Following three in-service reports of Bombardier Challenger 604s suffering fractured fastener heads on the inboard flap hinge-box, the Canadian Aviation Safety Board (CASB) has issued an emergency airworthiness directive requiring the refitting of incorrectly fitted fasteners.

The fasteners are the correct components, but were incorrectly orientated during fitting.

The CASB says the error may result in premature failure of the fasteners attaching the inboard flap hinge-box forward fitting. This could lead to the detachment of the flap hinge-box, and consequently detachment of the flap surface.

The release of a previous airworthiness directive (AD CF-2013-39) mandated a detailed visual inspection of each inboard flap hinge-box forward fitting on both wings of the large-cabin business jet, with rectification as required. The fasteners required repetitive inspection until corrective action was carried out.

However, since that directive was issued there has been one reported incident on a 604 where four fasteners were found fractured on the same flap hinge-box forward fitting.

An investigation determined that the fasteners were incorrectly installed.

The new CASB directive mandates replacement of the incorrectly oriented fasteners within 24 months. The CL-600-1A11, -2A12 and -2B16 (601-3A/-3R variant) models are addressed through AD CF-2013-39R1.

The directive has also been mandated by other national aviation authorities, including EASA.