialist confirmed that the spars, ribs, and glue joints were in excellent condition with no evidence of structural decay.
Findings
- The primary cause of the fabric detachment was an impact with a small bird, which destroyed the second rib of the left upper wing.
- The destruction of the rib initiated a rapid tearing of the fabric across the upper wing surface.
- Non-standard aerodynamic loads caused by the initial failure led to the remaining fabric pulling away from the ribs.
- The loss of the wing's aerodynamic profile and the detachment of the leading edge caused a significant loss of lift and increased drag.
- The pilot's extensive aerobatic experience and skillful handling prevented a total loss of control and enabled a successful emergency landing.