FLIGHT TEST INVESTIGATION ON THE EFFECTS OF AIRCRAFT FLAP CONFIGURATION CHANGE ON LONGITUDINAL TRIM
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Loss of Control (LOC) in general aviation has been a topic of extensive study and analysis in the United States and the European Union due to a long series of aircraft accidents in the past decades. The National Transportation Safety Board (NTSB) has identified over 1500 accidents in the traffic pattern involving LOC in general aviation aircraft between 1982 and 2015. LOC has often been attributed to deficiencies in pilot training, and the typical recommendation to reduce the number of accidents is to provide further training in stalls and spins. The European Aviation Safety Agency (EASA) has performed research that demonstrates a large amount of traffic pattern accidents are concentrated in specific areas, such as flap deployment or retraction. To appropriately test the hypothesis that flap changes can be a contributor to LOC, a series of flight tests have been performed on four aircraft: PA32-260, PA28-161, PA28-180, and C 172-M. These flights were conducted at both forward and aft CG locations and in both wings level and banked conditions. The case in which the pilot is distracted, either by looking outside for traffic, or simply attending to other matters in the cabin is assumed throughout this research. This assumption relates to James Reason’s chain of causation model, which states that an accident is caused by a series of failed defenses. Given this particular case, once the flap configuration change is applied, the flight test crew observed the behavior of the aircraft without the pilot intervening. The crew also recorded stick forces required to maintain trim airspeed after the configuration change. Using on-board cameras and a non-intrusive Data Acquisition System, data were recorded on the aircraft’s pitch attitude, altitude, airspeed, and bank angle. Stick forces were recorded with a hand-held force gage. The work presented in this thesis provides evidence that three of four aircraft tested, the deployment of flaps caused extremely dangerous pitch attitude and stick force changes, resulting in a loss of airspeed and consequential unusual attitudes. Tests were also conducted for go-around maneuvers, showing an increase in airspeed and sudden decrease in pitch attitude. Finally, recommendations are expressed to reduce the effects of flap configuration change on longitudinal trim, providing recommendations for future research and a proposed revised version of Federal Aviation Regulations 23.143 and 23.145. Regulatory recommendations include the limiting of stick forces to 10 lbs. (one hand on rim) during flap configuration changes, with an added recommendation for the pitching direction to be nose-up (push force on controls) for flap retraction and nose-down (pull force on controls) for a flap deployment. The final intent of this thesis is to provide valuable information to aviation authorities, in order to reduce the amount of accidents due to loss of control in traffic patterns.