FBW for the S-92

While offshore support operator Petroleum Helicopters Inc. (PHI) takes delivery this month of the first production Sikorsky S-92 at Heli-Expo in Las Vegas, engineers at BAE Systems’ Johnson City, N.Y., facility are busy developing flight controls for a future version of the medium-lift helicopter. Their task is to enable Sikorsky to remove the aircraft’s mechanical control system–bell cranks, push rods, mixer assembly, boost actuators, and tail-rotor cabling–and replace it with a lighter, workload-saving, fly-by-wire (FBW) flight control system that uses electronic circuits to send pilot inputs to the aircraft actuators, which move the main- and tail-rotor blades. When completed, and combined with Rockwell Collins’ integrated Avionics Management System (AMS), the S-92 will be one of the most advanced, civil-certified (FAR Part-129) helicopters.

Installed in fixed-wing aircraft since 1972–when an F-8 Crusader was modified at Dryden Flight Research Center, Edwards, Calif.–FBW now is making inroads in the rotary-wing world. The U.S. Army’s RAH-66 Comanche scout/attack helicopter has an FBW system. So do the U.S. Marine Corps’ V-22 and the civil BA609 tilt-rotor aircraft. And in December 2003 an NH-90 utility helicopter prototype with FBW controls made its maiden flight. Like the S-92, the early NH-90 prototypes were fitted with conventional, mechanical controls.

BAE Systems, with its some 25 years of experience in FBW technology, developed the systems for the Comanche, V-22 and BA609. This is in addition to FBW systems it produced for 17 commercial and military fixed-wing aircraft–ranging from the Boeing 777 to the JAS 39 Gripen. Under its arrangement with Sikorsky, BAE will be the preferred supplier of the S-92 FBW system.

The 19-passenger S-92, an aircraft that evolved from the S-70 International Black Hawk, currently is in three key competitions against the Anglo-Italian EH-101. The Canadian maritime forces seek a medium-lift helicopter to replace their fleet of 28 aging Sea Kings. The U.S. Marine Corps squadron assigned to transport the U.S. president (HMX-1) plans to replace its VH-3Ds. And the S-92 and EH-101 may go head to head in a U.S. Coast Guard helicopter competition. But Sikorsky wouldn’t disclose whether the S-92 they propose for these programs would include fly-by-wire. The S-92 equipped with mechanical controls was certified in 2002.

The S-92 with FBW is scheduled to make its first flight in 2005, seven years after the mechanical-control S-92 first took to the air. The FBW prototype probably will come from the S-92 production line, according to a Sikorsky official, as the three remaining mechanical-control prototypes are being used or prepared for demonstration flying.

Currently, the first production S-92 with mechanical controls is set for delivery to PHI, which is scheduled to receive another S-92 later this year. And four S-92s will go to Norsk Helikopter, a Norwegian offshore support operator. Sikorsky will be seeking corporate transport customers, as well, and has a five-year contract with Keystone Helicopter, West Chester, Pa., for VIP completion services.

FBW’s Benefits

There is little difference between FBW systems for rotorcraft and fixed-wing aircraft, according to BAE Systems officials. "The systems are much the same, but are different–you have to move different control surfaces," says John Husaim, rotary-wing integrated project team (IPT) lead at BAE Systems. "The level of hardware complexity is no greater [with helicopters], but there can be greater software complexity because helicopters are not as stable as fixed-wing� platforms." The software’s intricacy "depends on the complexity of the control laws that drive the stability of the helicopter," adds Joe Hart, S-92 FBW project engineer at BAE Systems.

The benefits of FBW are comparable in both fixed-wing and rotary-wing aircraft, however. The BAE system is expected to reduce the S-92’s weight (enough to accommodate the weight of another passenger), reduce pilot workload and life-cycle costs, and enhance aircraft performance and safety.

The S-92 FBW system comprises digital flight control computers, two control sticks for the pilot and copilot, electrohydraulic actuators to move flight surfaces, and redundant sensors for aircraft pitch, roll and yaw data. It is a triplex-redundant� system that has three flight control computers (each with two processing lanes) mounted in separate locations on the aircraft. This arrangement is to protect against common environmental or ballistic threats (for military applications). The computers will be PowerPC-based. BAE Systems officials say they are not prepared to describe the computers, other than to say they "have ample processing capacity in terms of memory and processing throughput in order to add functionality down the road."

Each computer has dual-lane processors, providing primary and secondary lanes. Each lane has self-checking processing pairs, and in the event of a fault in the primary lane, the computer defaults to the secondary lane, according to John Mayo, Sikorsky’s program manager for FBW.

The real-time operating system (RTOS) is BAE Systems’ CsLEOS, which is certified to the RTCA DO-178B, Level A, standard and is in flight control computers in aircraft ranging from the C-17 to the Pegasus unmanned air vehicle. For additional fault tolerance, it includes a built-in health monitor, which automatically checks the computer’s memory and processing. The S-92 FBW system is fault-tolerant, which means a critical function will continue to perform even in the presence of a failure.

Maintaining Integrity

BAE Systems’ CsLEOS ensures the integrity� of the flight control system in other ways, too. For example, the ARINC 653-compatible operating system provides time, space and resource partitioning between the applications operating on the same processing board. Each application has its own allocated execution time, allocated space in memory, and access rights to specific resources–such as serial data bus, event timer or other external hardware devices–so as not to interfere with other applications. "If one application has a problem, it can not lock out other applications," Husaim explains.

Through either analog links or digital serial links, the S-92’s flight control computers gather navigation data from the Avionics Management System and inputs from pilot and/or copilot inceptors, aircraft inertial sensors, the air data system and radar altimeter. The primary interface to the sensors meets ARINC 429 protocol standards.

The RTOS synchronizes all of the three computers’ redundant channels on a frame-by-frame, lock-step basis to ensure that all paths process coherent and compatible data concurrently. The incoming data is analyzed and compared among the three computers (actually voted on) to produce compatible output and ensure that the computers come up with the same results.

The RTOS’ "architecture is flexible, to accommodate any [input data] the airframer wants to provide," says Hart. Additional applications can be added, as well. Flight modes in the mechanical-control S-92–such as coupled hover, coupled approach, altitude hold, hover hold, etc.–are "rehosted" in the FBW flight control computer.

But these modes will function better in the FBW computer than in the stability augmentation system (SAS) in the mechanical-control variant. As with the SAS, the FBW flight control system automatically compensates for certain amounts of inherent instability and wind gusts. But with the FBW system’s higher bandwidth, the response is quicker, according to Mayo.

The FBW computer "will have control modes and functionality the SAS doesn’t have," he adds. "We have complete authority, and we can tailor the pilot inputs and sensor data to achieve a more predictable and stable control response.

"After a pilot makes, say, a speed adjustment, the system will automatically hold the new speed for him. Controllability is simplified for the pilot since the model-following system demands fewer control inputs to accomplish a given flight maneuver.

For both the civil and military versions of the S-92, Level 1 handling qualities are provided by the FBW flight control system. The pilot thus has less workload while flying in day, night or even instrument meteorological conditions (IMC). This software enhancement also allows the pilot to perform more aggressive maneuvers and push the aircraft to the edge of its envelope.

With software partitioning to separate the applications, modifications to enhance capability or performance can be added to any software partition without interfering with the embedded code or other partitions. This feature cuts life-cycle costs because the installation of a new application–even software of a different standard–would be tested independently. Testing would not necessarily be required for all of the software.

What the Pilot Sees

What the S-92 pilots see in the FBW-equipped S-92 is comparable to what they see in the mechanical variant. Sikorsky maintained the conventional collective control alongside the pilots’ seats. But the stick that controls the pitch of the S-92’s four main-rotor blades will incorporate sensors to communicate stick position and movement to the flight control computers.

The FBW-equipped S-92 will have foot pedals, too, but they will only be foot brakes, says Mayo. They will provide no tail-rotor control.

The principal flight control stick will be in the cyclic position, in front of the pilot, but it will function more like a joystick. Officially called the "right-hand controller," it is comparable to the Comanche’s sidearm controller, and it controls pitch, roll and yaw.

The right-hand controller doesn’t give the pilot the exact feedback of a conventional cyclic stick, but Mayo believes helicopter pilots will find operating the FBW-equipped S-92 "to be an easy transition because the automatic features simplify the job of flying."

"After 15 to 20 minutes in the [Comanche] simulator, I felt at home in the fly-by-wire cockpit," adds Husaim, who also is a U.S. Navy veteran and test pilot, with more than 5,000 rotary-wing hours, mostly in SH-60 Seahawks. "Training to fly the [FBW] aircraft should not be an issue. It reduces pilot workload, so you can pay more attention to the mission."

With electronic mixing in the flight control computer, S-92 pilots can maneuver the aircraft with single-axis inputs. According to Mayo, if the pilot desires a pitch attitude change, he simply pushes the right-hand control forward. The flight control computer calculates the appropriate off-axis inputs to eliminate the need for pilot compensation. This reduces pilot workload for controlling the helicopter’s highly coupled characteristic. Unlike mechanical flight control system mixing, the FBW system can optimize the electronic mixing with airspeed, which means the response is decoupled at all speeds and not just at a single design point.

"It is extremely responsive," says Husaim. He describes control response in conventional helicopters as "sloppy," compared with the much quicker responses of an FBW system.

The S-92 FBW system’s three computers require redundant power sources. Therefore, in addition to the S-92’s main gearbox and auxiliary power unit (APU), Sikorsky will tap additional onboard power from batteries and a permanent magnet generator (PMG), attached to the accessory gearbox.

S-92 Cockpit

In designing the S-92 Avionics Management System, Rockwell Collins shared BAE Systems’ intent of reducing pilot workload. The four 6-by-8-inch liquid crystal displays (LCDs) the company developed for the helicopter "provide an intuitive interface," says Jim Perkins, Collins Government Systems’ marketing manager for rotorcraft.

"The multifunction displays [MFDs] provide fixed-function keys across the top of each display, which give pilots direct access to primary flight information pages and engine instrumentation and crew alerting system [EICAS] pages," Perkins adds. "One press of a button and you go directly to one of those pages.

"This minimizes the need to search through menus or go heads down, looking for a control. All the keys are at eye level."

The displays also include 18 soft function keys to the left and right of each display. The pilots also can control the display, using the display control panel on the center console.

The S-92 comes standard with four fully independent, portrait-format MFDs, a primary flight display (PFD) and an EICAS/navigation display for each pilot. An optional, redundant fifth display would fill the gap on the panel between the four standard displays. The Collins system is integrated, so the displays are interchangeable, and if one MFD goes down, a display can be shown on another MFD.

Each display, which is night-vision goggle compatible, has its own internal data processing and graphic symbology generation. The S-92 display system uses Motorola PowerPCs running Green Hills Software’s Integrity-178B real-time operating system and GStart ADA runtime environment. The S-92 is the first helicopter to use the ARINC 653-compliant Integrity-178B operating system, according to Patrick Huyck, Green Hills’ certification manager. Integrity-178B, with an open systems architecture, also supports partitioning� and hardware memory protection. Collins’ AMS in the S-92 received FAA technical standard order (TSO) approval in November 2002, and an audit of the certification package verified Integrity-178B’s compliance with DO-178B, Level A. The computer hardware adheres to DO-254, Level A. The AMS’ primary interface is ARINC 429. "But the displays have [Mil-Std] 1553B hardware in them," says Perkins. So 1553B interfaces can be applied.

Showing Round Dials

Sikorsky applied much of the S-70’s display presentation to the S-92, according to Perkins. For example, like the S-70, the S-92’s PFD includes "a lot of round dials" to show altitude, indicated airspeed, turbine gas temperature, etc. "We spent a lot of time with Sikorsky pilots, and they wanted round dials for instrumentation," he adds. The primary engine instrumentation is in addition to the PFD’s attitude and heading indicators. Sikorsky’s objective with the AMS is to show as much flight information on one display as possible, "to reduce panel scanning and pilot workload," says Perkins.

Also, like the S-70, the EICAS includes vertical tapes to show main transmission oil pressure and temperature, gas generator speed and other engine instrumentation. "The tapes, arranged across the page, are ‘normalized’ such that the scaling of the tapes is all on the same height when all parameters are normal," says Perkins. "Anything that is out of limits will become immediately apparent to the pilot."

Instead of a dedicated warning, caution and advisory panel, the S-92’s AMS uses the EICAS display to alert pilots. Also, if the pilot is viewing the nav display, a caution or advisory message will pop up in a dedicated box, alleviating the need to maintain selection of a full-time EICAS display page. The messages are color-coded. For example, a caution would appear as black lettering in a yellow box. Once the pilot acknowledges the message, the box reverses, with yellow lettering in a black box. Advisory messages, which do not require crew acknowledgment, follow the same routine, except with white and black. Warning indicators, along with a master caution light, are located on the glareshield, within the pilot primary field of view. Cautions and warnings also are provided to the pilots in audio form.

The S-92 also comes standard with Collins ProLine 4 radios:

• Two VIR-432 VOR/ILS/marker beacons,

• One ADF-462 automatic direction finder (second optional),

• One DME-442 distance measuring equipment (second optional),

• Two VHF-422 communications radios (HF-9000 optional), and

• Dual RTU-4200 radio tuning units.

S-92 customers also can get the Honeywell Mk-22 enhanced ground proximity warning system (EGPWS), weather radar, and Universal Avionics flight management system with embedded GPS navigation. A Collins ProLine 21 radio suite is planned as the next-generation S-92 upgrade.

Sikorsky also is developing a moving map display, to appear typically in the fifth, center display. "The map display will be available later this year," says Perkins.

To further reduce pilot workload, the S-92 automatic flight control system contains an embedded, coupled flight director function in both the conventional and FBW variants. The pilot can select the heading mode on the flight director, then couple the autopilot to the flight director commands, and the autopilot will automatically fly the helicopter to the selected heading. Other approach and navigation modes–using sources such as ILS, VOR and GPS–are coupled to the autopilot, allowing hands-off terminal, approach and en-route navigation capability.

The S-92 has a health and usage monitoring system (HUMS) to check aircraft engines’ and transmissions’ critical parameters. Perkins says Sikorsky would like to include a communications downlink that would automatically transmit maintenance data to support facilities for improved, faster repair.