Is it airframe or is it avionics?

March 1, 1999


By Jim Sparks

March 1999

Top: Electronic Indicating Engine Display Bottom: Tester plugged into airframe test receptacle to allow fault diagnosis of airframe computers.

Back in the good old days, it was an easy task to decide whether an airframe technician or someone from the avionics shop was needed to solve a reported discrepancy. Typically the avionics group would handle problems related to radios, navigation, attitude heading sensing equipment, instruments, and autopilot. Everything else was the responsibility of the airframe or engine shop. Of course, there are those in the industry who do not call a specialist and must still contend with all reported discrepancies no matter what system is involved.

A good general knowledge of all the systems in the aircraft will usually give the technician the capability of deciding if they possess the qualifications to solve the problem and return the aircraft to an airworthy condition. Unfortunately, many believe "You can't teach an old dog new tricks," meaning airframe or powerplant technicians can't handle an avionics problem.

Aircraft that have been designed and built for the next millennium find that data received from various avionics systems can also be of great benefit to many airframe systems. The days when vertical gyros and directional gyros were considered state of the art have long since passed. Even though these devices are still widely used throughout the aviation industry, they cannot compete with the high reliability and compact size of today's laser gyro packages. Even the connection from the gyro to its primary display such as an Attitude Directional Indicator (ADI) or Heading Select Indicator (HSI) is most frequently an analog, three-phase syncro signal. With newer equipment, the digital language is spoken, allowing more components to benefit from the processed data. Plus, the flight deck display has changed greatly from an electromechanical unit subject to various mechanical malfunctions to an Electronic Flight Instrument (EFIS) with very few internal mechanical mechanisms. In addition to providing information to cockpit displays, these Attitude Heading Reference systems also supply information to auto flight systems including autopilot and flight guidance systems. Inertial Reference Units (IRU) are generally associated with flight deck. Inertial systems are based on sensing movement, in fact, Mr. Newton's Laws have a very important bearing. The laws of motion include:

1) A body at rest tends to stay at rest
2) A body in motion tends to stay in motion
3) For every action there is an equal but opposite reaction.

Motion can be measured in units of acceleration or Gravitational Forces called "Gs." A sustained One "G" acceleration requires an object to move 32 feet in the first second, 64 feet in the second second, and so on. Therefore, a one "G" acceleration is equal to 32 feet per second per second.

By using laser gyros and accelerometers, the acceleration about all axes of flight can be closely monitored by an Inertial Reference Unit (IRU).

Aircraft wheel braking systems have often included a system that will sense when one or more of the wheels equipped with brakes are decelerating too rapidly, which could mean a "skid" would occur. Such devices need to know the velocity of all wheels with brakes installed, as well as when a skid will occur during deceleration. Frequently an anti-skid or brake system computer will monitor an electrical signal produced by a wheel speed transducer or generator. In many cases, the velocity of the fastest wheel is used for the reference and all other wheels are judged on their speed compared to the fast one.

Should one wheel speed drop to a specific threshold value (maybe 80 percent of the fastest wheel), a brake release order is given by the brake system computer and the slow wheel should then speed up to the reference RPM. Many systems of this type have a pre-programmed deceleration slope within the computer.

Fuel computer receives data from IRS to correct fuel quantity and acts as an analog to digital converter for engine inputs.

Although effective, a more responsive braking effort could be delivered by a system that could observe the actual deceleration rate of the aircraft. This is a job for the Inertial Reference Unit. As the aircraft begins to slow, an Inertial Reference Systems (IRS) signal is delivered to the braking computers which can maximize the deceleration schedule by increasing braking. Many aircraft with a conventional anti-skid system have a maximum braking schedule of about .3 to .4 Gs. Aircraft that have an accurate aircraft deceleration value like those using IRS information, often provide a system that is considerably more effective and often have deceleration rates around .6 Gs.

This technology brings new challenges to service centers that support these "new" aircraft. In the past, in the event of a problem with a braking system, an airframe technician with some hydraulic knowledge was dispatched and the problem was usually corrected in short order. Many aircraft incorporating some type of anti-skid system may include a computer which receives electrical inputs from wheel speed sensors. This may require an electrician to be assigned to work with the hydraulic specialist and between them, a solution could be found.

In the new generation of aviation technology, the aircraft deceleration reference, instead of being a given pre-programmed reference, can be a variable supplied to a wheel braking computer by means of a digital data bus with information originating from an Inertial Reference Unit (IRU). Brake system computers also might receive an electronic input signal from a Linear Variable Differential Transducer (LVDT), sensing brake pedal position.

So, does this mean that in addition to a hydraulic specialist and an electrician, an avionics technician might be needed to resolve brake system problems?

Brake by wire technology is not really new but it is becoming more commonplace. For one reason, electronic systems are very reliable. Secondly, wires carrying electrons are usually much lighter than hydraulic lines carrying fluid. The device that converts brake pedal deflection into an electrical signal is also lighter in weight than its hydromechanical counterpart, plus it is much less likely to leak after several years in service.

Inertial Reference Systems (IRS), in addition to providing position information to the flight crew and navigation systems, are also used in some new generation aircraft to provide a higher degree of accuracy to Fuel Gauging Systems. Frequently, today's aircraft use Capacitance Fuel Quantity systems with a high degree of accuracy — often within five percent. However, during certain maneuvers, fuel loads may shift forward or aft. This will result in a change of level detected by the probes that have been strategically located throughout the fuel tanks. By correcting the indicated fuel quantity with information on aircraft movement, an even higher accuracy can be expected. These fuel quantity systems may also be used as a reference when using a single-point refueling system.

Unfortunately, if the aircraft is parked on a ramp that is not level, errors can occur in aircraft fuel loads. In a system where the IRS is used to correct fuel quantity, refueling errors due to ramp slopes can be significantly reduced.

Air Data Computers (ADC) are another area where airframe capabilities are tremendous. Pitot and static information have been used since the beginning of aviation and are most frequently thought of for supplying the Airspeed Indicator, Altimeter, and Vertical Speed Indicator (VSI). In the past, aneroid switches were used to sense pitot or static pressure and at a specific value would complete or open some electrical circuit. It was common to find these devices in systems such as landing gear warning where at a specific airspeed, a warning horn would be triggered if the power levers were pulled too far back and the landing gear was not locked down.

Other uses might include high lift systems such as a flap airspeed monitor or even a device to disarm stall systems at high speed. Many aircraft that utilize hydraulic power to operate flight controls will limit control range of movement or adjust the force the pilot must overcome when deflecting these controls at higher speeds. This is necessary to reduce the over control tendency.

The majority of the systems that accomplish hydraulic flight control regulation are concerned with airspeed or altitude or both. Rather than having pneumatic plumbing routed throughout the aircraft to all the various components, an air data computer can provide an electric or even a digital signal through a pair of wires to all concerned deflection limiting or feel force adjusting devices.

Engines, once the domain of the powerplant mechanic who could provide a "tweak" here and there to give optimum performance, are even using computer technology. Often with state of the art equipment, an engine computer might receive an electronic communication from an air data computer advising of temperature and pressure altitude. It can even be a controlling force with an auto-throttle system set to maintain a specific airspeed while the aircraft is maintaining altitude.

Systems are even available on high-speed turbine aircraft to automatically adjust engine thrust to maintain a constant Mach speed. These same turbine engines may also have their bleed air extraction controlled by computer. An air data computer can advise a bleed air system computer of such things as altitude, which enables the bleed computer to draw only the engine air that is needed by the environmental systems, and possibly anti-ice. By minimizing the air extraction from the powerplants, it is not necessary to run the engine as fast, which can mean a reduction in fuel burn. Even environmental and air conditioning systems will use air data computers to control heat exchanger operation and air blending.

Aircraft pressurization is now frequently regulated with information from an air data computer. From an operating perspective, having a pressurization computer, that is always "talking" with an air data computer, will require the flight crew to simply inform the pressurization computer of the elevation of the destination airport. Then, by sensing rate of climb/descent as well as present aircraft altitude, the pressurization computer can make very smooth transitions from pressurized to depressurized.

Commercial, as well as Business Aviation, have a vested interest in passenger comfort. Stereo systems as well as video cassette recorders have long been useful in providing in-flight entertainment. But, entertainment is now only part of what airline and executive aircraft passengers expect. In today's fast-paced environment, communication is a requirement. This requires elaborate telephone systems be installed. Of course, when telephone communication is possible from an aircraft, then the use of fax machines in flight is also a reality. Computer ports at each executive seat are also now commonplace, making today's business aircraft a virtual office in the sky complete with E-Mail access.

Airframe computers installed in a Falcon 2000. Top forward: brake computer; bottom forward: fuel computer; and top middle: environmental central computer.

Up to date Stock Market reports and current business news can be viewed on the video screens in the cabin as well as weather updates for the crew. This is accomplished by using dedicated radio systems. In addition, the passengers can observe aircraft position as it flies over the ground. This video display is often fed from Flight Management Systems, which determine position based on information from the aircraft navigation systems. Frequently, these position displays are complimented with air speed and altitude information — another job for the air data computer.

All of the interaction between avionics and airframe systems may in the not too distant future make distinguishing an Aircraft Technician from an Avionics Technician a very difficult task.

In fact, in this new age of aviation technology, computer skills are as important as mechanical skills. It is now as important to be able to operate a laptop computer as it is to operate a pair of safety wire pliers. A significant amount of diagnosing discrepancies is now being accomplished by attaching a laptop computer to airframe test connectors. Faults can then be downloaded and evaluated and if a problem had been encountered in flight, this means of diagnostics will usually point to the culprit.

To be an effective technician with new technology aircraft does require a willingness to learn new ways. Old dogs that are willing to learn new tricks generally receive more rewards.