Incident Overview
On 3 January 2016 at 14:10 local time, a SZD-9 Bocian D glider (registration SP-3616) experienced a structural failure during the final climb phase of a winch launch at Turbia airfield (EPST) near Stalowa Wola, Poland. The glider was privately owned and operated by the pilot, a 59-year-old instructor pilot with 1,259 hours of glider flight time. The flight was conducted under visual flight rules (VFR) in daytime conditions. Weather had no influence on the event.
Sequence of Events
The pilot arrived at the airfield intending to perform winch-launch flights. After a pre-flight inspection, the glider was positioned second in line for launch. The pilot entered the cockpit and the winch cable was attached. The takeoff run was normal, and the glider climbed. At approximately 450 meters altitude, the pilot heard a crack and the glider automatically released from the winch cable. The pilot executed an overhead pattern and landed safely. Post-flight inspection revealed a fractured keel strip and a torn-out lower tow hook.
Damage
The keel strip was broken and torn out together with the lower tow hook. The damage is shown in the report's illustrations. No other damage was reported.
Investigation Findings
The glider was examined at WIRKK Serwis Szybowcowy in Międzybrodzie Żywieckie with participation of specialists in wooden glider repairs. The investigation established:
- The keel strip fracture originated at the forward bolt holes of the lower tow hook attachment and propagated forward and aft.
- The reinforcing wall of the lower hook had poor glue quality.
- The glue joints of the keel strip stringers were good on the right side but showed incomplete bonding on the left side.
- The wood quality was good with no rot, but strength was likely reduced due to age.
- The design of the lower tow hook attachment was found to be poor and in need of reinforcement.
Causal Factors
The probable cause was attributed to:
- Improper design of the lower tow hook attachment.
- Poorly executed glue joints between the keel strip and the fuselage.
- Weakening of the wooden structure due to the age of the glider.
Safety Recommendation
The investigation recommended that the type certificate holder (Zakład Szybowcowy "Jeżów" Henryk Mynarski) suspend winch launches of SZD-9 Bocian D gliders until a modification of the tow hook attachment is implemented. The commission noted that a change analogous to the solution used in the SZD-9 Bis 1E Bocian would be optimal.