DEN02GA074

The airplane was maneuvering to deliver fire retardant when its left wing separated. Aircraft control was lost and the airplane crashed into mountainous terrain. A witness on the ground took a series of photographs that showed the airtanker's left wing separating at the wing root and the remaining airplane entering a 45-degree dive to the ground in a counterclockwise roll. An examination of the airplane wreckage revealed extensive areas of preexisting fatigue in the left wing's forward spar lower spar cap, the adjacent spar web, and the adjacent area of the lower wing skin. The portion of the wing containing the fatigue crack was obscured by the retardant tanks and would not have been detectable by an exterior visual inspection. An examination of two other airtankers of the same make and model revealed the area where the failure occurred on the accident airplane was in a location masked by the airplane's fuselage construction. The airplane was manufactured in 1945 and was in military service until 1956. It was not designed with the intention of operating as a firefighting airplane. In 1958, the airplane was converted to civilian use as an airtanker and served in that capacity until the time of the accident. The investigation revealed that the owner developed service and inspection procedures for the airtanker; however, the information contained in the procedures did not adequately describe where and how to inspect for critical fatigue cracks. The procedures were based on U.S Navy PB4Y-2 airplane structural repair manuals that had not been revised since 1948.

History of flight

On July 18, 2002, at 1840 mountain daylight time, a Consolidated-Vultee P4Y-2, N7620C, under contract to the U. S. Department of Agriculture Forest Service and piloted by a commercial pilot, was destroyed when it impacted into mountainous terrain 6 miles southwest of Estes Park, Colorado. A post-crash explosion and fire ensued. Prior to the impact, the airplane's left wing separated and aircraft control was lost while maneuvering to deliver fire retardant on the Big Elk wildfire, burning in an area northwest of Lyons, Colorado. Visual meteorological conditions prevailed at the time of the accident. The aerial attack flight was being conducted under the provisions of Title 14 CFR Part 91, public use. A company visual flight rules flight plan was on file. The pilot and second pilot on board the airplane were fatally injured. The flight originated at Broomfield, Colorado, at 1814.

According to the Airtanker Base Manager at Jeffco Airport, Broomfield, Colorado, the airplane, Tanker 123, was dispatched to join other airtankers, Tanker 161 and Tanker 22, a helitanker, number T-16, and a U. S. Forest Service lead airplane, Lead Bravo 8, to drop fire retardant on the Big Elk Fire. The airplane had flown seven previous air attack missions on the fire that day. Prior to the accident mission, the airplane was loaded with approximately 2,000 gallons of fire retardant, and 550 gallons of fuel.

The captain of T-161 said, "Tanker 22 had just completed his drop and Tanker 123 had observed the drop and was preparing to drop. All communication between 123 and Lead Bravo 8 was normal. I fell behind T-123 on downwind and base. I looked away momentarily and I again focused on T-123. I noticed his left wing was falling. The aircraft was in a 15 to 20 degree bank. I next saw fire near the fuselage as the wing failed inboard of the number 2 engine. The aircraft pitched nose down in a huge fireball and plunged into the ground vertically starting an immediate large fire."

The copilot of T-161 said Tanker 123 was in his base turn for the drop and in a "smooth 15 to 20 degree bank turn", when the left wing separated from the airplane. "The aircraft then went into a rotation and impacted the ground." The copilot said that operations were normal and the weather in the area consisted of "the smoothest, least turbulent conditions of the day."

The pilot in the Forest Service leadplane said, "The conditions were perfect for a tanker drop. No turbulence and no smoke in that area." The pilot said, "I had just made two runs with Tanker 22 on the same drop area. He had departed and I allowed helicopter 72D to make a water drop on the area. Tanker 123 was on scene when Tanker 22 made his split load drop, two different runs. I instructed Tanker 123 that we would be extending Tanker 22's first drop. Tanker 123 responded with something like, 'We can do that' or 'We see that," and that he was on downwind for the drop. I told him I was at his 8 o'clock and [he] said that he had me in sight. I then told him I would come up on his left side and continue downwind with him until he was ready to turn back. He then responded with 'I think I'm going to use this nice big valley to turn around in.' I told him that sounded like a good idea to me. We flew approximately 15 seconds before he began a gentle turn to final. We continued in the turn from downwind to final without squaring off for a base, which is normal on tanker runs." The pilot said that after he turned on final, he told the captain on Tanker 123 that his attack run would require a pitch over which was approximately 1/2 mile ahead. The pilot said after he finished that transmission, the captain of Tanker 161 called him and said that the left wing had just come off of Tanker 123 and the airplane had gone in.

A witness on the ground, approximately 6 miles east of the accident location, using a digital camera with a telephoto lens and automatic shutter, took a series of eight still photographs of the airplane at the time of the accident. The photo series shows the airplane's left wing separate at the wing root and fuselage, a fire ensues at the separation, and the airplane enters an approximately 45-degree dive to the ground. The photos also show the airplane in a counter-clockwise roll during its dive toward the ground. The witness said when he saw what was happening, he depressed the shutter button and held it until the airplane went behind a treed ridge. Copies of the eight photographs are provided as an attachment to this report.

Several witnesses on the ground in the vicinity of the accident location said they saw the airplane in a left banking turn. Some of the witnesses described the turn as "steep" or "tight." One witness said the airplane had rolled out of the turn after making a "hard banking turn." At this time, none of the witnesses indicated that there were any apparent problems with the airplane. The witnesses then said the left wing "folded upward" and severed at the "left wing-fuselage point." They all described "slurry" coming out and "fire erupting" from the area where the wing separated. The witnesses said the airplane rapidly descended and impacted into the hillside.

Radar information provided by the Denver Air Route Traffic Control Center showed the airplane come out of the Broomfield area and proceeding northwest to the Lyons-Estes Park area where the Big Elk fire was burning. The airplane is shown entering a series of counter-clockwise 3.5 to 4 nautical mile diameter orbits at an altitude averaging 8,900 feet mean sea level (msl), over Lion's Gulch, and in the vicinity of Moose Mountain. Terrain in this area is mountainous averaging from 7,300 feet msl to 8,500 feet msl. The radar information showed the airplane make five orbits over this area. On the last half of the fifth orbit, the radar information shows the airplane at an altitude of 8,500 feet msl and heading approximately 315 degrees. The airplane is shown on a 315-degree heading for 1.3 nautical miles. The airplane is then shown making a left turn to 260 degrees. The airplane is shown on a 260-degree heading for 1.1 nautical miles and at an altitude of 8,400 feet msl. The airplane is then shown making a left turn and rolling out on a 195-degree heading. The airplane remains on this heading for 0.6 nautical miles, when radar contact is lost. The accident site was approximately 0.1 nautical miles southwest of the last radar plot.

Personnel information

The pilot-in-command held a commercial pilot license with single and multiengine land, instrument airplane ratings, dated January 17, 1996. The pilot was type-rated in CV-P4Y airplanes. The pilot also held a flight instructor certificate with endorsements for instruction in single-engine land airplanes. The pilot-in-command held a second-class medical certificate dated January 18, 2002. The certificate contained no restrictions or limitations. The pilot-in-command met contract qualifications for the U. S. Department of Agriculture (USDA) and U. S. Department of the Interior (DOI) as a pilot-in-command for special mission, low level, airtanker initial attack in PB4Y-2 airplanes. The pilot-in-command completed a company pilot proficiency check in the P4Y-2 on January 31, 2002. At the time of his proficiency check, the pilot had 3,416 total flying hours and 1,094 hours in P4Y-2 airplanes. Information provided by the Forest Service indicated that at the time of the accident, the pilot-in-command had 3,658 total flying hours, and 1,328 hours in P4Y-2 airplanes.

The second pilot held a commercial pilot license with single-engine land and sea, multiengine land and sea, instrument airplane ratings, dated October 15, 1997. The second pilot was type-rated in CV-P4Y, DC-3, DC-B26, M-202, M-404, and N-B25 airplanes. The second pilot held a second-class medical certificate dated January 22, 2002. The c