Air Navigation

Instrument procedure design and obstacle clearance in Air Navigation

In the ATPL Air Law syllabus, Air Navigation focuses on the regulatory framework that shapes IFR departures, arrivals, and approaches. Using ICAO PANS-OPS (Doc 8168) criteria adopted by many authorities, including EASA, these procedures ensure obstacle clearance and predictable track-keeping so flight crews can fly safely using published routes, radio aids, and PBN/RNAV guidance. Understanding how tracks, protected airspace, and minima are defined is essential for interpreting instrument charts, complying with aviation regulations, and integrating aircraft systems such as FMS and GNSS with ATC clearances.

On departure, the concept of a straight departure limits the initial departure track deviation from the extended runway centreline to 15°. This small angular tolerance supports obstacle clearance assumptions and charted lateral protection. Pilots are expected to correct for known wind to remain within the protected area—whether flying a conventional SID based on VOR/DME or an RNAV SID flown via the FMS. Procedure design also accounts for speed, bank angle, and turn radii, but the crew’s job is to maintain the published track, apply appropriate wind correction, and meet any stated climb gradient while complying with altitude and speed restrictions in the clearance.

Approach design divides the instrument approach procedure into distinct segments—arrival, initial, intermediate, final, and missed approach—each with specified obstacle clearance margins and protected areas. A non-precision straight-in approach is acceptable when the angle between the final approach track and runway centreline is 30° or less; beyond this, alignment and visibility expectations change and circling may be required. Track-reversal techniques include the base turn (a turn executed during the initial approach between the outbound end and the start of the intermediate or final segment, with non-reciprocal tracks) and the procedure turn (a turn away from a designated track followed by a turn in the opposite direction to intercept the reciprocal). These manoeuvres create space to lose height, configure, and establish on final while staying inside protected airspace.

Obstacle clearance is expressed with clearly defined terms. Minimum Sector Altitude (MSA) provides at least 300 m (984 ft) obstacle clearance within 25 NM of the associated aid or fix for situational awareness and emergency use. Each segment’s primary area is symmetrically disposed about the nominal track and guarantees the required Minimum Obstacle Clearance (MOC), with additional secondary areas tapering protection. Minima terminology matters: Obstacle Clearance Altitude/Height (OCA/H) sets the reference for the lowest permissible descent—OCA is referenced to mean sea level, while OCH is a height above the runway threshold or aerodrome elevation as appropriate. For non-precision or circling approaches, the published Minimum Descent Height (MDH) cannot be lower than the OCH. Pilots translate these design values into operational minima (MDA/MDH or DA/DH) using the correct altimeter setting, approach category, and any procedural notes on the chart.

What this question bank covers

• IFR departures and arrivals: straight departures (≤15°), wind correction, SID track-keeping, and protected airspace.
• Approach segments and alignment: arrival, initial, intermediate, final, missed approach; straight-in criteria for NPAs (≤30°).
• Track-reversal manoeuvres: base turn and procedure turn, with their purposes and design assumptions.
• Obstacle protection: MSA radius (25 NM) and clearance (300 m), primary vs secondary areas, and MOC concepts.
• Minima and references: relationships between OCA/OCH and MDA/MDH, altimetry references to MSL and aerodrome/threshold elevation.
• Chart interpretation and compliance: applying aviation regulations, reading ATPL exam-style procedures, and integrating FMS/GNSS with published procedures.