ADF Qustion

[flighttest-engineer]

I am familiar with the „good old” Bendix T-12C/-D ADF system and I must confess that it was reliable equipment. I have repaired, installed and used this ADF system myself years ago.
Presuming that installed equipment shall work according to its specification, the allowable accuracy of ADF-bearing indication (not depending on the specific system) according to the ICAO Annex 10 is +/- 5° with a radio signal from any direction.

The rated coverage for NDB locator beacons is approx. 10-25 NM, where the rated coverage of NDB’s is approx. from 25-150 NM.

To determine the cause of your observed abnormal bearing deviation I would suggest the following procedure:

A good start would always be to have the ADF-receiver bench-checked by an avionics-shop to verify the proper function of the receiver parameters (sensitivity, selectivity, AGC, proper alignment etc.) and spend some money for it, even if you have GPS already available.

Thereafter perform some easy checks in the airplane (on gnd & in air).

• Check if the bearing-deviation is constant for all headings which means you have the same offset (+ / -) for every heading.

• Check if the bearing deviation depends also on the fieldstrenght of the received NDB-signal, .e.does a stronger signal give you more accuracy

As a first try you can check your ADF also on the ground, while taxiying around, but not in the vicinity of strong electrical fields like lightings etc.
For checking you shall perform it on a day with good weather conditions (no thunderstorm with lightning nearby etc, no twilight).

If you have the same offset deviation on all headings, it can also be a problem caused by the inaccurate installation of the Loop-Antenn (not aligned to the aircrafts centerline, if this problem was from the beginning).
If you observe different bearing-deviations on different headings (presuming you are receiving the same NDB, maybe there is a problem with the “quadrantal error correction” of the installed Loop Antenna.

The “HI-CAP” of your ADF sense antenna can also have an influence to the indication accuracy. It is importand, that the signal level of the sense-Antenna is matching the signal level also of the Loop Antenna.

Let me know if I can give you some more hints to get your system functioning correctly.

Have fun,

Armin