12. Terminal Area (CTA) Techniques
12.1 General Concepts
12.1.1 "Trombone-type" RNAV arrivals
Most STARs in the Arabian FIRs are RNAV “trombone” type arrivals, which allow aircraft to absorb arrival delay, and minimise the requirement for vectoring when sequencing aircraft, greatly reducing controller workload and frequency congestion during times of high traffic volume.
Aircraft are positioned on a 4 to 5-mile downwind on opposing sides of the runway, allowing aircraft to perform continuous descents, and resulting in consistent behaviour when vectoring aircraft for sequencing base-to-final.
The STARs also incorporate level and speed restrictions, which assure separation from departing traffic, as well as terrain and obstacles. Although, traffic permitting, the speed restrictions may be cancelled. Level restrictions may also be cancelled in compliance with the specific procedures for the terminal area.
12.1.2 RNAV departures
All airports in the Arabian FIRs (with the exception of Muscat FIR) utilise RNAV standard SIDs, incorporating altitude and speed restrictions to assure airspace containment, separation from arriving traffic and terrain and obstacle clearance.
Allowing aircraft to follow the published arrival reduces controller workload, but consideration must be given to the interaction between departure and arrival procedures, and appropriate climb instructions must be given.
Where airports are equipped for simultaneous instrument departures, allowing aircraft to follow the published procedure will assure separation between parallel departing aircraft under most circumstances.
12.1.3 Speed, turn radius, and turn rate
Turn radius and turn rate of an aircraft are proportional to their airspeed and bank angle. As the bank angle and/or airspeed increases, the turn rate and turn radius of an aircraft will increase. As most modern jet turbine aircraft autopilots are programmed to utilise a constant 25-degree bank angle, as the airspeed of an aircraft reduces, its turn rate and turn radius will increase. Controllers must have a sound understanding of this relationship, as it is particularly important during base to final sequencing.
12.1.4 Rules of thumb
Controllers shall keep the following in mind when providing vectors to final. For aircraft travelling at the same speed:
- “Aiming” an aircraft well ahead of the preceding aircraft will reduce separation rapidly
- “Aiming” an aircraft at the preceding aircraft will cause the separation to increase slowly
- “Aiming” an aircraft behind the preceding aircraft will cause the separation to increase rapidly.
12.2 Initial arrival sequencing
12.2.1 Sequencing before aircraft reach downwind
In some cases, aircraft may require vectors before reaching the downwind leg of the STAR. In these cases, an appropriate vector should be issued to the aircraft to achieve the target separation as appropriate.
In addition to vectors, aircraft may be issued direct routings from the upwind leg of the trombone to the downwind leg of the trombone, allowing more optimal sequencing of aircraft and reduced controller workload.
12.2.2 Vectoring multiple aircraft
When vectoring multiple aircraft off the upwind leg of the STAR, it is always good practice to issue slightly diverging heading instructions if they are within
12.3 Base to final sequencing
12.3.1 Use of standard vectors
When vectoring aircraft base to final, standard headings shall always be used. This permits consistent behaviour of aircraft when turning base and intercepting the final approach course, which allows the timing of the vectors to be adjusted to account for factors such as winds with a high degree of accuracy.
In general, the following standard vectors shall be used when sequencing aircraft from base to final:
- A 90-degree base vector, perpendicular to the runway course
- A 30-degree intercept vector to intercept the final approach course
12.3.2 Compression on final approach
In order to minimise the risk of a loss of separation on the final approach, a safety margin must be added to the required minimum spacing to account for a phenomenon known as “compression”.
Compression is a result of a combination of factors but is primarily due to the fact that aircraft final approach speeds are generally significantly lower (approximately 130 to 150 knots) than the speed control that is applied to the aircraft until 4 nautical miles from the threshold.
In addition, variation in winds between higher and lower levels, may result in unexpected variations in aircraft ground speed, possibly causing a trailing aircraft to have a higher than anticipated closure rate towards the preceding aircraft.
In general, a buffer of approximately 1 to 1.5 NM is sufficient to account for compression. Often, this buffer may be completely eroded by the time the leading aircraft crosses the landing threshold, but its application assures that no loss of separation will occur.
However, controllers must be careful not to apply buffers that are too large, as this results in a reduction in arrival rate and, as a result, a reduction in the overall capacity of the terminal area and increases the arrival delay required during peak arrival times.
12.3.3 Vectoring base ot final
12.3.3.1 Determining the required spacing
Before initiating the base turn, controllers must first determine the required spacing between aircraft, with due consideration given to performance, distance-based wake turbulence separation and compression.
As discussed previously, the target spacing will usually be the minimum required separation plus 1 to 1.5 NM to account for compression.
For example, if the minimum required separation is 4 NM, the target should be set at between 5 to 5.5 NM.
12.3.3.2 The base turn
Assuming aircraft are travelling at the same speed, the timing of the base turn depends primarily on the distance of the downwind from the final approach course. As discussed earlier, this is generally between 4 to 5 NM for most airports in the Arabian FIRs.
Allowance must also be made for the reaction time of the pilot, and autopilot response time. This generally requires the base turn to be anticipated by approximately one nautical mile.
Therefore, if the aircraft is on a 4.5 NM wide downwind, to achieve 4.5 NM spacing, the aircraft will need to be instructed to turn approximately one mile before they cross abeam each other. For 5.5 NM spacing on the same downwind, the base turn instruction should come as the aircraft pass abeam each other.
Using this technique, the timing of the base turn may be adjusted to account for differing wake turbulence-based radar separation requirements. Turning the aircraft earlier or later will result in a corresponding reduction or increase in separation on the final approach course, respectively.
12.3.3.3 The intercept turn
As with the base turn, adjusting the timing of the intercept turn will allow adjustments to be made to the final spacing.
If the base turn has come late, the intercept turn instruction shall be issued early to recover the spacing as quickly as possible.
Similarly, if the base turn has come too early, the final intercept instruction may be delayed until the aircraft have sufficient separation.
In some cases, aircraft may be required to fly through the localizer and re-intercept from the other side to achieve the required spacing. This, however, should be used sparingly, as the spacing can very easily become excessive using this technique. If this is the case, the timing of the initial base turn should be adjusted to come slightly later.
12.4 Departure sequencing
12.4.1 Airspeed profile
During the initial climb out, most jet turbine aircraft follow approximately the same airspeed profile. The initial climb speed immediately after take-off is typically dictated by speed restrictions on the SID, after which aircraft will accelerate to a speed of 250 knots.
Passing 10,000 ft, aircraft will accelerate to a cruise-climb speed (typically 270 to 320 knots) until approximately 30,000 ft, after which they transition to their planned cruise Mach number.
12.4.2 Speed control
Controllers must be aware of the expected acceleration points on the SID where the airspeed will rapidly increase. This may occasionally be undesirable and require assignment of a lower speed to maintain separation.
Conversely, aircraft may be instructed to accelerate early if separation with the leading aircraft is adequate. This allows for increased capacity of the departure procedure and minimises the need to vector traffic off the SID to increase separation.
12.4.3 Vectors
In some instances, vectors may be required in order to increase separation before aircraft transition to the en-route phase. When vectoring is applied, the aircraft shall always be turned away from the leading aircraft’s planned track.
The heading changes to achieve the desired separation should not exceed 30 degrees off track, as this technique results in a very rapid increase in the separation, and time must be allowed for the aircraft to carry out the heading necessary heading change manoeuvre.