History of Flight
On July 15, 2021, at about 1129 eastern daylight time, a Piper PA-23, registration N776MC, was substantially damaged during a forced landing in a cornfield near Easton, Maryland. The flight instructor and the pilot receiving instruction, who was also a flight instructor, were not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as an instructional flight.
The flight instructor reported that after takeoff, the pilot receiving instruction performed an approach to landing stall (flaps and gear down) and described the throttles as feeling like "bungee cords." The flight instructor then executed the same maneuver and agreed, noting that during recovery, the left engine spooled up "slowly."
The instructor directed the student to perform the maneuver again. The maneuver began at 3,000 ft msl by lowering the nose while the airplane was at 70 mph to achieve a 500 fpm descent at 80 mph and 1,500 rpm. When the airplane reached about 2,800 ft msl, the student initiated the stall by raising the nose and reducing engine power to idle. The airplane descended about 100 ft before the stall buffet occurred at about 62 mph. During recovery, full power was applied to both engines, but the left engine lost total power and the left propeller feathered uncommanded. At that time, the airspeed was about 57 mph, and the airplane immediately rolled about 45° to the left into a Vmc roll.
The flight instructor immediately took control. As he applied full right rudder, his seat slipped back, releasing pressure on the right rudder pedal. The airplane entered a second Vmc roll, from which he recovered, but by then the airplane was at about 1,200 ft msl (about 1,100 ft agl) and descending at about 800 fpm. The student was unable to restart the left engine, and the flaps and landing gear remained extended. Below 800 ft msl and descending fast, both pilots determined the airplane could not reach an airport. The instructor landed in a cornfield, resulting in substantial damage to the left wing and fuselage.
Aircraft Information
The airplane was equipped with hydraulically controlled flaps and a landing gear system primarily operated by an engine-driven hydraulic pump on the left engine, with a hand pump backup. With the left engine inoperative, manual pump operation required 30 to 40 pumps for gear and 12 pumps for flaps.
The airplane had an engine monitor recording cylinder head temperature (CHT) and exhaust gas temperature (EGT). Data review showed left and right engine EGT and CHT were normal until 1125:04, when left engine EGT began decreasing and CHT increased. From 1126:28 to the end of data at 1128:52, left engine EGT generally increased with occasional short decreases, and CHT slightly increased or held steady. At data end, right engine EGT ranged from 1,097°F to 1,200°F, and right engine CHT from 281°F to 326°F.
The airplane had been modified per Supplemental Type Certificate (STC) SA1306SW, issued May 4, 1971, authorizing installation of 200-horsepower Lycoming IO-360-C1C engines and Hartzell HC-C2YK-2 propellers with counterweight-equipped C7666A-4 blades (72-inch diameter). Feathering was accomplished by moving the propeller control to the feather position, draining oil from the propeller back to the engine, allowing air charge and counterweights to feather the blades. The CHT limit was 500°F. The FAA confirmed the airplane was modified in May 2009 per this STC.
Maintenance records showed after a gear-up landing in September 2020, both engines were inspected and reinstalled with Aeroshell 15W50 oil. Hartzell HC-C2YK-2RBF propellers with F7666A-2 blades (74-inch diameter) were installed; these blades lacked counterweights required by the STC and had a diameter 2 inches longer than approved. The left propeller was overhauled on November 3, 2020.
The Propeller Owner's Manual noted that propellers on certain aircraft/engine combinations have experienced inadvertent feathering at shutdown or low rpm, which could be reduced or eliminated "by lowering the air charge within the propeller cylinder." The air charge at 70°F was specified as 178 psi ± 2 psi.
During the last annual inspection (about 3 months before the accident), a technician inspected seat hardware for condition, security, and operation; no discrepancies were recorded for the right front seat, and no seat-related entries were found in airframe records.
Wreckage and Impact Information
Postaccident examination revealed the airplane upright with both propellers feathered. The rudder was positioned partially trailing edge left, and the rudder trim tab was positioned trailing edge right.
Examination of the right front seat and tracks showed the left and right seat catch plungers extended 0.442 and 0.120 inch from their housings. The right plunger would not extend into the track hole. The seat track structure showed no damage or deformity. The right seat tube-seat catch release had both ends bent or deformed with about 0.438 inch between them, reducing the plunger's ability to secure the seat. The forward portion of the right seat lower cushion was slightly deformed/wrinkled; the seat frame and pan were not deformed.
Both wings were examined. The left main landing gear was down and locked; the right main landing gear was extended. The left wing sustained substantial damage. The left fuel strainer bowl was empty; the right contained a small amount of liquid consistent with 100LL fuel, with a small bubble of undetermined debris. Fuel supply and vent systems showed no preimpact failure. Continuity of engine controls was confirmed.
The left engine had been removed for recovery. Left and right magneto timing were 21° and 20° BTDC (specified 20°). The fuel inlet screen showed previous corrosion consistent with water contamination. The engine was shipped to the manufacturer for operational testing. Upon arrival, left magneto timing was 0° BTDC, likely changed by a tightly secured shipping strap over the housing. The magneto was re-timed to the post-recovery value, and impact-damaged components were replaced. The engine was run in a test cell with a test propeller; idle speed was adjusted to about 700 rpm. It operated within limits at full power. Rapid throttle movement caused no hesitation. Left magneto drop was excessive; the reason was not determined.
Propeller oil control leak tests per Lycoming Service Instruction 1462A were performed on both engines with oil at ambient temperature because neither engine ran with oil temperature in the green arc. The left engine was tested twice at 40 psi, yielding 11 and 21 psi (within limits). The right engine produced 14 psi (within limits).
The left propeller had blade No. 2 bent aft about 30°, with leading-edge polishing and minor twisting, and chordwise paint abrasion at the tip. Blade No. 1 had chordwise erosion on the leading edge near the tip. With ambient temperature 72°F, air charge in the cylinder on start locks measured 175 psi (specified 176-180 psi at 70°F; interpolated at 72.5°F: 177-181 psi). The undamaged No. 1 blade was at specified blade angle; low-pitch setting was correct.
The right propeller had both blades feathered. On start locks at 72°F, air charge measured 169 psi. Start lock blade angles were within limits; No. 1 blade low-pitch angle was 0.1° greater than maximum limit, No. 2 was within limits.
Examination of both propeller governors revealed a slight leak at the left governor's control shaft O-ring and feather rod; no leakage on the right. Relief valve and feather checks were within limits. Pump capacity for both governors was 4.4 quarts per minute (minimum 5.0). High rpm check was 20 rpm high for both. Internal leakage tests were not performed due to test bench limitations.
Additional Information
The flight instructor reported a previous issue with another operator airplane, N76NA, a Piper PA-23-160 modified per STC SA899SW with 180-horsepower engines and Hartzell HC-C2YK-2RBF propellers with F7666A-2 blades (74-inch diameter). Before the accident flight, he noted the left propeller was feathered and was serviced. A few days later, on the ground at idle, the left propeller feathered uncommanded. Also, one week before the accident, another instructor reported that the right propeller on the accident airplane feathered uncommanded while on the ground. A mechanic cleared the feather, and the airplane flew uneventfully. The propellers from N76NA were subsequently removed, overhauled, and installed on the accident airplane.
The chief flight instructor reported no uncommanded feather events in over 13 years flying N76NA. He reported two uncommanded feather events on the ground (left and right propellers) and one in-flight (right propeller) in the accident airplane, all occurring when power was quickly reduced from approach to idle. The ground events resulted in normal restarts. Maintenance could not duplicate the problem after the first event; no entries regarding uncommanded feathering were found in maintenance records or work orders.
The flight instructor also reported that the right seat "was notorious for slipping back 2 or 3 notches when the right rudder was pushed hard," leading to the practice of physically pushing the locking pin into the notch to keep the seat in place. He stated that "as long as you don't slam on the rudders, putting too much force against the locking pin, the seat will remain in place." Other multiengine instructors experienced the same issue and complained to maintenance, but "nothing could be done to fix it." The chief flight instructor reported flying the accident airplane regularly with no seat issues.