Incident Overview
On 23 April 2018 at 17:15 local time, a Rolladen-Schneider LS 4-b glider (registration PH-974) was conducting a local flight in the vicinity of Larserveld, Netherlands. During the winch launch, the air brakes opened unexpectedly but were corrected by the pilot. After about one hour of flight, including mild turbulence, the problem of unintended air brake deployment recurred several times. At an altitude of approximately 1,100 meters, the air brake control rod detached with a loud bang. The pilot then had little to no control over the air brakes and decided to return to the airfield. Using his right hand to hold the remaining part of the control rod and his left hand to steer, the pilot completed a landing without further problems. No injuries were reported.
Aircraft and Pilot Information
The glider was a single-seat Rolladen-Schneider LS 4-b. The pilot held a Light Aircraft Pilot License (LAPL) and a valid medical certificate, with a total flying experience of 60 hours (295 flights), including 13 hours (59 flights) on the type. The flight was a local operation under daylight conditions.
Background and Airworthiness Directive
In 2017, at least six occurrences were reported to the manufacturer involving potential failure of the air brake control rod junction due to cracked welds or loose blind rivets. The junction is only visible during maintenance checks. Lateral forces on the air brake handle could induce a bending moment on the junction, potentially leading to weld or rivet failure and reduced air brake control. To address this, the manufacturer published Technical Note (TN) 4048 on 30 November 2017, providing inspection and modification instructions. The European Aviation Safety Agency (EASA) issued Airworthiness Directive AD 2018-0032 effective 1 March 2018, mandating compliance with TN 4048. The AD required inspection within 30 days and modification within four months if no discrepancies were found.
Modification and Findings
The involved glider was modified on 7 April 2018 at the flying club workshop in accordance with the Airworthiness Directive. Subsequent analysis by investigators determined that the wrong type of Loctite (243 instead of 638) was used, and the Loctite had expired two years prior. Additionally, the weld inside the air brake control rod was not removed using a 13.5 mm drill as required by TN 4048, but with a different drill. This resulted in limited movement of the control rod, introducing an additional bending moment and pressure on the threading of the bolt in the swivel. Insufficient thread locking, combined with repeated rotational movement of the air brake handle, caused the bolt to loosen and eventually led to complete detachment of the swivel. Consequently, the air brakes could not be fully retracted.
Maintenance Considerations
The aircraft manufacturer recommended performing TN 4048 in a certified maintenance organization (EASA Part-M Subpart F). However, EASA and the Royal Netherlands Aeronautical Association (KNVvL) stated that the modification and release could also be performed by an EASA M.A. 801 (c) organization (e.g., a flying club workshop). The manufacturer considered this formally incorrect. Feedback from mechanics who performed the modification indicated that the instructions were not always clear, though manufacturer support was adequate. The KNVvL suggested that future modification instructions include a complete material listing, including non-standard consumables and tools, or that such items be included in the retrofit kit.