The FAA and the United States Forest Service (USFS) supported Wichita State University in examining the factors that affect the structural integrity of aircraft flying in firefighting missions. This program was aimed at conducting operational loads analysis of the Beechcraft King Air fleet flown in support of various operations to evaluate typical operational in-service data and compare the results with the data used in the design and qualification of such aircraft. The ultimate goal was to provide a basis to improve the structural criteria and methods of operation and maintenance of these airplanes by providing processed data in statistical formats, which would enable the FAA, the USFS, and the operators to better understand and control the factors that influence their structural integrity.

Data were collected from a fleet of C90A, C90GT, and E90 airframes in actual operation during several fire seasons. A total of 3,829 distinct ground-air-ground (GAG) flights, consisting of approximately 7,100 flight hours, were identified. Only the data from the 2014 season contained information on flap deflection. Four basic missions were identified and, in firefighting cases, each mission was divided into various phases.

GAG (i.e. overall) airframe usage included examination of altitude and airspeed, and their comparisons with the published limits, as well as pitch and roll angles and overall load factors versus airspeed. In a number of cases, the VMO of 226 Knots Indicated Airspeed was exceeded, but by no more than 10% above the limit. Over-speeding was shown to be as likely during ferry flights as it was during other missions. With flaps retracted, load factors remained within the prescribed limits, except in one case in a C90A. However, in many firefighting operations, the maximum allowable load factor was exceeded when the flaps were deployed.

Exceedance spectra were developed for gust and maneuver loads and derived gust velocities for the overall flights and for specific flight phases. The results correlated much better with above ground level (AGL) altitude bands. The frequency of occurrence of incremental gust load factors for firefighting missions was higher than those of ferry missions. Maneuver load factors occurred in two orders of magnitude more frequently during firefighting cases than they did during ferry missions. Their frequency also increased with decreasing AGL altitude.

The report concludes with some recommendations for improved data acquisition for future similar efforts.