The AN/APN-171(V) radar altimeter system consists of a remote RT unit, two antennas, and one or more panel mounted indicators. This system dates from about 1975 and is primarily used in turbine engine aircraft with 115 VAC 400 cycle power systems. Later versions have been upgraded by replacing the vacuum tube based receiver and transmitter modules with solid state circuitry.
The most common version of AN/APN-171(V) has 0-5000 foot range and is typically used in fixed wing aircraft. For helicopters there is a 0-1000 foot version. Several different panel indicators are available with different expanded scale layouts. In this system, the scaling takes place in the indicator, and many different indicator designs will work with the same RT unit.
Additional optional capabilities can be added by installing one or two circuit cards in available slots. These optional circuit cards contain the necessary interface for autopilot systems and other auxiliary functions. A different “dash number” suffix of the part number indicates which cards are installed.
Since these units are quite old, deteriorated components, corrosion, loose connectors, vibration damage, and prior rough maintenance is the usual cause of failure. Typically there are multiple failures or intermittent conditions. During repair, MBT Electronics inspects, cleans, and performs preventative repairs on all modules. The condition of cavity tubes is determined by testing. Tubes are replaced with serviceable parts as necessary. The result is a reliable unit.