Study Guide:OBS

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This chapter of the Study Guide has been designed to give you all the information needed to start monitoring as Observer (OBS) with the intention to become a controller. It contains the fundamental basics so that you can understand, what happens. You won't learn how to control yet.

This version of the guide is of 24. April 2017.

Radio Communication - Basics

Radio Communication is the only way to make flying safe. So there is a lot of rules about it to make it so. As a starter, the following things apply:

  • The world's flying language is English, and there is exceptions.
  • All IFR flight communication in Austria is English.
  • For VFR communications, pilots decide if the want English or German (they do so by using their preferred language on first contact). There are some controllers at VACC Austria, who only speak English, and they may do so --> you don't need to learn German to control at VACC Austria.
  • To make radio communication safe, it is not complete English, but a defined phraseology. It is a form of simplified English, with certain words or phrases (and not others) and also a defined communication syntax.

Basic Rules

In order to achieve the goals set above the following rules important:

  1. Listen before you talk
    It's impossible for two radio stations to transmit on the same frequency at the same time. If this is done, the radio signal will be blocked and this will result in a nasty noise on the frequency. Therefore it's important that every station monitors the frequency for about 5 seconds before transmitting, to make sure there’s no ongoing radio traffic. If you hear an ongoing conversation, wait until the conversation is over before you begin to transmit. Don’t start your communication if there is a read-back expected on the last transmission even if there is a short pause.
  2. Think before you talk
    The radio traffic flow should be as smooth as possible. To achieve this it's vital to "think first" before transmitting so that a clear, concise and uninterrupted message can be sent.
  3. As far as possible use standard phraseology and syntax
    To prevent misunderstandings and to maintain the radio traffic as effective as possible, stick to standardized phraseology and skip slang and of course private messages. This might sound strange at the beginning, but the aim is simple: radio quality may be bad; controllers and pilots speak some hundred different languages; time to talk and listen may be scarce. Standard phraseology helps a great deal to shorten communication and prevent misunderstandings. In the course of this study guide and training documents, you will learn the relevant phrases. Stick to them - it's real life, and it's relevant on VATSIM, and you need to know it to pass your tests.

Callsigns and Principles

Every participant on the network has his own Callsign. Controller Positions are identified by their location and their Function (e.g. Wien Radar, Graz Tower), Aircraft either by their Registration (e.g. OE-ALB) or an Airline Callsign followed by a combination of numbers and letters (e.g. AUA25LM, SWR387). The airline is called by its real name (i.e. "Austrian" for AUA). Numbers and letters are spelled using the ICAO-Alphabet, found here in the Buchstabiertabelle.

Radio communication is safety-relevant. If anything goes wrong, [this] happens. Therefore, it follows four crucial principles:

1) Syntax

A call has the following structure:

Called station, calling station, Message
Example: LHA123: Wien Radar, Leipzig Air 123, Flight level 240.

That means: LHA123 calls Wien Radar to report at which altitude he flies.

2) Readback

When a controller (or aircraft) transmits a message to a station it is very important that the receiving station acknowledge the message. But saying "roger" is only in movies - what are you "roger"ing? The called station understood something, but what? Therefore, the message content has to be read back in relevant parts. If the receiving station does not acknowledge, the transmitted message is considered as a lost transmission and the sender should resend the message or check if the receiving station got the message. For a controller, this is extremely important to remember, since if a pilot's readback is incorrect, the controller has to ask for confirmation, i.e a new readback.

The Syntax for the readback is reversed: First the message, then the callsign.

LHA123: Wien Radar, Leipzig Air 123, Flight level 230.
LOVV_CTR: Leipzig Air 123, identified, continue.
LHA123: Continuing, Leipzig Air 123.

That means: LHA123 calls in the first time and tells which altitude in order to be found and identified by the controller (see part 3) below). The Controller acknowledges the contact, and then issues an order: to coninue (on the filed flight plan). This order is then read back by the pilot.

  • Items that must always be read back in full are all clearances (including altitudes, headings, speeds, radials etc), runway in use, altimeter setting (QNH or QFE) and transition level, and all frequencies.
  • There are also items that should not be read back to reduce unnesessary radio transmissions. In short, this includes everything not mentioned above, but a few examples are: wind, temperature and other weather information (except altimeter settings) and traffic information in detail.
LOWW_APP: AUA251, turn left heading 290, descend Altitude 5000 feet, QNH 1019.
AUA251: Turn left heading 290, descending altitude 5000 feet QNH 1019, AUA251 
LOWW_TWR: AUA251, traffic information: Cessna on rwy 16 downwind, report in sight.
AUA251: Traffic in sight, AUA251.
LOWW_GND: OE-DLT, taxi to Holding Point Runway 29 via Exit 12, M and A1, QNH 1019, give way
to Speedbird Airbus A320 crossing right to left on M.
OE-DLT: Taxiing to H/P Rwy 29 via Exit 12, M and A1, giving way on M, QNH 1019, OE-DLT.

3) Contact - messages - handoff

All conversation follows the following pattern:

  1. Initial call
  2. ... messages (there may be pauses, and other messages to other stations)
  3. handoff/handover.

Initial Call: An aircraft arrives in an airspace and needs to tell "Hi, I'm here!". To initiate the contact between two stations an initial call has to be made. Example - Austrian 251 is calling Wien Tower:

AUA251: Wien Delivery, Austrian 251, Radiocheck.
LOWW_DEL: Austrian 251, Wien Delivery, read you 5 by 5.

Messages: Once contact is established, you stay in contact, even if you don't talk to each other. This means: You know that the other station knows of you and counts on you to pick up conversation any time.

In ongoing conversation, two rules apply:

  • when giving an instruction the callsign is first (the called station needs to know that it is for him/for her.)
  • when reading back the callsign is at the end (although you are allowed to do it at the beginning too - the instructing station needs to know if the message has reached the right recipient).

Usually, ATC gives instructions and pilots read back, so in normal operations this means: ATC omits his own station (but says the called station), and pilots omit the calling station and report their callsign at the end.

Now one Example for a normal Clearence. You will learn in detail in the next section. Situation: Austrian 251 is requesting IFR clearance to München at Gate C34,Type of aircraft is a Fokker 70, Information C is the latest weather recieved.

AUA251: Wien Delivery, Austrian 251, Information C, Gate C34, request IFR clearence to München.
LOWW_DEL Austrian 251, cleared to München via SITNI4C departure, initial climb 5000 feet, Squawk 4612, QNH 1012.
AUA251: Cleared to München via SITNI4C departure, initial climb 5000 feet, Squawk 4612, QNH 1012, AUA251.

Handover/Handoff: At some point, it's time to say good bye - that is a handoff or handover. It is vital that no aircraft disappears from the radio. Handover is a transfer to another station. Handoff is dropping contact into uncontrolled airspace (like UNICOM).

LOWW_DEL Austrian 251, readback correct, for push and start contact Wien Ground on 121.600.
AUA251: Contacting Wien Ground on 121.600, Austrian 251.

4) Reserved words

Some words are reserved and should only be used, if they are meant:

  • mayday and Pan-pan: Only use it, when you declare it.
  • takeoff and landing: Only use it, when you clear (or read back) for takeoff or to land. If you report ready for t... no: you report ready for departure.
  • Affirm(ative) means "yes". Negative means "no". Unable means that the pilot can't do what the ATC just instructed.


As a controller, METAR and TAF are the base for clearances: They determine procedures, active runways and even airport closures. When you become a controller, you should be able to understand METAR and TAF. Startign with Study Guide:TWR, you will learn what it means for your decisions as controller.

References for detailed information: METAR, TAF 

How is an Aerodrome organized

As airports grew bigger over time also the workload for the Air Traffic Controller handling the traffic got bigger. Soon it was necessary to distribute this workload onto more than one controller in order to be able to cope with the traffic.

So the Tower Position got divided into three basic types with different areas of responsibility.

  1. Clearance delivery (DEL), responsible for checking flightplans and issuing IFR clearances to departing aircraft.
  2. Ground (GND), responsible for all traffic on the apron and the taxiways.
  3. Tower (TWR), responsible for movements on the runway and within its associated Control Zone.

Because Tower and Ground controllers rely very strongly on what they see out of their window, these are the positions which are situated within the airports control tower.

Apart from that there are the controllers who manage the traffic once it has left the control zone. They are again divided into:

  1. APP Positions, managing the traffic within the airports vicinity (the so called TMA, Terminal Area). In Austria they are situated directly at the airports.
  2. ACC (Area Control Center, on VATSIM the abbreviation CTR is used) positions, which are responsible for enroute traffic. They reside in Vienna.

If the air is too crowded, APP and CTR can be divided horizontally (lower and higher APP), vertically (north APP and south APP), or a director responsible for approach spacing - but we won't want to make it too complicated for now. Local procedures and agreements regulate it, how this is done.

  • Since they all use their radar to control air traffic, they are also called Radar positions.

Login as OBS

With this preparation, you are fit to login as Observer. You can listen and understand what happens and learn from others doing. Important info and the Euroscope software can be found [here]. (You must be accepted as controller to access the page. If you are not yet, contact your mentor).

Please abide to the following rules:

  • Logged on as Observer, your callsign should be a letter code (best are your initials) and "_OBS", like "CF_OBS". In the "connect" dialogue, set the visual range to 100 - the maximum allowed range for observers. If you exceed, you will get a nasty whack from a VATSIM supervisor.
  • You can log onto the VACC Austria Teamspeak server. Software and access instructions are [here]. Teamspeak version 3 is current.
  • Teamspek is recommended - vital ATC coordination happens there. If you want to listen to controller coordination then switch channel to where those controllers are who you monitor. Say hello, say that you observe and learn, and then shut up. Don't even ask if intense coordination is going on - wait for a pause in traffic and conversation. Do not change into a channel where an exam is going on.
  • If you ask a question, then wait for a pause and ask first, if you can ask a question. You can do so by PM in Euroscope or via Teamspeak. Do not ask if traffic is intense, if a training or an exam is going on (in trainings, you might want to ask the trainer in a PM first). Controllers are happy to explain, but ATC is first.

More Information

To download them, register yourself as user with VACC-SAG, it's free.

  • a very explained in detail documentation is the Pilots Guide check here also the METAR section Meteorologie

More information on Radiotelephony

... are not really an option at VATSIM. We take the real thing as reference, and this reference is really thick. Some links for further reading:

Overview: Study Guide - Next: Study Guide:Delivery