Synopsis
The pilot of a Schempp-Hirth Duo Discus T glider (registration G-SAXT) deployed its self-sustaining engine while crossing a stretch of water. After about five minutes of running, a loud bang occurred, accompanied by severe vibration as one of the five propeller blades detached. The pilot also sensed a restriction in aileron movement and decided to force-land in a field, which was accomplished without further incident or damage. Subsequent examination revealed that the blade separation resulted from metal fatigue cracking in the propeller hub blocks.
History of the Flight
On 6 September 2015 at 1437 UTC, the glider departed from RAF Halton with the intention of flying to Bembridge on the Isle of Wight and returning. Shortly after takeoff, the engine was deployed and tested, then the flight continued unpowered to the Lee-on-the-Solent area without issue. Due to poor soaring conditions, the pilot started the engine at 2,500 ft in preparation for crossing the Solent. After approximately five minutes at 3,000 ft, a loud bang and substantial vibration occurred. The ignition was turned off; once the propeller stopped, the pilot and passenger—also a pilot—observed that a blade was missing. Control was briefly transferred to the passenger while the pilot secured loose items. Upon resuming control, the pilot felt restricted aileron movement and selected a suitable stubble field about 10–12 km away, into wind. He stowed the engine and landed without further damage or injury.
Aircraft Examination
The detached propeller blade had embedded itself in the left wing root, damaging the aileron control circuit in that area. The adjacent blade was also damaged, apparently struck by the departing blade. The propeller hub, an aluminium alloy component, exhibited failure of the metal blocks that retained the blade. Laboratory analysis confirmed that the blade detached due to pre-existing fatigue cracking of these hub blocks. The initial crack originated on the outer surface of one block and propagated inward toward the blade trunnion bore, generating additional cracks that eventually released the blade. Cracks were also found in the blocks retaining two other blades; these were confirmed as fatigue cracks consistent with high-frequency oscillations, not associated with material defects.
Previous Occurrences and Safety Actions
The propeller model (part number FL5.110/83AV, manufactured by Technoflug) had logged 36.8 flight hours at the time of failure. Hub cracking had been recognized by the glider manufacturer since at least 2006, when Technical Note No. 890-8/868-11 was issued, requiring visual checks and submission of high-time propellers to the manufacturer for inspection and revision. In November 2011, the Flight Manual was amended to include a daily inspection of the propeller hub and blades for cracks. The BGA estimates about 47 self-sustaining gliders in the UK use this propeller type; cracks have been found in about six units. Propellers with cracked hubs returned to the manufacturer were reportedly replaced with a new hub design (all hub blocks equal size), and no cracking has been reported with the later standard hub, available since late 2013.