N776MCPiper PA-23 · 1957

On July 15, 2021, about 1129 eastern daylight time, a Piper PA-23, N776MC, was substantially damaged when it was involved in an accident near Easton, Maryland. The flight instructor and pilot receiving instruction were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.

The flight instructor stated that, after takeoff, the pilot receiving instruction, who was also a flight instructor, completed an approach to landing stall (flaps and gear down) and reported that the throttles felt “like bungee cords.” The flight instructor then executed the same maneuver and agreed, reporting that, during recovery, the left engine spooled up “slowly.”

The flight instructor told the pilot receiving instruction to perform the maneuver again, which began at 3,000 ft msl by lowering the nose while the airplane was operating at 70 mph to achieve a 500 fpm descent at 80 mph and 1,500 rpm. When the airplane was at an altitude of about 2,800 ft msl, the pilot receiving instruction initiated the stall maneuver by raising the nose and reducing engine power to idle. The airplane then descended about 100 ft before the stall buffet occurred at an airspeed of about 62 mph. During the subsequent 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 flight instructor stated that the airplane immediately rolled about 45° to the left into a Vmc [velocity minimum control airspeed] roll.

The flight instructor immediately took control of the airplane to recover, but, as he applied full right rudder, his seat slipped back, which released pressure on the right rudder pedal. The airplane entered a second VMC roll, from which he was able to recover, but the airplane was at an altitude of about 1,200 ft msl (or about 1,100 agl) and in a descent of about 800 fpm. The pilot receiving instruction was unable to restart the left engine, and the flaps and landing gear remained extended. The airplane was below 800 ft msl and descending fast when both pilots determined that the airplane would be unable to reach an airport. The flight instructor subsequently landed the airplane in a cornfield, which resulted in substantial damage to the left wing and fuselage.

The airplane was equipped with hydraulically controlled flaps and a landing gear system that was primarily operated by an engine-driven hydraulic pump installed on the left engine. The airplane was also equipped with a hand pump in the cockpit. With an inoperative left engine, extension or retraction of the flaps and landing gear was done manually using the hand pump. According to the Owner’s Handbook for Operation and Maintenance of the Piper Apache Models PA-23 and PA-23-160 Airplanes, the landing gear required 30 to 40 pumps of the handle to be raised or lowered, and the flaps required 12 pumps of the handle to be raised or extended.

The airplane was also equipped with an engine monitor that recorded parameters for both engines, including cylinder head temperature (CHT) and exhaust gas temperature (EGT). A review of the downloaded engine monitor data revealed that the left and right engine EGT and CHT were normal and showed increasing and decreasing values for both engines, consistent with airwork. From 1125:04 to 1126:28, the left engine EGT began decreasing, and the CHT for all cylinders increased. From 1126:28 to the end of the recorded data at 1128:52, the left engine EGT showed a general increase with an occasional short-term decrease in temperature, and the CHT for all left engine cylinders showed slightly increasing or steady temperatures. The end of the recorded data for the flight showed the right engine EGT between 1,097°F and 1,200°F and the right engine CHT between 281°F and 326°F (the highest value recorded).

Supplemental Type Certificate (STC) SA1306SW, issued on May 4, 1971, to Apache Modification Aircraft Corporation, approved a change to the airplane type design of Piper PA-23 and Piper PA-23-160 airplanes involving the installation of 200horsepower Lycoming IO-360-C1C engines and Hartzell HC-C2YK-2 model propellers with counterweight-equipped C7666A-4 propeller blades that had a basic propeller diameter of 72 inches. With this STC, feathering of the propeller was accomplished by moving the propeller control in the cockpit to the feather position, which drained oil from the propeller back to the engine, thus allowing the air charge in the propeller cylinder and the counterweight on each blade to move the blades to the feather position. According to the airplane flight manual supplement associated with the STC, the CHT limitation was 500°F. According to the Federal Aviation Administration (FAA), the airplane was modified in May 2009 in accordance with STC SA1306SW.

According to maintenance records and work orders, after the airplane made a gear-up landing in September 2020, both engines were inspected and then reinstalled on the airplane, and Aeroshell 15W50 oil was added to both engines. Hartzell HCC2YK2RBF propellers, with F7666A-2 model propeller blades, were installed. A review of the left propeller logbook revealed that the propeller was overhauled on November 3, 2020. The propeller blades did not have the counterweights required by the STC and the basic propeller diameter of 74 inches was 2 inches longer than the STC-approved propeller.

According to the Propeller Owner’s Manual and Logbook, No. 115N, a note in the maintenance practices section indicated that propellers on certain aircraft and engine combinations have experienced inadvertent feathering that occurred either at the time of shutdown or at low engine rpm. The manual further stated that the feathering could be reduced or eliminated “by lowering the air charge within the propeller cylinder.” The manual showed that the air charge in the cylinder at 70°F was required to be 178 pounds per square inch (psi) ± 2 psi.

Further review of the maintenance records and work orders showed that, during the airplane’s last annual inspection (about 3 months before the accident), a technician inspected the seat hardware for condition, security, and operation. The records contained no written discrepancies related to the right front seat, and there was no entry in the airframe maintenance records regarding the right front seat.

The airplane was equipped with hydraulically controlled flaps and a landing gear system that was primarily operated by an engine-driven hydraulic pump installed on the left engine. The airplane was also equipped with a hand pump in the cockpit. With an inoperative left engine, extension or retraction of the flaps and landing gear was done manually using the hand pump. According to the Owner’s Handbook for Operation and Maintenance of the Piper Apache Models PA-23 and PA-23-160 Airplanes, the landing gear required 30 to 40 pumps of the handle to be raised or lowered, and the flaps required 12 pumps of the handle to be raised or extended.

The airplane was also equipped with an engine monitor that recorded parameters for both engines, including cylinder head temperature (CHT) and exhaust gas temperature (EGT). A review of the downloaded engine monitor data revealed that the left and right engine EGT and CHT were normal and showed increasing and decreasing values for both engines, consistent with airwork. From 1125:04 to 1126:28, the left engine EGT began decreasing, and the CHT for all cylinders increased. From 1126:28 to the end of the recorded data at 1128:52, the left engine EGT showed a general increase with an occasional short-term decrease in temperature, and the CHT for all left engine cylinders showed slightly increasing or steady temperatures. The end of the recorded data for the flight showed the right engine EGT between 1,097°F and 1,200°F and the right engine CHT between 281°F and 326°F (the highest value recorded).

Supplemental Type Certificate (STC) SA1306SW, issued on May 4, 1971, to Apache Modification Aircraft Corporation, approved a change to the airplane type design of Piper PA-23 and Piper PA-23-160 airplanes involving the installation of 200horsepower Lycoming IO-360-C1C engines and Hartzell HC-C2YK-2 model propellers with counterweight-equipped C7666A-4 propeller blades that had a basic propeller diameter of 72 inches. With this STC, feathering of the propeller was accomplished by moving the propeller control in the cockpit to the feather position, which drained oil from the propeller back to the engine, thus allowing the air charge in the propeller cylinder and the counterweight on each blade to move the blades to the feather position. According to the airplane flight manual supplement associated with the STC, the CHT limitation was 500°F. According to the Federal Aviation Administration (FAA), the airplane was modified in May 2009 in accordance with STC SA1306SW.

According to maintenance records and work orders, after the airplane made a gear-up landing in September 2020, both engines were inspected and then reinstalled on the airplane, and Aeroshell 15W50 oil was added to both engines. Hartzell HCC2YK2RBF propellers, with F7666A-2 model propeller blades, were installed. A review of the left propeller logbook revealed that the propeller was overhauled on November 3, 2020. The propeller blades did not have the counterweights required by the STC and the basic propeller diameter of 74 inches was 2 inches longer than the STC-approved propeller.

According to the Propeller Owner’s Manual and Logbook, No. 115N, a note in the maintenance practices section indicated that propellers on certain aircraft and engine combinations have experienced inadvertent feathering that occurred either at the time of shutdown or at low engine rpm. The manual further stated that the feathering could be reduced or eliminated “by lowering the air charge within the propeller cylinder.” The manual showed that the air charge in the cylinder at 70°F was required to be 178 pounds per square inch (psi) ± 2 psi.

Further review of the maintenance records and work orders showed that, during the airplane’s last annual inspection (about 3 months before the accident), a technician inspected the seat hardware for condition, security, and operation. The records contained no written discrepancies related to the right front seat, and there was no entry in the airframe maintenance records regarding the right front seat.

Postaccident examination of the airplane revealed that the airplane came to rest upright with both propellers feathered. The rudder was positioned partially trailing edge left, and the rudder trim tab was positioned trailing edge right (tail right).

Examination of the right front seat and seat tracks revealed that the left and right seat catch plungers extended 0.442 and 0.120 inch, respectively, from their housings. The right seat catch plunger would not extend into the track hole. The seat track surrounding structure revealed no evidence of damage or deformity. The right seat tube-seat catch release had both ends bent or deformed with about 0.438 inch between them, which reduced the length of the right seat catch plunger to secure the seat into position. The forward portion of the right seat lower seat cushion was slightly deformed/wrinkled, and the seat frame and seat pan did not appear to be deformed.

Examination of both wings revealed that the left main landing gear was in the down and locked position and that the right main landing gear was in the extended position; the left wing sustained substantial damage. The left fuel strainer bowl was empty while the right fuel strainer bowl had a small amount of liquid, consistent in odor and color to 100 low lead (100LL) aviation fuel. A small bubble of undetermined debris, which remained separate from the fuel, was visible at the bottom of the right fuel strainer bowl. Examination of the left and right fuel supply and vent systems revealed no evidence of preimpact failure or malfunction. Continuity of the engine controls for both engines (from the cockpit to the end fittings in each engine compartment) was confirmed.

Examination of the left engine which had been removed for recovery revealed the left and right magneto to engine timing were 21° and 20° before top dead center (BTDC), respectively, while the specified timing is 20° BTDC. Examination of the fuel inlet screen of the servo fuel injector revealed evidence of previous corrosion consistent with water contamination. The engine was shipped to the manufacturer’s facility for operational testing. After arriving, inspection revealed that the left magneto timing was at 0° BTDC. A tightly secured shipping strap was over the left magneto housing which likely changed the magneto timing position. The left magneto was re-timed to the value found following recovery and impact damaged components needed for an engine run were removed and replaced. The left engine was placed in a test cell with a test propeller installed, and the idle speed was adjusted to about 700 rpm. The engine operated within the limits for full-rated power and manifold pressure. The engine was subjected to a rapid throttle movement, and the engine did not hesitate in response. The rpm drop when checking the left magneto was greater than the allowed amount. The reason for the excessive magneto drop was not determined.

The propeller oil control leak test procedure was performed for both engines in accordance with Lycoming Service Instruction 1462A. The procedure specified that it was to be performed whenever the engine feathers during the landing rollout with a reduced throttle setting. The procedure specified that the engine was to be started and that the engine oil temperature should move into the green arc, but neither engine ran with those conditions. As a result, the procedure was performed on both engines while the engine oil was at ambient temperature. Testing of the left engine was performed twice at an air pressure of 40 psi, and both test results of 11 and 21 psi were within allowable limits. According to the engine manufacturer, the 21 psi is the reading obtained from a cold factory engine. Testing of the right engine using the same air pressure produced a value of 14 psi, which was also within allowable limits.

Examination of the left propeller revealed that blade No. 2 was bent aft about 30° and that it exhibited leading-edge polishing and minor twisting. A chordwise paint abrasion was at the tip. The cambered side of blade No. 1 exhibited chordwise erosion on the leading edge near the tip. While on the start locks with an ambient temperature of 72°F, the air charge in the cylinder measured 175 psi. According to the Propeller Owner’s Manual and Logbook Manual, the specified air charge in the cylinder at 70°F for that model propeller and propeller blades was between 176 and 180 psi. With the use of interpolation, the pressure at 72.5°F was between 177 and 181 psi. While on the start locks, the undamaged No. 1 blade was at the specified blade angle, and the low-pitch setting was correct.

Examination of the right propeller revealed that both propeller blades were in the feathered position. While on the start locks with an ambient temperature of 72°F, the air charge in the cylinder measured 169 psi. The start lock blade angle for both blades was within limits, the low-pitch blade angle for the No. 1 blade was 0.1° greater than maximum limit specified, and the No. 2 blade low-pitch blade angle was within limits.

Examination and testing of both propeller governors was performed. For the left governor, a slight leak was noted at the control shaft O-ring and at the feather rod. No leakage was noted for the right governor. The relief valve and feather checks of both propeller governors were within limits, and the pump capacity for both governors was 4.4 quarts per minute (minimum specification was 5.0 quarts per minute). The high rpm check for both governors was 20 rpm higher than the high-value limit. Both governors were not subjected to an internal leakage test because the test bench did not have that capability at the time of the testing.

The flight instructor reported an issue with another one of the operator’s airplanes, N76NA, a Piper PA-23-160. According to the FAA airworthiness records, the airplane was modified in December 1979 in accordance with STC SA899SW, which authorized the installation of 180-horsepower Lycoming O-360-A1D engines, Hartzell HCC2YK2RBF propellers, and F7666A-2 propeller blades, resulting in a basic propeller diameter of 74 inches. The flight instructor reported that, before the accident flight, he noted that the left propeller was feathered and that it was serviced afterward. A few days later while on the ground with the throttle at idle, the left propeller feathered uncommanded. The flight instructor also reported that, 1 week before the accident, another flight instructor told him that the right propeller on the accident airplane had feathered uncommanded while on the ground. A mechanic was able to get the blades out of feather, and the airplane was flown uneventfully. The propellers from N76NA were subsequently removed, overhauled, and then reinstalled onto the accident airplane.

The chief flight instructor for the operator reported that he had not experienced an uncommanded propeller feather event during his more than 13 years of flying N76NA. He did report two uncommanded feathering events in the accident airplane during landing and one that occurred during approach (not including the accident flight). The ground events involved the left and right propeller, and the in-flight event involved the right propeller. For all three cases, the feathering occurred when power was quickly reduced from an approach power setting to full idle. For both ground cases, the engines restarted normally. Maintenance could not duplicate the problem during troubleshooting after the first event. There was no entry in the provided maintenance records or in any work order regarding any inspection or testing regarding the uncommanded feathering.

In addition, the flight instructor reported that the right seat “was notorious for slipping back 2 or 3 notches when the right rudder was pushed hard.” As a result, it was common practice to physically push the locking pin into the notch to keep the seat in place. The flight instructor indicated 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.” He reported that other multiengine flight instructors also had the seat slip and that they complained to maintenance, but “nothing could be done to fix it.” The chief flight instructor reported that he flew the accident airplane on a regular basis with no seat issues.