Procedure Design and Departures

PANS-OPS and its vocabulary

Doc 8168 comes in two long-standing volumes:

Volume I, Flight Procedures — describing operational procedures for pilots and operations personnel.

Volume II — the construction manual for the procedure designers themselves. A third volume, of aircraft operating procedures, was split out of Volume I in 2018.

The exam lives in Volume I, and it opens with a chapter of definitions and a chapter of abbreviations that the rest of the book leans on. The working set this module uses constantly:

DER — departure end of the runway: the end of the area declared suitable for take-off; the point where every departure procedure begins its arithmetic.

PDG — procedure design gradient: the published climb gradient a departure is designed around.

OIS — obstacle identification surface: the sloping surface used to identify the obstacles that matter to a departure.

MSA — minimum sector altitude: the lowest altitude giving prescribed obstacle clearance within a sector around a significant point or the aerodrome.

MOC — minimum obstacle clearance: the vertical margin a procedure guarantees between the aircraft and the controlling obstacle, the obstacle that drives the calculation.

OCA/H, DA/H, MDA/H — the minima family (obstacle clearance altitude/height, decision altitude/height, minimum descent altitude/height), unpacked at the end of this lesson.

IAF, IF, FAF, FAP, MAPt, TP — the named fixes and points of an approach (initial approach fix, intermediate fix, final approach fix, final approach point, missed approach point, turning point), defined in their own section below.

One reading convention pays for itself immediately: wherever a term pairs an altitude with a height (OCA/H, DA/H, MDA/H), the altitude (A) is referenced to mean sea level, and the height (H) to the threshold elevation, or for non-precision approaches to the aerodrome elevation (or to the threshold, where the threshold lies more than 2 m below the aerodrome elevation). Same position in space, two rulers.

How departures are designed

Three things dictate the design of an instrument departure:

the terrain and obstacles surrounding the aerodrome;

the ATC and airspace requirements the route must respect;

the navigation aids available to guide the aircraft.

And one assumption stands above every departure the book designs: all procedures assume all engines operating. PANS-OPS protects the climbing aircraft from rock, not from engine failure: the development of contingency procedures for the engine-out case is the responsibility of the operator, and the exam asks for exactly that division of labour.

The obstacle protection works with two slopes. From a point 5 m (16 ft) above the DER, the obstacle identification surface climbs at 2.5 per cent; any obstacle that pierces it becomes part of the procedure's arithmetic. The aircraft is then asked to climb at the procedure design gradient of 3.3 per cent unless a steeper gradient is published: the 0.8 per cent between the two slopes is the growing minimum obstacle clearance: thin at the runway, thickening with every kilometre flown. A steeper PDG is published when an obstacle pierces the 2.5 per cent surface, and it applies until the height where the standard clearance is regained.

Straight, turning and omnidirectional departures

A standard instrument departure (SID) comes in two flavours, and the boundary between them is a number. A straight departure keeps the initial departure track within 15 degrees of the runway centre-line alignment. Anything requiring more is a turning departure, and the turn is specified at a point, a fix, an altitude or a height, with the design assuming no turn below 120 m (394 ft) above the elevation of the DER: the aircraft is given room to climb straight before it is asked to bank. The figure shows both, and the third case besides.

The omnidirectional departure is the case without a drawn line: a departure designed so the aircraft may turn in any direction after reaching a specified altitude or height, used where no track guidance is provided (no suitable navigation aid exists to define a departure route). The design protects the whole circle instead of a corridor; where terrain makes that impossible in some directions, the procedure specifies sectors to be avoided or sectors with minimum gradients of their own.

The five segments and the protected areas

An instrument approach procedure may have five separate segments: arrival, initial, intermediate, final and missed approach, in that order, each handing the aircraft to the next. Their tasks, boundaries and the fixes that join them are the whole subject of the next lesson; this lesson takes what all five share: the protected airspace wrapped around them.

Each segment's obstacle clearance area, seen in cross-section, splits into two regions whose promises differ:

Primary area — straddling the nominal track: the full minimum obstacle clearance is provided across the whole width.
Secondary areas — along each edge, each occupying the outer quarter of the total width: the clearance tapers from the full value at the inner edge to zero at the outer edge.

A pilot holding the track sits above the strongest guarantee; the further the navigation strays toward the edge of the protected area, the thinner the guarantee becomes. The figure draws exactly this.

Fixes, accuracy and gradients

The named fixes and points stitch the segments together, and the exam wants their definitions:

IAF — initial approach fix: where the initial approach segment begins.
IF — intermediate fix: where the initial segment ends and the intermediate segment begins.
FAF — final approach fix: where the final approach segment of a non-precision approach begins.
FAP — final approach point: the final approach's beginning on an ILS: the point where the intermediate approach altitude intercepts the glide path, the ILS's sloping descent-guidance beam.
MAPt — missed approach point: the point at or before which the missed approach must be initiated.
TP — turning point: a point at which a turn is specified, in a departure or a missed approach.

A fix is only as good as the beam that defines it, so the areas around fixes are sized from fix tolerance: the region where the aircraft might actually be when the instruments say it is at the fix. Where a fix is formed by the intersection of two radials or bearings, the tolerance area is the diamond the crossing beams enclose. The tracking accuracies the design assumes are exam numbers: VOR plus or minus 5.2 degrees, ILS plus or minus 2.4 degrees, NDB plus or minus 6.9 degrees. The order to remember: the ILS localizer, the ILS's lateral guidance beam, is the sharpest, and the NDB the bluntest, which is why NDB procedures carry the widest protected areas.

Descent, too, has design numbers. For the final descent of a precision approach the optimum descent gradient is 5.2 per cent (3 degrees): the slope of the standard ILS glide path. Procedures use the optimum wherever terrain allows and steepen only where it must; a steeper published gradient means a higher rate of descent for the same ground speed, and the pilot should expect it from the chart, not discover it on the glide.

Aircraft categories, straight-in limits and sector altitudes

One procedure must serve aeroplanes of very different manoeuvring capability, and speed is the measure of that capability, so PANS-OPS sorts aircraft into categories A to E by indicated airspeed at threshold, a speed defined as 1.3 times the stalling speed in the landing configuration at maximum certificated landing mass. A faster aeroplane turns wider and needs more protected room; the category decides which set of areas, minima and circling radii applies. That is the reason categories exist: the manoeuvring room a procedure must protect grows with approach speed.

Two more design numbers close the section. A non-precision approach still counts as a straight-in approach while the angle between the final approach track and the extended runway centre line is no more than 30 degrees; beyond that the procedure must end in a visual circle to the runway. And the minimum sector altitudes published around an aerodrome, normally four sectors within 46 km (25 NM), each provide at least 300 m (1,000 ft) of obstacle clearance over everything in the sector: the pilot's coarse safe altitude before any procedure is joined.

OCA/H and the minima family

Everything the designer computed condenses into one number per procedure and category: the obstacle clearance altitude, or height (OCA/H). It is the lowest altitude (OCA) or height (OCH) at which the required obstacle clearance is still assured:

Precision approach — the lowest at which a missed approach must be initiated.
Non-precision approach — beneath which the aircraft may not descend on instruments.
Visual circling — beneath which the circling manoeuvre may not go.

Descending below it without the required visual reference forfeits the only guarantee the procedure gives: below OCA/H, obstacle clearance is no longer assured — for precision and APV approaches, for non-precision approaches, and for circling alike. APV (approach procedure with vertical guidance) is the family between the two: vertical guidance without full precision-approach performance.

The operational minima the crew actually flies are built ON the OCA/H, never below it:

Decision altitude/height (DA/H) — the minimum for a precision or APV approach: the point where the decision is made — visual reference established, continue; not established, go around.
Minimum descent altitude/height (MDA/H) — the minimum for a non-precision approach or for circling: a floor the aircraft may not descend through without visual reference, level flight above it permitted while looking.

The relationship the exam tests: MOC is the designer's clearance inside the OCA/H; DA/H and MDA/H are operational minima at or above the OCA/H, raised further by the operator's factors (aircraft equipment and category, crew qualification and experience, the aerodrome's aids and lighting, and the meteorological limits of the day). One expectation runs beneath the whole system: PANS-OPS expects wind corrections to be applied throughout an instrument approach, because the protected areas are drawn around tracks, not headings. And beneath all of it sits the single most significant factor in the conduct of instrument approach procedures: obstacle clearance. The ladder in the figure stacks the whole family.