Chalk's Ocean Airways Flight 101

Probable cause

The NTSB determined that the probable cause of the accident was a fatigue failure in the right-wing initiated by a crack in a span-wise stringer close to the wing root. The crack had been detected running through a slosh hole (an aperture in the wall of the stringer that allows fuel to flow from one side of the stringer to the other) and seemingly repaired earlier, but the repair was ineffective. Chalk's Ocean Airways conducted several repairs to the area which did not restore "the load-carrying capability of the wing structure." The company failed to identify the repairs as ineffective. The NTSB found the maintenance programme of Chalk's Ocean Airways to be ineffective at addressing the longstanding structural problems which led to the accident.

The Mallard was designed in the 1940s with a so-called 'wet wing' where the fuel tanks, instead of being separate items within the wing, are constructed from sealed-off portions of the wing structure itself. This eliminates the additional weight of the tanks and also allows more fuel to be contained within a given wing size. The drawback of this form of construction is that all the joints around the tank seams must be sealed in order to ensure the tank is fuel-tight. In addition, the normal flexing of the wing in flight tends to open seams over extended periods, leading to fuel leaks. Grumman, the manufacturer, had issued warnings as early as 1963 about fuel leaks from the Mallard's wing being indicative of possible structural problems.

The accident aircraft had, for several months, suffered repeated fuel leaks from both wings that had been remedied by the operator by repeatedly applying a sealant to the inside seams of the fuel tanks. In applying the sealant inside the right wing tank the operator inadvertently applied the material over a damaged lower stringer (an important, load-bearing structural member of the wing) in such a way as to conceal a fatigue crack. The crack was discovered during previous maintenance, and an attempt was made to repair the crack by grinding it out. In applying the sealant, the operator was required to access the inside of the fuel tank using small removable inspection hatches on the top surface of the wing. This resulted in poor visibility and awkward conditions for working inside the tank. The sealant concealed the previous repair to the stringer and made subsequent checking for further damage to this component (i.e., any crack growth) impossible.

The first outward sign of possible significant problems with the accident aircraft was when a chord-wise crack (running from the front of the wing towards the rear) was noticed in the skin of the lower surface of the right-wing at the root. After several failed attempts at stopping the crack by drilling stop-holes, the crack was repeatedly repaired by affixing doublers (metal patches intended to take over the load from the damaged part, a normal repair procedure for minor skin damage), which were riveted over the cracked skin. However, the crack continued to grow, requiring longer and longer doublers to be fitted. Although the skin crack was slowly getting longer it was not thought to be anything other than a skin problem, which could be dealt with by affixing a doubler.

Like most aircraft of the period, the Mallard was built using an aluminum stressed skin construction. Unlike earlier fabric covered aircraft where the fabric covering is merely for aerodynamic purposes and contributes little to the overall strength of the airframe, the metal skin itself carries part of the flight loads and is stressed during flight. Many such aircraft develop minor skin cracks over time; provided appropriate action is taken to repair any cracking, the safety of the aircraft is not compromised.

The unseen cracked stringer on the accident aircraft allowed the right wing to flex more during flight which increased the bending forces at the root such that the visible skin crack slowly increased in length with each subsequent flight.

The crack in the underside of the wing grew in length until the wing was so weakened it was unable to support the flight loads during the accident flight, and the wing separated. This caused the fuel contained within the wing to be released and ignited, resulting in the fire seen by witnesses. By sheer chance, a tourist from New York was able to capture video of Flight 101's final moments on his cell phone as the plane went plunging towards the ocean. The footage clearly showed the airplane and the right burning wing falling on two separate spots as they hit the water.

On examination of the wreckage, investigators discovered that, in addition to the external doublers, internal doublers had also been affixed to the root area of both wings. However, maintenance records for these repairs were not available. Investigators also concluded that the cracked stringer that initiated the wing loss had probably failed completely some considerable amount of time prior to the accident, leading to a substantial weakening of the wing structure.