The VHF Direction Finder

CHAPTER ONE

T-6A INSTRUMENT NAVIGATION

INOPERATIVE ILS COMPONENTS:

When the LOC fails, an ILS approach is not authorized.

When the glideslope fails, the ILS reverts to a nonprecision LOC approach.

All aircrew should be aware that disturbances to ILS LOC and glideslope courses may occur

when surface vehicles or aircraft are operated near the LOC or glideslope antennas. Most ILS

installations are subject to signal interference surface vehicles, aircraft, or both. ILS critical

areas are established near each LOC and glideslope antenna.

During periods of routine or emergency maintenance, coded identification (or code and voice,

where applicable) is removed from certain FAA NAVAIDs. Removal of identification serves as

a warning to aircrews that the facility is officially off the air for tune-up or repair and may be

unreliable even though intermittent or constant signals are received.

NOTE

During periods of maintenance VHF ranges may radiate a T-E-S-T

code (-●●●●-).

Voice equipped enroute radio NAVAIDs are under the operational control of either an FAA

Automated FSS or an approach control facility. The voice communication is available on some

facilities. The Hazardous In-flight Weather Advisory Service (HIWAS) broadcast capability on

selected VOR sites is in the process of being implemented throughout the conterminous U.S. and

does not provide voice communication. The availability of two-way voice communication and

HIWAS is indicated in the IFR Supplement and Aeronautical Charts.

In aircraft that have more than one receiver, there are many combinations of possible

interference between units. This can cause either erroneous navigation indications or complete

or partial blanking out of the communications. Aircrews should be familiar enough with the

radio installation of the particular airplanes they fly to recognize this type of interference.

THE VHF DIRECTION FINDER

The VHF Direction Finder (VHF/DF) is one of the common systems that help aircrews without

their being aware of its operation. It is a ground-based radio receiver used by the ground station

operator. FAA facilities that provide VHF/DF service are identified in the IFR Supplement.

The equipment consists of a directional antenna system and a VHF radio receiver. The VHF/DF

receiver display indicates the magnetic direction of the aircraft from the ground station each time

the aircraft transmits. DF equipment is of particular value in locating lost aircraft and in helping

to identify aircraft on radio detection and ranging (RADAR) equipment.

1-10 INTRODUCTION TO NAVIGATION SYSTEMS