There is no Substitute for
Thorough Inspections
By Geoffrey Pence,
Customer Service Manager, Twin Commander Aircraft
A routine 150-hour inspection of a 690B conducted recently by an authorized Twin Commander Service Center turned up a few surprises that are proving to be instructive about the importance of being resourceful and thorough.
The first surprise came when technicians conducting the inspection decided to prism check the picture windows in the passenger cabin. Though not required, the prism check can help detect small cracks in the edges of the window that are hidden from normal view. Sure enough, cracks were found along the top edge of the left-side picture window. The real significance of the discovery, however, was yet to come.
To try and determine what caused the window cracking, the service center technician went into the passenger cabin to expose and examine the metal structure that forms the window frame. That inspection revealed a 20-inch-long crack in the upper support channel. This called for further sleuthing.
Interior panels, soundproofing, insulation, and sealant were removed to expose the entire channel back to where it attaches to the main spar frame support channel at the aft pressure bulkhead. When the technician removed the clip that joins the window frame channel to the main spar frame support channel, more cracks were found.
Further investigation uncovered cracking in the aft pressure bulkhead web, which is attached to the main spar web and cap. The good news was that non-destructive testing confirmed that there was no damage to the spar web or cap.
The discovery of the cracks led the FAA to issue an Airworthiness Concern Sheet on November 27 asking for reports from the field on similar problems involving 685, 690, 690A, and 690B Twin Commanders. (Later models are already subject to an inspection in that area of the airframe.)
Twin Commander Aircraft surveyed the service center network to see if similar problems were being seen on affected models that were undergoing inspections. As of this writing, service centers have responded with information on a dozen aircraft. No evidence of the kind of cracking seen on the 690B has been found among the 12, according to the service centers.
Twin Commander is supporting the engineering effort to design a repair for the cracking on the affected aircraft, which is fitted with a belly camera port. The cracking appears to be due to wing- and fuselage bending moments. An inspection procedure likely will result in a service publication for affected models.
The problems with the 690B were discovered thanks to experienced, naturally skeptical technicians at an authorized Twin Commander Service Center who went the extra mile to be thorough. A telltale sign of problems in the aft pressure bulkhead area�working rivets or gaps in fuselage skin laps�were not evident on the airplane they were inspecting. However, they did notice that both upper and lower fuselage-to-wing fairings had cracks, even though the fairings had doublers installed.
A second clue that convinced them to undertake further investigation was the discovery of working fasteners where the main spar frame support channel attaches to the aft pressure bulkhead cap. That area can be inspected from the baggage compartment looking forward at the aft pressure bulkhead. Their inspection further concluded that the fasteners were not the correct type for that application.
Just as there is no substitute for horsepower when it comes to performance, there is no substitute for experience, thoroughness, and a healthy dose of skepticism when it comes to inspecting and maintaining aircraft. You get all of that at an authorized Twin Commander Service Center.
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