Investigators are homing in on metal fatigue, probably aggravated by corrosion or sloppy repair, as a primary cause of the Chalk’s Ocean Airways fatal accident on Dec. 19, 2005.
The right wing separated from the seaplane shortly after takeoff from Miami, and the crash into the coastal water killed all 20 aboard. The wing was recovered from the water on Dec. 20 and National Transportation Safety Board (NTSB) engineers and metallurgists quickly observed that the wreckage indicated fatigue fractures. As an immediate result, Chalk’s grounded its remaining fleet of Gulfstream G-73 airplanes. The Federal Aviation Administration (FAA) did not order grounding, preferring instead to advise operators of what happened but confessing it does not know what action it should mandate at this time.
The FAA action seems puzzling, as the wreckage reveals that the failure likely started at the location where two offset drill holes are located. Inspecting other airplanes for such holes seems a prudent first order of business. The term “offset drill holes” is perhaps a gentlemanly way of referring to a carelessly drilled second hole that ruins the first. When this occurs, the appropriate step is to open the hole to a larger size so that only a clean new hole is left. This can be a tedious procedure with little forgiveness for ruining any of the holes in the mating surface.
There are limits to this stratagem. If the standard engineering criteria for hole edge distance to the edge of the structure (usually three to four times the fastener diameter), or to any next fastener hole edge, and a doubler cannot be utilized, normally the fitting must be replaced.
When the fasteners (bolts, Hi-Locs or rivets) are reinstalled, the sloppy repair (“offset drill holes”) is hidden until cracks develop and migrate outward. From the shading in the cross-sectional view on the NTSB photograph, it appears that the cracking existed for some time – as evidenced by the large area of darker colored metal.
The fracture photographs suggest also that stress was raised as a result of the offset drill holes, and salt-water corrosion would exacerbate the structural weakening. Corrosion can occur anyplace, but it can manifest especially in drilled holes after production, where the surface may have a coarse texture (as in an improperly drilled out hole). The two different sides of the hole shown in the NTSB photographs appear to have two different types of fatigue initiations at their surface, which could possibly constitute abusive machining or corrosion on one side and secondary initiation on the other.
What is evident are two separate drill holes, not parallel to each other and apparently merging together. It would seem that such an action would create a stress concentration.
The inexpert drill work should raise an obvious question in the engineering community’s mind, specifically, why are there two offset drill holes in this particular location which have removed maybe 15 percent or more from the horizontal leg of the spar cap?
The resulting fatigue induced failure (whether corrosion-related or not) would seem to have inevitable consequences in just a matter of time and flight cycles after those offset drill holes were inexpertly made.
Recall that there have been spar problems recently on the Beech T-34 (see ASW, Dec. 8, 2003). Also, several firefighting aircraft were found to have cracked or broken spars. As in the case of the G-73, these aircraft were designed many years ago, before designers fully understood metal fatigue.
Not Sure What To Do At This Point
Extracts from the FAA’s Special Airworthiness Information Bulletin, No. NM-06-18, of Dec. 23:
“On Dec. 19, 2005, a G-73 modified by STC [supplemental type certificate] ? to increase the passenger count [from 12] to 17 experienced a wing separation on takeoff ? The wing separated between the fuselage attachment and the engine attachment. Preliminary indications ? reveal occurrences of fatigue failure of a wing spar. This particular airplane was operated in passenger service and in a salt-water environment.
“At this time, the initiating cause of the failure is unknown. The FAA is closely monitoring the investigation to determine the existence of a condition on the accident airplane that may be manifest on other G-73 airplanes in the existing fleet.
“At this time, the FAA is not able to identify an inspection or other action that can be taken to determine whether other aircraft may have the same condition. FAA is considering the need for mandatory detailed inspections of parts or all of the wing structure on the fleet of G-73 airplanes.”
Source: FAA