The FAA has issued an Airworthiness Directive (AD) for Twin Commander 690s, 690As and 690Bs requiring inspection for cracking of the outer fuselage attachments, the lower wing main spar, the vertical channels, the upper picture window channels, aft cabin pressure web, external wing to fuselage fillets and fasteners. It requires modification of the structure with reinforced parts. According to the AD, the condition, if not corrected, could result in structural failure of the airplane.

“About three years ago a routine scheduled inspection of a Twin Commander 690B conducted by a Twin Commander Service Center revealed cracking in the aft pressure bulkhead area of the aircraft. Subsequent inspections of several other 690A and 690B Twin Commanders turned up evidence of similar cracking,” Twin Commander Aircraft president Matt Isley told AIN.

Isley said he has been working closely with the FAA since the discovery and Twin Commander Aircraft has developed a service bulletin (SB241) that calls for inspection and modification of the affected area on the 690/A/B.

“These cracks developed over time due to wing loads and pressure cycle stresses concentrating in a single area on the aircraft, thus the need for a service bulletin to inspect the affected area and make necessary modifications so that no further inspections will be required. Later-model Twin Commanders [690C/D and 695/A/B] have a different wing-to-fuselage design and are not affected by the service bulletin,” he said.

The May 14 AD, which specifies an effective date of May 29, mandates compliance with the inspection procedure and modifications specified in SB241. The compliance schedule in the AD is much more aggressive than that specified in the service bulletin, said Isley.

As per the AD, compliance is based upon the aircraft’s specific time-in-service (TIS).

“The AD deadline places an extreme burden on owners of many of the approximately 240 aircraft affected. We’re working with our Service Centers to accelerate our response to customers in the fleet,” he added.

Isley said that in addition to originally identifying the problem at one of Twin Commander’s own Service Centers, the Twin Commander engineering team, in conjunction with experts in the field, quickly developed a solution.

“We have spent the last three years aggressively communicating the matter to aircraft owners and perfecting the modification procedure, which is now in its third revision stage. To ensure that modifications are performed to the highest standards, we developed a factory training course specific to SB241, and to date have trained more than 30 technicians to perform the work. Once properly implemented, this modification will permanently address problems caused by dynamic and cyclic stresses in the aft pressure bulkhead area,” he said.

According to Isley, nearly 40 Twin Commanders have been modified to date. –D.A.L