Frederick, MD, USA
N6645Q
Grumman-Schweizer G-164B
During cruise flight, the pilot of a Grumman-Schweizer G-164B airplane noted that the engine began to run rough, and shake violently. He also noted smoke coming from the exhaust. The engine subsequently lost power, and the pilot performed an emergency landing to a cornfield. During the landing, the airplane came to rest inverted, resulting in substantial damage. A post-accident examination of the engine revealed a fractured masterod. Deformation and damage to the masterod was consistent with being exposed to a hydraulic lock prior to the accident flight.
On July 11, 2006, at 1500 eastern daylight time, a Grumman-Schweizer G-164B, N6645Q, was substantially damaged when it impacted terrain during a forced landing in Frederick, Maryland. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the flight that originated at Fallston Airport (W42), Fallston, Maryland, and was destined for a private airstrip in Martinsburg, West Virginia. The positioning flight was conducted under 14 CFR Part 91. According to the pilot, he was returning to his base airport after completing a crop dusting assignment near Fallston, Maryland. While in cruise flight, the engine started to run rough, and began to shake violently. The pilot also noted smoke coming from the exhaust. The engine subsequently lost power, and the pilot deviated to Frederick Municipal Airport (FDK), Frederick, Maryland for an emergency landing. The pilot was unable to reach the airport, and subsequently performed an emergency landing in a cornfield about 1 mile north of FDK. During the landing, the airplane came to rest inverted. An on-site examination of the airplane was conducted by a Federal Aviation Administration (FAA) inspector. All four wings and wing struts sustained damage, and the upper left wing spar was bent. The vertical stabilizer was crushed, and the elevators were substantially damaged. Under the supervision of the FAA inspector, the pilot attempted to rotate the propeller and engine crankshaft by hand, and loose pieces of metal were heard moving inside the engine compartment. The Pratt & Whitney R-985-AN-14B radial engine was subsequently removed, and shipped to a facility in Okmulgee, Oklahoma, for teardown. The teardown facility conducted an engine disassembly and parts inspection on September 10, 2006, and the facility's Quality Assurance Manager provided a copy of the findings report to the Safety Board. According to the report, the number 5 masterod (located in the lower, number 5 cylinder) was found fractured, and the intact portion of the masterod was bent. The report further stated, "The bent condition of the intact portion of the masterod (number 5 cylinder location), and the rod's fracture surface, are typical of damage known to have resulted from a liquid hydraulic lock of the cylinder / piston. The burnished area on the skirt of the number 5 piston indicates operation, for an undetermined period of time, with the piston misaligned with the cylinder bore. Stresses placed on the masterod during operation in the condition described above resulted in eventual fracture of the rod..." "With the masterod fractured, the masterod assembly was free to 'ratchet' on the throw of the shaft in response to power impulses from the surviving, functioning cylinders. The skirt damage on the number 5 cylinder allowed the intact portion of the masterod to move into the bore of the number 4 cylinder. Engine rotation moved the two rods toward the top of the number 4 cylinder bore. The widening shape of the shank portion of the masterod resulted in insufficient clearance and resulted in seizure of the engine." Engine maintenance records indicated that the engine had been overhauled in 2001. The engine had accumulated 250 hours of operating time since overhaul, and 9,525 hours of total operating time. All pistons were replaced during overhaul, and no abnormalities were noted. In addition, the most recent annual inspection was performed in November 2005, with no abnormalities noted. FAA Advisory Circular (AC) 65-12A, Airframe and Powerplant Mechanics Powerplant Handbook, described the dynamics, causes and effects, of hydraulic lock as follows: "Whenever a radial engine remains shut down for any length of time beyond a few minutes, oil or fuel may drain into the combustion chambers of the lower cylinders or accumulate in the lower intake pipes when the engine starts. As the piston approaches top dead center of the compression stroke (both valves closed), this liquid, being incompressible, stops piston movement. If the crankshaft continues to rotate, something must give. Therefore, starting or attempting to start an engine with a hydraulic lock of this nature...may result in a bent or broken connecting rod. A complete hydraulic lock - one that stops crankshaft rotation - can result in serious damage to the engine. Still more serious, however, is the slight damage resulting from a partial hydraulic lock which goes undetected at the time it occurs. The piston meets extremely high resistance but is not completely stopped. The engine falters but starts and continues to run as other cylinders fire. The slightly bent connecting rod resulting from the partial lock also goes unnoticed at the time it is damaged but is sure to fail later. The eventual failure is almost certain to occur at a time when it can be least tolerated, since it is during such critical operations as takeoff and go-around that maximum power is demanded of the engine and maximum stresses are imposed on its parts. Before starting any radial engine that has been shut down for more than 30 minutes, check the ignition switches for 'off' and then pull the propeller through in the direction of rotation a minimum of two complete turns to make sure there is no hydraulic lock or to detect the hydraulic lock if one is present." According to the pilot, during his pre-flight inspection, he rotated the propeller and crankshaft by hand to detect a hydraulic lock, and none was noted. The pilot further stated that he performed this procedure before every flight, and had never encountered a hydraulic lock.
A failure of the masterod, due to a hydraulic lock on a previous flight, which resulted in a total loss of engine power.
Source: NTSB Aviation Accident Database
Aviation Accidents App
In-Depth Access to Aviation Accident Reports
