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Can the RT-300 be used as a direct replacement for the RT-220 Radar Altimeter?
The Sperry/Honeywell RT-300 can be used as a replacement for the RT-220 as long as Pin E in the RT-220 mating connector is jumpered to Pin N. Of course, the mounting trays would need to be changed to compensate for the size differences between the two units.
My radio altimeter is operating erratically, what could be the problem?
Dirty antennas can sometimes cause poor or erratic operation of the radio altimeter system. Especially during winter months, some aircraft's nose wheel's throw dirt and slush onto the antenna. As a preventative measure to keep the antennas clean, a quality aircraft wax can be applied to the antenna.
My radio altimeter indicator is locked up between 5-50 feet, what could be the problem?
Display lockup is a commonly encountered problem where the indicator locks up at a low indicated altitude between 5-50 feet. Some pilots will report that the radio altimeter will work normally at low altitudes but after climbing above a certain altitude the indicator locks up. Once the aircraft descends past the locked altitude, the radio altimeter works normally again.
In almost all cases, the cause of lockup is excessive signal leakage between the receive and transmit antennas. Signal leakage is commonly caused by the following situations:
- Antenna reflections hitting an object below the fuselage such as landing gear, skids, or searchlights.
- Poor ground plane between the antennas and the airframe, antennas not mounted on the same piece of metal, or antennas mounted on composite material
My radio altimeter indicator is indicating between 0-2500 feet at altitude between 18000-20000 feet, what could be the problem?
Sometimes a pilot may report that the radio altimeter may start indicating altitudes in the rad alt range (0-2500 ft.) at altitudes between 18000-20000 feet. This frequently occurs when flying over a reflective surface such as calm water. What occurs with second time around acquisition is that the receiver detects a return signal that was transmitted two pulses earlier not the transmitted signal it should be receiving. The time between transmitted pulses is equal to a distance in the 18000-20000 ft. range. Therefore, the receiver may process it as though it was signal it was actually trying to receive and then display the incorrect altitude.
The needle on the radio altimeter is jumping by hundreds of feet, what could be the problem?
Large needle fluctuations is a problem commonly seen when a helicopter is hovering over a soft, diffused surface such as grass. Needle jumps are usually less than 50 feet but can range from hundreds to a thousand feet. Once the helicopter moves forward, the radio altimeter should operate normally again. This needle jumping occurs because the radio altimeter is experiencing decreases in received signal strength. Over a soft, diffused surface, the signal reflection consists of hundreds of small weak reflections with different time and phase, directional qualities. These differences cause cancellations in the total return signal and cause the needle to jump.
To correct this needle jumping, some radio altimeters have an extended dB STC range. This extended range provides an additional receiver gain that can reduce the decreases in signal strength. However, the additional dB gain increase can subsequently increase the potential for display lockup caused by poor isolation between the antennas.