FAA Repair Station #QYMR098B

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MBT Electronics offers Repair and Overhaul Service for the RT-1115/APN-209 Radar Altimeter System.

MBT Electronics specializes in servicing components of the APN-209 radar altimeter system. This equipment has been installed in Army helicopters since the late 1970s. It is a self contained, panel mounted instrument with the display and receiver-transmitter contained in one unit. There is an optional remote (co-pilots) display which repeats the output of the master unit. Several remote instruments can be installed in a system if necessary.

The APN-209 system is a pulse type radar altimeter utilizing two antennas for the transmit and receive functions. It transmits a short pulse and receives the reflected signal while the tracking system measures the time delay. The aircraft antennas point straight down and the signal bounces off the ground under the aircraft. The time delay is converted to a digital and analog readout in feet. Depending on the height above ground and the received signal level, the tracking circuitry controls the transmit power, pulse width, and receiver sensitivity. It will maintain the correct power and gain for reliable operation over all types of terrain. When the signal is reliable, the off flag pulls out of view and the digital display is enabled.

A problem with all radar altimeters is the tendency to lock-on to stray signals. If a signal is reflected from one antenna directly to the other without bouncing off the ground, the tracking system may be fooled. Usually this results in a constant reading of zero feet. The skids or other items under a helicopter can reflect the signal directly from one antenna to the other. The APN-209 has extensive circuitry to prevent this problem. This equipment was designed for helicopters. Internal adjustments can be customized to compensate for situations such as external (sling) loads or poor installation.

RT-1115C/APN-209 & JG1209AC01

Components on test bench.

These can be repaired.

Fabricated installation wiring. Ready to install.

MBT Electronics Offers the Following Services

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APN-209 Family History

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The following information is not from official sources. It is observations and experience that I have collected while repairing APN-209 units. Many different part numbers have been assigned to these units. It is confusing. I have grouped them according to the RT number suffix which they resemble.

All the APN-209 units are interchangeable in the aircraft and will provide similar performance. The difference is the internal technology which has changed greatly over the 30 year existence of this equipment. The early units contain a vacuum tube and the latest units have micro-processor circuitry.

Detailed breakdown of part numbers:

Straight (No Suffix)

The “straight” model seems to be the earliest example of APN-209 and dates from about 1975. It can be identified by the gold coax connector on the back. One connector is gold and one is steel. This model incorporates a vacuum tube transmitter and requires a warm up period when first turned on. The LED display and lighting is red.

A Suffix

The A model has a solid state transmitter and turns on instantly. The receiver AGC, transmit power control, and pulse width control circuits have been improved. The LED display and lighting is red.

B Suffix

The B model is similar to the A model except it is night vision compatible. Most units have a green LED display and the High and Low warning lamps are blue / green. This makes the unit more compatible with NVIG (night vision) equipment. Strangely, some units also have red instrument lamps. NVIG lamps are available to replace these red bulbs if required.

C Suffix

The C model is similar to the B model in form and function. These units date from about 1982. The LED display and lighting is green. There are circuitry improvements including a change in the analog pointer servo system to give better resolution. The servo motor gives off an audible tone when the unit is turned on in a quiet environment.

Note: In the A, B, and C models, the power supply runs very hot and requires some airflow around the instrument case. In some failed units, the solder has melted on the power supply board. For best reliability, be sure that the aircraft installation provides sufficient airflow.

D Suffix

The D model is completely redesigned internally. These date from about 1993. They can be identified by the position of the analog and digital “zero” trimpot openings on the sides of the unit. The D unit has one opening on the side and one on the top. The previous models have both openings on the side. Otherwise the form and function is the same as the earlier units. They have slightly better performance on the bench than the older models and I suspect that they work a little better in the aircraft. There are noticeable improvements in the transmitter power control and the receiver AGC. The heat problem of the earlier units has been solved and these units are very reliable. The display section has been redesigned for NVIG operations. The lighting is very carefully isolated with rubber boots and green filters. In my opinion this is the best model of the APN-209.

J Suffix

The J model is a completely new design internally. This is the current model, as far as I know. It has a bare metal case which makes it easy to recognize. The internal construction consists of multi-layer circuit boards with surface mount components. It incorporates micro-processor controllers with proprietary software installed. This model has a verbal warning system. When wired into the aircraft audio system, a voice will sound off when the “Low” or “High” lights come on. This unit runs cool and the power drain is lower than the previous units.

In my opinion, this unit seems to be less robust than the earlier units. The electronic components are very vulnerable to ESD damage. The circuit cards, with tiny gauge circuit runs, are easily damaged when any short circuit occurs, whether inside or outside of the instrument. This damage is difficult to repair when the burned run is deep inside a multilayer board. The micro-controllers are easily destroyed by stray voltages or accidental shorts which may exist in the aircraft installation. Extreme care should be exercised to check for shorts or stray voltages on the connectors in the aircraft before plugging the unit in.

Remotely Mounted RT Units

These units contain the receiver-transmitter section of the APN-209 without the instrument section. They are compatible with any of the ID-1917/APN-209 remote indicators. Any number of remote indicators can be connected in a daisy chain fashion. The case is designed to mount in a remote location in the aircraft and thereby simplify the installation. The unit is controlled by the ID-1917 instrument on/off switch and push-to-test switch. The antennas and electrical connections are identical to the APN-209 units.

Remote Indicators

The remote indicators are all compatible electrically and work with any model of the APN-209. The difference is the internal design and the NVIG lighting.

  • ID-1917/APN-209
  • SM-C-707893
  • 5841-01-058-7994

This is the earliest version of the remote indicator. Normally it would be installed in the co-pilots position. It repeats the information from the RT unit. In the aircraft, it looks the same as the RT unit and can control the on-off and test functions. It has its own “high” and “low” bugs and dimming system. This version has a red LED display and lighting.

  • ID-1917A/APN-209
  • SM-C-967240
  • 5841-01-137-8445

Remote indicator similar to “straight” model but probably incorporates subtle improvements.

  • JG1209AB04
  • JG1209AB02

Remote indicator similar to the ID-1917 units. This model has green LED display and lighting and normally would be used with the B or C model RT units.

  • JG1209AC01

This remote indicator is completely redesigned internally. It uses more modern components and incorporates better NVIG design. It is used with the D or J model RT units.

  • ID-1917C/APN-209
  • A3060591
  • 5841-01-245-9093

This remote indicator is similar to the JG1209AC01 and is used with the D or J models.


  • AS-2595/APN-194
  • LG81J1
  • 5841-00-181-0330

Antenna. This antenna mounts in a round hole and has a pyramid structure.

  • AS-3831/APN-209
  • LG81AT01
  • SM-D-967308

Antenna. This antenna mounts in a round hole or flat on a surface and has a flat (micro-strip) construction. The diameter is about 6 inches.

  • LG81AT2

Antenna. This antenna is a flat (micro-strip) antenna and mounts flat on a surface. The shape is square.

Can other antenna types be used?

All radar altimeters operate at 4.3 Ghz and have similar requirements for antenna performance. The APN-209 radar altimeter will electrically match, and function with, any 4.3 Ghz radar altimeter antenna. Any good quality antenna should work with the APN-209 system but for optimum performance, other factors should be considered.

The beam shape and gain is different with different types of antennas and should be selected to match the type and mission of the aircraft. Some aircraft missions involve roll and pitch maneuvers and would work best with a wide beam antenna. Other missions will do better with a narrow beam antenna.

Another factor is the style of mounting the antenna. The pyramid shaped antennas require a large reinforced hole in the aircraft while the flat micro-strip style can be installed on the skin with a simple backing plate and minimal structural changes.

If a previous system was installed in an aircraft, and it worked well, that antenna installation should work with the APN-209 system. The safest procedure is to use the type of antenna and the mounting location which is successful in another of the same type of aircraft.

The APN-209 system has no installation manual available. Refer to the manuals for other high quality radar altimeters such as Collins and King for information on antenna installation. The suggested location and spacing of antennas will be applicable to the APN-209 system.

Reinstalling APN-209 Radar Altimeter After Maintenance

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Two adjustment ports are available on the side of the APN-209 unit. These are small “doors” that slide open to allow access to screwdriver adjustments. These should be adjusted before putting the system back in service.

If these are not adjusted, the radar altimeter will indicate a height above ground when the aircraft is actually on the ground. This represents the electrical delay in the coax cables and the distance from the antennas to the ground. This can be as much as 50 feet in some aircraft.

The two adjustments are independent. One adjustment controls the analog pointer and the other controls the digital readout. Unfortunately the adjustment ports, on the sides of the unit, are hard to reach in a typical aircraft installation. Before installation, the unit can be plugged in to the aircraft connectors and held in the hand while adjusting the trimpots. This also makes it easier for the technician to see the dial while doing the adjustment.

The analog dial is adjusted to read zero. In most units, the dial will hit a mechanical stop just past zero, so be sure that it is not adjusted on the negative side of zero and against the stop.

The digital zero control will adjust the readout to zero and a negative sign will appear if it is adjusted too far. The numbers that would represent the negative direction will not appear. If you see a negative sign, you don’t know how far on the negative side the adjustment actually is. For this reason it is best to adjust it for 1 foot or ticking between 1 and zero. This will insure that it is not adjusted past zero.

If the reading is unstable, it might mean that the ground under the aircraft has poor RF reflective properties. You can wet the ground under the aircraft to get around this problem.

After adjustment, the digital and analog readouts should agree. To test the system, hold a sheet of metal under the antennas at an angle which causes the signal to reflect against a nearby car or other object. The altimeter will indicate a distance to this object.

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