Civil Development and Certification of a Helicopter Automatic Approach and Hover System on the Sikorsky S-76

Sikorsky Aircraft and Honeywell incorporated, in response to customer requirements, began development of an all weather Search and Rescue (SAR) capable S-76 helicopter in April 1989. To accomplish this primarily over water rescue mission, an automatic approach and hover capability was added to the standard Digital Automatic Flight Control System (DAFCS). A key element to the system was the automatic approach and hover with doppler velocity control, and new approach-to-hover and hover Electronic Flight Instrument System (EFIS) displays.

The SPZ-7600 DAFCS autopilot with the added SAR modes required the most development effort. Similar autopilots had been developed by Honeywell in other helicopters, but none had been taken to full FAA certification. To aid in mission success, the system was designed to remain coupled to the remaining autopilot in the event of a single autopilot failure, a significant change from the standard DAFCS autopilot where single autopilot coupling is not permitted. This technical approach required a careful analysis of coupled single autopilot control actuator gains, pilot delay times after autopilot malfunctions, and synchronization of each autopilot flight director so that automatic switching after a failure would be accomplished smoothly and safely. A large portion of the flight testing was dedicated to achieving this highly important goal.

A total of 125 flight test hours were dedicated to development of the SAR autopilot. Each individual SAR mode, APPROACH 1, APPROACH 2, MARK ON TARGET (MOT), VELOCITY HOLD, RAD ALT HOLD, and CLIMB was flown repeatedly until helicopter response was optimized. After, dual autopilot optimization, the process was repeated coupled to a single autopilot. Failure mode testing followed successful development of the SAR modes. Control hardovers in all axes were tested to insure that dual autopilot minimum recovery times of 3.5 seconds and single autopilot times of 1.5 seconds could be met. Pitch hard-overs and collective down failures proved to be the most critical and were tested at the extremes of the weight and center of gravity envelope.

After completion of company tests, the helicopter was flown by FAA pilots of varied background and helicopter experience, over 50 automatic approaches were flown successfully during the certification flight tests, including night overwater approaches. Extensive evaluation of the system failure modes, including flight control hardovers, was completed successfully. On 16 October 1990, Sikorsky received the first FAA certification of an automatic approach and hover system on a civil helicopter. The system allows unrestricted over water automatic approach and hover operations in Instrument Meteorological Conditions (IMC), single or dual autopilot.

The Sikorsky S-76 with the Honeywell SPZ-7600 autopilot with automatic approach and hover capability, provides a significant increase in the capability for civil operators to prosecute search and rescue operations under all weather conditions.