FAA Repair Station #QYMR098B

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MBT Electronics offers repair and overhaul of the King KRA-10A and KRA-10 systems.

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The King KI-250 radar altimeter indicator.

The King KRA-10A radar altimeter system is designed for small general aviation aircraft. It radiates a microwave signal downward and measures the time required for the signal to reflect from the terrain and return to the aircraft. This time interval is converted to feet and displayed on an analog dial with a maximum range of 2500 feet. This continuous measurement of altitude above ground level is not dependent on pressure altitude settings or GPS database. A bug on the instrument can be set to alert the pilot when the aircraft descends below the set level. This is useful during approach to landing in IFR conditions. The DH warning illuminates a lamp on the instrument bezel and produces a tone in the aircraft audio system.

The microwave RT module
This module is often blamed for poor operation when the problem is elsewhere. Unlike more expensive two antenna designs, reflection of the transmitter power from an imperfect antenna is directed by the microwave circulator (round black object) directly to the receiver.

The radar altimeter system consists of three assemblies: RT (receiver-transmitter), antenna, and pilot’s indicator instrument. The RT and antenna are usually installed in the tail or belly of the aircraft. Except for the DH bug setting, the system requires no tuning or adjustment by the pilot. When the system does not receive a valid signal, because of poor signal reflection from the terrain, the system hides the indicator pointer behind a mask at the top end of the scale. This “stow” position is also used to indicate a height above terrain greater than 2500 feet. If the system loses power, the pointer mechanically springs back to the zero feet position on the indicator which is labeled “off”.

This type of radar altimeter cannot read below about 50 feet. When the aircraft is on the ground, the indicator readout is disabled by a signal from the landing gear switch. This prevents the confusing readouts, during taxi, when the system signal bounces off of nearby objects and intermittently indicates a distance. The earlier KRA-10 units do not have this feature.

Many low cost radar altimeters have been designed for general aviation and most have failed to live up to expectations. Compromises in design cause these low cost units to lack adequate performance in the aircraft. Intermittent operation, with the pointer flipping in and out of the stow position, annoys the pilot and causes loss of confidence in the equipment. Since there is nothing blocking the signal between the aircraft and the ground, the pilot blames the equipment and questions whether to trust it during actual IFR conditions. For this reason, many poor performing designs have become extinct in the active fleet.

The King KRA-10A system has performed well enough to remain in production for many years. Although it is a low cost unit, which means compromises in design, it has adequate performance to satisfy most aircraft operators. It works when installed and functioning properly but any loss of performance will put it below the usefulness threshold. The correctness of the installation and the quality of maintenance make the difference.

To keep the cost low, the King KRA-10A outputs only about 15 mW. This is the power that is directed toward to the ground and reflected back to the receiver. When the aircraft is above terrain with poor reflective properties, and the reflected signal is below the minimum necessary to activate the receiver, the indicator needle goes to the stow position. All radar altimeters suffer from this situation sometimes, but more power results in better reliability. As an example, a professional corporate level radar altimeter, the Collins ALT-50, transmits 200 mW of power; more than ten times greater than the King KRA-10A. Naturally, the professional unit can be expected to operate more reliably over adverse terrain. This is only one parameter of the system but it illustrates the situation where low cost dictates minimal margins of performance.

With minimal power available, the antenna and coax cable become important issues. The effective radiated power and sensitivity of the system (loop gain) is strongly influenced by the antenna system. If there is a loss of efficiency in the coax or antenna, it detracts from both the transmitted and received signals.

The King KA-131 antenna with its special coax cable
Opposite the coax connector is the tuning screw which fine tunes the antenna system match to the RT unit.

The simple RF section in the King KRA-10A uses only one antenna and depends on correct antenna matching for proper operation. In this system, the transmitter and receiver operate simultaneously using a single input /output port. The transmit signal normally flows from the RT unit to the antenna without interacting with the received signal flowing back to the RT unit. This depends on the antenna and coax being correctly matched to the transmitter. According to transmission line theory, with correct matching, all the transmitter energy is radiated from the antenna and no transmitter energy returns to the receiver except the received signal reflected from the terrain. This is what we want. A poor antenna or coax causes strong transmit power to reflect back to the receiver. The unwanted reflected transmitter power overloads and de-sensitizes the receiver and distorts the receiver response. The distorted output from the receiver presents a broken-up signal to the processor system resulting in inaccurate and unsteady indicator readout.

The King KRA-10A or KRA-10 RT unit
The single RF port carries both transmit and receive signals simultaneously resulting in critical antenna requirements.

King warns against altering or lengthening the special coax cable supplied with the antenna. The antenna and coax must be tested and adjusted for minimum reflected power as a unit. As explained above, excessive reflected power will reduce system performance. This test and adjustment is impossible to do in the aircraft. The reflected energy from the ground, under the aircraft, is indistinguishable from the possible reflected energy due to a bad antenna or coax. A special test set-up is necessary. Many aircraft operators live with less than optimum performance because the antenna system has deteriorated and no longer presents the correct match to the RT unit. A bench test of the RT unit alone will not reveal this situation.

A related issue is the gain and beam shape of the antenna. If there is corrosion or other damage to the antenna, the gain or beam shape may be compromised. The coax may also be bad due to age, loose connectors, or water intrusion. These conditions will reduce performance of the system due to attenuation and loss of gain. With any of these problems, the performance of the system will be poor.

The test procedure included in the King manual is written for facilities with minimal microwave test equipment. Although the antenna and coax are critical, there is no provision for a system test including the antenna. The microwave circuitry in the RT is treated as a replaceable component and not tested in detail. Using only the bench test procedure, it is easy to be fooled into replacing expensive components while following a process of elimination.

MBT Electronics has developed the capability to test and adjust the King KRA-10A system in detail including the antenna, coax, RT, and indicator. The components can be tested and operated together as a system on a special test setup. By testing and adjusting the system as a whole, the efficiency of the system can be assured and any abnormal conditions fixed. When a system with a poor antenna is operated on the test stand, abnormal test point readings will be seen within the RT. This is a sensitive test of antenna quality. The antenna system can be repaired or adjusted until the test point readings are normal.

The general procedure is to test, repair, and adjust the RT unit and indicator for correct operation. Then the components are installed on a test stand to verify the antenna and coax quality and impedance match. When the antenna is pointed skyward and no reflection from terrain is possible, any reflected energy reaching the receiver indicates a mismatched antenna. When the system antenna is pointed at the calibrated pickup antenna, the radiated power and antenna beam characteristics can be checked.

MBT Electronics recommends removing the antenna from the aircraft anytime poor operation of the KRA-10A is suspected. Removing the antenna is the only way to reveal possible poor bonding to the aircraft skin. This can be a source of intermittent operation. Like all bottom mounted aircraft antennas, the KRA-10A antenna is subject to water that sometimes finds its way into the belly or tail. The rubber and steel-mesh RF gasket gets wet and corrodes against the dissimilar aluminum skin. Sometimes the antenna is sealed around the outside perimeter by the installer. This is guaranteed to trap water between the gasket and skin which never dries out. Water can enter the antenna through cracked and warped plastic radome.

The earlier model King KRA-10

The KRA-10 is very similar to the KRA-10A. The only operational difference is the provision, in the KRA-10A, for a disabling signal to suppress operation when on the ground. This usually comes from a switch on the landing gear. Internally, one circuit card is different and completely redesigned in the KRA-10A. Presumably the designers at King corrected some defects in the original version by replacing the circuit card. MBT Electronics believes that with careful repair and adjustment, the KRA-10 units can be completely satisfactory. The same test setup is used and the issues with the antenna are the same.

The King KI-250 Instrument

The KI-250 contains the DH bug circuitry, the tone generator, and the meter drive circuitry. The meter drive circuit is a voltage to current converter. The analog altitude voltage from the RT unit is converted to current and applied to the meter drive motor. This motor torque counteracts a spring and results in a pointer position proportional to input voltage. When power is removed, the pointer springs back to the zero / off position. The indicators are compatible between the KRA-10 and KRA-10A units. The later serial number KI-250 indicators have modified and improved circuitry although the part number is the same. Many subtle faults can exist in the KI-250. It should be tested and adjusted with the system to insure best possible accuracy.

The antenna system match can only be tested in a simulated installation which is not permanently pointed at the ground. The free space situation, where the antenna match should be perfect, is simulated by pointing the antenna at the sky. Measurements are then made to the RT unit to verify a satisfactory antenna match and the absence of reflected power entering the receiver. The reflected power check is a sensitive measurement of antenna quality. To check the radiated power and antenna beam shape, the calibrated pickup antenna and power meter are used.

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