Airspace and Procedure Design
Airspace and Procedure Design
Description
The proper planning and design of routes, holding patterns, airspace structure and ATC sectorisation in both terminal and en-route airspace can be effective in reducing the likelihood of level bust incidents. The converse is also true: poorly designed airspace can create situations where a level bust incident is more likely to occur within an air traffic management (ATM) system.
In an ideal world, airspace design would make it possible for arriving, departing and en-route flights to operate so that they did not have to cross one another, or climb and descend through each other’s levels. Furthermore, approach and take-off flight paths would be free of obstacles. Unfortunately, however, this “ideal” design environment is seldom possible, which means that airspace designers need to take steps to reduce the likelihood of level busts in the above circumstances.
Effects
While not actually causing them, poorly designed airspace can increase the risk of loss of separation, level bust, and Controlled Flight Into Terrain (CFIT).
Typical Scenarios
- A standard instrument departure (SID) requires pilots to level at an intermediate altitude until passing a specified point. The pilot fails to stop at this intermediate altitude and loses separation with an aircraft above.
- A SID is based on an off-airfield DME. The pilot does not notice this and becomes confused when the DME range decreases rather than increases.
- A non-precision approach begins at a fix defined by bearing and distance from a radio facility. The pilot has difficulty flying to this fix and descends the aircraft outside the calculated safety zone.
- A non-precision approach specifies a descent path by means of a series of fixes and corresponding check heights. The descent path consists of a series of steps rather than a continuous descent and the pilot has difficulty in establishing a stabilised approach.
Contributory Factors
- The extent of the navigation, communication and surveillance infrastructure;
- Terrain surrounding the aerodrome;
- Other ATS routes, prohibited, danger and restricted areas;
- Proximity of other aerodromes and other airspace structures;
- Requirements to ensure environmental mitigation (e.g. noise abatement procedures).
Defences
- Airspace and procedure design should follow the principles laid down in ICAO Doc 8168 (PANS-OPS) and Doc 4444 (PANS-ATM). PANS-OPS provides criteria for the design of instrument approach, holding and departure procedures. PANS-OPS provisions also cover en-route procedures where obstacle clearance is a consideration. PANS-ATM provides procedures for air navigation services, whose basic tenets form the basis of airspace design.
- EUROCONTROL Manual for Airspace Planning provides guidance material for airspace design and PANS-OPS Procedure design. This is supplemented by Guidance Material for the design of Terminal Procedures for Area Navigation (DME, GNSS, Baro-VNAV and RNP RNAV).
- The introduction of airspace procedures and design should be planned. This implies identifying and addressing all relevant stakeholder interests; carrying out an impact assessment; and carrying out a safety assessment. This also implies the introduction of changes in an organised manner which reduces the likelihood of design solutions creating operational difficulties for either pilots or ATCOs.
- Changes introduced to existing terminal area procedures as well as SIDs and STARs should be properly validated, prior to implementation.
- Sufficient time should be allowed in the planning process to allow for necessary ATCO and pilot training.
- When RNAV terminal area procedures are designed (excluding the final approach and missed approach segment), procedures should be designed using P-RNAV criteria in accordance with guidance material published by EUROCONTROL.
- Make use of procedure-design tools.
Accidents and Incidents
This sections contains occurrences where airspace design has been considered to have contributed to the event.
On 29 January 2025, a Mitsubishi CRJ700, operating a scheduled airline flight, on short final approach to land on runway 33 at Washington National in night VMC as cleared collided with a U.S. Army Sikorsky UH60 helicopter on a training flight using a designated restricted-height route which intersected the runway 33 extended centreline with ‘own separation’ from the other traffic requested and obtained. This route had a maximum height of 200 feet agl but no lateral limits. Both aircraft were rendered unable to continue flight and there were no survivors.
On 28 September 2022, a Boeing 787-9 and an Airbus A330-200 were successively cleared for takeoff from Sydney having been instructed to follow the same SID and climb to the same level - FL280. The A330 climbed faster than the controller anticipated and turned towards the next waypoint inside the preceding aircraft, resulting in a loss of separation. The Investigation found that the SID concerned did not provide separation assurance to aircraft with different performance characteristics because aircraft had to satisfy two separate conditions prior to turning which meant the turning point was not a fixed position.
On 16 July 2019, a Boeing 737-800 inbound to Malaga and another Boeing 738-800 inbound to Seville and under area radar control lost separation after the Malaga-bound aircraft was unexpectedly given radar headings to extend its destination track miles after early handover to a control sector which it had not yet entered. With no time to achieve resolution, the two aircraft, both descending, came within 1.3 nm of each other at the same level. The Investigation attributed the conflict to an overly-permissive Letter of Agreement between Seville Centre and Malaga Approach and recommended that it be revised to improve risk management.
On 23 February 2018, an Embraer 195LR and an Airbus A320 on SIDs departing Brussels lost separation after the 195 was given a radar heading to resolve a perceived third aircraft conflict which led to loss of separation between the two departing aircraft. STCA and coordinated TCAS RA activations followed but only one TCAS RA was followed and the estimated minimum separation was 400 feet vertically when 1.36 nm apart. The Investigation found that conflict followed an error by an OJTI-supervised trainee controller receiving extended revalidation training despite gaining his licence and having almost 10 years similar experience in Latvia.
On 12 May 2019, a Boeing 737-800 making its second procedural ILS approach to runway 25 at Reus came into conflict with an opposite direction light aircraft as the latter approached one of the designated VFR entry points having been instructed to remain well above the altitude which normally ensures separation of IFR and VFR traffic. The collision risk was resolved by TCAS RA promptly followed by the 737. The Investigation concluded that limiting the TWR radar display to the ATZ for controller training purposes had resulted in neither the trainee controller nor their supervisor being aware of the risk.
Further Reading
ICAO Documentation:
- Doc 8168 (PANS-OPS);
- Doc 4444 (PANS-ATM);
EUROCONTROL Guidance material
EUROCONTROL Level Bust Toolkit
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