Cessna warms to fly-by-wire for Columbus, but system is a hybrid

Aviation International News » April 2008
March 25, 2008, 5:52 AM

Whenever a manufacturer develops a new airplane, engineers have the opportunity to incorporate new technology into the design. With the large-cabin Columbus, Cessna engineers didn’t opt for a composite airframe or an all-electric systems architecture, but they have chosen an innovative approach to fly-by-wire flight controls. The Columbus system is Cessna’s first application of fly-by-wire flight controls, and the design of the system has features that are unique enough that the company has applied for patents on the system architecture.

In “traditional” fly-by-wire systems, cockpit controls–generally sidesticks–send signals to computers and then to actuators that move the flight control surfaces. The actuators are normally hydraulic, but this makes for heavier systems installations because of the need for redundant hydraulics.

Of course, fly-by-wire systems, so vitally dependent on a reliable source of electricity, also have stringent requirements for backup electricity generation and storage (batteries) as well as redundant computer systems. The worst-case scenario for a traditional fly-by-wire system is complete loss of all electrical generation and storage capability, which likely means loss of the aircraft.

Reliable Fly-by-Wire

Known to be conservative when it comes to adoption of new technology, Cessna has figured out a way to provide the benefits of fly-by-wire flight controls while retaining the ultra-reliable cable-operated flight control technology that has served aviation well for more than 100 years.

The Columbus flight control system is a “hybrid fly-by-wire” system. Hybrid means that part of each primary flight control is operated by the fly-by-wire system and another part by cables. This design delivers two major benefits: the performance and safety improvements offered by fly-by-wire and a simpler, less electrically and electronically complex backup system.

Cessna’s hybrid fly-by-wire system weighs significantly less than either a traditional fly-by-wire system or a completely mechanical flight control system. All Columbus primary flight controls–ailerons, elevators, rudder–are moved by hydraulic actuators. Five spoiler/speed-brake panels on the wings are also hydraulically actuated and controlled by the fly-by-wire system. The Columbus has two hydraulic systems instead of the three normally needed by a full fly-by-wire jet. The fly-by-wire signals drive one set of actuators, and the cables– attached to a yoke and not a sidestick–drive another set. Each flight control is split into two, so that one hydraulic actuator moves one part of, say, the aileron and the other actuator moves the other part.

In normal flying both flight control segments move, but each is large enough for safe control of the airplane. If a cable breaks, for example, the remaining fly-by-wire-controlled surface is sufficient to control the airplane. Each aileron is split
into two unequal sections; the fly-by-wire section is one-third smaller than the cable-controlled section, and the outboard aileron panel is the larger section. The elevators and two-panel rudder are split evenly.

In normal flying, both sets of flight control panels “work together to provide tailored roll or pitch performance,” said Rolf Anderson, a supervisor on the Columbus flight control system.

A side benefit of fly-by-wire systems is that engineers can program the computer to make the flight controls perform functions that normally require extra hardware. Autopilots in aircraft with mechanically controlled flight controls require a servo, attached to the control cables, with at least one for each axis. Trim tabs and separate cockpit controls and cables to drive them are another example, as are yaw- and roll-damping stability augmentation mechanisms. But fly-by-wire computers can mimic those functions, thus saving more weight and complexity.

Cessna engineers have been working on the system for a number of years, according to Anderson, including extensive ground testing and flight tests last year on a Citation Sovereign. To his knowledge, no other aircraft manufacturer has developed a similar hybrid fly-by-wire system. “We’ve designed a system that gives you the benefits of a manual system for safety and uses the benefits of fly-by-wire for tailored performance. Either is adequate to control the airplane. Together they provide reliable superior performance.”

While the Columbus fly-by-wire system will offer some envelope protection features, Anderson said that Cessna is trying to keep the design as simple as possible and not overly complex with high-level functions. One feature that will be incorporated is diagnostic and rigging capabilities, something that is difficult to provide on a pure mechanical flight control system.

Cessna marketers and engineers spent about six years interviewing and polling potential Columbus customers, and Anderson said that the hybrid fly-by-wire system has been well received. “The main customer wish list,” he said, “is that our airplanes are reliable, easy to maintain and safe.”

Parker Aerospace’s Control Systems Division will help design and manufacture the primary and secondary flight control system.

Cessna is contemplating derivative versions of the Columbus, and the hybrid fly-by-wire system will be the control system of choice. The Columbus should receive FAA certification in 2013 and enter service in 2014. Configured for 10 passengers, the Columbus offers 4,000 nm range at Mach 0.80, Mmo of Mach 0.86 and a full-fuel payload of 1,950 pounds.

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