FAA is advising operators of various Hawker 750s, 800s, 900s and 1000s to check the aircraft aileron and aileron tab assemblies for potential problems that could cause oscillations at high altitudes.

The agency issued a Special Airworthiness Information Bulletin (SAIB) in late November encouraging the checks, saying it had received reports of wing/aileron oscillations from operators of Hawker 800XP and 850XP airplanes at altitudes above 33,000 ft. and at speeds of more than 0.73 Mach.

Unlike an airworthiness directive (AD), an SAIB is not mandatory. The agency says “at this time, the airworthiness concern does not warrant an AD action.” Neither has Beechcraft issued a service bulletin on the issue. FAA, however, is gathering information on wing oscillations and asking for operators to report them to wichita-cos@faa.gov.

While the reports stem from Hawker 800XP and 850XP aircraft, FAA is advising the inspections on a range of Hawkers, including Beechcraft Model BAe.125 Series 800A (including C-29A and U-125), 800B, 1000A and 1000B airplanes and Model Hawker 750, 800 (including variant U-125A), 800XP, 850XP, 900XP and 1000 aircraft.

The SAIB, however, does not cover aircraft modified by Aviation Partners, Inc. The agency had already issued an AD to address “severe vibration and potentially divergent wing/aileron oscillations” on those aircraft. The AD, 2013-11-16, was issued in late May, limiting the affected Hawkers to flight below 34,000 ft. That AD covered slightly more than 100 Hawker 800, 800A and 850 business jets.

Beechcraft in May cited that AD as one of the reasons for a letter sent to its operators that it would not support aircraft incorporating major modifications conducted by facilities outside its authorized network.

Unlike reports of oscillations involving the aircraft modified by Aviation Partners, Inc., the reports concerning the other Hawkers did not involve “divergent” oscillations, FAA says. When speed was reduced and the aircraft flew below 30,000 ft., the oscillations stopped, the agency adds.

Follow-up investigations of the aircraft that experienced the oscillations revealed missing aileron bushings, low cable tension and improperly installed brackets. Once the problems were fixed, the oscillations ceased altogether, FAA says.

But the agency worries that continued oscillations can reduce the aircraft’s fatigue life and weaken the wing structure. “It is imperative that the wing structure be thoroughly checked after any such severe oscillations,” FAA says.

FAA is recommending a one-time check “at the earliest opportunity” of the aileron and aileron tab assemblies to ensure the bushings, brackets and cable tensions are all correct. The agency also recommends maintenance be accomplished at regular intervals.