Radio Altitude: The instrument of choice

Automatic flight control systems may also take advantage of information provided by radio altitude. The gain or sensitivity of the auto pilot is often compensated as a result of the aircraft descending to lower altitudes and during approach to landing. With landings requiring slow flight conditions flight controls often require an increased deflection as well as responsiveness. In many installations the radio altimeter RT unit has discrete altitude presets. This data can be supplied to the auto flight computer to accomplish the flight control biasing. Should this bias not occur the result may be the auto pilot's inability to steer an accurate approach to the runway but all other auto flight functions work as they should.

Testing and operation
Avionics should be tested, ideally, as close to a flying or operational configuration as possible. This means the aircraft is on the ground, with engines and all systems operating. It is not unusual for systems to interfere with each other. Radio transmitters, for example, are a common source of interference, as are electric motors, ignition, and poor electrical grounds. Transmitter radiation is often so great that it is difficult to eliminate.

In the case of operating engines, which supply electrical power buses, there is an engine-driven generator or alternator. Output can vary significantly with engine rpm. A ground power supply, on the other hand, provides a steady, clean power and may not expose problems. An area of significant difference is the radio signal environment. Aircraft in flight are bombarded with radio signals that can cause problems. An aircraft on the ground, however, is out of range of most signals and a ramp test may not exhibit the problem.

Major items that cause problems are the interconnecting wiring, connectors, and LRU trays or mounts. Antennas and coax cables are also candidates for a ramp test. It is important to evaluate these items because antennas mounted on the outside of aircraft are subject to environmental damage. Because of problems with interconnecting wiring and antennas, it is imperative that a ramp check be performed on newly installed systems. Thorough ramp tests reduce the need for flight tests, which are expensive.

Most testers for radio altimeters plug into the RT unit and will provide various altitude simulations. This will allow most of the flight deck displays to be verified plus various interfaced systems such as auto flight can be checked. This type of test does not check the integrity of the antennas or the coax transmission lines. Unfortunately these are the areas where problems often arise. Bonding issues on either the transmit or receive antenna can result in deterioration of the system. In fact paint applied to the antenna surface will often block the transmitted electrical energy as well as cause degraded reception. In some systems the transmit and receive antennas have to be positioned in a specific situation relative to one another in the aircraft. System manufacturer installation instructions should always be checked to verify proper position. Degradation of the coax cables can result in similar conditions. A frequent occurrence with coax cables is overtightening of cable clamps. This causes a change in the di-electric properties and will result in signal loss.

Troubleshooting the radio altitude system as with many avionics systems begins in the flight deck. Verifying proper operation of all, some, or none of the systems affected by radio altitude including visual flight queues such as EGPWS, digital altitude readout below 2,500 feet, decision height, and the rising runway not to mention auto flight interface will provide the line technician with significant information in solving problems regarding system operation.

New regulations that make EGPWS a requirement in certain types of aircraft will elevate radio altitude systems to a new position of prominence. With interfacing of radio altitude with airframe systems, the fun associated with troubleshooting these devices may soon be removed from the avionics technician and placed on the shoulders of the A&P.