Is it airframe or is it avionics?

IS IT AIRFRAME OR IS IT AVIONICS ? 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...


By Jim Sparks

March 1999

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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.

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