Old Age is catching up with the McDonnell Douglas DC-9 fleet as the US Federal Aviation Administration issues directives on two electrical modifications and a revision on the safe life of structural components.
Reporting incidents in which short-circuiting has caused "severe smoke and burn damage which could result in inflight electrical fires", the FAA has issued a notice of proposed rulemaking (NPRM) requiring major modification to the electrical power centre to an estimated 924 DC-9s and C-9s (military variant). Modification or overhaul of some of the AC power relays is also demanded.
The NPRM reports incidents where "the AC cross-tie relay shorted out internally...which caused severe smoke and burn damage to the relay, aircraft wiring and adjacent panels. Investigation reveals that the electrical fire originated within the cross-tie relay of the power distribution system." This has been attributed to a phase-to-phase short within the relay, and the NPRM calls for their replacement or overhaul.
Also required by a separate NPRM is a one-time visual inspection of fuel-quantity indicating system wiring in the forward cargo compartment because incorrect wiring has been found in 12 aircraft during random checks.
Finally, an airworthiness directive (AD), despite objections to the associated NPRM, requires airlines to incorporate in their "Instructions for Continued Airworthiness" additional requirements for safe-life limited structural components for about 150 DC-9-30s.
The reason, says the AD, is the risk of fatigue cracking "adversely affecting the structural integrity of the aircraft".
Source: Flight International
