Study Guide:Tower
This study guide is still work in progress. Stay tuned for further chapters.
Introduction
This Study Guide is designed to give you all the information you need to become a Tower Controller within VACC Austria. We assume that you have already read the Delivery and Ground Study Guides and that you have some experience controlling on VATSIM. Since you will handle aircraft in the air for the first time, we want to discuss some basic principles of flying before actually talking about procedures. Also we'll have to talk about some organisational issues. The fourth chapter of this article will then familiarize you with the procedures you need for controlling tower positions.
Aircraft and basic Flying Principles
Producing Lift
For an aircraft to fly the lift force produced by (mostly) the wings has to outweigh the gravitational force that affects the aircraft.
Basically a wing produces lift by deflecting the air it moves through into one direction. According to Newton's third law of motion the lift is produced into the opposite direction. This lift grows with the speed the aircraft has in relation to the air and with the angle the wing draws with the direction of movement. This angle is called Angle of Attack (AoA).
The principle only works as long as a steady airflow around the wing exists. As soon as the airflow seperates from the wings surface the lift starts to decerease. The AoA at which this occurs is called critical Angle of Attack. It depends on the profile of the wing and it's dimensions but for subsonic aircrafts it typically lies between 8 and 21 degrees.
Think of an level flying aircraft that reduces it speed. In order to compensate the reducing lift the pilot has to raise the nose. However at some point the Angle of Attack will cross the critical angle of Attack and the pilot will find himself in a stall. So the speed of an aircraft is limited on the lower side by the so called stall speed. Because the stall speed depends on the profile most aircraft are equipped with devices that alter the profile during flight such as flaps or slats.
On approach pilots have to fly in a certain speed range in order to conduct a safe landing. The lower boundary is called landing reference speed and is often a fixed multiple of the stall speed. As a result of this the approach speed also depends on weight an aircraft configuration (Flap/Slat setting). For safety the Approach Vapp is higher than Vref and the difference depends mostly on the weather conditions.
Generally you can say that bigger aircraft also have a bigger approach speed however at some point this rule does not work anymore because the Vref depends largely on the aircrafts weight in relation to it's maximum takeoff weight (MTOW). The speed ranges from 50 knots in a C150 up to 170 knots with a fully loaded 747. However for example it is possible that a light 747 is slower than a fully loaded 737.
Aircraft Categories
The most important ways of categorizing aircraft in aviation are by weight or by approach speed.
Weight Categories
Aircraft are categorized into three weight categories:
| Category | MTOW |
| Light Aircraft (L) | < 7 000 kg |
| Medium Aircraft (M) | 7 000 – 136 000 kg |
| Heavy Aircraft (H) | >136 000 kg |
You can find a list of aircrafts in this link [1]
Weight depicted is MTOW.
Approach Speed
Aircraft are categorized by their reference approach speed (Vref) at maximum landing weight:
| Category | Vref |
| A | <= 90 knots |
| B | 91 - 120 knots |
| C | 121 - 140 knots |
| D | 141 - 165 knots |
| E | >= 165 knots |
Before you start controlling
Tower is responsible for all movements on the runways as well as for all movements within the control zone. He decides which runways are in use and maintains the ATIS. Tower is also responsible for ground and delivery if they are not online.
Airspace Structure around Major Airports
ATIS
ATIS stands for Automatic Terminal Information Service and is a usually automatically generated broadcast that contains essential informations for pilots. It is continuously broadcasted on a dedicated frequency. On initial contact with the controller, pilots should already have listened to the ATIS and state the identifying letter.
A ATIS broadcast has to consist of:
- Name of the Airport
- Identification Letter
- Time of Observation
- Active Runways
- Transition Level
- Wind direction and velocity
- Visibilities
- Special weather conditions (such as rain)
- Cloud ceiling
- Temperature and Dewpoint
- QNH
- Trends
It is updated every 30 minutes or as soon as significant changes occur.
Transition Altitude/Transition Level
Knowing the altitude you are flying is one of the most important informations you need in order to safely operate an airplane. Aircraft Altimeters use the air pressure around them to determine their actual altitude. In order to get correct readings you have to use the actual local pressure in your area. As a memory hook you can use this: The altimeter needle moves in the same direction you turn the rotary knob to adjust the pressure. If you turn it counterclockwise, the needle also turns counterclockwise and therefor indicates a lower altitude.
On the other hand it would not be very practical to use the local pressure while flying at higher altitudes, since terrain is not an issue here and you would have to set a new pressure setting in your altimeter every few minutes.
To avoid this pilots use the local pressure when departing from an airport until they pass the so called Transition Altitude (TA), where they set the so called standard pressure (QNH 1013 hpa or Altimeter 29.92 inHg). They continue to use this setting until they descend through the Transition Level (TRL) at their destination airport (or an airport on their route), where they set the local pressure again.
In airport charts only TA is given, whereas TRL has to be determined by ATC. Use the following table to calculated your TRL:
QNH < 0977: TA + 3000 ft. QNH 0978 - 1012: TA + 2000 ft. QNH 1013 - 1050: TA + 1000 ft. QNH 1051 > : TA = TL
The room between TA and TRL is called Transition layer. It ensures that the minimum spacing of 1000 ft between aircraft flying in lower part (with local pressure) and the upper part (using Standard pressure).
Working as a Tower Controller
Runway Separation
The runways are one of the most dangerous spots on an airport because aircraft are travelling at high speed with little room to maneuver and most of the time no ability to stop at a reasonable distance. Because of this the general rule is that only one aircaft may be cleared to use a runway at the same time. What this means practically and exceptions from this rule are explained in the following chapters.
Departing Traffic
So now we are at the point where the pilot reaches the Holding Point of his departure runway and reports ready for departure. What are the things you should check before issuing the takeoff clearance?
- Have a look at the flightplan. Take note of the type of aircraft and the Departure Route.
- Check the traffic approaching the runway.
To give him the takeoff clearance the following phrase should be used:
e.g.: TWR: AUA2CM, wind 320 degerees at 7 knots, Runway 29, cleared for takeoff. AUA2CM: Cleared for takeoff Runway 29, AUA2CM
The pilot lines up on the runway, advances the throttle and takes off. When he is well established in climb check he is squawking Mode C and the right Code. Afterwards he is handed off to the next Controller, in this case a radar position:
TWR: AUA2CM, contact Wien Radar on frequency 128.20, bye bye! AUA2CM: Contacting Wien Radar on frequency 128.20, AUA2CM.
The next aircraft reports ready for departure. Again check the points above, but this time we cannot give the takeoff clearance straight away because the preceeding aircraft is still occupying the runway. Now you get to know the first exception to the Runway Seperation rule above. To speed things up you can instruct the next aircraft to line up behind the first one while this one is still in the takeoff roll occupying the runway:
TWR: AZA639, behind departing Austrian Airbus A319, line-up rwy 29 behind and wait. AZA639: behind departing Airbus lining up runway 29 and waiting behind, AZA639. Note: The two times behind in this instruction is not a typing error but was implemented to emphasize that part of the clearance.
This type of clearance is called a conditional clearance.
The earliest possible point where you can issue the next takeoff clearance is, when the preceeding aircraft has overflown the opposite runway end or has clearly turned onto either side of it.
However in some cases this could be very close which leads us to the next chapter but before lets have a look on helicopters.
Helicopters are sometimes able to start from there current position like a Helipad or a normal stand, if he want to depart from a Runway you can use the normal Phrases for VFR Traffic.
e.g.: OEATD: Wien Tower, OEATD at General Aviation Parking ready for departure. TWR: OEATD, Wien Tower, after departure leave control zone via Freudenau and Donauturm, 2500 feet or below, Wind 290° 6 Knots, present position cleared for take-off. OEATD: After departure leaving the control zone via Freudenau and Donauturm not above 2500 feet, present position cleared for take-off.
Departure Seperation
Based on Type of Aircraft and departure route
One of the main tasks of air traffic control is to keep aircraft at a safe distance to each other. So imagine the following situation:
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Obviously it would not take long until the B767 catches up with the Cessna, a potentially very dangerous situation! You can see, that it is very important to check the flightplan of the aircraft you are about to clear for takeoff.
The minimum radar seperation in the area around an airport is 3 nm or 1000 feet. These are the limits radar stations have to obey. Tower Controllers should aim to achieve the following seperation for departing aircraft following departure routes which share a common part:
| Fast followed by slow | 3 nm |
| Matching Types | 5 nm |
| Slow followed by fast | 10 nm |
In extreme examples like the one above it is often more advisable to coordinate with APP to find another solution. Often this involves clearing the aircraft to a non standard altitude or departure route:
TWR: DLH2441, after departure maintain runway heading, climb initially to 3000 ft DLH2441: After departure maintaining runway heading, climbing to 3000 ft, DLH2441 TWR: DLH2441, wind 320 degrees at 9 knots, runway 29, cleared for takeoff DLH2441: Cleared for takeoff runway 29, DLH2441
The other main task of ATC is to expedite the flow of traffic. Situation:
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It would benificial to use the gaps that arise between the aircraft using similar Departure Routes, so in close coordination with ground you should try to distribute aircraft over the holding points in a way to be able to fill those gaps.
Based on Wake Turbulence Category
There are two ways aircraft influence the air around them when passing through it:
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This turbulence can cause severe problems or even loss of control for following aircraft. The wake turbulence categories are based on the Maximum Takeoff weight (MTOW) of the aircraft:
| Light Aircraft (L) | < 7 000 kg |
| Medium Aircraft (M) | 7 000 – 136 000 kg |
| Heavy Aircraft (H) | >136 000 kg |
For departing aircraft, 2 minutes separation (3 minutes if the succeeding aircraft departs from an intersection) is applied when an aircraft in wake turbulence category LIGHT or MEDIUM departs behind an aircraft in wake turbulence category HEAVY, or when a LIGHT category aircraft departs behind a MEDIUM category aircraft.
You may issue a take-off clearance to an aircraft that has waived wake turbulence separation, except, if it's a light or medium aircraft departing as follows:
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To point out this hazard to a pilot the following phrase should be used:
TWR:ESK32C, behind departing heavy B777 line up runway 16 behind and wait, caution wake turbulence. ESK32C: behind departing B777 lining up rwy 29 and waiting, ESK32C.
Arriving Traffic
Arriving Aircraft call you when they are established on an approach to a runway. Most of the time this is an ILS Approach but also other kinds are possible.
MAH224:Linz Tower, MAH224 established ILS Approach rwy 27.
Again you are not allowed to clear more than one aircraft onto the same runway at the same time.
In order to issue a landing clearance
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If these conditions are met use the following phrase to clear the aircraft:
TWR:MAH224, Linz Tower, wind 300 degerees at 16 knots, runway 27, cleared to land. MAH224:cleared to land runway 27, MAH224.
During periods of high traffic it is likely that you have more than one aircraft approaching the same runway at the same time. Approach has to ensure the minimum radar seperation of 3 nm and additionally increased seperation due to wake turbulence.
AUA26T:Linz Tower, AUA26T established ILS 27. TWR:AUA26T, Linz Tower, continue approach, wind 300 degrees at 16 knots. AUA26T:continuing approach, AUA26T.
| Meanwhile MAH224 has left the runway. |
TWR:AUA26T wind 310 degrees at 14 knots, runway 27 cleared to land. AUA26T:Runway 27, cleared to land, MAH224.
Often it is useful to give pilots additional information, such as traffic information or wind:
| CSA276 is following NLY7751 (A320): |
CSA276: Wien Tower, CSA276 established ILS 34. TWR:CSA276, Wien Tower, preceeding traffic is a NLY Airbus A320 3,5 nm ahead of you, continue approach runway 34, wind 010 degrees at 4 knots. CSA276:We have the airbus in sight continuing approach, CSA276.
| AUA81 is approaching runway 16, OE-AGA is on left base runway 16 and there is a rescue helicopter operating in the area around Freudenau. |
AUA81:Wien Tower, AUA81 established ILS 16 TWR:AUA81, Wien Tower, VFR traffic is on left base rwy 16, continue approach, wind 140 degrees at 7 knots. AUA81:continuing approach, AUA81. TWR:AUA81, There is an helicopter operating west of the extended centerline, presently at your one o'clock position, 5 nm, 1400 ft. AUA81: Thank you, looking out, AUA81. AUA81: traffic in sight, AUA81.
Helicopters don't need a Runway for the approach, sometimes they are able to land at their parking position, lets have a look on the Phrases.
eg. the rescue helicopter from the example above needs to land on your airport: OEATD: Wien Tower, request landing at the General Aviation Terminal. TWR: OEATD, wind 010 degreees 4 knots direct General Aviation Terminal, cleared to land.
To give you an idea how dense traffic can get in real life consider that during peak times and good weather the seperation is reduced to 2,5 nm. This equals to one landing every 75 seconds. However on VATSIM the minimum seperation is 3 nm which already requires good cooperation from all the pilots involved.
Merging Departing and Arriving Traffic
And now to the most fun part of being a Tower Controller. Sometimes you get into the situation that you use the same runway for departures and arrivals. Either your airport has only one runway or weather demand this configuration.
Still the above rule of only one aircraft at the same time applies, however we also use conditional clearances which look very similar to those above in the departing traffic section.
LOWW_TWR: AUA123, Traffic short final RWY 29, C750, report in sight AUA123: Traffic in sight, AUA123 LOWW_TWR: AUA123, behind landing C750 line up RWY 29 behind and wait AUA123: Behind landing C750 lining up RWY 29 behdind and waiting, AUA123
To avoid misunderstandings, this time we make sure that the Pilot has the the landing aircraft in sight. You don't have to worry about wake turbulence seperation between landing and departing aircraft since they never cross through each others wake.
To depart an aircraft in front of an approaching aircraft at the time of the departure clearance given the arriving aircraft should not be closer than 4 nm to touchdown. To squeeze a departing aircraft between two arrivals you normally need a minimum of 6 nm between them. It is important for you to check carefully if you have the necessary gap, so have a close look at the distance between the arrivals and their speed. If the second one comes in faster than normal consider this in your calculation. Also you should make sure, that the pilot will be ready for departure when you need him to depart. To check this use the following phrase:
Callsign, are you ready for immediate departure?
Again it is a good idea to give the pilot an idea of the traffic situation around him.
Example:
| You are the Tower Controller at Vienna airport. Runway 29 is active for departures and arrivals. One aircraft is on a 5 nm final, one at 12 nm out. Additionally you have two departures waiting at the holding point of ruwnay 29. |
TWR:CAL275, are you ready for immediate departure? CAL275:Affirmitive, ready for immediate departure, CAL275 TWR:Traffic is now at a 4 nm final, wind 300 degrees at 7 knots, runway 29 cleared for immediate takeoff. CAL275:cleared for immediate takeoff runway 29, CAL275
| After the CAL B747 has taken off. |
TWR:AUA289, wind 300 degrees at 7 knots, runway 29, cleared to land. AUA289:Runway 29, cleared to land, AUA289. TWR:AUA2LT, traffic is an AUA Airbus A320 on a 2 nm final rwy 29, do you have traffic in sight? AUA2LT:Traffic in sight, AUA2LT. TWR:AUA2LT, behind landing traffic line up runway 29 behind and wait. AUA2LT:Behind the landing Airbus, lining up runway 29 behind and waiting, AUA2LT.
| AUA289 has vacated the runway. |
TWR:AUA2LT, wind 300 degrees at 8 knots, runway 29 cleared for takeoff, landing traffic is now on a 3,5 nm final. AUA2LT:cleread for takeoff runway 29, AUA2LT.
VFR Traffic
Differences to handling of IFR Traffic
The essential collision safety principle guiding the VFR pilot is "see and avoid." Pilots flying under VFR assume responsibility for their separation from all other aircraft and are generally not assigned routes or altitudes by air traffic control. Governing agencies establish specific requirements for VFR flight, consisting of minimum visibility, distance from clouds, and altitude to ensure that aircraft operating under VFR can be seen from a far enough distance to ensure safety.
To guide VFR TRaffic through your airspace you make use of VFR Routes, Sectors and reporting Points. Used phrases:
TWR:OE-AGA, enter control zone via VFR route Klosterneuburg – Freudenau, 1500ft or below, QNH 1020, Squawk 4604, report XXXX (i.e. Freudenau), expect runway 29. TWR:OE-AGA hold (orbit) overhead XXXX (i.e. Freudenau) in XXXX (i.e. 2500ft)
VFR flights should be guided into downwind, base and final leg for landing.
TWR:OE-AGA, enter downwind for runway 29, report on downwind TWR:OE-AGA, enter base for runway 29, report on base
VFR Flights get their Clearance from Tower. After startup, they will contact Ground for taxi, thereafter the Tower will issue the clearance. A possible VFR clearance could be:
TWR:OE-AGA, verlassen Sie die Kontrollzone über Sichtflugstrecke Klosterneuburg, 1500 Fuß oder darunter, QNH 1014, Squawk 4607, Rechtskurve nach dem Abheben so bald als möglich. TWR:OE-AGA, leave controlzone via VFR-route Klosterneuburg, 1500 feet or below, QNH 1014, Squawk 4607, right turn after departure as soon as possible.
TWR:OE-AGA, steigen sie auf 3500 Fuß, melden Sie Donauturm. TWR:OE-AGA, climb 3500 feet, report Donauturm.
In the air ATC provides traffic information.
TWR:OE-AGA, Traffic at your 12 o'clock position, 2100 feet, a PA28 on VFR inbound route Klosterneuburg-Freudenau.
When the aircraft leaves the controlzone.
TWR:OE-AGA, set Sqauwk 7000, leaving frequency is approved.
Wien Tower/Turm can also be contacted in German.
Merging in VFR Traffic
To manage VFR Traffic efficiently you have to use traffic information and visual seperation.
TWR: OE-ANX, traffic at your 3 o´clock position, moving right to left, B767, distance 2.5 miles, report mentioned traffic in sight OE-ANX: Traffic in sight, OE-ANX
Because of other traffic it might be necessary for the aircraft to remain in the downwind leg until the traffic has passed:
TWR:OE-AGA, fly extended right downwind, standby for base. OE-AGA: Extending right downwind, OE-AGA
To instruct the aircraft to continue it's approach use the following procedure:
TWR: OE-ANX, traffic at your 3 o´clock position, moving right to left, B767, distance 2.5 miles, report mentioned traffic in sight OE-ANX: Traffic in sight, OE-ANX TWR:OE-AGA, behind B767 traffic, enter final RWY 29, caution wake turbulence OE-AGA: Behind B767, enter final RWY 29 behind, caution wake turbulence, OE-ANX
When using an extended downwind you should always consider that the aircrafts speed might be considerably lower than the speed of other aircrafts involved. So if an aircraft has to fly a long way out it might take some time for it to come all the way back, generating a big gap in the arrival sequence. Instead you should aim to keep the plane within the vicinity of the airfield:
TWR: OE-AGA, Make a right three-sixty. OE-AGA: Making three-sixty to the right. TWR: OE-AGA, Orbit left OE-AGA: Orbiting left, OE-AGA
The second instructions means, that the pilot should make orbits until further advice.
Information Positions
Special Situations (High Traffic, Slots, ...)
High traffic situations
During high traffic situations communication with adjacent approach sectors is very important. Especially during single runway operations you might have to ask for increased inbound spacing to be able to fit in departing aircraft.
Additional phrases during periods of high traffic
In order to expedite the flow of traffic use the following phrases:
Austrian 125, wind is xxx/xx runway 29 cleared for takeoff, expedite Austrian 125, wind is xxx/xx runway 34 cleared to land, expedite vacating OE-ABC, wind xxx/xx, runway 29 cleared for takeoff, after departure right turn as soon as practicable
Opposite runway operations
This is one of the more difficult situtions for a Tower controller. You have to consider the departure route of each aircraft to estimate the required spacing to arriving traffic. Again close coordination with approach is very important.
Ressourcen