Controlling

    8.1 Introduction

    We will end this theoretical manual with a description about what you need to know about call-signs, transponder, charts and things that are related to your radarscope. The more practical aspects will be found in the guide and this section is closely related to some of that material.

     

    8.2 Radar Clients [S]

    The radar client is the software that you will work with as a controller. 
    At the time this manual is written there are three clients available; ASRC, VRC and Euroscope. It is not in the scope of this manual to describe them in detail, since both already have excellent manuals. 

    The “radar-part” of the clients is maybe the most important and it gives you information about the traffics movement through the sky and on the ground. But there are numerous other functions built in to the software that will help you control and give information to the traffic. It is well spent time to read the manual of the client that you will be using closely. 

    8.3 Clearance [S]

    All instructions regarding the movement of aircraft in air or on ground are called clearances. You can issue clearances both en-route and before the aircraft is airborne.
    A pilot who wants to fly in controlled air space (except for Class E)  is required to get permission from a controller. To be able to give permission, you as a controller need to know what intention the pilot has. The pilot can send this information to the ATC in a so called flight plan. Aircraft flying VFR can ask for clearance without having sent a flight plan. The pilot must in that case send all relevant information via radio. This is very unusual in our on line environment. Clearances can vary in content and can contain restrictions of different sort. Before issuing a clearance, you need to ascertain that it doesn't lead to a conflict between two aircraft. A good strategy is to give as few restrictions as possible in the clearance. More about clearance will be found in the GUIDE.


    8.4 Call sign [S]

    All aircraft need to have a call sign in order to establish radio contact.
There are difference forms of callsigns.
SAS345, KLM574, DLH1771 are examples of large companies' call signs. DLH1171 isn't the name of a specific aircraft but rather the ICAO code  of a company (Lufthansa) followed by a number that is specific for flight path.
British Airways uses the acronym BAW, but this is read as “Speed bird”. There are tables over these acronyms; you don't have to know them by heart. SE-GTD and OH-SLT are examples of specific aircraft. SE stands for Sweden and OH stands for Finland. The two country letters are followed by three letters to designate the aircraft. Even though all aircrafts have specific call signs like the one above, they are almost only used when flying private and not for a company. Smaller aircraft that flies VFR are one example where the aircraft specific call sign is used.
When writing call sign into the flight plan, the pilot can either use the full aircraft specific name (SEGTD, OHSLT) or the acronym for the flight-operator followed by the flight-specific numbers/letters (BAW554D, KLM574).
When using the call sign on radio, you are allowed to make some abbreviations after the first contact has been established and the quality of the radio transmission is good; When reading OHSLT you can omit the second letter or both the second and third letter, if the above criteria are met and there’s no risk for confusion with other aircrafts in your airspace.
  • Oscar Hotel, Sierra, Lima, Tango
  • Oscar, Sierra, Lima, Tango3 Oscar, Lima, Tango


    8.5 Transponder [S]

    On ordinary radar, you can see the position of the aircraft, but not their height. You also can't differentiate one blip from the other. This has been solved by installing one (or often more than one) transponder in the aircraft. This box transmits a signal which contains information on the height together with a four digit code.

     

     

    As controller, you give the pilot this unique four digit transponder code. This is done at clearance, but the transponder code can also be changed en route. Two aircraft cannot have the same transponder code if they are in the same area. This is not a problem on line, but you should try to give every aircraft a unique transponder code. You will get an error message (CODE) is this isn’t done, but you will still be able to see the correct call sign on your scope.
    Transponders can be set in different modes:

  1. Stb (Standby) – Means that the transponder doesn’t give information about the code entered or the  height. (Default mode on VATSIM when on ground)
  2. Mode A – Only the code, and not the height is transmitted.
  3. Mode C – Code and height is transmitted (Should be used on VATSIM when flying)
  4. Mode S – Used together with TCAS*. Gives same information as mode C
VATEUD have reserved a range of codes for each FIR in Europe. This has been done in order to minimize the risk of two planes being assigned the same code. The list can be found on on the main website
 
The exception to unique code is VFR aircraft which sometimes are given the same transponder code (7000 in most European countries, 1200 in some). 
 
A transponder works with binary digits and can’t use the digits “8” and “9”. Hence a transponder code can’t contain these two digits. 

* Traffic alert and Collision Avoidance System (or TCAS) is an implementation of the Airborne Collision Avoidance System mandated by ICAO to be fitted to all aircraft over 5700 kg or authorised to carry more than 19 passengers, designed to reduce mid-air collisions.



    8.5.1 More about transponders [S] 

Primary radar works best with large all-metal aircraft, but not so well on small, composite aircraft. Its range is also limited by terrain and rain or snow and also detects unwanted objects such as automobiles, hills and trees. Furthermore it cannot estimate the altitude of an aircraft. Secondary radar overcomes these limitations but it depends on a transponder in the aircraft to respond to interrogations from the ground station to make the plane more visible. 
An airborne Transponder transmits a reply signal on a frequency of 1,090 MHz in response to the SSR interrogation which is transmitted on a frequency of 1,030 MHz.

Due to the technique that the transponder is built with, only the digits 0 to 7 can be used (8 and 9 can’t be entered). This means that there are 8x8x8x8 = 4096 unique codes. Some codes are reserved for special use. 


    8.6 Understanding Charts [S+]

    Charts are the maps of the skies. They contain information about airports, airways, airspace and much more. Hence, they are essential for both flying and controlling.
All official vACCs are required to publish carts over the airspace they govern. These charts might be simplified, but should at least contain the essential information needed to control and fly in the area. Many countries have published the real charts for their airspace on Internet for easy reference. There are also companies like Jeppsen that are selling charts for profit.   Regardless of how you find the charts you must be able to interpret them. An extensive guide is published for reference and can be found here: understanding charts
A short description about some essential parts is given below. 


    8.6.1 Standard Instrument Departure SID [S] 

SID stands for Standard Instrument Departure. 
It is a pre defined route which has been named using a special system. SIE 1A is an example of a SID where SIE (SIEDLCE) is the navigation beacon where the SID ends. 1 is a version number. Next time the SID is updated it gets version number 2. Changes are quite rare and when done they are mostly minor adjustments. 
If you want a pilot to fly ARS 3C but the pilot only has charts for ARS 2C, then this is normally not a problem, but you have to make sure that no big changes has been made between the two versions. The ending letter of the SID is usually, but not always connected to a specific runway. For example, all SIDs ending with ”G” at Arlanda depart from runway 19R. but in EKCH for example all "A" designators would be valid both for 04R and 04L.
There are obvious advantages with the SID system. Most SID are quite complex, and to give the instructions to fly them step by step would indeed be time consuming. Since the autopilot usually is used to fly the SID, all aircraft flying the same SID will do it in the (close to) exact same way, making it predictable. Moreover, the SIDs and STARSs (see bellow) are designed in a way to minimizes potential conflict situations.


    8.6.2 Standard Instrument Arrival STAR [S+]

As the name implies, this is a standard procedure when arriving at an airport. It's like a route to the airport. This road has a name that has three parts. The first part is the navigational point where the route starts, the second is the version number, and the third is usually but again not always coupled to a certain runway(s). An example is OSKOR 1J. The point at which the STAR ends is called Initial Approach Fix (IAF). in some cases  the STARs continue  and  end at the Final Approach Fix ( FAF), and that means that you as controller don't need to vector the aircraft unless there is other traffic in the way. The only thing you have to do is to instruct the pilot how to descend the aircraft. This simplifies the arrival considerably for both pilots and controllers. 

    In Zurich though, (as an example) the situation is different, as here it is the first letter of the IAF where the STAR ends. RILAX1A for instance ands at AMIKI and BERSUG1G ends at GIPOL. and neither AMIKI not GIPOL are linked to any runway.

There are exceptions of course, where the STARs don't end at the final, but at a navigational point some distance away from the runway. You as a controller must give vectors the last part to the runway. If you for some reason don’t give vectors, the pilot must enter holding at the STAR's ending point (clearance limit). Make sure to avoid this.

     

    8.6.3 Transition [C] 

There is a defined transition point at which an airway and a SID or STAR intersect. Some STARs and SIDs have more then one transition that are best thought of as branch routes feeding the main procedure. 


    8.6.4 Routes [S+]

ATS routes are pre-determined routes connecting waypoints to each other, which the aircraft will follow. ATS routes are named by a character followed by two or three numbers. If the route is used in the upper airspace it is also given the prefix “Upper”. For example UN872 (“Upper November 872”). 
Some ATS routes are for aircrafts flying in one direction only.


     8.6.5 Approach [S+] 

There is much information on the charts regarding the approach. 
There will be some kind of navigational aiding system in use if the approach isn’t visual. When talking about approach aids, they are often divided into two categories: precision and non-precision. A non-precision approach only gives you guidance in one axis; mostly horizontally but on rare ocations can also be vertically, while a precision approach gives you guidance in both the horizontal and vertical axis.


    8.6.6 Precision approaches [S+]

The most usual type of precision approaches is an ILS approach, and that is the only precision approach covered in this manual. An ILS guides a pilot on the approach by indicating the vertical and horizontal deviation  from the correct approach path.
Information about how the ILS is constructed, frequencies, glide slope etc can be found on the chart. Please note that not all airports are equipped with ILS. Other types of approaches must then be used. See below. 

     

    8.6.7 Non-precision approaches [C] 

The most usual types of non-precision approaches are VOR, VOR/DME and NDB approaches. 
The VOR approach is performed by flying towards a VOR beacon. The VOR is in this case located at the airport. 
A VOR/DME approach is also an approach into a VOR, but the pilot can use the distance to the airport given to him from the DME. 
A NDB approach is done by flying to a NDB beacon which is located on the runway extension and then flying on a certain heading which directs the pilot to the runway. We have only covered the non precision approaches briefly here, but it is important to know that they exist since not all airports are equipped with ILS. 
All information about how to perform the non-precision approach is found on the speciphic charts for the airport. 



8.6.8 Visual approach [S+]

     A visual approach is an approach by an IFR flight when either part or all of an instrument approach procedure is not completed and the approach is executed in visual reference to terrain.


     8.6.9 Minima [C]

    At the approach, it is required that the pilot has visual contact with the runway or runway lights at a certain height specified at the chart, to be able to continue the approach and eventually land. The reason for this is that the pilot needs visual reference in order to make a safe landing. Depending on type of approach and approach aids used, the lowest height a pilot can descend to will vary. This height is called the decision height (or decision altitude).

At this height the pilot must have visual contact with the runway or runway lights. If not, a missed approach must be executed. 
Since the different approach aids leads the aircraft to the runway with varying precision, the different approach types will have different “minima’s”. For example, a NDB approach has relatively high minima, approximately a height of 400- 500 ft, while a typical ILS approach has minima of 200 ft. A VOR approach has normally a minima of 300-400 ft. Note that “height” is ft over ground and “altitude” is ft over sea level. 
Decision height/altitude (DH/DA) is used for precision approaches and Minimum Descent Height/Altitude (MDH/MDA) is used for non-precision approaches.



     8.6.10 Runway Visual Range (RVR) [C] 

RVR is the Runway Visual Range.

The Pilot may commence an instrument approach regardless of the reported RVR/Visibility but the approach shall not be continued beyond the outer marker, or equivalent position, if the reported RVR/visibility is less than the applicable minima.

If, after passing the outer marker or equivalent position in accordance with the above, the reported RVR/visibility falls below the applicable minimum, the approach may be continued to DA/H or MDA/H.

The approach may be continued below DA/H or MDA/H and the landing may be completed provided that the required visual reference is established at the DA/H or MDA/H and is maintained.
RVR is measuredonly if the visibility is below 1500 m. RVR indicates the visibility of the runway lights, and will thus often give a larger distance than actual meteorological visibility.



     8.6.11 Missed approach Point (MAP) [C] 

For non precision approaches, a Missed Approach Point (MAP) is indicated. MAP is the point where the pilot at latest must increase thrusters (i.e. a go-around) if he hasn't a visual on the runway. The MAP can be a DME distance, a timed distance or a navigational aid.


     8.6.12 ILS categories [C]

The reason for going back to ILS approach is that we have now covered RVR and visibility, which is needed to understand the different ILS categories. 
The different ILS categories have different precision, and there are different visibility requirements when using them. The categories are named CAT I, CAT II, CAT III a, CAT III b and CAT III c. The category for each runway is given on the airport charts. 
During normal ILS CAT I, minimum RVR is 550 m. If RVR is below 550 meter, the visibility is too low, and an approach can not be initiated unless the airfield is equipped with a higher precision ILS. If there isn't a higher precision ILS, the pilot can either wait or see if the weather improves, or land on an alternative airfield where the visibility is better.

ILS Classification is used to determine the accuracy of the landing system. Category one (CAT I) is the least accurate, and CAT III is the best. This means you can fly the approach to lower limits (decision heights) on a CAT III ILS than on a CAT I ILS. When you reach the appropriate limit you need to see the runway, otherwise you have to go around and fly the missed approach procedure. We are only talking about the ground facilities here. In real life there are more factors which can change your lowest limit.
  1. Pilot qualification
  2. Airplane qualification
  3. Ground facility qualification
The one which has the highest limit is the limiting factor for an approach. Standard limits are:

     

     

    CAT I

    CAT II

    CAT III-A

    CAT III-B

    CAT III-C

    Limit

    200 ft *

    100 ft

    50 ft

    0 ft

    0 ft

    Visibility/RVR

    550 m

    300 m

    100 m

    50 m

    0 m

The visibility is the limiting factor in an approach. 
If the Cloud base is at 50 ft, but the visibility is 600 meters, you may fly and land with a CAT I ILS. The chances that you can see the runway at 200 ft are very limited, but maybe the approach lights are very bright.
So, if a pilot is not qualified for CAT II, the visibility is 400 meters, the ILS (ground) is CAT III-B, the airplane is CAT III-A, the limiting factor will be the pilot, so CAT I is your lowest limit.
If the pilot is CAT III-B qualified, the airplane as well, visibility is 100 meters, but the ILS (Ground) is CAT II Only, CAT II is your limit.
If the pilot is CAT II qualified, the airplane CAT III-A, visibility is 10 km, but the ILS (Ground) is CAT II Only, you have no problems, because the weather is perfect!



    8.7 Flight plan and Route [S]

    The main reason for filing a flight plan and route is the pilot is informing the ATC units along the way his/her requests to complete the journey; this will include the speed, cruise level waypoint and airways to be flown.

    And one other major reason in non-Radar equipped regions (these are becoming much less in the real world and non existent in the simulated environment) if you haven't arrived at your destination in the allowed time limit, the emergency services will have a good idea where to look for you. Flying isn't as easy as jumping into your car and going from point A to B.

    The Basic format for a f/p route is;
Waypoint RouteDesignator Waypoint DCT Waypoint RouteDesignator.... etc.
     In some circumstances were a Waypoint doesn't cross from a designated route the letters "DCT" meaning direct are used (there are no waypoints in the world with this name or code). Some countries do not allow deviation from designated routes, or if allowed for short distances only. 


    8.7.1 Example of route [S+]

A simple Flight Plan between LTBA (Atatürk) and LGAV (Venizelos) might look like this:
 
BIG UG8 AMANI UΝ604 KEPIR

BIG is the first navigational aid and depending on the pilot's and ATC's choice this can be a vector or SID departure (i.e.BIG1S). Remember that a pilot has the choice of refusing a SID/STAR and may request vectors. 
Airway UG8 goes from BIG to AMANI and from here the route will continue on UN604 to LSV, OLIDA and KEPIR. 
The complete list of waypoints does not have to be specified as shown in the route above, but if included this will also be acceptable, but will usually take very long to fill and read and is not good practice. 


     8.7.2 Speed and cruising level [S+]

You will also find information about speed and cruising level in the flight plan that the pilot sends. This information can be given in different formats as specified in the table below.

    Cruising Speeds

    e.g.

    Km/h

    Kxxxx

    K0830

    Knots

    Nxxxx

    N0456

    Mach

    Mxxx

    M075

    Cruising Level

    e.g.

    Flight Level

    Fxxx

    F320

    Altitude

    Axxx

    A045

    Standard Metric Level in tens of meters

    Sxxxx

    S1100

    Metric Altitude in tens of meters

    Mxxxx

    M0120

    VFR (unspecified)

    VFR

    VFR

     


 

    8.7.3 Changes in speed and cruising level [C+]

Speed and Level change format is:
RouteDesignator Waypoint/SpeedLevel...
either if a speed or a cruise change is requested both are supplied.
 
Cruise Climb format is:
RouteDesignator C/Waypoint/SpeedLevelLevel... 
Begins with the letter "C" and waypoint, the speed is the intended cruise speed to be maintained during the cruise climb and the layer of the two levels during the climb. If the second level is specified by the letters "PLUS" this indicates the level above which the cruise climb is planed for.

The speed and level formats should be quite obvious depending in what part of the world you are e.g. Km/h and metric cruise levels are used in Russian Federation Countries.

A route from LTBA (Atatürk) to EGLL (Heathrow) might look like this:
N0440F300 FENER A16 VADEN UL610 BATTY UL608 LOGAN 

If a climb was requested, the route may appear like this:
FENER A16 VADEN UL610 ABETI/M075F340 UL610 BATTY UL608 LOGAN
 
If the waypoint where a speed/level change is required the following airway designator will be supplied, even if on the same route designator UL610 in this case.

A long haul route from LTBA(Atatürk) to KORD(Chicago) might be similar to this:

N460F280 FENER A16 VADEN UL610 C/TIMOT/N0455F300F320 UL610 BATTY UL608 C/BUB/M080F340PLUS UA24 NIK UL610 LAM/M080F360 UB29 CPT UG1 STU UN546 DEVOL UN544 DOGAL/M081F360 54N020W/M081F370 55N030W 55N040W/M081F390 54N050W CARPE REDBY YNA YRI YXI ECK J94 FNT PMM4
 

     
     

     8.7.4 RVSM Tansitions [C+]

Since the introduction of RVSM (Reduced Vertical Separation Minima) in Europe, the pilot flying in and out of RVSM airspace will require a cruise level change to comply with correct Flight Levels for the airspace in which they are operating.

A Route from OEJN(Jeddah) a non-RVSM airspace to LTBA(Atatürk) a RVSM approve airspace, there will be a need for a transition between the two:

N0460F310 EPLOM A424 PMA B544 TUSYR/M080F340 VB36 GAZ DCT TOROS VW75 BAG UL614 YAA

In the above route the initial cruise level is FL310, this is the correct level for non-RVSM airspace at TUSYR the entry/exit point into RVSM, the level requested is FL340, also correct for RVSM airspace. 

The speed and level change format is exactly the same as for step climb. 


 

    8.7.5 A Real Flight Plan [ref]

A real world ICAO coded flight plan contains much more information than what we are usually accustomed to in the simulated environment, here's an example.

    CODED ICAO FLIGHT PLAN

     

    (FPL-N100A-IG

    -GLF4/M-SXWHIGRY/S

    -KEWR2315

    -N0465F370 DCT MERIT DCT HFD J42 BOS DCT VITOL/M080F410 N27A

     NANSO/N0459F410 N27A RAFIN/M080F410 DCT 45N050W 47N040W 49N030W

     49N020W DCT BEDRA/M080F410 UN491 TAKAS/N0459F410 UN491 VMP UL851

     MELKO UM606 BLM DCT

    -LSZH0652 LSGG

    -EET/KZBW0003 KZNY0040 CZQM0041 CZQX0141 EGGX0342 EISN0457

     EGTT0531 LFRR0534 LFFF0606 LFEE0631 EDFF0645 LSAZ0646

     RAFIN0156 45N050W0204 47N040W0253 49N030W0342 49N020W0432

     REG/N100A SEL/GQEK DOF/020214 RMK/TCAS EQUIPPED AGCS EQUIPPED)

    KZNYZQZX KZBWZQZX CZQMZQZX CZQXZQZX EGGXZOZX EBBDZMFP LFPYZMFP

    This can be decoded as (FPL-N100A(Aircraft Call Sign or Flight Number)-I(FR)G(eneral flight)

  • GLF4(Gulfstream 4)/M(edium Wake Category)-Equipment/S (transponder equipment do not confuse with equipment suffix)
  • Departure Airport and Time in Zulu
  • Route N0465F370 (KTAS465 initial speed and FL370).... at VITOL/M080F410 a climb to FL410.. etc
  • Arrival Airport (Zurich) and duration of Flight 6hours and 52minutes Alternate Airport (Geneva)
  • Estimated enroute time for crossing FIR regions... EGTT0531 London FIR in 5hours and 31minutes and other info the pilot wants you to know.

     
    8.8 Separation [S]

    As mentioned before, this is your most important task. How much should you separate? What should be done in order to avoid accidents, or as it is called in aviation, conflicts? Since this is such an important task it will be covered here and in the GUIDE.

  1. Have a clear strategy what you want the pilot to do. Order and contrary orders leads to confusion and frustration.
  2. Consider what implications your instructions have. It's not a good idea to give a pilot clearance to land if you at the moment before gave another pilot instruction to line up on the same runway.
  3. Talk clearly and not too fast. It may sound “cool” talking fast but it often leads to misunderstanding which makes it slower.
  4. Use standard phraseology. This reduces the risk of misunderstanding and confusion.
  5. Listen to the readback carefully as it was the first time the instruction was given. Mistakes happen easily.
  6. Act immediately if a conflict can occur. Don't wait until the conflict is developing. An aircraft doesn't turn immediately when given the instruction, the pilot needs to hear the instruction, act on it and then the aircraft starts turning.
  7. Don't take on more than you can manage. Take a position which you feel you manage and ask for help if you need it and there is someone available. That was the ”software” which always is the most important.


 

    8.8.1 Vertical separation [S]

Vertical separation should at least be: 
  • RVSM: 1000 ft
  • Non-RVSM: 2000ft
You are allowed to climb or descend an aircraft to a level previously occupied by another aircraft provided that vertical separation is maintained. This is done by observing the transponder echo in mode C. 
To make sure vertical separation is maintained, it has been decided that aircraft eastbound use odd flight levels and aircraft westbound use even flight levels.
This, so called semi-circle-rule applies when no other rules override it. 
Some airways/routes have specific flight levels assigned to them that contradict the semi-circle-rule. 
There are areas where the rule isn’t applicable due to local restrictions etc. Please refer to your local vACC and charts for this local information.

     

     8.8.2 Horizontal separation [S]

There are several ways of maintaining horizontal separation, but as long as aircrafts are in radar covered area and use a transponder that transmits pressure altitude (mode C (Charlie)) the following rules apply.

There are other conditions not covered here that applies for example when crossing oceans. 

The basic rule is that there should be at least 5 nm horizontal separation in all directions. You can therefore imagine a circle around all aircraft with 2.5 nm radius to reach the 5 nm requirement.

There are situations when the 5 nm separation can be overruled. One situation is when two aircraft are on the final for landing. In this case 3 nm separation is sufficient. (not regarding wake turbulence separation)
Other rules may apply on national level. 
It is however not recommended to use this small separation even in this situation.

Depending on the airspace class you are in most instances required to separate IFR traffic from all other traffic in controlled airspace, so it's often your responsibility to separate IFR from VFR and vice versa. 
See 4.3 for more details. 


 

    8.8.3 Separation between departing traffic [S+]

It is often difficult to know the speed and vertical climb rate of a departing aircraft. It depends of course on the type of aircraft, but also on current weight and weather. You should avoid giving departing aircraft speed restrictions. Instead, use climb rate and vectoring as means of separation during climb. 

A rule of thumb is to always separate at least traffic by one minute for departing aircraft, in some cases even more. This depends on the performance of the aircraft. If you cannot separate using radar, use the below table as reference.

    Minimum Separation

    Condition

    1 Minute

    The first aircraft turns more than 45° compared to following traffic headings.

    2 Minutes

    The first aircraft's speed is 40 KTs higher than following traffic.

    3 Minutes

    The heading difference is less than 40° and the first aircraft'  is similar or slower than thefollowing aircraft.

     
     

     8.8.4 Wake turbulence spacing minima [C]

All aircraft generate turbulence called vortex wake. Large aircraft flying at slow speeds create the most severe wake turbulence. This turbulence can cause problems for following aircraft, which in severe cases can cause the pilot of the following aircraft to loose control. 
In addition to separation minimum above, another spacing minimum therefore needs to be taken into account. 
The wake turbulence categories are based on the maximum take-off mass of the aircraft. See 7.2.1 for more details. 

     

    8.8.5 Speed and height [S+]

Speed can be used for separation, but it should be used with restriction. The only exception is when separating traffic inbound for arrival. Climbing traffic and en route traffic should instead be separated using vectors and height. 
Using speed for separation for inbound traffic is however important since all inbound aircraft sooner or later will have the same height at the same place, but not at the same time. 
To descend and at the same time reduce speed can be difficult, especially for turbo jets. Therefore, it is essential to inform the pilot which of the two instructions has priority.

     

    8.8.6 Minimum Speed [C]

An aircraft needs to maintain a certain speed not to fall to the ground. 
The minimum speed is mainly dependent on the weight of the aircraft. There are also other factors, so it is not always possible for a pilot to slow down or speed up to the instructed speed. It is the pilot's responsibility to inform you of this. 
In that case, you must separate him from other aircraft by other means. In aircraft performance tables, there are several speed restrictions given, but only two are of interest for the controller. 
The first is “minimum clean” which is the lowest speed an aircraft can maintain without using flaps or spoilers. 
The second is “minimum approach speed” which is the lowest speed an aircraft can maintain using both flaps and spoiler. 
Avoid giving a pilot, who is flying using his flaps, a speed instruction which forces him to again retract his flaps.

Apart from the specific aircraft's speed restrictions; there are speed restrictions common for all aircrafts. By following these, you need not study the specific aircraft's specifications:

Aircraft at FL280 – FL100:
Do not give a speed restriction below 250 knots or corresponding Mach.

Aircraft below FL100:
Turbo jet: not slower than 210 knots, except when within 20 nm from runway, in that case not slower than 170 knots. 
Turbo prop: not slower than 200 knots, except when within 20 nm from runway, in that case not slower than 150 knots.

Departing traffic (if speed restrictions really are necessary):
Turbo jet: not slower than 230 knots.
Turbo prop: not slower than 150 knots.
Helicopter: not slower than 60 knots.

     

    8.9 ATIS [S]

    ATIS (Automatic Terminal Information System) is in real life a recorded message that is transmitted on a specific radio frequency. All major airports have ATIS and all pilots approaching the airport are required to monitor the ATIS. 

    What information that is put into the ATIS depends on which position you man, but please note that an ATIS must never be more than 4 rows with a suggested maximum at 3 rows. This is a VATSIM requirement.

    Information that should be included regardless of what position you man:

  • Voice channel (Note that this is now automatic for users of VRC or ASRC 1.3)
  • The name of your position
  • The ATIS version. You start with ALPHA, and every time you change anything you use next letter in the alphabet. When you reach ZULU you start over with ALPHA. When pilot calls you for the first time he should inform you which ATIS version he has received. This way you know he has the latest information.
  • Date and time (more about this below)
  • Active runways used. Always specify landing (arrival) first. (the exception being a DEP controller who does not need to include this information