Fly-by-Wire Control Testing

With its weight savings, reliability and potential to reduce flight crew workload, digital fly-by-wire control systems are entering more and more new aircraft. Dassault soon will begin delivering the first fly-by-wire (FBW) business jet, the Falcon 7X. Boeing and Airbus both have introduced air transport aircraft with FBW controls and plan to install fly-by-wire in the B787 and the A380, respectively. And military aircraft, from the A400M utility aircraft to the F-35 Joint Strike Fighter, also forego pulleys and cables for electronic signals and processors to control flight.

On Board Testing

Maintenance testing for FBW takes place both on board the aircraft and outside the aircraft on a bench. FBW systems are at least triply redundant (the Falcon 7X is quadruple redundant), and that includes their built-in test equipment (BITE). Generally, within each channel, or primary flight control processor (PFCP), there exist redundant computer lanes, each with microprocessors that monitor each other. The microprocessors are crosslinked to "talk" to each other; should the data from one microprocessor conflict with those of the other two, it will be thrown out.

Each electronic FBW component, located on a shelf in the electronics equipment bay, has its own built-in test capability. On the B777, according to Boeing engineers, the PFCP is monitored continuously throughout the flight. Then, in addition, a comprehensive self-test is automatically executed at the end of the flight.

Describing the onboard FBW fault detection and isolation in the Falcon 7X, Gerry Goguen, senior vice president for customer support, says, "The [control] actuators are heavily instrumented with sensors, which are tied to the [flight control] computers, so the [BITE] system knows the [control surface's] tension and movement along with all the physical properties, and it stores that information for analysis."

Should a failure occur that requires flight crew alert, the PFCP would instantaneously send a message to the aircraft's central warning system, or display function, for transmission to the engine indication and crew alerting system (EICAS). Simultaneously, the processor would feed data to the central maintenance computer (Airbus calls it the central fault display system), where it is stored in non-volatile memory to facilitate troubleshooting and component replacement, if needed. On the B777 the on-board central maintenance computer (CMC) and the primary flight computer share the task of consolidating fault information and tagging that information with diagnostic data.

The information provided to the flight crew can vary, depending on the aircraft. Dassault FalconJet, for example, has divided the seriousness of a failure into three levels and provides the pilots with data relating to just two levels--in other words, no more information than is needed for continued safe operation. "If [the failure] is actionable, the system provides what the issue is and gives the recommendations of what to do," says Goguen.

Airbus, which has been building fly-by-wire aircraft since introducing the A320 in 1988, takes a slightly different approach. In the case of a FBW component failure, the flight crew "will get an emergency check list on the screen [MFD]. It provides just the information you need for correction," says Robin Wohnsigl, president and chief operating officer of Airbus North America Customer Services. "But the pilot can enter into the menu and pull up the detailed information, too."

The CMC also can "tag" failure messages with data from other sensors to provide maintenance technicians a more complete picture of an event, including the time of failure, intermittency, flight phase, etc. Should an even more complete picture be needed, the operator can tap flight data recorder data with a quick access recorder. This time-consuming process rarely is required and negates one primary benefit of built-in testing, which is to quickly repair and turn around the aircraft.

In fact, for the fastest turnaround and to give maintenance technicians optimum advanced notice, operators of aircraft equipped with the Airborne Addressing and Reporting System (ACARS) data link often have on-board diagnostic data transmitted automatically and instantaneously from the CMC to the ground while the aircraft is in flight. Many operators, however, select data transfer time and have it done near the end of the flight.

Test and monitoring information for a FBW system are gathered and distributed usually over an ARINC 429 bus on airliners (ARINC 629 bus on the B777). Some still use an analog bus. Future aircraft, such as B787 and A380 will include variants of an Ethernet bus.

The robust on-board diagnostics and testing in FBW aircraft produces reliability that allows components to be replaced on-condition and not according to a designated life cycle. Most, if not all, electronic line replaceable units (LRUs) have a 30-minute remove and replace requirement to allow quick dispatch, according to a BAE Systems engineer.

BITE and the CMC also preclude the use of extra test equipment to troubleshoot the FBW system. Technicians use the maintenance access terminal on the aircraft to access data from the CMC and to ground-test subsystems and LRUs (using the CMC). The data can be decoded for read out and further analysis, according to officials with Lufthansa Technik, a FBW test and repair provider.

On Ground Testing

When failures do occur, FBW components must be removed and bench tested. A failed actuator would need to be tested for leakage, slew rates, pressurization, force and accuracy. Other components that may need testing are electronic LRUs (black boxes), pumps (for hydraulic pressure), valves, pressure switches, proximity switches, temperature sensors and linear variable or rotary variable voltage differential transducers (LVDTs and RVDTs). Tests of these components--some of which are comparable to those on a mechanically controlled aircraft--are described in the component maintenance manual (CMM). All testing must meet test specifications spelled out in the CMM.

What is unique to FBW is testing the primary flight control processor. The A320, the first FBW airliner, has seven PFCPs: two elevator/aileron computers, three spoiler/elevator computers and two flight augmentation computers. The premise applied to PFCP testing may be quite conventional: simulate the inputs and measure the outputs. But the process is a bit more complex, requiring an automated test equipment (ATE) stand along with appropriate adapters and software for each unit. With the software, usually written in the ATLAS (Abbreviated Test Language for All Systems), the ATE will simulate digital signals, then measure their value and compare them to specifications in the CMM. The software does much more during a test, however, confirming the correct operation of a processor's internal components.

"For a flight control computer, there are about 4,000 to 5,000 individual test results," says a Lufthansa Technik engineer. "Test on the automated test equipment takes about two to four hours."

The ATE examines all of the primary flight control processor's inner workings, including power control, memory, processing, internal monitoring functions, input/output circuitry and connector filters, for high-frequency and lightning strike protection. This is done down to the card level.

Further, automated testing of the errant card can be performed, though it is done almost exclusively by the original equipment manufacturer (OEM). One of the few exceptions is Lufthansa Technik. "We also can repair down to the component level," says an engineer with the company. "The defective component is isolated with the use of a [oscillo]scope and ohm meter." Lufthansa Technik is somewhat of an exception; the OEM, according to a Boeing official, performs most component level maintenance.

Various ATE manufacturers can supply test stands for FBW systems and other avionics. The manufacturer must receive the test specifications in plain English for development of the test software. The OEM provides technician training to properly use the test stand.

"All electronic boxes can be tested on one ATE," says the Lufthansa Technik engineer. "We have an ATEC 6000, a two-cabinet test stand that has the power sources, generators for analog and digital signals, measuring devices, such as voltmeter, ohm meter, timer and digital signals, inside."

Maintenance centers, such as Lufthansa Technik, have quite complete FBW test capability; others are evaluating whether to expand their in-house testing or outsource. For example, American Airlines is considering in-house testing of FBW electronic LRUs now that its B777s are emerging from warranty, according to an official with the carrier. The norm, say industry officials, is to remove failed equipment and have the original equipment manufacturer perform test and repair.

Dassault FalconJet believes the reliability of its Falcon 7X will preclude the ability of an outside maintenance center to test FBW components profitably. "You would need individual test systems for the servos, for the computers, for the cards and for the actuators," says Goguen. "It would be very expensive for someone other than FalconJet to set that up."

For Falcon 7X operators, the manufacturer plans to establish just two centers for FBW test and repair, one in Little Rock, Ark., and one in France. Goguen sees no need for additional service. "In fact, we expect our [test] technicians to be like the lonely Maytag repairman."