(ADF) Finder Direction Automatic 7

Quadrantal Error

The theoretical reception polar diagram of the loop aerial is distorted by the airframe which produces a strong electrical field aligned fore and aft. Incoming NDB signals are thus refracted towards the fore and aft airframe axis. The maximum refraction occurs in the quadrants (i.e. on relative bearings of 045°, 135°, 225° & 315°.) Older ADF systems are regularly ‘swung’ to assess the value of quadrantal error. In modern aircraft the error is determined by the manufacturer and corrections are put into the equipment to reduce the effect to a minimum.

Angle of Bank (dip)

A loop aerial is designed to use vertically polarized waves for direction finding. If the incoming wave has any horizontal component of polarization it will induce currents in the top and bottom horizontal members of the loop resulting in a circulating current. This would destroy the nulls of polar diagram (similar to night effect) and reduce the accuracy of the bearings. The angle of bank during a turn causes currents to be induced in the horizontal elements of the loop thereby leading to a bearing error which is referred to as dip error. This error is only present when the aircraft is not in level flight.

Lack of FailureWarning System

False indications due to a failure in the system are not readily detectable because of the absence of failure warning on most ADF instruments. Particular care should therefore be exercised in identifying and monitoring the NDB and independent cross-checks made with other navigational aids where possible. It is essential that when using the ADF as the primary navigation aid, for example for a runway approach procedure, that it is continuously monitored to detect any failure.

Factors Affecting ADF Range

The major factors which affect the range of NDB/ADF equipment are listed below:

NDB transmission power; the range is proportional to the square root of the power output i.e. to double the NDB range, quadruple the power output of the transmitter.

NDB range is greater over water:

Note: Using ranges calculated by these formulae does not guarantee that the aircraft will be within the DOC.

The lower the frequency, the greater the surface wave (greater diffraction, lower attenuation).

All precipitation, including falling snow, reduces the effective range and accuracy of ADF bearings.

N0NA1A NDBs have greater ranges than N0NA2A. But note that ICAO Annex 10 recommends the use of N0NA2A for long range beacons.

Receiver quality.

Accuracy

The accuracy of ADF is +/-5° within the designated operational coverage, by day only. This refers to the measured bearing and does not include any compass error.

ADF Summary

Figure 7.23

Automatic Direction Finder (ADF) 7

Questions 7

7.An aircraft is tracking away from an NDB on a track of 023°(T). If the drift is 8° port and variation 10° west, which of the RMIs illustrated below shows the correct indications?

8.The BFO facility on ADF equipment should be used as follows when an NDB having N0NA1A type emission is to be used:

a.BFO on for tuning and identification but may be turned off for monitoring

b.BFO on for tuning but can be turned off for monitoring and identification purpose

c.BFO off during tuning, identification and monitoring because this type of emission is not modulated

d.BFO should be switched on for tuning, ident and monitoring

9.The protection ratio of 3:1 that is provided within the promulgated range/designated operational coverage of an NDB by day cannot be guaranteed at night because of:

a.long range sky wave interference from other transmitters

b.sky wave signals from the NDB to which you are tuned

c.the increased skip distance that occurs at night

d.the possibility of sporadic E returns occurring at night