Not just for big aircraft
In light airplanes, panel space, weight, and especially cost are major factors in the decision to install new avionics equipment. Often, when a useful piece of avionics is being considered by a general aviation aircraft owner or operator, one of the criteria cited above all but eliminates its chances of being installed.
Traditionally, radar altimeters (or radio altimeters as they are sometimes called) have seen limited use in light single-engine aircraft. Many owners and operators figure that the money could go to "more important things" such as engine reserves or a new paint job. Competition for panel space may also come from a weather radar package, a lightning-detection unit (Strike Finder or Stormscope), or that fancy new GPS everyone's talking about. Truth is, on a day-to-day basis, the radar altimeter is probably less useful to most pilots than is a GPS or a Stormscope. However, for pilots who often fly IFR, a radar altimeter can provide useful information and critical reminders during approach and low-level operations.
By bouncing a radio signal down at the ground and timing the signal's return, radar altimeters measure the airplane's absolute height above ground level. Like airborne weather radar, the radar altimeter works best when bouncing off water (listen up, seaplane drivers) but loses effectiveness over snow; plowed fields; and, particularly, dry sand.
In the past, radar altimeters were reserved for high-dollar corporate or charter aircraft, and many units' prices reflected that belief. About 20 years ago, however, several manufacturers saw a thriving general aviation market and offered boxes that were compact in size and weight, and priced within the grasp of many operators and individual owners. Today, since the market is exponentially less thriving, there are only two manufacturers targeting general aviation with their radar altimeters: Terra and AlliedSignal.
Why would anyone want a radar altimeter? For starters, if you fly a lot under IFR, the more information you have on terrain clearance, the better. For example, as you're collecting ice in the soup on the final leg of an approach to a valley airport surrounded by jagged mountains, an additional reminder of reaching the decision height would certainly be welcome.
Today's airliners utilize radar altimeters and autoland-capable autopilots to qualify for Category IIIb approaches in visibilities as low as a runway visual range (RVR) of about 150 feet. Airliners must have at least two operable radar altimeters to shoot the approach legally. The altimeters measure the height of the main wheels above touchdown. Airliners and high-end corporate jets utilize radar altimeters for their ground proximity warning systems (GPWS), which give a "pull up" warning at a predetermined altitude and closure rate. AlliedSignal is currently exploring the feasibility of a simplified terrain warning system for general aviation airplanes.
For now, about the best the average general aviation aircraft can get is Category II ILS approval, by jumping through some hurdles at the local FAA flight standards district office. For Cat II, where minimums get down to 100 feet agl and an RVR of 1,200 feet, the radar altimeter must be accurate to within five feet or five percent of true altitude above ground, whichever is greater. It must maintain that accuracy, given variations of the aircraft's pitch, speed, and sink rate.
Perhaps the best application for a radar altimeter is in helicopters that spend the majority of their time at low altitudes. In remote areas at night, the radar altimeter can assure terrain clearance until the helicopter's landing lights or floodlights provide a visual picture. One medevac pilot we spoke to likes to set the RA's decision height at 100 feet. This gives him audible and visual indications that he can shift his eyes toward the ground and look for a suitable landing spot. In normal or VFR operations he also sets the RA at 100 feet as a "reality checkpoint," the point at which all landing checklist items should be completed.
The relatively low-cost general aviation radar altimeters are AlliedSignal's Bendix/King KRA 10A and Terra's TRA 3000 and 3500. The Bendix/King unit has only an analog display head (KI 250), while Terra's TRA 3000 has the choice of three display heads — the TRI 20 (digital), the TRI 30 (analog), and TRI 40 (digital). The Bendix/King unit (using a converter) and Terra's TRA 3000 with the TRI 20 head are compatible with 14-volt systems, while the rest of the Terra indicators are for 28-volt systems. The TRA 3000 can interface with any of Terra's indicators, but the TRA 3500 must be used with the TRI 40 head.
There's not much to operating a radar altimeter, and that's a good thing — it does its job without making the pilot think about it. Before takeoff, the system can be tested to ensure that all of the lights, chimes, needles, or digits are working correctly. The Bendix/King KRA 10A and all but the top-of-the-line Terra units accurately depict altitudes between about 50 feet and 2,500 feet agl. Units employing only one antenna generally cannot accurately give altitude information below about 50 feet. Dual-antenna units, like Terra's TRA 3500, can function all the way to the ground. In single-antenna systems, because of the extremely short duration of the signal's return, the RA needles or numerals may jump around when the aircraft is on the ground. Terra's two-antenna TRA 3500 adds more than $2,000 to the price of its single-antenna system.
Once airborne, the radar altimeter should behave normally after entering its operation window (40 to 50 feet agl), with altitude indications corresponding to terrain features such as hills and valleys. Steep turns can cause loss of signal because the antenna(s), generally mounted on the aircraft's belly, will no longer have any ground for signal reflection. After the aircraft passes through 2,500 feet, the accuracy begins to fall off and the needle displays (TRI 30, KRA 10A) should park themselves. The display of Terra's digital TRI 20 will be blank if it unlocks (loses ground reception), and the digital TRI 40 will display a "U" if it unlocks. If such indications become distracting, the unit can simply be turned off while en route. Since Terra's TRI 20 and TRI 40 indicators are digital, they probably won't be as distracting as the analog TRI 30's and KRA 10A's swinging needles. Linking the RA's power source to the airplane's strut squat switch can be a handy feature. This will turn the unit on as soon as the wheels leave the ground on takeoff and turn it off upon landing.
In setting up for an approach, the RA requires only that you set the decision height agl of that approach, whether the DH will be for straight-in or circling minima or a preselected altitude of your choice. Let's say that you've never flown a circling approach to a runway to which you've flown hundreds of straight-ins. The radar altimeter will remind you where to stop the descent, in effect breaking the temptation to continue to the straight-in minimums.
The RA works well as an altitude checker and alerter during the approach. At the outer marker, cross-checking the RA's depiction with the aircraft's altimeter and the chart's published glideslope intercept altitude is extra assurance that the terrain is cleared and that on-board equipment is functioning properly.
You can run into problems with radar altimeters when airports are located on a hilltop. As shown in the accompanying diagram, the RA can lead you astray unless you're familiar with the airport's surroundings. It is imperative in cases like this to use the standard altimeter's information first and use the RA as a backup.
If used properly, the RA shines in mountainous areas as well as in flat terrain. Especially for a newcomer to the mountains, where standard altimeters are reading high numbers (usually associated with plenty of terrain clearance), the RA brings a sense of actual height agl. The needle or digits will probably be active as the radar signal bounces off the peaks and valleys; but when the peaks reach up for your airplane, the information is much appreciated, especially during night or IFR operations. In the flat regions, the RA offers accurate and solid indications at nearly all times within its operating window.
Seaplane pilots may also find the RA a useful tool during glassy-water or night operations where height above touchdown is nearly impossible to determine. In these cases, a dual-antenna unit like the TRA 3500, which can read all the way to the surface, would be of great help. Flying boat pilots will appreciate the fact that the antennas for this unit can be mounted on the underside of the wings.
When the preselected altitude is reached on an approach, the RA indicator will illuminate the DH lamp and sound a chime indicating that the airplane is at the selected height. If your aircraft has a flight director, multifunction display, or a flight management system with a DH lamp or radar altitude readout, the altitude information can be displayed there for a more heads-up indication. Terra's TRI 40 indicator has audible and visual landing-gear reminders that alert the pilot to a forgotten (but important) item on the checklist. If this feature saves a pilot from bellying in just once, the unit will have paid for itself several times.
Although it may be tempting, the radar altimeter does not give you an invitation to continue the approach to less than Category I minimums without approval. The RA is strictly an awareness device and a reassuring backup to the standard aircraft altimeter. If you decide to pursue Cat II approval, a radar altimeter will be required for a descent below 150 feet.
The radar altimeter will add little weight to the airplane — less than five pounds installed — but enough panel acreage should be available to accept a 3.25-inch round analog indicator or the slimmer, rectangular Terra TRI 20 and TRI 40. The fuselage is the preferred location for the antenna, although Terra says wing installations are OK. Bill Needy of Hagerstown (Maryland) Aviation Services prefers not to mount antennas on the wings, for fear of disturbing the shape of the airfoil. "Some local FSDOs might have a problem with that," he said.
The fuselage area should be well prepped to prevent corrosion, which can eventually cause erroneous readings, said Needy. He also cited bad grounding and interference as common bugaboos for radar altimeters. Unless your flight director or attitude indicator has its own DH light, it would be advisable to put the RA indicator in a spot within the usual instrument scan. There's no sense in having this source of information buried in a corner of the panel where it can't be seen during an approach.
With prices starting at $3,270 for the entry-level Terra TRA 3000 combined with the TRI 20 indicator, the radar altimeter is not inexpensive. The top-of-the-line Terra TRA 3500 and TRI 40 checks in at $5,920, while the King KRA 10A is $5,875. In looking at the cost of an ADF receiver, one might seriously consider which is more useful in the panel.
Overall, the radar altimeter provides useful information during the most critical phases of an IFR flight. It probably won't win panel acreage from a Stormscope/Strike Finder or a new GPS unit, but it can supplement the well-rounded panel of the serious IFR airplane.
For more information, contact Bendix/ King division of AlliedSignal General Aviation Avionics, 400 North Rogers Road, Olathe, Kansas 66062; 913/768-3000, fax 913/791-1302 or Terra Corporation, 3520 Pan Am Freeway N.E., Albuquerque, New Mexico 87107; 800/328-1995.
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