Repair Service for AN/ASN-128 and AN/ASN-128A Doppler Radar System Components

The ASN-128 is a navigation system designed for military helicopters. In the age before GPS, cross-country navigation of a helicopter, in military situations, was difficult. Most forms of electronic air navigation involved receiving a line-of-sight signal and required the pilot to do plotting and calculation. The pilot of a low flying helicopter has no time for plotting and the signals from navigation stations are unreliable at low flight levels. In military situations, external navigation signals may be disrupted. The solution requires a self contained, automatic, system that will calculate the present position of the aircraft, in real time, and display navigation information to the pilot.

The ASN-128 uses Doppler radar as its primary sensor. The movement of the aircraft across the earth is detected by four microwave beams which are radiated from the antenna and reflect back from the ground. The beams radiate out in four directions and reflect back from the ground with a slightly altered frequency, do to the relative movement of the aircraft. The right front, left front, right rear, and left rear beams strike the earth under the helicopter at an angle which causes a change in the frequency of the reflecting energy if the aircraft is moving. The physical laws of the Doppler-effect relate the actual change in frequency to the velocity of the aircraft. Thus the true velocity of the aircraft across the earth can be known regardless of wind speed and direction.

The four beams are periodically sampled, one at a time, and analyzed for frequency shift. When flying normally, the two forward beams will show an increase in frequency and the two aft beams will show a decrease. If sideways or even rearward movement exists, the frequency shift of the beams will tell the story. The computer combines this information with other information available in the aircraft system.

The other inputs utilized include: heading, pitch, roll, and true air speed. These are available, in most aircraft, from the air data computer and heading reference systems. The computer in the ASN-128 system combines these inputs with the analysis of the four beams and derives a complete navigation picture. The pilot only has to select which function is required and fly the helicopter.

The ASN-128 is a dead-reckoning system because it has no absolute knowledge of position. The initial position must be entered before flight. The relative movement of the aircraft is then maintained within the accuracy tolerance of the system.

The ASN-128 system is organized in three units. The Computer Display Unit is located in the cockpit and contains the computation and display circuitry. The pilot controls the mode of operation and reads the information from this unit.

The Signal Data Converter is a remotely mounted black box containing auxiliary circuitry including: the power supply, the Doppler frequency tracking circuitry, and the synchro to digital converter. The inputs from the aircraft systems, in synchro form, are converted to digital form in this unit.

The Receiver-Transmitter-Antenna unit contains the RF circuitry including: the transmitter, the receiver, the beam switching system, the video amplifier, and the calibration factors memory. The large flat area on the bottom is the radome and is mounted flush with the belly of the aircraft. The four beams radiate from this surface.

The Computer Display Unit Part Numbers

• CP-1252/ASN-128
• SM-D-858970
• 5841-01-063-1918
• CP-1252B/ASN-128

MBT Offers the following services

Repairing Military Avionics Equipment

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MBT electronics is FAA approved repair station # QYMR098B. This means that the shop is approved, by the FAA, to repair components that are itemized on a capability list. The capability list details the part numbers of items that the shop is authorized to repair. When the item is repaired, a tag is attached which certifies that the item has been repaired by an approved facility. The most common tag is the form 8130-3 which is issued by the FAA. This form is basically a formalized serviceable tag, which the repair station fills out, with some details of the repair, and the chief inspector signs. The form tells anyone receiving the part about the repair and who did it. Generally, a tag is necessary for a component to be installed in a certificated aircraft. The installer determines the legitimacy of the tag and the component.

At different times and in different places, the FAA has either allowed or not allowed repair stations to place 8130-3 tags on military avionics components. The policy has not been consistent. A military avionics component may be similar or even identical to a general aviation component, but it has no status with the FAA. It is not “airworthy”. The difference is the documentation. To repair a component to FAA standards, you must have documentation with the most up-to-date changes and revisions, issued by the manufacturer. Military equipment also has manuals and updates issued, but these are not necessarily available outside the military and the FAA does not recognize these as fulfilling the requirements.

The FAA requirements insure that the repair facility has the latest information from the manufacturer. Even when the equipment is very old, and no changes have been made to it for years, the FAA still requires your data to be the latest available. A situation sometimes happens where the service manual evolves to the point where it no longer covers early versions of a piece of equipment.

Strangely, if you can show that the manufacturer no longer exists, the FAA will allow you to keep repairing the item based on the last available documentation. For example, you can repair a Lear autopilot, which hasn’t been built since about 1960, if you write a repair procedure and have it approved by the FAA. The procedure that you get approved will probably be the contents of the old service manual.

Can you write a procedure for a military component and have it approved by the FAA? In the past you could, but my FAA inspector says that the policy has tightened up and no FAA 8130-3 forms can be applied to military components. The FAA doesn’t care if you fix military components, but you can’t use an FAA 8130-3 form.

Another complication is the former military aircraft being used by state and county law enforcement and other agencies. These aircraft are full of components which the FAA does not consider “airworthy”. To get around this problem, the FAA has special rules which designate these aircraft “public service” and allows the operating agencies to determine the airworthiness of the aircraft. They can use any components that they deem suitable.

When MBT Electronics repairs military avionics components, no FAA 8130-3 form is issued. MBT Electronics issues a similar form which has all the same information but does not imply that the item is approved by the FAA. The form is a certificate of compliance which identifies the item and details the information used as a basis for the repair. In most cases it refers to the available military manuals. A test data sheet is also issued. This is a final test check-off list with important parameters recorded. This is intended to show that the testing is thorough and minor defects have not slipped by.

MBT Electronics maintains the highest standards for bench repair of military avionics components. Although no FAA tag is issued, the quality of service is the same as the FAA requires for certificated components.