On June 17, 2017, about 1434 Alaska daylight time, a float-equipped Piper PA-12 airplane, N4018H, was destroyed when it was involved in an accident near Skwentna, Alaska. The commercial pilot was uninjured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 external cargo load flight.

According to the pilot, the purpose of the flight was to transport plywood to his cabin. The cabin was located off Indian Creek, a tributary of the Yentna River, about 14.5 miles southeast of Skwentna, Alaska. About 120 lbs of plywood was secured with various cargo straps to the struts that attached the floats to the fuselage. As the pilot was approaching to land to the south on the north-to-south oriented waterway with a "light wind" from the south, about 200 ft above the trees, the airplane made an uncommanded turn to the left. The pilot did not recall the airspeed at the time. The pilot reported that he applied left rudder with no response and that the rudder was "locked to the left." The pilot was unable to maintain control, and the airplane impacted trees and came to rest in an open meadow. The pilot was able to egress without further incident.

The pilot reported no preimpact mechanical malfunctions or failures with the engine or the airframe prior to the uncommanded turn to the left during the landing approach. Plywood secured to the float struts of a Piper PA-12 would be angled such that the plywood would follow the downward angle of the struts. The floats on the airplane were the Edo 089-2000 type.

The water rudder cables on the airplane descend from the fuselage along the support struts to which the plywood was secured. The water rudders are directly linked to the rudder.

The airplane's gross weight and center of gravity for the accident flight could not be determined. According to FAA Order 8400.34, for an airplane to legally and safely conduct external load operations under Part 91 or Part 135, the completion of several steps of a comprehensive process are required. This process is required for each individual airplane and is handled through a local FSDO. The airplane must meet certain eligibility requirements, the external load must meet certain eligibility requirements, the airplane must be inspected by the FAA, various forms must be reviewed and completed, and then a FAA Form 8310-7 Special Airworthiness Certificate is issued as well as operating limitations. Operational flight checks are also required to ensure that the airplane is safely controllable and has no adverse flight characteristics while carrying an external load. The pilot reported that he had an external load permit on file with the FAA and that he had been conducting external loads for several years with the accident airplane; however, review of FAA records revealed no external load permit for the accident airplane. The pilot previously held an external load permit for a Cessna A185F airplane.

FAA Order 8400.34 states that plywood installed as an external load is consider a major alteration to the airframe. Additionally, this document discusses the aerodynamic effects of external loads and states in part:

Aerodynamic forces and the weight of an external load change an airplane's handling and flight characteristics. These forces can negatively affect airplane performance (takeoff, climb, cruise, and landing), airplane stability, flight control effectiveness, vibration, fuel consumption, and engine cooling, among other characteristics.