8. Navigational Instruments

Question 1

What navigation aids are available to pilots? (6)

Answer 1

• ADF - Automatic Direction Finder
• NDB - Non-Directional Beacon
• VOR - VHF Omni-directional Radio Range
• DME - Distance Measuring Equipment
• TACAN - Tactical Air Navigation (a military system)
• ILS - Instrument Landing System

Question 2

What is ADF?

Answer 2

Automatic Direction Finder
On board equipment used in conjunction with an NDB.
ADF indicates the relative bearing of the station tuned (NDB) and/or its QDM / QDR in the cockpit.

Question 3

What is VOR?

Answer 3

VHF Omni-directional Radio Range
It neither indicates position on the radial (the aircraft could be right next to the station or miles away), nor does it indicate anything about the aircraft’s heading.

It is important to remember that the VOR indicates to the pilot only which radial the aircraft is on

Question 4

Issues with VOR?

Answer 4

• Site error - Around uneven terrain such as hills and man-made structures, trees
• Propagation Error
  At a considerable range from the VOR, the transmitted signal bends due to reflections.
• Airborne equipment error
  Caused by aircraft equipment assessing and converting the phase difference into a radial.

Question 5

What are the errors of VOR?

Answer 5

Aircraft error – 1 degree

Aggregate total errors – 5 degrees

Question 6

What is DME?

Answer 6

Distance Measuring Equipment
The distance to the station is a slant range, in
nautical miles.

“DME suffers from slant range”

Question 7

ADVANTADGES of DME?

Answer 7

It can be ‘Frequency Paired’ with its associated VOR.

A VOR/DME installation can be used to make an Instrument Approach to an airfield.

Simple to use and it can calculate Groundspeed - pilots can give accurate estimates of arrival times over positions.
A very accurate ‘fix’ can be obtained using two DME
stations.

Question 8

Disadvantages of DME?

Answer 8

Only gives a slant range, not accurate at high altitudes over the DME station! (e.g. 0nm DME is not possible).

Question 9

What is ILS?

Answer 9

Instrument landing system

Question 10

What are the components for an ILS?

Answer 10

• The localiser transmitter;
• The glide path transmitter;
• The outer marker (or an NDB or other fix); and

In association with a suitable runway, the approach
lighting system.

Question 11

ILS Categories?

Answer 11

Cat 1 - DH of not less than 200ft
Cat 2 - DH of not less than 100ft
Cat 3 - DH 0ft

Question 12

ILS Frequencies

Answer 12

• 108.10MHZ – 111.95MHZ - Localiser (VHF)
  - provides lateral guidance
• 329.30MHz – 335.00MHz - Glide Path (UHF)
  - provides vertical guidance

Question 13

ADVANTADGES of ILS (3)

Answer 13

• Pilot interpreted and simple to use.
• ‘Precision’ approaches possible, i.e., provides full guidance in azimuth and elevation, in some cases during the landing.
• Can be coupled to auto-pilot for automatic approach and landings.

Question 14

DISADVANTADGES of ILS (3)

Answer 14

• Subject to interference from commercial VHF radio stations.
• Reflections from other ground facilities can create ‘false’ glide paths.
• Shortage of frequencies.

Question 15

What is RBI?

Answer 15

Radio Bearing Indicator

Question 16

What are the issue with ADF? (7)

Answer 16

• Errors and abnormal indications
• Designated operational coverage
  - errors increase at range
• Mountain effect
  - reflection of the waves propagation along with
  - natural magnetic disturbance,
  - increase at low altitude
• Static interference
  - static charge
  - Thunderstorms can discharge static electricity
  - Sky waves during the night
• Station interference
  - Congestion of stations in the LF and MF band
  - Alternating between two interfering stations
• Coastal refractions
  - The ground waves travel in one direction over land, but another direction over water
• Lack of failure warning system
  - False indications due to a failure in the system are not readily detectable