Erickson Air-Crane and Stilwell Baker Inc. (SBI) have jointly developed and certified a modernized automatic flight control system (AFCS) for the Sikorsky S-64F heavy-lift helicopter, using an unusual reversal of the all-digital approach to avionics. Erickson has contracted with SBI for the latter to supply production AFCS hardware to equp the 16 S-64s in the Erickson fleet.
Erickson, Central Point, Ore., and SBI of Vancouver, Wash., updated the original analog AFCS in the S-64F using current-generation digital sensors, flight control system components and electronic displays in the existing S-64F system architecture. With stability augmentation system (SAS) redundancy, integrated autotrim and hardover detection, the AFCS replaces the original Sikorsky-designed and -built flight control system. Erickson (Booth No. 6224) has held the type certificate for the S-64 since 1992.
The unique approach was dictated by launch customer San Diego Gas & Electric’s (SDG&E) schedule requirement of 18 months from contract to delivery. SBI president Darrel Baker called the project “reverse engineering of a legacy product.” He explained, “Given the tight contract timeframe, we chose to implement all of the control functions in analog for simplicity to avoid software certification issues and complex hardware certification.” The result is a hybrid analog/digital system, following original Sikorsky control paths, with Arinc 429 outputs from a digital air data attitude-heading reference system (ADAHRS) processed through a digital-to-analog converter.
The VFR-only AFCS implements the S-64F’s unique fly-by-wire external load control through a joystick at an aft station. The Erickson/SBI team has installed the modernized AFCS in two S-64F helicopters to date: the SDG&E machine and one operated by Erickson.
SBI undertook the electrical and electronic systems design and engineering, while Erickson, as system integrator, selected the ADAHRS and EFIS displays, handled contract administration and oversaw FAA supplemental type certification, which was issued in February 2011. First customer delivery took place “18 months to the day from the contract date,” Baker said.
Erickson selected the AHR-150 ADAHRS from Archangel Systems, Auburn, Ala., which is based on micro-electromechanical sensor technology. Weighing just over than three pounds, the system consists of an inertial reference unit and remote-mounted magnetic flux valve heading sensor, replacing the Sperry “spinning iron” VG-14 vertical and C-14 directional gyros in the original S-64F installation. Brian Terhune, Erickson senior project engineer, said removal of the “original complex mechanical gyro system” contributed to a 33-percent reduction in system weight, with “significantly improved aircraft performance and stability, especially for precision placement of external loads.”
Terhune said rising support costs for the legacy AFCS dictated the modernization. “Parts were getting harder to find, and with these highly specialized aircraft operating all over the globe in critical missions like firefighting and heavy lifting, we needed a long-term solution.” Baker added, “Our customers now have a spare parts supplier and product support provider through the foreseeable future.”
Erickson selected electronic engine and mode annunciation units from Sandel Avionics (Booth No. 8940), while Rogerson Kratos (Booth No. 8659) provided primary flight and navigation EFIS displays linked to the AFCS.
Working within the framework of existing S-64 control laws, the Erickson/SBI team achieved a 50-percent improvement in S-64 AFCS reliability through circuit redesign and component selection, said Baker. The task entailed “a lot of head-scratching and some old-fashioned engineering. We had to learn why Sikorsky did what it did, and thoroughly understand each design element. To duplicate functions with current equipment required time to become fully familiar with the core flight control paths.”
Still, SBI needed just eight months to design and fabricate the first AFCS flight article. Flight test followed to assure the same or better levels of hardover protection and performance. Baker noted that the updated autotrim function earned praise. “The pilots liked that the helicopter sticks where they put it.”
He concluded, “The S-61 has a very similar control system, and it would be possible to do a similar solution there, perhaps even a little easier because the S-61 has no fly-by-wire stick in the aft station. It’s technically feasible to achieve a streamlined path to reverse engineering of that legacy system.”