| Home | Privacy | Contact |

Instrument Flying Handbook
Navigation Systems
Traditional Navigation Systems

| First | Previous | Next | Last |

Instrument Flying
Handbook

Preface

Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Flying
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Flying
Using an Electronic Flight
Display

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight
Display

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency
Operations

VOR Accuracy
The effectiveness of the VOR depends upon proper use and
adjustment of both ground and airborne equipment.

The accuracy of course alignment of the VOR is generally
plus or minus 1°. On some VORs, minor course roughness
may be observed, evidenced by course needle or brief flag
alarm. At a few stations, usually in mountainous terrain,
the pilot may occasionally observe a brief course needle
oscillation, similar to the indication of "approaching station."
Pilots flying over unfamiliar routes are cautioned to be on
the alert for these vagaries and in particular, to use the TO/
FROM indicator to determine positive station passage.

Certain propeller revolutions. per minute (RPM) settings
or helicopter rotor speeds can cause the VOR CDI to
fluctuate as much as plus or minus 6°. Slight changes to
the RPM setting will normally smooth out this roughness.
Pilots are urged to check for this modulation phenomenon
prior to reporting a VOR station or aircraft equipment for
unsatisfactory operation.

VOR Receiver Accuracy Check
VOR system course sensitivity may he checked by noting
the number of degrees of change as the OBS is rotated to
move the CDI from center to the last dot on either side. The
course selected should not exceed 10° or 12° either side. In
addition, Title 14 of the code of Federal Regulations (14
CFR) part 91 provides for certain VOR equipment accuracy
checks, and an appropriate endorsement, within 30 days prior
to flight under IFR. To comply with this requirement and to
ensure satisfactory operation of the airborne system, use the
following means for checking VOR receiver accuracy:

1. VOR test facility (VOT) or a radiated test signal from
an appropriately rated radio repair station.

2. Certified checkpoints on the airport surface.

3. Certified airborne checkpoints.

VOR Test Facility (VOT)
The Federal Aviation Administration (FAA) VOT transmits
a test signal which~ provides users a convenient means to
determine the operational status and accuracy of a VOR
receiver while on the ground Where a VOT is located.
Locations of VOTs are published in the A/FD. Two means of
identification are used. One is a series of dots and the oilier is
a continuous tone. Information concerning an individual test
signal can be obtained from the local flight service station
(FSS.) The airborne use of VOT is permitted; however, its
use is strictly limited to those areas/altitudes specifically
authorized in the A/FD or appropriate supplement.

To use the VOT service, tune in the VOT frequency 108,0
MHz on the VOR receiver. With the CDI centered, the
OBS should read 0° with the TO/FROM indication showing
FROM or the OBS should read 180° with the TO/FROM
indication showing TO. Should the VOR receiver operate an
RMI, it. would indicate 180° on any OBS. setting.

A radiated VOT from an appropriately rated radio repair
station serves the same purpose as an FAA VOT signal, and
the check is made in much the same manner as a VOT with
some differences.

The frequency normally approved by the Federal
Communications Commission (FCC) is 108.0 MHz
however, repair stations are not permitted to radiate the
VOR test signal continuously. The owner or operator of the
aircraft must make arrangements with the repair station to
have the test signal transmitted. A representative of the repair
station must make an entry into the aircraft logbook or other
permanent record certifying to the radial accuracy and the
date of transmission.

Certified Checkpoints
Airborne and ground checkpoints consist of certified radials
that should be received at specific points on the airport surface
or over specific landmarks while airborne in the immediate
vicinity of the airport. Locations of these checkpoints are
published in the A/PD.

Should an error in excess of ±4° be indicated through use of
a ground check, or ±6° using the airborne check, IFR flight
shall not be attempted without first correcting the source of
the error. No correction other than the correction card figures
supplied by the manufacturer should be applied in making
these VOR receiver checks.

If a dual system VOR (units independent of each other except
for the antenna) is installed in the aircraft, one system may
be checked against the other. Turn both systems to the same
VOR ground facility and note the indicated bearing to that
station. The maximum permissible variation between the two
indicated bearings is 4°.

Distance Measuring Equipment (DME)
When used in conjunction with the VOR system. DME makes
it possible for pilots to determine an accurate geographic
position of the aircraft, Including the bearing and distance.
TO or FROM the station. The aircraft DME transmits
interrogating radio frequency (RF) pulses, which are received
by the DME antenna at the ground facility. The signal triggers
ground receiver equipment to respond to the interrogating
aircraft. The airborne DME equipment measures the elapsed
time between the interrogation signal sent by the aircraft and
reception of the reply pulses from the ground station. This
time measurement is converted into distance in nautical miles
(NM) from the station.

 
7-16