NTSB Factual Report

HISTORY OF FLIGHT

On May 9, 2004, at 1400 eastern daylight time, a Piper PA-31-350, N151LL, was substantially damaged following an explosion in the right wing during takeoff from the Morrisville-Stow State Airport, Morrisville (MVL), Vermont. The certificated airline transport pilot and six passengers were not injured. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed to the Groton/New London Airport (GON), Groton, Connecticut. The personal flight was conducted under 14 CFR Part 91.

According to the pilot, he had departed from Westerly, Rhode Island, earlier in the day, and proceeded uneventfully to MVL. During the flight, the pilot managed the airplane's fuel supply by alternately burning between the outboard and inboard fuel tanks. Upon arriving at MVL, at 1145, the pilot estimated that the outboard tanks were half full.

After a two-hour wait on the ground at MVL, the pilot observed that the cloud cover had dissipated, and estimated that the temperature had risen about 20 degrees Fahrenheit. At 1345, the passengers arrived and were boarded. The pilot then started the airplane and taxied onto runway 02, a 3,700-foot long asphalt runway.

About 700 feet into the takeoff roll, the left engine "seemed to lose a little manifold pressure," and the pilot aborted the takeoff. While decreasing power during the aborted takeoff, the outboard section of the right wing "exploded." The pilot then stopped the airplane on the runway, and the occupants were evacuated.

AIRCRAFT INFORMATION

The airplane was equipped with four flexible Goodyear Aerospace model BTC-67 rubber fuel cells, two in each wing panel. Each fuel cell was vented by a NACA fuel tank vent. The airplane was also modified with a "Colemill Conversion." The conversion included the installation of wingtip landing lights on both wings.

Review of the airplane's maintenance logbooks by a FAA inspector, from 1994-2004, did not reveal any work conducted on the fuel cells, with the exception of the replacement of the right wing outboard fuel cell on April 19, 2004. There was no mention of any electrical work being done on either wing with the exception of light bulb replacements. The only work performed on the entire fuel system of the airplane, was of routine maintenance.

The rubber fuel cell involved in the accident was produced in 1977, and installed during the production of the airplane, which was completed in 1978.

METEOROLOGICAL INFORMATION

The pilot reported the outside air temperature at the time of the accident was 79 degrees Fahrenheit.

WRECKAGE INFORMATION

Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the upper and lower portions of the outboard wing skin had separated from the wing structure. There was no evidence of fire within the wing structure observed by the inspector.

According to a mechanic who replaced the damaged wing, he observed that the inboard fuel cell vent line nipple was broken off from the cell. Examination of area where the nipple had broken off revealed that the rubber was brittle and deteriorated.

Further examination of the damaged wing by the mechanic revealed that 28-volt power wire used for the "Colemill Conversion" wingtip recognition light, was routed from the wing root area to the wing tip following an aft stringer and passing thru ribs via stringer cutouts with no chaffing protection. The wire was found to be shorted and burned off with evidence of arching to the wing structure at approximately wing station 128.

Examination of the left wing revealed that the inboard fuel cell vent line nipple was also brittle and deteriorated. When the fuel cell was removed from the wing cavity, the nipple separated from the cell. It was also noted that the wingtip recognition light wiring routed through the wing cavity had cracked insulation and the conductor was exposed at numerous locations.

ADDITIONAL INFORMATION

According to the PA-31-350 Service Manual Inspection Report Checklist, a mechanic was to:

Inspect fuel cells and lines for leaks and water, then drain both main fuel cells to check the tension and knots of the nylon support cords, inspect security of baffles and free operation of flapper valve in each main fuel cell, inspect the condition of the fuel cell material (every two years), check that the fuel cells are marked for capacity and minimum octane rating, and inspect the fuel cell vents (replacing the fuel tank vent line as required, or every five years, whichever comes first).

According to FAA Advisory Circular, AC-43.13-1B, ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICES, AIRCRAFT INSPECTION AND REPAIR:

"...Flexible Fuel Cells. Inspect the interior for checking, cracking, porosity, or other signs of deterioration. Make sure the cell retaining fasteners are properly positioned. If repair or further inspection is required, follow the manufacturer's instructions for cell removal, repair, and installation. Do not allow flexible fuel cells to dry out. Preserve them in accordance with the manufacturer's instructions....Check tank vents and overflow lines thoroughly for condition, obstructions, correct installation, and proper operation of any check valves and ice protection units. Pay particular attention to the location of the tank vents when such information is provided in the manufacturer's service instructions. Inspect for cracked or deteriorated filler opening recess drains, which may allow spilled fuel to accumulate within the wing or fuselage. One method of inspection is to plug the fuel line at the outlet and observe fuel placed in the filler opening recess. If drainage takes place, investigate condition of the line and purge any excess fuel from the wing..."

According to a representative of Colemill Enterprises, Piper Aircraft provided grommet holes along the forward spar area of the PA-31-350 to run wiring through. Due to the complexity and cost involved with being able to access the forward spar area, Colemill Enterprises would run wiring for the winglet recognition light through cavities located in the wing near inspection holes. The representative added that all wiring was protected from chafing in the wing, at the time of installation.

On September 16, 2005, Colemill Enterprises published a service bulletin regarding a "One time inspection of winglets recognition light wiring," for security and chafing. It instructed to remove the inspection panels in the aileron/bell crank areas, and visually inspect the winglet recognition light wiring for security and chafing. If chaffed wiring was observed, it was to be replaced and secured in accordance with Part 43.13-1B.

NTSB Probable Cause

The improper installation of an electrical wire, which resulted in arching and the initiation of a fuel vapor explosion. A factor related to the accident was the failure of maintenance personnel to detect a cracked rubber fuel vent line nipple.