What happened
On 23 April 2018, a single-seat Rolladen-Schneider LS 4-b glider, registration PH-974, was conducting a local flight near Larserveld, the Netherlands. During the initial winch launch, the pilot noticed the air brakes opening unexpectedly, a problem that recurred several times during the flight. While at an altitude of approximately 1,100 metres, the air brake control rod detached from the mechanism with a loud bang.
Following the detachment, the pilot lost the ability to control the air brakes effectively. To ensure a safe return to the airfield, the pilot used his right hand to hold the remaining portion of the control rod while steering the aircraft with his left hand. The aircraft landed at the airfield without further incident, and no injuries were reported.
The investigation
The Dutch Safety Board investigated the mechanical failure, focusing on recent maintenance performed on the aircraft. The glider had undergone a modification on 7 April 2018, just weeks before the incident, to comply with EASA Airworthiness Directive 2018-0032. This directive was issued to address a known safety risk regarding the failure of the air brake control rod junction.
Investigators examined the work performed at a flying club workshop. They discovered that the modification process had deviated from the manufacturer's technical instructions. Specifically, the investigation looked into the tools and consumables used during the installation of the new air brake swivel.
Findings
The investigation established that the failure was caused by improperly executed maintenance work during the recent mandatory modification. Several critical errors were identified:
- The technicians used the incorrect type of Loctite (type 243 instead of the required 6TB 638).
- The Loctite used had expired two years prior to the maintenance.
- The required 13.5 mm drill was not used to remove the existing weld in the control rod; instead, a different drill bit was used.
These errors caused the control rod to have restricted movement, creating a bending moment on the junction. Additionally, the remaining weld material exerted pressure on the bolt threading. Because the thread-locking compound was both the wrong type and expired, the repeated movement of the air brake handle caused the bolt to loosen, eventually leading to the complete detachment of the swivel.