COMPTON, CA, USA
N6110Q
Cessna 152
The instructor experienced a sudden reduction in engine rpm during climb out. After checking the engine and electrical systems, he took the controls and made a forced landing to a dry flood control canal. His attempts to advance the throttle resulted in a further loss of power. During the last portion of the approach he saw utility lines and attempted to avoid them but the aircraft struck an overhead cable. Witnesses heard popping sounds and saw smoke trailing the aircraft. An examination of the engine revealed a broken exhaust push rod. An examination found the push rod failed due to the manufacturing process in which it was artificially aged. This process made the push rod susceptible to stress corrosion cracking (SCC) with a lower stress threshold. The ASM handbook states that the preferred method is to maintain the material at the aging temperature for a longer period of time. The interference pins that are inserted in the ends of the push rod most likely produced hoop stress at a level sufficient to initiate SCC once the push rod is exposed to an environment of humid and/or salty air. SCC most likely began after manufacture, but before the push rod was placed into service during a period in which it was exposed to adverse environmental conditions.
HISTORY OF FLIGHT On December 26, 1995, at 1222 hours Pacific standard time, a Cessna 152, N6110Q, collided with utility lines and crashed during a forced landing while on initial climb from the Compton, California, airport. The aircraft was destroyed; however, the instructor and his student received only minor injuries. The aircraft was being operated as an instructional flight by Jack's Aircraft when the accident occurred. The flight originated in Long Beach, California, at 1145. Visual meteorological conditions prevailed at the time and no flight plan had been filed. The instructor and his student were on climb out from runway 7R at the Compton airport when the engine power dropped to about 1,300 rpm. The instructor reacted by checking the engine controls and electrical switches which included the application of carburetor heat. When this failed to correct or identify the problem, he told his student that he was taking the controls. With his altitude below 400 feet, the pilot opted for a forced landing area immediately ahead of the aircraft. The instructor selected a dry, concrete lined, flood control canal and established the aircraft on a straight-in approach. At approximately 40 to 50 feet agl, the pilot saw utility lines stretched along a bridge which crossed his approach path. He attempted to duck under them but the aircraft struck the lowest strand, an overhead TV cable. The pilot estimated his airspeed as about 70 knots when the aircraft struck the cable, which was about 8 to 9 feet above the adjacent bridge roadway. After striking the cable, the aircraft continued into the canal striking the bottom and east embankment. The aircraft remained upright throughout and came to rest on a north-northwesterly heading. After coming to rest the instructor noticed fuel leaking from the right wing. He turned off the master and magneto switches and assisted his student out and away from the aircraft. The pilot stated that, during the emergency, any attempts to advance the throttle resulted in a further loss of power. He estimated the engine was producing about 1,000 rpm just prior to striking the TV cable. Witnesses to the accident reported hearing popping sounds and seeing smoke trailing the aircraft as it passed overhead. They reported that the aircraft struck and became entangled in an overhead line. The aircraft spun around as the aircraft stretched the cable whose opposite end had wrapped around a utility pole. They observed the aircraft striking the side of the canal as it slid to a stop. Upon obtaining a closer view they noticed that there were two occupants in the aircraft and ran to help them. PERSONNEL INFORMATION The pilot is a certificated commercial pilot and flight instructor. The student pilot had not soloed in this make and model aircraft. AIRCRAFT INFORMATION According to available, the aircraft accumulated a total of 8,764 flight hours. After an inspection of the aircraft and engine logbooks, FAA airworthiness inspectors found the aircraft had not been in an airworthy condition at the time of the accident. Although the aircraft was being operated for flight instruction for hire, the last required annual inspection expired on September 30, 1995. The maintenance records included an STC for automotive fuel; however, there was no logbook entry authorizing automobile fuel. The required placards indicating fuel capacity were missing from each fuel tank. (A copy of the FAA form 337 is appended to this report.) No weight and balance form existed for the aircraft's current equipment configuration. The aircraft was being operated with an unapproved Pilot's Operating Handbook (POH). Maintenance records showed an engine mount had been replaced on April 4, 1995, but the required FAA form 337, which according to the logbook had been completed, was not included with the maintenance records. The last seat rail inspection, which is required every 100 hours by Airworthiness Directive (AD) 87-20-03R2, was performed on April 3, 1990, when the aircraft had a total of 4,918.7 flight hours. Since then, the operator performed four inspections on the aircraft: July 21, 1994, August 31, 1994, September 6, 1994, and April 3, 1995, covering 397.1 flight hours. The maintenance records did not reflect a list of applicable and recurring AD's showing the status of each one. According to the engine logbook, the aircraft's engine had been overhauled but a description of the work performed was not provided. A mandatory service bulletin from the engine manufacturer dated March 31, 1992, requires that all exhaust valves be replaced with current exhaust valves at overhaul. There was no reference to this bulletin in either the aircraft or engine records. (A copy of the service bulletin is appended to this report.) Prior to the accident flight the aircraft had been refueled with 15.4 gallons of 80/87 octane aviation fuel by Airflite Inc.'s No. 2 refueling truck. After the accident fuel samples were taken from both the truck and the aircraft. (Excerpts from Airflite Inc.'s refueling log are appended to this report.) WRECKAGE AND IMPACT INFORMATION The aircraft landed in a drainage canal about .3 miles southeast of the departure end of runway 7R. An on-scene examination revealed that the TV cable was wrapped around the propeller hub. The propeller did not show any evidence of leading edge damage, scratches, gouges or bending, nor did the spinner, which was crushed longitudinally, show any rotational scratches or scoring. The engine tachometer meter read 7,042.1 hours at the accident site. The instructor recorded a start time of 7,041.6 at the beginning of the rental period. The center anchor points for both front seat lap belts were found pulled loose from their mounted positions. Both fuel tanks were about 3/4 full. The fuel was blue in color and had the odor of aviation fuel. A leak was evident from a broken fuel line from the right fuel tank near the right front door post. All the fuel lines were clear and unobstructed except for a broken right fuel line which was allowing fuel to pool under the aircraft and then to trickle toward the center of the channel. The fuel selector indicator was found in the "both" position. The air induction box and the exhaust system were clear and unobstructed. The carburetor was attached to the engine manifold. The interior of the gascolator was clean, but showed the presence of evaporated water distillates. Throttle and mixture control continuity was established between the carburetor and the cockpit. The carburetor was removed and disassembled. The fuel in the bowl was checked with water finding paste without result. The main discharge nozzle was open and unobstructed. The accelerator pumped fuel when actuated by hand and the plunger displayed flexibility. The fuel bowl inlet screen was clean and in place. The metal floats were set to move between the 5/8 inch down and 3/16 inch up limits. The one-piece venturi was in place; however, the mixture control arm was found bent and separated. The carburetor bowl gasket showed evidence of leaking with stains on the exterior of the bowl. The throttle plate shaft was loose in its bore with axial and radial play noted. The carburetor heat control was located in the air intake box and was found in the open (on) position. The right and left magneto were securely mounted and sparked all four cylinders when the crankshaft was hand rotated. The left impulse coupler could be heard clicking during crankshaft rotation. Timing was found set at 20 degrees by means of a timing light as the crankshaft was hand rotated. The top and bottom spark plugs were removed and examined. They contained visible lead deposits (clinkers) around the electrodes. According to the Champion Aviation Check-A-Plug chart, some plugs exhibited evidence of exposure to high operating temperatures. The engine exhibited impact damage to the No. 1 cylinder and the right side of the intake/exhaust systems. Thumb compression was established in the proper order for all four cylinders. The valve covers were removed from all four cylinders. An examination of the No. 4 cylinder revealed partial movement of the exhaust valve. Further examination revealed that the push rod for the exhaust valve was broken near the rocker end. During hand rotation the exhaust valve was observed to open about 50 percent. TESTS AND RESEARCH The operator of Jack's Aircraft told the Safety Board investigators that he felt his aircraft performance could possibly have been the result of sabotage since sand and water had been discovered in the fuel systems of the remaining aircraft on his flight line as they were being pre-flighted on the morning following the accident. When asked about the disposition of the contaminated fuel samples he reported that he had been told about the incident and had not drained the sumps himself, had not seen the samples, and to his knowledge, none of the contaminated samples had been retained. The operator also voiced the opinion that the aircraft's fuel system could have been contaminated during the refueling process. Safety Board investigators drained the sumps of all of the operator's remaining aircraft. About 1 tablespoon of reddish-brown sediment, as well as between 1 or 2 tablespoons of clear liquid were drained from the right main tank of a Piper PA-28-140, N55093. The clear liquid immediately precipitated out and collected in the bottom of the sample bottle along with the sediment. No other visually detectable contamination of either liquid or particulate material was noted. The sample was submitted to Core Laboratories for spectrographic analysis. The analysis revealed that the sample contained 30 percent silicon. (Core Laboratories spectrographic analysis is appended to this report.) A fuel sample was taken from the No. 2 refueling truck operated by Airflite, Inc. The sample was analyzed by Exxon, the bulk fuel vendor. No abnormalities were noted. (A letter from Exxon stating the results of testing is appended to this report.) A fuel sample from the accident aircraft was also taken by Safety Board investigators and submitted to Core Laboratories in Long Beach, California, for analysis. The sample was found to be without any abnormalities. (Core Laboratories fuel analysis report is appended to this report.) The broken No. 4 exhaust push rod was submitted to the Materials Laboratory Division of the Safety Board for failure analysis. The push rod separated at the upper end of the tube showing that the longitudinal fractures intersected the tube on diametrically opposite sides approximately 0.5 inch from the pin end. A scanning electron microscope (SEM) examination revealed the longitudinal fractures of the broken end of the push rod contained intergranular fracture features. The circumferential fracture that intersected the first two fragments exhibited severe rubbing damage; however, in small isolated areas that were not damaged the circumferential fractures exhibited fatigue crack features. A metallurgically prepared and etched specimen of a longitudinal tangential section of the third fragment contained intergranular fractures which confirmed that the longitudinal portion of the fracture was also intergranular. Textron Lycoming engineering drawing No. 73808 for the push rod assembly indicated that the tube portion is to be made from 2024-T3 aluminum alloy, and that the push rod after machining is artificially aged to obtain the minimum properties of 65 KSI ultimate tensile strength, 48 KSI yield strength, and 7 percent elongation. Microhardness testing of the section produced a hardness value of between 144 and 149 HB. According to the ASM Metals Handbook Desk Edition, the typical hardness for artificially aged 2024 aluminum alloy is 120 HB or higher, indicating that the hardness of the push rod was consistent with an artificially aged 2024 aluminum alloy. An EDS spectrum of the section displayed peaks of aluminum, magnesium, silicon, iron, copper, manganese, and chromium consistent with a 2000 series aluminum alloy. Interference pins are inserted into each end of the push rod The opposite end of the push rod was also examined with a stereo microscope. The examination revealed two longitudinal cracks, located on almost diametrically opposite sides of the push rod, which extended to as much as 0.3 inch from where the interference pin is located. The crack was sawed open and an SEM examination of the cracks revealed that they also contained intergranular fracture features similar to the cracks on the separated end. (The Materials Laboratory Division Factual Report is appended to this report.) ADDITIONAL INFORMATION An examination of the emergency locator transmitter revealed that the alkaline batteries expired on October 31, 1995. The aircraft was recovered by Aero Retrieval to their storage facility in Compton, California. The aircraft wreckage was released to a representative of the registered owner on February 8, 1996.
a push rod failed from fatigue as a result of longitudinal stress corrosion cracking.
Source: NTSB Aviation Accident Database
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