Operators have reported partial debonding of trim tabs on a small number of main rotor blades.
Trim tab assemblies on VHA main rotor blades consist of two identical trim tabs that are bonded together and then bonded to the trailing edge of the main rotor blade structure. The trim tabs incorporate a joggle to allow the tab to fit over the trailing edge of the blade.
Early 206L MRB trim tabs had a shallow joggle that inadvertently allowed the trim tabs to be installed too far onto the blade assembly, thereby creating a preload in the bondline of the tab-to-blade bond regions. The preload created the potential for microcracking of the adhesive bondline when the trim tabs were adjusted.
Under extreme conditions, environmental contaminants were found to gradually penetrate the bondline and slowly cause delaminations of the trim tab assembly.
Note that this is not a safety of flight issue as trim tabs are used to adjust vibration levels caused by manufacturing variables to enhance ride comfort. The blades are shipped with trim tabs set neutral to the chord line, thus not affecting the rotor system, and many operators find that they don’t need to adjust the trim tabs during track and balance.
Any debonded trim tabs must be removed and rebonded to the trailing edge of the rotor blade. This can generally be done at the customer’s facility by personnel authorized in composite structures repair. Refer to DRAFT VHA Service Bulletin 33000-3R2 for complete information and instructions.
Operators should inspect the trim tab assemblies at 100 hour intervals for any bondline cracking or separation/debonding of trim tabs.
- When operators contacted us about this issue, we immediately dispatched personnel from Van Horn Repair to repair the blades under warranty.
- We created new trim tab assembly kits to provide to operators.
- We redesigned the trim tabs to modify the joggle and eliminate the fit issue.
- We are working toward publishing a service bulletin regarding this issue, and posted a draft bulletin to alert operators while the bulletin is finalized and approved by the FAA.