GRANITE, OK, USA
N4473A
Piper PA-18-150
The airplane was destroyed upon impact with terrain following a loss of control during the takeoff/initial climb. Severe corrosion was found at two structural tube clusters (one with a windshield pillar) located directly below the right wing forward and aft attach fittings. The portion of the forward structure available for carrying loads prior to the accident was less than 30% of the circumference of the pillar tube. Deformation associated with the separation of the pillar tube indicates that the pillar failed under compression loads. The manufacturer's representative stated that the 'windshield pillar is normally subjected to a tensile load during flight.' It is possible that compression loads generated during the takeoff roll led to a compression failure of the pillar. This failure would have changed the lift characteristics of the wing since the separation point was directly below the right wing forward attach fitting. The most recent annual inspection was performed in January 1999; the maintenance entry for the annual stated, in part, 'treated steel tubing as required.' In June 1999, a major repair was made to a tube cluster in the aft fuselage area 'due to external rust through from past acid spill.'
On October 10, 1999, at 1905 central daylight time, a Piper PA-18-150, single-engine airplane, N4473A, was destroyed when it impacted the terrain following a loss of control during the takeoff/initial climb from a private grass airstrip near Granite, Oklahoma. The aircraft was owned and operated by Great Plains Aviation LLC, of Granite, Oklahoma, under 14 Code of Federal Regulations Part 91. The private pilot and the passenger received serious injuries. Visual meteorological conditions prevailed for the local personal flight, and a flight plan was not filed. The flight was originating at the time of the accident. On the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the operator reported that the pre-flight, run-up, takeoff on runway 34, and initial climb were normal. Wind was calm. The aircraft had climbed to an altitude of 50 to 100 feet above ground level, and "Suddenly, there was no control of the aircraft - neither aileron nor elevator. The plane descended, the left wing was low and brushed/impacted the ground (not hard), [and] apparently started the plane to turn counterclockwise and pushed the left wing up and the right wing down which then impacted the ground with sufficient force to then pull the engine into the ground. The plane then rotated approximately 180 degrees and came to rest approximately 50 feet from the point of impact with the ground." In a written statement, the pilot reported the following: "After liftoff I initiated a left turn to the west. I soon realized that I might not be able to clear terrain, so I initiated a right turn back to the north. It was at this time that I realized I could not even maintain altitude and was in fact descending. I remember checking airspeed (I was indicating 65mph) not wanting to stall, and heading to open terrain (no trees). I told my passenger that we were going down, and I cannot recall anything after that." During an on site investigation, the FAA inspector established flight control continuity from the elevators to the aft and forward control sticks, the ailerons to the aft control stick, and the rudder to the rudder pedals. He also noted that the elevator push-pull rod connecting the two control sticks was separated near the aft stick. Both flight control sticks and the push-pull rod were removed and forwarded to the NTSB South Central Regional Office for further examination. On November 16, 1999, the airplane was conditionally released for recovery and transport from the accident site to Air Salvage of Dallas, Lancaster, Texas, for further examination by the NTSB. The wings were removed from the airframe during the recovery process. On January 4, 2000, the airplane was examined at Lancaster, Texas, under the supervision of the NTSB investigator-in-charge (IIC). Flight control continuity was confirmed via the presence of cables from the elevators to the aft and forward control stick attachment areas, the ailerons to the aft control stick attachment area, and the rudder to the rudder pedals. The flight control sticks and push-pull rod, as received from the FAA, were also examined. The separation area in the push-pull rod, previously identified by the on site FAA inspector, exhibited uneven surfaces consistent with overload separation. According to the manufacturer representative, the "control system appeared to be operable prior to the accident." The representative noted that the "small pulley mounted just below the aft control stick where elevator cable normally runs shows no indication of cable presence. Pulley was covered with dust and lint but keeper cotter pin below pulley was shiny and slightly worn indicating the cable may have been misrouted over the pin." He further stated that the "aluminum tube housings used for routing the tow hook (2) release cables were not properly secured (Tyrap on one end) and chaff marks and wear was noted in several areas from contact with the lower elevator cable." Two areas of fuselage tube corrosion were noted on the forward right side of the aircraft. Both corroded areas were located directly below the right wing attachment fittings. At the forward right windshield pillar lower tube cluster, numerous holes were found through the tubing and the pillar tubing was separated from the cluster. At the tube cluster below the right wing trailing edge at the door mating seam, internal corrosion was noted and the upper tube was separated from this cluster. Both clusters were forwarded to the NTSB Materials Laboratory for metallurgical examination. Additionally, several areas of major repairs were noted to tube clusters in the aft fuselage area. Metallurgical examination of the forward cluster revealed that the windshield pillar consisted of a round tube tack welded to a three-sided flat sheet formed around the tube. "Severe" corrosion was found in both the tube and the sheet in the vicinity of the pillar fracture. In numerous locations, the corrosion had completely penetrated the thickness of the tube and sheet. The corrosion damage had reduced the material to a "knife edge over about 70 percent of the circumference of the tube." Compression buckling, indicative of separation under compression loading, was noted on the sheet material. In addition, the separated end of the tube contained significant bending deformation, also consistent with compression loading. The manufacturer representative stated that the "windshield pillar is normally subjected to a tensile load during flight." Examination of the rear cluster revealed features "typical of a separation as a result of bending loads." The mating surfaces of the fracture "showed the presence of what appeared to be long-term corrosion." The corrosion had reduced the tube wall thickness over about 10 to 20 percent of the circumference of the tube. Maintenance records were reviewed by the FAA inspector, and portions of the records were reviewed by the NTSB IIC. The entire aircraft was cleaned and painted in December 1991. The last annual inspection was performed on January 12, 1999, at a total aircraft time of 5,414.99 hours, and the maintenance entry stated in part: "Freed and lubed pulleys, treated cables. Treated steel tubing as required." A June 10, 1999, maintenance entry stated: "cut out and replaced 3-ft section of right bottom longeron at 3rd cluster forward of tail post due to external rust through from past acid spill. Internal sleeve used on longeron and external sleeves used on all diagonal tubing, then entire cluster welded, left bottom longeron externally reinforced with 4-inch external patch at same cluster location due to small weak area found during punch testing. All work performed using 4130 chromoly steel, gas welding in accordance with AC 43.13."
Corrosion failure of the fuselage support tubing below the right wing forward attach fitting due to inadequate maintenance.
Source: NTSB Aviation Accident Database
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