A test feature is included and will provide the flight crew with an indication regarding the operational status and in some cases the accuracy of the radio altimeter. This test will not check the operation of the two antennas or the condition of the transmission lines.
Flight deck displays of radio altitude are also varied. Some systems have a stand-alone instrument while in other cases height above ground level can be displayed on an attitude indicator or an electronic flight instrument.
GPWS and TAWS information
Radio altitude is not only an important flight deck stand alone, it is also a primary source of information for ground proximity warning systems (GPWS) or terrain awareness warning systems (TAWS).
The Class A TAWS/EGPWS is an excellent safeguard against Controlled Flight Into Terrain. This is accomplished by using available data from the radio altimeter, air data computer, and aircraft systems along with satellite navigation. By applying information from the flight management system and an additional terrain database there is the ability to supply the flight crew with a very reliable and realistic image of surrounding geographical conditions. This internal database will be used with selected flight plans and aircraft position to provide the pilot with a visual image of any obstacles in the aircraft flight path.
Class A TAWS is a sophisticated system, however the Class B version will be much simpler. It will not include a radio altimeter or an air data computer. Instead it will rely almost entirely on the external satellite navigation position. There is an acknowledged weakness in the Class B TAWS system and questions arise about its ability to provide adequate levels of protection. With a Class A system if an error occurs and the aircraft approaches terrain, the radio altimeter is still available to provide the system with the capability to advise the flight crew.
Instrument landing systems (ILS) are another area where radio altitude can play an important role. Instrument landings are classified based on ceiling height and visibility with Category I (Cat. I) requiring a minimum distance of 200 feet from the ground to the break out point of the lowest cloud layer plus a visibility enabling the pilot to see almost one half of a mile. Category II (Cat. II) has a ceiling reduced to 100 feet and requires around one quarter mile visual range and while Cat. III will allow an aircraft to land with a zero foot ceiling there are several different stipulations on visibility based on the type Cat. III landing performed. Radio altitude can play an important role in all of these conditions.
In the case of both Cat. I and Cat. II, the decision height (DH) (that is the point where the pilot has to have a visual reference of runway position) can be selected into the radio altimeter and an aural or visual indicator will occur at that specific point. In the event DH is reached and the pilot does not have visual contact with the runway, a go around should be executed. Decision height was often associated with the Middle Marker portion of the Marker Beacon system. In fact the regulations governing instrument landings state that either the marker beacon or radio altimeter can be used as the determination for a landing decision.
Another feature of radio altitude is the control of the rising runway that appears in conjunction with the ILS displays on many attitude indicators. In aircraft so equipped, whenever a navigation radio frequency falls into odd channels between 108 and 111.95 MHz, it will cause runway symbols to appear. This includes a vertical Glide slope pointer and a lateral moving runway symbol. The left and right movement of the runway is controlled by the localizer function of the ILS. This gives the pilot a reference as to aircraft position relative to runway centerline. Once the aircraft gets within 100 feet above the ground, the runway symbol will begin to rise. At the point where the aircraft main wheels just touch the ground the rising runway symbol will just contact the aircraft icon.
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