Statement to the AMOC (Alternative Method of Compliance)
Since a couple of weeks an AMOC (Alternative Method of Compliance) exists for AD 2018-0032 concerning air brake control of the LS4-b, which was not developed by DG Flugzeugbau.
DG Flugzeugbau recommends against the use of this AMOC. The DG design organization has considered to develop alternative inspection intervals for the LS4-b, but has decided against this, to achieve best possible safety. The LS4-b is widely used as a training glider, typically in club use and the strict compliance of changed inspection intervals might prove to be difficult in clubs. The DG design organization assesses the risks of non-compliance to the inspection intervals as too large, especially since these intervals would be uncommon for sailplanes.
In the year 2017 alone, 6 cases were reported where the air brake control of the LS4-b was about to fail, either by failed welding or by loose rivets. As a consequence, the DG Design organization developed TN4048, which was published on December 22, 2017.
The design that is now selected for the LS4-b, has been in service in over 1500 LS sailplanes (LS1-f, LS3, LS4-a, LS6-a, -b), and has been successful for over 40 years, hence can be considered to be a mature design. In this particular case, extensive stress analysis of the swivel in the air brake control were made, as a basis for the compliance to the certification specifications of the LS4-b.
After revision of the documents, EASA has decided an „unsafe condition“ existed for the LS4-b and has issued AD 2018-0032 as the logical consequence.
Proper Implementation of TN4048
In this instance, it’s once more recommended to have the work on the air brake control of the LS4-b executed in a certified maintenance organization. A case was reported to us where TN4048 was executed in the club workshop. Shortly after, the bolted connection of the air brake swivel loosened in flight and the air brakes could not be retracted completely anymore. During the subsequent analysis it was concluded the wrong type of Loctite (243 instead of 638) was used and furthermore the Loctite was expired for quite a long time. Furthermore, the weld in the control rod were not removed as required by the TN, so the control rod was limited in its movement. Due to this example, we would like to ask all inspectors to carefully study the details of TN4048. Should anything be unclear, please contact the technical support of DG.