NTSB Factual Report

HISTORY OF FLIGHT

On May 16, 1998, approximately 0750 central daylight time, a Beagle B-206 twin-engine airplane, N1885S, was destroyed when it impacted terrain in an uncotrolled descent following a loss of left engine power shortly after takeoff from the Wilbarger County Municipal Airport, Vernon, Texas. The airline transport pilot and her two passengers sustained fatal injuries. The airplane was registered to and operated by Bradson Properties of Vernon, Texas. Visual meteorological conditions prevailed for the Title 14 Code of Federal Regulations Part 91 personal flight for which a visual flight rules flight plan was filed. The flight was destined for Austin, Texas.

According to witnesses, the airplane departed from runway 02, and the landing gear was retracted during the initial "normal climb." Soon after takeoff, a reduction in engine power was heard. The airplane was observed leveling off, and turning left back toward the airport. The airplane was observed in level flight for a few seconds and then it "rapidly" rolled to the left and impacted State Highway 283 in a nose low attitude.

PERSONNEL INFORMATION

The pilot, who was occupying the left front seat of the airplane, obtained her private pilot certificate with an airplane single-engine land rating in 1985. She obtained her commercial certificate with an airplane single-engine land rating in December 1985. In 1986, the pilot obtained a commercial multi-engine rating along with instrument single and multi-engine airplane ratings. In 1987, the pilot obtained a certificated flight instructor rating in single-engine airplanes. In 1990, the pilot received a commercial rating in helicopters, and later that same year obtained a flight instructor rating in helicopters. In November 1991, the pilot obtained an airline transport pilot certificate in multi-engine airplanes. In 1992, the pilot received a flight instructor rating in multi-engine airplanes. The pilot was issued a second-class medical certificate with a limitation to wear corrective lenses on September 4, 1997. Her last biennial flight review was completed on January 17, 1998, in the accident airplane. The pilot had accumulated a total of 3,506 flight hours, of which 1,707 hours were in multi-engine airplanes, and 41 hours were in the same make and model as the accident airplane. Within the 30 days prior to the accident, the pilot had flown 22 hours, of which 16 hours were in the same make and model as the accident airplane.

AIRCRAFT INFORMATION

The 1969-model six-place un-pressurized twin-engine airplane was equipped with two Teledyne Continental GTSIO-520-C 340-horsepower engines. Each engine had an Allied Signal Model T11 turbocharger (part number 404360-9004) installed. The 3-blade McCauley propellers had full feathering capabilities and un-feathering accumulators. The airplane was registered to Bradson Properties on April 7, 1998. The logbooks for the airplane and engines were not made available to the NTSB investigator-in-charge (IIC). Work orders, invoices, and other related paperwork, dated back to 1989, indicated the following information concerning the engines:

On April 4, 1989, the left engine (serial number 150739-7-C) was "repaired for damage to [the] crankcase from starter adapter," and "each cylinder cracked in exhaust port." An inventory list attached to the April 4, 1989, work order indicated that 6 pistons, 6 ring sets, 5 chromed cylinders, and 1 valve guide (part number C641951P005) had been purchased.

An invoice, dated March 3, 1994, indicated that 5 piston pins, 5 pistons, and 1 gasket set for top overhaul had been purchased. The engine that these parts were intended for was not specified on the invoice.

An invoice, dated June 28, 1994, indicated that a technician "troubleshot oil blow by problem" and "all cylinders" had been removed and replaced with newly overhauled cylinders. That same invoice indicated that a turbocharger was removed, overhauled, and reinstalled. It was not clear whether the cylinder and turbocharger replacements were conducted on one or both engines.

A work order invoice, dated January 4, 1995 (for work conducted between November 15, 1994, and November 30, 1994), indicated that the left engine had an "oil leak." According to the invoice, the engine was ground run and the #2 and #6 cylinders were found cracked. The #2 and #6 cylinders and piston assemblies were removed and replaced with "2 overhauled cylinder assemblies with piston and ring assemblies." That invoice reported the aircraft total time as 2,711.9 hours.

An invoice, dated January 12, 1995, indicated that 1 cylinder ring kit, 1 cylinder gasket kit, 1 valve guide, and a serviceable piston were purchased. The invoice also indicated that a technician "replaced valve guide and reground all valves and seats." The engine, on which this work was conducted, was not specified. The aircraft and engine total times were not recorded on this invoice.

A letter, dated March 15, 1995, indicated that work had been performed on one of the engines' turbochargers; however, the serial number listed on the letter did not match either of the two serial numbers found at the accident site.

A note, dated January 24, 1998, reported that an annual and 100-hour inspection had been conducted on the aircraft at a total time of 2,905 hours. According to this note, the left engine had accumulated 581 hours since its last overhaul, and the right engine had accumulated 1,054 hours since its last overhaul.

An invoice, dated May 1, 1998 (for work conducted between February 17, 1998, and April 30, 1998), reported a maintenance discrepancy noting a left engine oil leak. The ensuing engine run-up revealed that "bolt...on starter adapter leaks. Dampner shaft leaks, oil cooler leaks bad, vacuum pump leaks. Found loose fitting on vacuum. Tightened and sealed fitting and 2 plugs. Replaced push rod housing seals on #2, #4, #6 cylinder. Replaced rocker catch gaskets on #2, #4, #6 cylinder. Replaced oil pan gasket. Replaced starter o-ring...Removed oil cooler and replaced gasket." An additional maintenance item reported that left engine's propeller governor cable was evaluated and determined to be "O.K." It was not noted why the cable needed evaluating. The endorsement for the aforementioned work on the left engine indicated that the airplane had accumulated a total of 2,942 flight hours, and the left engine had accumulated a total of 714.9 hours since its last overhaul.

An undated document, which appeared to be a sales advertisement for the aircraft, listed the aircraft total time as 2,875 hours, and a right engine total time since its last overhaul as 1,024 hours. The aircraft total time from the advertisement was subtracted from the aircraft total time from the May 1, 1998, invoice to obtain a total of 67 hours. That time was then added to the right engine time since overhaul to obtain an estimated total time since overhaul of 1,091 hours for the right engine at the time of the May 1, 1998, invoice.

The May 1, 1998, invoice was the latest piece of maintenance information provided to the NTSB. The total time of the aircraft and engines at the time of the accident could not be determined.

The airplane was last topped off with fuel during the morning of the accident, when 85 gallons of 100LL aviation fuel were added at the Wilbarger Airport. The total fuel capacity of the airplane was 235 gallons.

The minimum controllable airspeed (Vmc) was listed at 76 mph in a brochure provided by the previous owner of the airplane.

METEOROLOGICAL INFORMATION

Wichita Falls, Texas (located 42 miles southeast of the accident site) was the nearest weather reporting station to the accident site. At 0756, Wichita Falls was reporting the wind from 150 degrees at 4 knots, visibility 10 statute miles, scattered clouds at 25,000 feet, temperature 17 degrees Celsius, dew point 9 degrees Celsius, and an altimeter setting of 30.12 inches of mercury.

WRECKAGE AND IMPACT INFORMATION

The aircraft impacted near the center of State Highway 283. The wreckage came to rest in a wheat field adjacent to the highway on a measured magnetic heading of 270 degrees. Fire destroyed the airframe. The entire left wing skin, top and bottom, was burned through. The left fuel tank was found burnt but intact. The right wing was completely destroyed by fire. All the tail skin, with the exception of a small area located on the outboard left side of the elevator and horizontal stabilizer, was destroyed by fire. The fuselage and the cockpit were completely destroyed by fire. Flight control continuity could not be established. The position of the wing flaps could not be determined due to fire damage, and the landing gear was found in the extended position. The elevator trim was found in the nose up position.

Examination of the engine controls revealed that both throttles and mixture controls were found in the full forward positions, the left propeller control was found in the aft position, and the right propeller control was found in the full forward position.

The left engine was intact with all the accessories separated except for the starter, fuel pump, and both magnetos. It sustained moderate fire damage in all areas and the exhaust pipes were separated. The number five cylinder head was found partly separated. The left propeller assembly was separated from the engine. All three blades remained attached to the hub, and appeared to have sustained minimal damage. Blade one was loose in the hub, bent toward the non-cambered side, and displayed chord-wise rubbing on the cambered side. Blade two was found in the feather position, and was bent slightly toward the cambered side. Blade three was stuck in the low pitch position, and displayed very light rubbing on the leading edge near the tip. The propeller dome was crushed and displayed no rotational damage. The left propeller was not exposed to fire.

The right engine remained intact with all of its accessories separated. It sustained fire damage, and the number five cylinder and the oil pump displayed impact damage. All intake and exhaust pipes were found separated. The right propeller assembly was separated from the engine. The propeller hub of the right propeller was shattered and all three blades were found separated from the hub. Two of the propeller blades were burned and both were twisted toward the direction of rotation and exhibited leading edge impact damage. The remaining blade was not burned; however it was also twisted in the direction of rotation, and exhibited chord wise scarring on the camber side. Its tip was folded toward the non-cambered side.

MEDICAL AND PATHOLOGICAL INFORMATION

An autopsy and toxicological tests were requested on the pilot. An autopsy could not be performed due to the condition of the remains. The FAA's Civil Aero Medical Institute (CAMI) in Oklahoma City, Oklahoma performed toxicological tests. Toxicological tests for drugs and alcohol revealed that no drugs were detected; however, 19 mg/dL of ethanol was detected in the pilot's muscle tissue. The ethanol was attributed to "postmortem ethanol production."

TEST AND RESEARCH

Both engines and their respective turbochargers were examined under the supervision of the NTSB at the Teledyne Continental Motors facility in Mobile, Alabama, from July 23-29, 1998.

Right Engine (serial number 155481-R)

All of the cylinders were Cermi-Chrome type cylinders. All 6 cylinders and pistons displayed even wear and "normal" combustion deposits. The crankcase main bearings and connecting rod journals exhibited "normal operational signatures" with no evidence of main bearing movement. Other than massive impact and fire damage, the engine "exhibited normal signatures throughout." No evidence of a pre-impact malfunction was noted.

Examination of the right turbocharger revealed evidence of rotational scoring on the turbine components. It was also noted that the compressor blades were bent opposite of the direction of rotation. There was no evidence of pre-impact anomalies that would have prevented the right turbocharger's operation.

Left Engine (serial number 150739-7-C)

The examination revealed that the #1, #2, and #6 cylinders were Cermi-Chrome type cylinders, and the #3, #4, and #5 cylinders were Channel-Chrome type cylinders. It was noted that the #2 cylinder's exhaust valve was missing its head. The stem of the exhaust valve remained inside the exhaust valve guide. The exhaust valve head was found in the engine. Portions of the head were broken away. The #2 cylinder's combustion chamber displayed numerous impact damage marks. The #2 piston also displayed numerous impact damage marks and had a hole punched through the crown structure. The remainder of the left engine's components displayed "normal operational signatures."

Closer examination of the left engine's #2 exhaust valve revealed that it was broken at the valve underhead to valve stem transition. Both keepers were in place at the valve tip end. The fractured valve stem was removed from the guide and it was noted that the valve stem exhibited extensive carbon deposits near the valve keeper end of the stem. A detailed examination of the valve stem's fracture surface revealed that it displayed characteristics consistent with a fatigue fracture.

A "normal" valve guide is a one-piece cylinder with a flange around its outside circumference positioned approximately half way down the guide. Closer examination of the #2 cylinder's exhaust valve guide revealed that it was fractured above the flange. The upper portion of the valve guide was not in place and was not found in the engine. The valve guide clearance was measured with the remaining section of valve guide in the cylinder (normal loose clearance should be approximately 0.003 to 0.0047 inches). The measured clearance at the top, middle, and bottom ends of the remaining guide were 0.023, 0.034, and 0.064 inches, respectively. The valve guide part number could not be ascertained due to its missing upper portion.

As noted previously, the valve stem exhibited extensive carbon deposits on the stem, which will form from exhaust leakage through a worn or broken valve guide. These carbon deposits confirmed that the valve operated for some time without the upper portion of the valve guide. The actual operating time with the upper portion of the guide missing could not be determined.

There was a noted absence of rotational scoring on the compressor and turbine components on the left turbocharger, and an absence of bending of compressor wheel blades opposite to the direction of rotation. The left turbocharger's impeller remained intact. The journal bearings remained intact and were measured and found to be within the manufacturer's serviceable limits. The journal bearing lubrication holes were found undamaged and unobstructed. The turbine wheel blades were separated at the tips. The fractured turbine blades were examined and it was noted that their fracture surfaces displayed characteristics of separation due to "overload." In addition, it was noted that, at the fracture, each of the blades was deformed in a direction opposite of rotation. There were no pre-accident conditions identified which would have interfered with the normal operation of the left turbocharger.

The two McCauley Propellers were shipped to the manufacturer's facility in Dayton, Ohio, for further examination and teardown which took place on July 26, 1999. The examination revealed that "neither propeller was in the feather position at impact." Counterweight and pitch change mechanism signature marks revealed that the "left propeller was operating in the vicinity of low pitch at impact." The absence of evidence of rotational damage revealed that the left propeller was being operated with "no or low power at impact, and at low RPM." Overall propeller damage, including blade bending, twisting and retention damage indicated that the "right propeller was operating at high power and at high rotational speed (RPM) at impact." There were no indications of any type of propeller failure prior to the impact.

NTSB Probable Cause

The pilot's failure to maintain the airplane's minimum controllable airspeed following a loss of left engine power after takeoff, which resulted in a loss of control. Contributing factors were the fractured exhaust valve guide on the left engine's # 2 cylinder, the fatigue failure of the #2 cylinder's exhaust valve, and subsequent failure of the left engine's turbocharger resulting from foreign object damage.