Vintage heavy lifter gets new-generation avionics

 - December 19, 2007, 6:03 AM

Erickson Air-Crane is giving its star heavy lifter, the S-64 Aircrane, a makeover. The company is gradually retrofitting the 19 aircraft in its fleet with new-generation cockpit displays, attitude/heading reference systems (AHRS) and three-axis autopilots. With an increase in demand for the type from around the world, particularly for the firefighting role, the Oregon-based manufacturer/operator is also gearing up to start building new airframes.

Erickson is both the manufacturer and the largest operator of the S-64–a stalwart of helicopter heavy lift since its introduction, as the CH-54 Tarhe, to the U.S. Army in the early 1960s. At the same time Sikorsky produced a civil version, the S-64E Skycrane. Pilot and forestry entrepreneur Jack Erickson was quick to realize its potential: he bought the first four S-64Es and put them to work in timber harvesting and construction. They were an immediate success.

In 1992 the company purchased the type certificate from Sikorsky and renamed the helicopter the Erickson Aircrane. Since then, Erickson has continued to expand the machine’s roles and mission capabilities. In 1993 the company produced the first commercial model of the S-64F with a lift capacity of 25,000 pounds.

The helicopter has been used worldwide in timber harvesting, electrical tower construction, firefighting, hydro-seeding and general construction roles. Erickson has further expanded its capabilities with innovations such as a patented anti-rotation device, to help stabilize external loads; and a dedicated firefighting variant, the Helitanker, with a 2,650-gallon water tank, optional water cannon and snorkel attachments.

Heritage Aviation, based in Grand Prairie, Texas, has just finished developing, testing and certifying new-generation cockpit displays for the Aircrane. Six supplemental type certificate (STC) packages cover a host of new sensors, avionics and panel display enhancements. The improvements include a vibration-isolated instrument panel, Collins Pro Line 21 avionics suite, two 10.4-inch LCD multifunction displays and a new LED warning panel.

Heritage will eventually produce modification kits that will allow Erickson to do the installations on its own.

Erickson’s primary motivation for the upgrade was to enhance the overall reliability of the helicopter by moving to more modern, solid-state avionics and system sensors. According to chief pilot Paul Mavrinac, this became a concern as governmental emergency response and civil protection agencies began to ask for the S-64 to take on firefighting missions. “They need an aircraft they can depend on, to be ready every time they need it.

“We have one of the highest performance eliability rates in our industry, and our goal is to have the absolute highest. We need 100-percent availability.” Erickson is eager to increase not only revenue work for the S-64, but also sales of the airframe around the world. “If we expect to compete in a world market, the S-64 needs to offer the same, if not better, level of performance and reliability as the newest types rolling off the assembly line,” Mavrinac explained.

Erickson is finding some success in selling the Aircrane package–the latest was a recent delivery (of unmodified aircraft) to the South Korean Forest Service. Erickson has sold the first four modified S-64Fs to the state forestry corps of Italy (Corpo Forestale Dello Stato), which also took an option on two more. The second of the four on order is currently in production.

Automatic Flight Control System
The centerpiece of the new upgrade program is the Sagem (formerly SFIM) PA 155 three-axis analog automatic flight control system (AFCS), which is essentially the same AFCS that is in the Eurocopter AS 332 Super Puma and AS 365 Dauphin. Mavrinac said, “We chose the PA 155 because of its history of reliability, and because of favorable reports that we received from Super Puma operators.”

Jake Hart, project test pilot for the development and certification of the AFCS and cockpit multifunction displays and FAA-designated flight-test engineering representative, acknowledges that the PA 155 is not the newest AFCS on the market. However, he says, it is a good fit for the Aircrane. “The requirement for the autopilot to work through the electro-valves on the S-64, which pass commanded control inputs from the AFCS computer to the main hydraulic servos, narrowed the selection process,” he said.

Economics was another consideration. Hart explained that the development costs for a new digital AFCS would have been several times higher because of their increased test, certification and documentation requirements.

The standard S-64 stability system consists of a simplex architecture Hamilton Standard autopilot, or a duplex AFCS on the military CH-54B (which became the S-64F). These systems provided for basic attitude hold and two “coupled” modes, one for heading hold and the second providing a barometric altitude hold through the collective axis.

Randy Erwin, a 12-year veteran Erickson pilot, says that with the older Sikorsky system, “The pilot couldn’t really let the controls go in cruise flight. The new one is great. I love it.” Erickson believes that the PA 155 will not only provide enhanced stability, but also herald a step-change improvement in reliability.

The PA 155 AFCS has duplex architecture, with two AFCS computers, designated Lane 1 and Lane 2, located in a common box in the nose of the helicopter. The duplex feature gives the system a fail-passive and fail-operational capability. The AFCS provides attitude retention and automatic heading hold in a hover. For cruise flight modes the pilot can opt for basic attitude retention or select to “couple” to heading select, and altitude or airspeed hold for true “hands off” flight. The list of STCs includes a Garmin 500-series GPS and, while the AFCS is fully capable of navigation coupling, Erickson chose not to pursue that option. Consistent with previous approvals, the certification covers VFR flight only.

The AFCS also provides a stability augmentation system (SAS) mode, to allow increased pilot input during takeoff, approach and hover. The SAS mode is most likely to be used during the “hands-on” flying required by logging and firefighting ops.

The installation of a new aluminum instrument panel, mounted through two vibration-dampening dynamic mounts, has significantly reduced panel vibration. The previous panel was “hard” mounted directly to the airframe, allowing vibration to travel directly into the instruments.

The aircraft exhibits a fairly pronounced translational shudder when passing through 30 knots on approach, and Erwin says it shuddered so much “that you could hardly read the attitude indicator, let alone any of the gauges.” This phenomenon led to a succession of premature gauge replacements. “I was able to fly a few approaches with the new dynamic panel installed and, while the shudder was still apparent, the gauges and displays remained stable and readable,” Erwin explained.

During flight testing, Sagem Avionics pilot and sales engineer Jim Shirey spent some time at Heritage, talking to the Erickson people involved in developing and certifying the modifications. He participated in several of the developmental test flights, thus gaining an “up close and personal” impression of the upgrade package.

“During my first trip, it felt like I was trying to maneuver a small house around the sky. The veteran Erickson pilots, on the other hand, demonstrated their mastery over every nuance of this unique heavy lifter.”

Avionics Upgrades
Erickson has made some significant upgrades to the S-64 avionics. They include Rockwell Collins dual navcoms, dual transponders, a single ADF and a single DME, all controlled by dual radio tuning units. In addition to the Garmin GPS, a Collins TWR-840 weather radar, Honeywell enhanced ground proximity warning system, a Universal Avionics cockpit voice recorder, a stand-alone attitude indicator and an additional audio panel augment the existing three-panel system.

Aircraft attitude and heading information is transmitted to the AFCS computers via the AHRS, with added inputs from the air data sensor and dual magnetometers. These digital, solid-state units operate without the older, often unreliable spinning mass vertical and directional gyros and their associated sensors.

In an effort to improve crew comfort, Erickson has installed a new environmental control system (ECS). The center of the system is a Honeywell unit that provides heating, demist/frost and air conditioning functions. The old bleed-air system allowed only heating and defogging. Erwin said that, while the system was functional, “You needed 20 knots of airspeed just to get ambient air into the cockpit.” He tested the new system during the past Texas summer and gives it a definite thumbs up.

Heritage has developed a caution advisory panel (CAP) as a “plug and play” replacement for its predecessor. The system displays a text message in the form of a lighted display segment for every event, color-coded for urgency. In addition, it generates individually resettable master caution outputs for every cautionary (amber) event, which in turn drives an amber master caution warning at each crew station. This allows crewmembers to clear and acknowledge a caution event, while keeping the system armed for a re-occurrence or new event.

The caution panel is designed so that each display segment is powered directly by the signal that triggers the alert, so the system cannot completely fail due to a central power or circuit loss. A power loss directly to the CAP system affects only the common master caution outputs. If unit power is lost, a dedicated display circuit activates the master caution fail segment, so that the crew is aware the master caution function has been lost.

FAA certification for all six new STCs on the S-64F was completed during the last week of July, allowing deliveries to the Italian Corpo Forestale to begin.

As well as installing the new upgrades, Erickson now plans to build new Aircranes at a rate of three per year. According to Mavrinac, “We are capable of building the S-64 from the rivets up.” In May the company reached a deal with Ducommun AeroStructures for the California-based company to make new main rotor blades for the S-64.