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Testing, testing, and more testing
by Mike Keirnan, Manager, Installations & Certification When S-TEC designs an autopilot installation for a specific aircraft our challenge is to assure that the product will meet our high standard of performance and safety throughout the aircraft's performance and CG envelope. As part of the STC approval process, S-TEC tests the autopilot in every situation that the pilot might encounter while using the autopilot in both normal and extreme situations. Many safety features have been designed into S-TEC autopilot systems to prevent hazardous conditions. For instance, the locking mechanism for the servo bridle cable is designed to fail at a certain force, and the cable guards are designed to shear. Both actions prevent the possibility of jamming the aircraft's primary controls. The electronics that perform the control functions and drive the servos are designed to prevent the possibility of a malfunction or hardover. Even with careful design, the possibility of a failure while the autopilot is functioning still exists. Because the autopilot computer could cause a servo hardover or runaway, S-TEC must test the autopilot for this type of malfunction in each axis of the autopilot (roll, pitch, trim, yaw) in the aircraft's least stable configuration, normally at the most aft cg limit. The malfunction tests are also performed with the highest possible servo clutch setting-given the environmental tolerances of the servo clutches. The environmental tolerances for a standard S-TEC servo using a Delrin clutch assembly are 40 percent above and 10 percent below the nominal setting for the aircraft. If the nominal setting was 30 inlb, the malfunctions would be tested with a clutch setting of 42 inlb. Clutch settings are measured before and after each flight test to ensure a valid test result. Servo malfunction flight tests begin by applying full aircraft bus voltage to the servo (stick free) for a specified time and then recovering the aircraft to the starting conditions, and during the hardover test, aircraft attitude, accelerations, airspeed and altitude changes are measured and recorded. Malfunction tests are performed in all flight conditions to include climb, cruise (low and high altitude), descent, approach (wheels and flaps down), and maneuvering. In order to assure that a pilot can recover from a hardover, in all these conditions except approach, the test pilot must allow the hardover to continue for three seconds beyond recognition of the failure before initiating a recovery. An approach malfunction only requires a 1 second delay. When testing the autopilot, the aircraft's bank angles cannot exceed 60 degrees, and pitch attitude cannot exceed 30 degrees. The aircraft's acceleration, during the hardcover and recovery, must be within a 0- to 2-g limit-that is no more than +- 1g from the normal condition of 1g. The testing described here is only a brief description of what is typically performed for a given certification program. Variations in test programs can vary widely. Regardless of the aircraft or the autopilot system being certified, the test process is thorough to ensure the best performanceand the highest level of safety. Typical Autopilot Tests
Roll Axis Testing
Pitch Axis Testing
Trim Testing
Yaw Testing |