Radio Navigation Aids – VOR/DME

Navigation is the directing of aircraft from one place to another along a particular line of travel. To navigate a pilot uses radio navigation aids. There are a variety of different types of radio navigation aids. Here are some of them.

Vor (omnidirectional radio range) and DME (distance measuring equipment) are often located at the same site. They operate on VHF (very high frequency) and UHF (ultra high frequency) respectively and are not affected by static or other interferences. The maximum range of VOR is about 200 nautical miles. By flying VOR the pilot ensures he is flying directly to the station. Also by measuring radials from more than one VOR station, a pilot can check his position.

The function of DME is to measure distance. The DME measures, electronically, the time it takes for a signal, transmitted from an aircraft interrogator, to reach the ground base station transponder, and return. This elapsed time is converted to miles and appears on a digital indicator on the flight deck. The indicator actually seems to rapidly count the number of miles between the aircraft and the station giving the pilot a continuous digital reading of how far he is from or to a station.

With the many VOR/DME stations along the route, a pilot can make good his desired track; is constantly aware of his distance to or from a DME station; or, by using two VOR radials, establish his exact position.

 

ILS (Instrument Landing System)

 

The ILS is designed to provide an approach path for exact alignment and descent of an aircraft on final approach to a runway.

The ground equipment consists of two highly directional transmitting systems along with three (or fewer) marker beacons. The directional transmitters are known as the localizer and glide path transmitters.

The system may be divided functionally into three parts: guidance information – localizer, glidepath Range information – marker beacons, Visual information – approach lights, touchdown and centerline lights, runway lights.

1. The localizer transmitter, operating on one of the twenty ILS channels emits signals which provide the pilot with course guidance to the runway centerline.

2. The UHF (ultra high frequency) glidepath transmitter, operating on one of the twenty ILS channels radiates the signals principally in the direction of the final approach.

3. Ordinarily, there are two marker beacons associated with an ILS; the outer marker and middle marker. However, some locations may employ a third beacon – the inner marker.

The outer marker normally indicates a position at which an aircraft at the appropriate altitude on the localizer course will intercept the ILS glide path.

The middle marker indicates a position at which an aircraft is approximately 3500 feet from the landing threshold. This will also be the position at which an aircraft on the glidepath will be at an altitude of approximately 200 feet above the elevation of the touchdown zone.

The inner marker, where installed, will indicate a point at which an aircraft is at a designated decision height on the glidepath between the middle marker and landing threshold.

 

Radar

The principles of radar are not new: in fact, some early experiments were made back in 1880s. In 1904 a German engineer had invented, as he explained, a “radio-echo collision prevention device”

The word “radar” was originally derived from the descriptive phrase “Radio Detection and Ranging”.

The application of radar in the air traffic control system consists of two basic designs. The initial type of radar, called primary radar, began to be used for advanced air traffic control. When the word “radar” is used alone it usually includes both primary and secondary radar.

There are three additional forms associated with primary and secondary radar:

Radar Echo – the visual indication on display of a radar signal transmitted from an object.

Radar Response – the visual indication on display of a radar signal transmitted from an object in reply to an interrogation.

Radar Blip – the collective term meaning either echo or response.

 

Primary Radar

In primary radar a beam of individual pulses of energy is transmitted from the ground equipment. These pulses hit the aircraft from 16 to 34 times each scan. An aircraft in the path of this radar beam will reflect back some of the pulses which are picked up by a receiver. This reflected energy produces a bright “echo” or “target” on a cathode ray tube.

 

Secondary Surveillance Radar (SSR)

The SSR system provides for six modes; only two modes are used in civil aviation:

Mode A for civil and military identification.

Mode C for automatic pressure altitude information.

The SSR is a valuable tool for automatically identifying aircraft. Identification is achieved by providing the controller with a specific radar beacon target identity of aircraft. A total of 4096 discrete reply codes are available for special position identification to be transmitted on request of a controller.

With SSR display, the controller sees aircraft returns on his PPI (plan position indicator) as two slashes, clearly distinguishing them from primary targets which are single blips.

In modern systems different synthetic symbols are used to indicate a lot of additional information.

 

Visual Aids for Navigation

Additional visual aids to navigation consist of markings on the aerodromes. These markings comprise single lines or rows of lines which, for the pilot, are very important for holding positions, runway thresholds, the runway centre lines, the sides of the runways, etc.

However, at night or during poor visibility by day, lights are required. To be effective lights must be of adequate intensity. At certain aerodromes the controller can vary the intensity of some of the lights so that they can be reduced not to blind the pilot and strong enough so that he can see them in bad weather.

The first lights a pilot sees on approach is generally the aerodrome beacon. It may rotate and can be seen at a great distance. There might be an identification beacon which shows green flashes of light. Red lights, the usual danger signal, warn pilots of the obstacles such as hangars and other high buildings, telephone poles, etc. Runway edge lights identify the runway and approach lights assist the pilot to align himself with the runway.

Lights may also be used to provide a glidepath similar to what an ILS provide electronically. The Visual Approach Slope Indicator System (VASIS) is a beam of light having a white colour in its upper part and a red colour in its lower part. A pilot of an aeroplane during an approach will:

a) when above the approach slope, see the lights to be white in colour;

b) when on the approach slope, see the lights to be pink in colour; and

c) when below the approach slope, see the lights to be red in colour.

By reference to vasis, combined with ILS, the pilot can bring an aircraft down safely almost to touchdown by day or night.

After landing, he follows the blue taxi lights along the taxiway to the apron and the service areas.

At the service area a marshaller, with illuminated wands, directs the aircraft with signals to its proper position for unloading and, finally, signals pilot to cut the engines.

 

 

Airport

There are airports in every country. In theory, an aircraft can fly an infinite number of paths through the air from any surface point to any other. In practice, paths of flights lead from airport to airport. As a rule the airport is to be situated not far from the city. If it is a long way to the airport there is special bus service to take passengers from the city Agency to the airport.

Aircraft not only need proper landing and take off facilities. Moreover, those who use aircraft need services and accommodations which the airport must provide. The modern airport is a complex structure, a centre of most diversified services. Millions of passengers and thousands of tons of air freight are handled by modern airports. Thousands of people are working at airports.

Any airport can be divided into main parts: the landing area (runways and taxiways) and the terminal area (aprons, buildings, car parking areas, hangars etc.). The number of runways, their length and location depend on the volume and character of traffic, the prevailing wind directions and other factors.

The runways and taxiways should be arranged so that to prevent delays on landing, taxying and take off operations.

Aprons are required for aircraft to make final checks prior to departure. The main function of the terminal buildings is to handle departing and arriving passengers and their baggage. In the reception halls at the check-in desks passengers register their tickets, their suitcases are weighed and labelled here too. Baggage check-in facilities utilize conveyors to move baggage without delays.

In the terminal there is an electronic flight information board to list departure and arrival times. If any delay takes place such information is also indicated on the board.

The airport has to maintain a number of supplementary services. There must be an airport clinic, fire brigade, special vehicles and equipment units (water and catering trucks, tow tractors, refuellers, etc.).

Other services include maintenance, overhaul and repair of stationary and mobile equipment, the supply of electricity, water, heat and air conditioning.

Among the airport services are: flight assistance service, air traffic control, airport traffic control, approach control, air route traffic control; radio communications and weather service observation and forecasting.

Nowadays there exists one more pressing problem – that of air piracy. Now every airport has new specific detection systems capable to screen passengers and their baggage, cargo parcels and mail.

 

Emergency

Emergency is a serious event that needs immediate action. The type of emergency that may occur is completely unpredictable. No official documents examine the classification of emergencies. Each of them is an event on its own. It may be similar to other emergencies, but it is rare to have two which are identical in every respect. The exception to this for working radar controllers is a mid-air explosion, and although the actual cause of the explosion might well differ, its effect on the controller will be the same.

It is impossible to define instructions for all cases and write such a document as phraseology for emergencies. Nevertheless there are some standard procedures which help to prevent chaos and make controller’s work organized and regulated. Some types of emergencies have specific instructions as to the actions which the pilot and ATC controller must make.

An aircraft under emergency gets priority over other aircraft. There exist instructions concerning using special radiotelephony signals. Pilots must inform ATC by sending established signals (May Day, PAN, Securite) and the controller must impose silence.

There are certain actions which are common to a controller handling of all occurrences.

1. Don’t keep it to yourself.

2. Get help. And get it early enough to be of practical value.

3. Inform your supervisor. In most cases he will be able to do most of the liaison which will be needed.

4. Do not forget your other traffic. It may become necessary to transfer all traffic except the emergency flight to another frequency. The whole of the air traffic team on duty will be very busy to provide the best possible service to the flight in the difficulty. Emergencies are where all of the controllers training and expertise are vital.

5. Keep calm. Never let your voice portray nervousness or unease.

Sometimes the controller does not fully understand what the precise problem is. That’s why a controller (as well as a pilot) must know not only radiotelephony phraseology but also possess knowledge of the general English. Reading aviation magazines and accident reports can greatly help to understand problems which may occur.

 

Emergency Definitions

ICAO has some definitions concerning emergency procedures.

Emergency phase. A generic term meaning, as the case may be, uncertainty phase, alert phase or distress phase.

Uncertainty phase. A situation wherein uncertainty exists as to the safety of an aircraft and its occupants.

Alerting phase. A situation wherein apprehension exists as to the safety of an aircraft or its occupants.

Distress phase. A situation wherein there is reasonable certainty that an aircraft and its occupants are threatened by grave and imminent danger or require immediate assistance.

Emergency procedures.

Emergency is a serious event that needs immediate action.

Summarizing aeronautical experience a list of most common reasons for the crew to declare an emergency can be made: mid-air explosion, serious fire in the cabin or engine, oil or door warning lights, loss of an engine, bird strikes, illness on board. However, this list will never be comprehensive and complete. Thus, each emergency must be treated as an event of its own. It may be similar to other emergencies, but there hardly could be two identical in every respect. That is why it is totally impossible to define instructions for all cases and write such a document as phraseology for emergencies. Nevertheless, there are some standard procedures which help to prevent chaos and make controller's work organized and regulated.

An aircraft under emergency gets priority over other aircraft. An aircraft in distress informs ATC using radiotelephony signal MAYDAY, radiotelegraphy signal SOS. The aircraft in distress sets its transponder mode A code 7700.

An aircraft having some difficulties but which does not need immediate assistance can inform about it switching on and off its landing lights or flashing its navigation lights in a way different from the normal one.

An aircraft which has an urgent message concerning people safety, other aircraft or vehicle transmits radiotelegraphy signal XXX or radiotelephony signal PAN.

In some cases it can be difficult to determine into which of the categories a particular incident falls and in other cases it is quite clear. The English used in these events can be confusing and often does not give the information a controller needs to make a reasonable assessment of the situation. The pilot may not be proficient in the use of English outside the standard laid down phraseology. And there are no laid down phraseologies for emergencies. If in doubt as to exact nature of the problem, then ask for clarification. Never forget that one unusual situation can lead to another, and they can overlap.

Inform your supervisor. He will be able to do most of the liaison which will be needed. Do not forget your other traffic. The necessity of transferring all the rest traffic to another frequency may arise. Radio silence may be imposed on all traffic except the flight in emergency.