Accident Summary
On February 25, 2024, at approximately 1610 central standard time, a Piper PA-28-181 airplane, registration N38696, was substantially damaged during a forced landing near Watertown, South Dakota. The flight instructor and student pilot were uninjured. The aircraft was operating under Title 14 Code of Federal Regulations Part 91 as an instructional flight.
Sequence of Events
The flight instructor reported that while the student was performing a touch-and-go landing, the engine lost partial power shortly after takeoff. The instructor assumed control and executed a forced landing on the remaining runway. Engine monitoring data indicated that during the takeoff, engine power increased from 851 rpm to a maximum of 2,505 rpm. Approximately five seconds after rotation, power dropped to 1,446 rpm.
During the forced landing, the airplane departed the runway surface and struck a fence and trees, resulting in substantial damage to both wings.
Engine Examination
Postaccident inspection of the engine revealed a partially open, stuck exhaust valve on the No. 2 cylinder. Removal of the valve from the cylinder assembly required penetrating oil and a mallet. Deposits were observed on the valve head and stem: reddish-brown deposits on the neck area and adjacent surfaces, and gray-colored, raised deposits on the side of the valve stem. The piston crown had similar deposits.
Energy-dispersive X-ray spectroscopy performed by the NTSB Materials Laboratory on deposits from the No. 2 cylinder intake valve showed relatively high levels of lead, carbon, and bromine. Deposits from the exhaust valve showed high lead content with lower levels of bromine and carbon.
Valve Clearance Measurements
With deposits present, the clearance between the No. 2 cylinder exhaust valve guide and valve stem measured between 0.0007 and 0.0009 inches at the middle of the guide, and between 0.0011 and 0.0029 inches at the inboard (piston side) end. The engine manufacturer's specification (Lycoming Service Table of Limits and Torque Value Recommendations, SSP-1176-5) requires exhaust valve stem-to-guide clearance between 0.0040 and 0.0060 inches.
After cleaning deposits from the exhaust valve guides of all four cylinders, the inside diameter of the No. 2 cylinder exhaust valve guide measured 0.4975 inches at the outboard (rocker side) end, 0.4977 inches at the middle, and 0.4983 inches at the inboard end—all below the manufacturer's minimum of 0.4985 inches. The inside diameters of the exhaust valve guides on the other three cylinders were also below the specified minimum in at least two locations each.
Maintenance History and Engine Data
The cylinders were factory-new when installed on the engine during overhaul by the engine manufacturer on January 11, 2022. No maintenance work had been performed on the cylinders or valves since installation. As of February 23, 2024, the engine had accumulated 646.6 hours since overhaul.
Recorded engine monitoring data from the accident flight showed the No. 2 cylinder peak temperature was 336°F. A review of 109 previous flights totaling about 165 hours indicated a maximum No. 2 cylinder temperature of 364°F and a median of 293°F.
The engine manufacturer recommends cylinder head temperatures (CHT) of 400°F or less during normal operations for normally-aspirated engines. Additionally, Lycoming's document on valve sticking (SSP-525) advises proper leaning at cruise power settings to promote complete fuel burning and to avoid long periods of ground operation, as improper leaning or extended low-temperature operation can lead to oil-lead sludge formation.