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On May 31, 2008, about 1115 eastern daylight time, a Piper PA-18-135, N802DA, was substantially damaged when it impacted trees during a forced landing near Myrtle Beach, South Carolina. The certificated commercial pilot received minor injuries. The aerial banner tow flight was operated by SC Sky Signs under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the flight, and no flight plan was filed.
According to the pilot, he departed the operator's private grass airstrip in Conway, South Carolina, about 1058, and planned to fly to, and then along, the shoreline. The pilot leveled the airplane at an altitude of approximately 1,000 feet for the transit to the shoreline. Just after he informed the controller at the Myrtle Beach International Airport (MYR) air traffic control tower (ATCT) that he had crossed the extended centerline of MYR runway 36, the pilot noticed that his altitude had decreased to 900 feet. He applied power and climbed back to 1,000 feet.
The pilot then observed the engine oil temperature increasing and felt the engine "losing power." He communicated with MYR ATCT and requested a 180-degree turn back to his home airport. When the pilot completed the turn, he noted that the oil pressure was falling, and that the oil temperature was continuing to increase. At this point the pilot knew that he could not get the airplane safely to either the departure airport or MYR, and that he would have to conduct an off-airport landing. He observed a street in a housing development that he thought would serve as a suitable landing site, and set up for a right traffic pattern. When the airplane was approximately 400 to 500 feet above the ground, the pilot spotted a clearing and released the banner that he was towing.
The pilot then requested that MYR ATCT notify fire/rescue authorities that he was making a forced landing. At a position about "midfield downwind" for his selected landing site, the engine "quit," and the pilot realized that he did not have sufficient altitude to reach that site. He then selected and headed for a second landing site, and the wings impacted trees prior to the touchdown.
According to information provided by the pilot, he had approximately 840 total hours of flight experience, with approximately 120 hours in the accident airplane make and model. The pilot reported 126 hours of flight experience in the 90 days preceding the accident. His most recent flight review was conducted on February 28, 2008. His most recent second-class medical certificate was issued on April 1, 2008.
According to Federal Aviation Administration (FAA) records, the airplane was manufactured in 1953. At that time, it was equipped with a Lycoming O-290 engine. At the time of the accident, the airplane was equipped with a Lycoming O-320 engine, but the most current airplane registration indicated that an O-290 engine was installed. A supplemental type certificate (STC SA00522SE) was commercially available to permit the installation of a Lycoming O-320 engine in the accident airplane make and model.
The operator did not make any airframe, engine, or propeller logbooks available for the investigation. Examination of the airplane's airworthiness documents that were maintained by the FAA revealed that the documentation did not provide a continuous chronology of the engines installed in the airplane. No documentation of the incorporation of STC SA00522SE, or the replacement of the original O-290 engine with the O-320 engine, could be located. The earliest document that referenced a Lycoming O-320 engine was dated February 17, 1992. That document related to a modification to the pistons in the engine. The engine serial number on that document did not match the serial number of the accident engine.
An additional document, dated May 25, 2005, also referenced an O-320 engine, but did not contain any engine serial numbers. That document referenced an inspection compliance with STC SA00522SE.
The 1145 surface weather observation at MYR reported winds from 230 degrees at 11 knots, visibility 7 miles, clear skies, temperature 29 degrees Celsius (C), dew point 20 degrees C, and an altimeter setting of 30.06 inches of mercury.
WRECKAGE AND IMPACT INFORMATION
The operator's airstrip was located approximately 10 miles west-northwest of MYR, and the accident site was located approximately midway between MYR and the operator's airstrip. The airplane struck trees and came to rest in a flat area adjacent to a residential neighborhood.
An FAA inspector conducted a preliminary examination of the airplane at the accident site. The left wing was partially separated from the fuselage, and the right wing exhibited leading edge crush damage. The fuselage was essentially intact, with some crumpling of the structure just aft of the left wing. The engine and propeller remained attached to the airplane. One propeller blade appeared intact and undamaged, and the other blade was bent aft approximately 50 degrees at the one-third span point. The landing gear was deformed upwards, and the empennage exhibited multiple impact damage sites.
The airplane was transported to the operator's facility, and a subsequent engine examination was conducted under the supervision of the FAA.
The engine was examined at the operator's facility by an FAA inspector, assisted by a mechanic employed by the operator. According to the mechanic, the engine had a total time in service of 5,457 hours, and had accumulated approximately 3,200 hours in service since the last overhaul. The engine manufacturer's recommended time between overhaul for the accident engine was 2,000 hours.
The intake and exhaust systems were not installed on the engine at the time of the examination. The examination did not note any damage to the magnetos, ignition harnesses, spark plugs, accessory section, or crankcase. The fuel and induction systems were not examined.
The camshaft exhibited "normal wear." The main bearings were not examined, the crankshaft exhibited minor scoring, the connecting rods were "ok,", and the connecting rod bearings exhibited scoring and pitting. The general condition of the lubrication system and sump was "ok." The oil screens were clean, with no metal found. The oil pump was "frozen" initially, but rotated freely on the second day of the examination.
The No. 3 intake and exhaust valves "were leaking, but not due to any heat damage," and no other damage to the valve train was noted. The No. 4 cylinder, piston and rings were heat damaged, and the piston was sent to the National Transportation Safety Board for additional examination.
No. 4 Piston
The piston head was essentially intact, with a distinct coloration pattern. The juncture of the piston head and piston side was eroded along an arc approximately 1 1/4 inches long. The side of the piston below the arc exhibited significant erosion, scoring and metal transfer. The erosion extended from the piston head to below the oil scraper ring. All three piston rings were fractured, and partly eroded in the area. A triangular hole, approximately 1/2 inch by 1/4 inch, penetrated the piston wall adjacent to one wrist pin boss. The wrist pin exhibited bluing of its finish, and the piston interior was blackened.
According to Lycoming, the coloration pattern on the head was typical of detonation, and the wrist pin bluing, as well as the blackening of the piston interior, were indications that the piston had been exposed to abnormally high operating temperatures.
Fuel Mixture Guidance
According to the current STC holder, the STC which installed the O-320 engine in the PA-18 (STC SA00522SE) did not contain any guidance or limitations concerning fuel mixture.
The Piper-produced PA-18 equipped with an O-320 engine was designated the "PA-18-150 Super-Cub" by Piper. The only fuel mixture guidance in the Piper Super Cub Owner's Handbook was the sentence "A considerable saving in fuel usage can be effected...by judicious use of the mixture control during cruising flight."
Lycoming Service Instruction 1094D was entitled "Fuel Mixture Leaning Procedures," and was applicable to "All Textron Lycoming Opposed Series Engines." The document was dated March 25, 1994, and it stated that "For engines equipped with fixed pitch propellers, gradually lean the mixture until either the tachometer or the airspeed indicator reading peaks," and "For a given power setting, slowly lean the mixture until engine operation becomes rough or until engine power rapidly diminishes as noted by an undesirable decrease in airspeed. When either condition occurs, enrich the mixture sufficiently to obtain an evenly firing engine or to regain most of the lost airspeed or engine RPM." The guidance also contained the cautionary note "TEXTRON LYCOMING DOES NOT RECOMMEND OPERATING ON THE LEAN SIDE OF PEAK EGT."
Another product support document published by Lycoming was the "Flyer." The Maintenance edition of the document stated the following: "As far as the pilot is concerned, operating on the lean side of peak EGT can only be accomplished with fuel-injected engines of at least 250 HP...It isn't possible with float-type carburetors because of the fuel/air distribution problem. In any case, leaning past the peak is not recommended."
Mixture Leaning Procedures
According to the pilot, the airplane was not equipped with either a fuel flow gauge or an exhaust gas temperature gauge, and the airplane manual did not contain any guidance regarding engine leaning procedures. He said that the operator instructed him on the procedure that he wanted the pilot to use to lean the mixture. The pilot stated that he was instructed to first reduce the mixture until the engine began to miss, then enrichen the mixture slowly until the engine smoothed out, and then give the mixture a "little bump" to enrichen it for the final setting. The pilot further stated that the operator instructed him to set the cruise power by adjusting the throttle to achieve an indicated airspeed of "45 to 50 mph." The actual engine rpm varied as a function of the size of the banner being towed; smaller banners required approximately 2,350 rpm to maintain the target airspeed, while larger banners required approximately 2,450 rpm. The pilot noted that the operator monitored the overall fuel consumption of his airplanes "pretty closely."
According to the FAA inspector, the pilot of the accident airplane generally leaned the mixture to achieve a fuel consumption of 6 gallons per hour (gph) in cruise flight. The O-320 fuel consumption chart in the Piper Super Cub Owner's Handbook indicated that a "best lean mixture" fuel consumption rate of 6 gph was obtained an engine speed of 2,100 rpm. The chart indicated that best lean mixture fuel consumption rates of 8 gph and 9 gph were obtained at engine speeds of 2,350 rpm and 2,450 rpm, respectively.
The Lycoming O-320-B Operator's Manual sea level performance chart presented fuel consumption rates as a function of engine rpm and manifold pressure (MP). The airplane was equipped with a fixed-pitch propeller, and therefore the pilot could not independently adjust MP. The chart did not contain performance data for a fuel consumption rate less than approximately 6.3 gph, which was obtained at 2,000 rpm and 17 inches of MP. According to this chart, a fuel consumption rate of approximately 6.5 gph would be obtained at an engine speed of 2,100 rpm, and an MP of 17 inches. Minimum fuel consumption rates of 7 gph and 7.2 gph were obtained with 17 inches of MP at engine speeds of 2,350 rpm and 2,450 rpm, respectively. Maximum fuel consumption rates of 12.5 gph and 13.4 gph were obtained with 28 inches of MP at engine speeds of 2,350 rpm and 2,450 rpm, respectively.