The Approach Controller

Approach Sequencing and Speed Control

While the Tower (TWR) controller primarily focuses on departures, the Approach (APP) controller is responsible for managing arrivals, ensuring they are sequenced safely and efficiently. This involves preventing arrivals from being too close together (which could force a go-around due to an occupied runway) or too far apart (which could result in unnecessary airborne holding and increased fuel consumption).

Approach Sequencing

Sequencing can be managed using several key concepts:

Separation: The minimum vertical or lateral distance required between two aircraft. This includes radar separation and wake turbulence separation, both of which are critical in approach operations.
Spacing: The desired distance between aircraft on final approach, which depends on factors such as weather conditions, airport layout, traffic volume, and pilot proficiency.
Compression: A phenomenon that occurs when a leading aircraft reduces speed on final approach while trailing aircraft continue at a higher speed, causing them to close the gap. Controllers must anticipate this effect and adjust spacing accordingly.

For example, if the required spacing on final approach is 7 NM, an additional 1 NM can be added to account for compression, aiming for 8 NM final spacing when no additional wake turbulence separation is required.

Factors Affecting Final Approach Spacing

The APP controller must consider several factors when determining final approach spacing:

Airport Layout: The number of runways, their configurations, and operational capabilities.
Runway Exit Design: High-speed exits allow aircraft to vacate the runway more quickly, reducing spacing requirements.
Traffic Volume: Depending on demand, priority may be given to either arrivals or departures, requiring close coordination with TWR.
Low Visibility Procedures (LVPs): Increased spacing is necessary during reduced visibility conditions.
Ground Situational Awareness: Monitoring ground movements to adjust spacing for optimal traffic flow.

Speed Control for Approach Management

To establish and maintain proper spacing, controllers should first:

Reduce the speed of trailing aircraft or
Increase the speed of leading aircraft, then adjust the speeds of other aircraft accordingly.

Aircraft may be assigned specific speed instructions, such as:

Maximum speed
Minimum clean speed (minimum speed without flaps, speed brakes, or landing gear deployed)
Minimum approach speed
A specified IAS (Indicated Airspeed)

General Speed Control Guidelines

Controllers should avoid instructing aircraft to reduce speed while maintaining a high descent rate, as these maneuvers are often incompatible.
Aircraft should be allowed to remain in a clean configuration for as long as possible.
Above FL150, turbojet aircraft should not be reduced to less than 220 knots IAS, which is close to their minimum clean speed.
On intermediate and final approach, only minor speed adjustments (not exceeding ±20 knots IAS) should be used.

Standard Speed Reductions on Final Approach

The following speed recommendations ensure efficient sequencing and predictable spacing:

Distance from Runway	Maximum IAS
15 NM	250 knots
12 NM	220 knots
10 NM (Glideslope Intercept)	200 knots
7 NM	190 knots
6 NM	180 knots
5 NM	170 knots
4 NM	160 knots

Pilots should not exceed 200 knots upon reaching the glideslope (approximately 10 NM out).
The approach clearance does not cancel speed restrictions, unless explicitly stated by the controller.
If unsure whether a pilot is aware of the speed restrictions, it is best to reissue them rather than assume the pilot will adjust preemptively.
Assigning 180 knots to 6 NM can lead to less precise approaches, as different aircraft types decelerate at different rates. Using 160 knots to 4 NM or 170 knots to 5 NM provides more consistency, reducing spacing deviations to around 0.3–0.4 NM.

Best Practices for APP Controllers

When workload increases, reduce aircraft speeds earlier to maintain control over sequencing.
Avoid shortening aircraft paths too much, as this can disrupt the flow and spacing.
Use standard speeds consistently to maintain an organized sequence.
Prioritize situational awareness and proactive adjustments to prevent unnecessary go-arounds.

By applying these principles, an approach controller can effectively manage arrivals, ensuring safe and efficient sequencing while maintaining smooth coordination with Tower.

Climb and Descent Clearances

ATC issues climb and descent clearances to facilitate departures, arrivals, or to help aircraft avoid adverse weather conditions. As a rule of thumb, controllers can estimate that an aircraft descends at approximately 300 feet per NM (or 1,000 feet per 3 NM), commonly referred to as the 3:1 rule.

For example, when guiding an aircraft over the downwind leg, it should not be higher than 8,000 feet abeam the field; otherwise, it will be too high to turn onto a 10 NM final. To compensate for excessive altitude, pilots may adjust their descent rate at their discretion. However, controllers may assign a specific descent rate if necessary—but should act promptly, as even with speed brakes, an aircraft’s descent rate has its limits.

ATC can also manage vertical speed during both climb and descent to ensure separation between successive or crossing aircraft. This is particularly useful in high-traffic scenarios. In TopSky radar, the assigned rate of climb/descent (ARC function) is marked in the aircraft’s radar label.

When issuing an approach clearance, pilots are expected to descend to the published altitude for the approach. If the controller requires a different altitude, this must be explicitly stated.

Radar Vectoring

Radar vectoring is the process of guiding an aircraft using ATC-assigned headings instead of standard IFR procedures (SID/STAR/Instrument Approach). Controllers must adhere to the Minimum Vectoring Altitude (MVA), which ensures obstacle clearance while vectoring aircraft.

When issuing radar vectors, controllers must:

Inform the pilot of the purpose of the vector and specify the limit of the vector (e.g., "Vectoring for ILS approach Runway 36").
When terminating vectoring, instruct the pilot to resume own navigation.
Aircraft must not be vectored closer than half of the separation minimum (i.e. closer than 2.5 NM if the separation minimum is 5 NM) from the limit of the airspace which the controller is responsible for, unless otherwise specified in local arrangements.
Avoid vectoring aircraft into uncontrolled airspace, except in emergencies or to circumvent severe weather.
If an aircraft reports unreliable directional instruments, instruct the pilot to make all turns at an agreed rate and to comply with instructions immediately upon receipt.

Obstacle Clearance

When vectoring an IFR flight or issuing a direct routing that takes an aircraft off an ATS route, controllers must ensure that prescribed obstacle clearance is maintained at all times until the pilot resumes navigation. If necessary, the minimum vectoring altitude (MVA) must be adjusted for low-temperature corrections.

Radar vectors can be provided in two ways:

Heading Assignment: e.g., "Turn left heading 180°."
Relative Turn Instruction: e.g., "Turn right by 10°."
- This should only be used when there is insufficient time to request a specific heading.

If a radar vector is not self-explanatory (e.g., for final approach), the reason should always be provided (e.g., "Turn left heading 180° for spacing").

Important Considerations:

When an aircraft is already in a turn, avoid ambiguous instructions like "Turn left/right by..." since the aircraft may not know which heading this refers to. Instead, use "Stop turn" if an immediate heading correction is needed.
For ILS or localizer approaches, vectoring should be within 30° of the final approach course.
- Example: Runway 36 → Heading 330° or 030° for intercept.

RNAV Arrivals and Point Merge System

RNAV arrivals are predefined sequences of navigation points that an aircraft must pass over (or near) during its descent. The flight path is often deliberately curved to allow controllers flexibility in managing traffic flow. Controllers may issue shortcuts to reduce flight distance or allow the aircraft to follow the full STAR to delay its arrival. This method significantly reduces both controller workload and frequency congestion, which is why an increasing number of aerodromes are implementing it.

The Point Merge System is a specific RNAV-based arrival structure that consists of:

A merging point
An arc that arriving aircraft follow until further instruction

Controllers issue a "direct to" clearance to the merging point when appropriate. Since the distance from any point along the arc to the merging point remains constant, this method allows for precise sequencing with minimal workload.

However, RNAV arrivals alone may not always provide sufficient spacing, especially in high-traffic situations. In such cases, controllers may still need to apply vectoring to ensure optimal sequencing and separation.

Achieving the Desired Spacing

While it is important to consider an aircraft's distance from the extended centerline or ILS feather, controllers should avoid relying too heavily on leader lines and the heading tool. These tools can assist in understanding aircraft performance, but developing a holistic approach—including judging headings by eye and using standard headings—will improve overall control of the approach sequence.

Final approach spacing should be measured aircraft-to-aircraft rather than focusing solely on how far an aircraft is from the extended centerline. The key consideration is the relative position of aircraft along the approach path. As the trailing aircraft nears the localizer, the crucial factor is how far ahead the leading aircraft is when determining when to turn the next aircraft onto the localizer.

Ensuring that aircraft intercept at the correct point for their altitude is important. When managing a continuous flow of inbound traffic, controllers should generally aim to establish aircraft outside of 10 miles whenever possible. However, once this baseline is set, the relative positioning of aircraft matters more than their absolute distance from the localizer.

Key Considerations for Effective Spacing

Relative Speed Matters: Instead of only checking how far an aircraft is from the extended centerline, focus on its speed relative to the aircraft ahead.

Perpendicular Base Leg Advantage: Using a perpendicular base leg can help maintain consistent spacing and improve sequencing.

Avoid Fixation on Identical Intercept Points: Aircraft do not need to intercept at exactly the same point every time. The focus should be on maintaining appropriate spacing between aircraft.

Localizer Turn Spacing: When an aircraft is a reasonable distance from the extended centerline, its turn onto the localizer will naturally create around 1 NM of spacing. Therefore, aircraft should typically be turned 1 NM before reaching the required spacing down the ILS.

By prioritizing relative positioning and speed differentials rather than strictly measuring distance to the localizer, controllers can achieve more efficient sequencing and smoother approach operations.

Intercept Headings and Base Leg Management

Consistency is key when assigning intercept headings. In still wind conditions, a 30-degree intercept heading should be the standard. Any deviation from this should have a clear justification, such as:

The aircraft is closer or further from the ILS feather than usual, requiring an adjustment to establish them at an appropriate point for their altitude (typically within ±5°).

The controller needs to increase spacing by assigning a wider intercept heading or reduce spacing by using a tighter intercept heading (again, usually within ±5°).

Wind conditions require an adjustment to maintain a true 30-degree track to the ILS.

The Importance of the Base Leg

The previous section highlighted the usefulness of the base leg, but it is important to emphasize its role in maintaining efficient sequencing. The next aircraft must always be ready to turn onto final when required.

Once the required spacing down the approach is achieved, the following aircraft should be positioned correctly relative to the extended centerline, ready to be turned onto final.

If an aircraft is on a non-perpendicular base leg, heading away from the airport, it becomes harder to judge spacing along the ILS. The turn toward the localizer will take longer, creating more than a mile of additional spacing. In such cases, controllers should initiate the turn earlier than they would for an aircraft on a perpendicular base leg.

However, if an aircraft is already in position for final, there is no need to extend them onto a base leg—simply turn them onto final immediately.

It is also acceptable to issue an intercept heading before the aircraft has fully rolled out on base. This demonstrates good situational awareness and ensures spacing down the ILS is maintained efficiently.

Calculating Track Distance from Touchdown

To facilitate a Continuous Descent Approach (CDA), pilots should be informed of their track distance from touchdown along with their initial descent clearance. This helps them plan a smooth and efficient descent.

How to Calculate Track Distance

The easiest way to determine track distance is by counting backward from an aircraft already established on final approach:

Identify an aircraft already on the final approach course.

Count backward along the approach path to estimate the distance of aircraft still on base or downwind.

For example, if you are aiming for 7 NM spacing on final approach in a stream of A320s:

If the leading aircraft is established at 10 NM, then the aircraft on base leg will be at approximately 17 NM.

The aircraft following that will be at around 24 NM.

For VATSIM operations, it is recommended to add an extra NM to account for the additional track miles flown in turns. So, if targeting 7 NM spacing, plan for 8 NM to ensure a consistent separation.

Adjusting for Wind Conditions

Wind significantly affects intercept headings, especially at airfields where aircraft establish from both sides of the extended centerline.

Example: East-West Runway with a Northerly Wind

(A wind coming from the north)

Aircraft establishing from the south will require a wider intercept heading to maintain a 30° intercept track to the runway.

Aircraft establishing from the north will require a tighter intercept heading to maintain the same 30° intercept track.

Controllers must adjust headings accordingly to ensure consistent and predictable approaches, taking wind direction and strength into account.

Wind Effects on Base Leg and Final Approach

Wind conditions can also affect the headings used for a perpendicular base leg, requiring adjustments to ensure proper alignment with the localizer.

Example: East-West Runway with a Westerly Wind

(A wind pushing aircraft to the east)

A 360° track may require a heading of 350°–355°.

A 180° track may require a heading of 185°–190°.

Adjusting for Wind Conditions

Assess the adjustments needed based on wind direction and strength.

Monitor your initial aircraft to see how these adjustments are working.

Communicate wind effects during controller handovers to maintain consistency.

Impact of Headwinds on Final Approach

If there is a strong headwind on final approach, precise timing of the turn onto final is crucial:

Once an aircraft is turned onto the intercept heading, its ground speed will decrease due to the headwind.

Turning the trailing aircraft too early could lead to spacing issues. While speed control may help, its effectiveness is limited.

Turning the trailing aircraft too late means that traffic turning into a strong headwind will cover more track miles in the turn than in still wind conditions.

When adjusting speeds:

If an aircraft is slowed to 160 knots to 4 NM (or even 150 to 4 NM) too early, it can significantly affect the aircraft behind.

Avoid speeding up aircraft to close gaps—it rarely works effectively.

With a strong crosswind on base leg, consider how it impacts aircraft momentum, even when all aircraft are assigned 180 knots.

By carefully managing wind effects on base leg headings, final approach turns, and speed assignments, controllers can maintain efficient sequencing and stable approach operations.

- Holding stacks