Erwin, NC, USA
N2WZ
BEECH A36TC
After what he described as an uneventful engine run-up, the pilot took off from the 5,000-foot-long runway. The pilot stated that when the airplane reached an altitude of about 50 feet, he retracted the landing gear; the engine lost partial power. The airplane yawed to the left and touched down again, impacting a thicket. According to a witness who observed the takeoff, the engine power never decreased until the engine stopped suddenly upon impacting the ground. Postaccident examination of the engine revealed that the No. 4 cylinder exhaust valve springs had fractured in fatigue. Detailed examination of the springs showed that the fatigue cracking likely originated from one or more of the iron oxide (rust) pits present on the surface of the springs. The springs had been installed in the engine about 1 year before the accident flight. According to the mechanic who installed the springs, he had retrieved them from a similar engine’s cylinder that was removed from another airplane; there was no documentation of the springs’ service history or serviceability.
HISTORY OF FLIGHT On May 12, 2012, about 1245 eastern daylight time, a Beech A36TC, N2WZ, was substantially damaged when it impacted terrain during an attempted takeoff from Harnett Regional Jetport (HRJ), Erwin, North Carolina. The certificated private pilot and four passengers were not injured, and one passenger incurred minor injuries. Visual meteorological conditions prevailed, and no flight plan was for the flight, which was destined for Darlington County Jetport (UDG), Darlington, South Carolina. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. During a telephone interview, the pilot stated that following an uneventful run up of the engine, he initiated the takeoff from runway 23 at HRJ. After lifting off from the runway, and as the airplane was accelerating beyond the best angle of climb speed, the engine began "skipping and popping." The pilot also felt that the engine had lost a significant amount of power, but stated that none of the engine gauges showed anything abnormal. The airplane yawed left, descended, and impacted brush-covered terrain off of the left side of the runway with the landing gear retracted. During a separate interview, one of the passengers stated that the takeoff appeared normal until the airplane lifted off from the runway. The airplane’s engine then “fluttered”, the airplane lost airspeed, and drifted off the left side of the runway before impacting the ground. A pilot-rated witness, who was preparing his glider for a later flight, observed the airplane prior to and during the accident takeoff. According to the witness, the airplane’s taxi and engine run up appeared normal, with no abnormal engine sounds. The airplane then taxied onto the runway, increased engine power and began the takeoff roll. The airplane rotated into a normal takeoff attitude, but continued to roll down the runway without climbing. As the airplane reached the mid-point of the runway, the witness thought that the airplane might have been loaded in excess of its maximum gross weight, as it had not yet lifted off from the runway. When the main landing gear finally lifted off from the runway, the airplane appeared to “wallow”, and upon reaching an altitude of about 50 feet, the landing gear retracted. The nose of the airplane then pitched down slightly, and the airplane began to sink in a “stall-like mush descent.” The engine continued to sound as though it was producing full power as the airplane drifted off the left side of the runway. The airplane continued in its nose-high descent until it disappeared from the witness’s view, off the left side of and below the level of the runway. The witness then heard the sound of impact, and the sound of the engine stopping instantly. At no time during the takeoff did the witness hear the engine “sputter” or otherwise lose power. PERSONNEL INFORMATION The pilot held a private pilot certificate with a rating for airplane single engine land. He reported 280 total hours of flight experience, 96 hours of which were in the accident airplane make and model. AIRCRAFT INFORMATION The pilot provided his preflight weight and balance computations to the Federal Aviation Administration (FAA) following the accident. According to the computations, the pilot listed a combined weight of 340 pounds for the front seat occupants, 245 pounds for the third and fourth seat occupants, 230 pounds for the fifth and sixth seat occupants, no allowance for baggage or cargo, and 185 pounds of fuel. The calculated total ramp weight was 3,668 pounds. The pilot also calculated a 20-pound decrement to the takeoff weight for fuel consumed during engine start, taxi and run-up. The calculated takeoff weight was 3,648 pounds. The airplane’s maximum gross takeoff weight was 3,650 pounds. The calculated takeoff distance required given the airplane’s weight and the conditions present at the time of the accident was 900 feet, with 1,900 feet required to clear a 50-foot obstacle. The accident pilot’s mechanic stated that he had performed maintenance on the accident airplane about one year prior to the accident flight. The mechanic recalled that the pilot contacted him to request assistance following a partial loss of power while on approach to a nearby airport. The pilot reported that he had removed the rocker covers from the engine and determined that the loss of power had been caused by a broken exhaust valve spring. The mechanic had previously removed two similar engine cylinders from another airplane, and subsequently retained the cylinders and their related internal components. The mechanic removed the required exhaust valve springs from one of the cylinders and provided the part to the pilot at no charge. After installing the valve spring, the mechanic could not recall if he had made an immediate entry in the engine log. Review of the airplane’s engine log showed an entry made on a sheet of paper stapled to a logbook page, dated November 14, 2010. The entry stated that the mechanic had, “Replaced the #4 exhaust valve spring set with a serviceable set of springs.” No other documentation of the maintenance was available, and no service history of the exhaust valve spring set could be found. METEOROLOGICAL INFORMATION The weather conditions reported at HRJ, at 1255, included clear skies, winds from 130 degrees at 5 knots, temperature 23 degrees C, dew point 6 degrees C, and an altimeter setting of 30.34 inches of mercury. The calculated density altitude was 708 feet. AIRPORT INFORMATION The single asphalt runway at HRJ, oriented in a 05/23 configuration, was 5,000 feet long by 75 feet wide. The airport elevation was 202 feet. WRECKAGE AND IMPACT INFORMATION The airplane came to rest in a thicket of briars off the left side of the runway. Examination of the airplane following the accident by FAA inspectors revealed substantial damage to the fuselage structure. The inspectors also noted that the left fuel tank was completely filled with fuel, while the right fuel tank appeared nearly empty. The fuel selector, magneto, and battery switches were all found set to the “off” position. The landing gear handle was selected to the down position, the landing gear circuit breaker was found tripped, and the flaps were set to the “approach” position. When power was applied to the airplane’s electrical system, the red landing-gear-in-transit light illuminated. The inspectors subsequently instructed the owner to have the airplane removed from the accident site for further evaluation. After repositioning the airplane, the mechanic assessed the engine at the request of the FAA inspectors to see if the engine could be test run. During the assessment, the mechanic found that the number 4 cylinder exhaust valve spring had broken in two places. The assessment was subsequently suspended and the engine was shipped to its manufacturer for further examination. The crankshaft to camshaft timing was verified by observation of the alignment of the gear timing marks. Both magnetos were placed on a test bench, and when rotated produced blue spark at all terminal leads. Each of the spark plugs were removed and examined, and all displayed normal wear signatures. The engine-driven fuel pump, fuel control/metering assembly, and fuel manifold valve were tested on a bench and found to function properly through their full range of operation. Examination of each of the engine’s six cylinders showed that they each exhibited a normal amount of combustion deposits and their bore conditions were free of scoring and undamaged. The cylinder skirts were intact and undamaged and there were no hone marks visible in the cylinder bore ring travel area. Each of the intake and exhaust valve heads exhibited normal deposits and operating signatures, and the rocker box areas of the cylinders had an oil residue present, indicating lubrication to the upper portions of the cylinders. The cylinder overhead components including the valves, rocker arms, valve guides, retainers, and shafts were lubricated and undamaged. Each of the valve springs were intact and undamaged, with the exception of the number 4 cylinder exhaust valve springs, of which both the inner and outer springs were fractured. The number 4 cylinder exhaust valve springs were subsequently removed and examined in detail. The springs were tested for material hardness, with no anomalies noted. Both springs exhibited rust pitting on their exterior surfaces. The inner spring was fractured in one location, while the outer spring had fractured twice. Examination of each of the fracture surfaces showed evidence of post-separation damage, which obscured some of the fracture features. The outer surface of the inner spring exhibited evidence of a fatigue crack, with crack arrest lines that appeared to originate from one or more of the pits on the surface of the spring. Detailed examination of one origination pit showed that it contained iron oxide.
The partial loss of engine power shortly after takeoff due to fatigue-fractured exhaust valve springs. Contributing to the accident was the mechanic’s decision to install exhaust valve springs with an unknown service history in the engine’s No. 4 cylinder.
Source: NTSB Aviation Accident Database
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