Approach design and minima

Speed determines the protected geometry

Higher speeds need more space and can change the applicable minima.

Approach categories

Aeroplane approach categories are based on Vat: 1.3 times Vso or 1.23 times Vs1g in landing configuration at maximum certificated landing mass, as specified. The categories are A below 91 kt; B 91 to below 121; C 121 to below 141; D 141 to below 166; E 166 to below 211. Apply the relevant procedure category and segment speed limitations, including use of higher-category protection when required.

Procedure design includes the navigation aid’s accuracy, fix tolerances, obstacle survey and turn/track performance assumptions. A strong wind changes heading, groundspeed and turn geometry; apply wind correction to maintain the protected track. Flying a published heading when the chart requires a track can take the aircraft out of protection.

Straight-in alignment and sector safety

For conventional non-precision straight-in criteria, final approach alignment is normally within 30 degrees of runway centreline for categories A/B and 15 degrees for C/D/E, subject to the procedure design conditions. “Straight-in” does not mean the aircraft is already perfectly runway-aligned at every point.

MSA normally provides at least 300 m (1,000 ft) obstacle clearance within 25 NM of the specified reference, with the applicable terrain and sector provisions. It is not a clearance to descend anywhere to that altitude and does not necessarily assure navigation signal reception. Use the relevant published procedure altitude and ATC clearance.

OCA, DA and MDA are not interchangeable

Obstacle clearance is an input to operating minima, not the whole decision.

References and decisions

OCA/H is the lowest altitude or height used to establish compliance with obstacle-clearance criteria for the procedure. OCA is above mean sea level; OCH uses the stated runway threshold or aerodrome reference as applicable. MOC is the required obstacle-clearance margin within the defined protected area. DA/DH is the decision altitude/height at which a missed approach is initiated if the required visual reference is absent.

MDA/MDH is the minimum descent altitude/height for the relevant non-precision or circling operation; do not descend below it without the required visual reference and other conditions. The missed approach point and the MDA are separate horizontal and vertical limits. A CDFA technique uses the applicable operational decision method without authorising descent below a published limit.

Operating minima consider more than obstacles

Determine operational minima using the applicable Air Operations rule, procedure type, aircraft and equipment, crew qualification, aerodrome facilities and lighting, obstacle assessment and required visual reference. They cannot be assumed equal to OCA/H. Precision, APV and non-precision procedures use their relevant decision/minimum-descent concepts; circling has its own protected area and category minima.

An approach clearance does not authorise descent below a safety altitude or disregard of a chart restriction. If operational minima exceed the procedure’s obstacle-clearance value, use the higher applicable minimum. Keep altitude and height datums consistent when comparing values.

Fix tolerance and protected areas

A charted fix has a tolerance region, not mathematical zero uncertainty.

Primary and secondary protection

Where secondary areas are used, the primary area provides full MOC across its width; the secondary area reduces from full clearance at its inner edge to zero at its outer edge according to the design criteria. That taper is not permission to deliberately fly the edge. Navigation and tracking tolerances are already part of the design.

Conventional PANS-OPS system-use accuracy assumptions include VOR ±5.2 degrees, NDB ±6.9 degrees and ILS localiser ±2.4 degrees for the relevant track-guidance criteria. Fix tolerances also depend on range, intersection angle, along-track facilities and the type of fix. These design assumptions are not a cockpit deviation limit for every receiver or procedure.

Segments and descent gradient

An IAF starts an initial approach segment; IF starts an intermediate segment; FAF starts the final segment for a conventional non-precision approach. For an ILS, the FAP is normally where the nominal glide path intersects the minimum intermediate altitude. MAPt is the point at or before which a missed approach must be initiated to ensure the specified protection. TP is a turning point.

The preferred precision descent path is 3 degrees, approximately 5.2 percent. Do not confuse a 3-degree path with a 3-percent gradient. At a given path angle, groundspeed controls the required descent rate; wind correction and stable tracking remain essential.

Can you apply the rules?

Four original questions. Choose from the facts, then review the explanation.