6.06 Final Approach

01. PRECISION/NON-PRECISION, STRAIGHT-IN/CIRCLING

Final approach can be a precision or non-precision approach for a straight-in landing on a specific runway. A circling approach may be available to other runways of the airport. An instrument approach to each the circling MDA must be available before circling approaches can be executed. The 3 typical approaches, precision, non-precision and circling are shown in the paragraphs 03, 04 and 05.

02. DESCENT LIMIT

For every instrument approach a descent limit is established. The descent limit can be considered as the lowest IMC crossing altitude on final approach. When descending below the descent limit you are leaving protected airspace and as such are responsible for the visual assessment of navigation. If that is not feasible, go around. It is called the see to land concept.

There is only one exception to this on CAT III approaches protected airspace extends to the runway even though the company has established a descent limit.

A descent limit is calculated to ensure both obstacle clearance and adequate manoeuvring space for the operation below the descent limit.

To ensure adequate obstacle clearance an OCH is established. The descent limit can not be lower than this OCH. For ILS approaches this OCH is calculated by accounting for the theoretical height loss the aircraft would have before a positive rate of climb can be achieved. It is based on a manual go-around. Values may be found in the table below. This height loss value is then added to the critical obstruction to find the OCH.

For non-precision approach 250 ft (FAF avail) or 300 ft (no FAF avail) is added to the critical obstruction to find the OCH.

03. THE ILS APPROACH

04. NON-PRECISION APPROACH

NOTE: Sometimes location of a FAF may be impossible, (refer e.g. the example in 6.1.5/02). In that case final approach begins when established on final approach track. For more information including the company method to fly a non-precision approach, see next pages.

ICAO recommendations for non-precision approach procedures

ICAO has issued recommendations for the States how to design and graphically present non-precision approach procedures.

Two of these recommendations affect the company’s Standard Operating Procedures:

1. The optimum length of the final approach segment is 5 NM. This often results in the location of the FAF at a locator at 4 or 5 NM before the threshold. The crossing altitude is low and the final approach segment is short. A point ‘D’ is normally lacking.
2. A missed approach point should be located preferably not further than the threshold. This results often in the location of the missed approach point AT the threshold.

The ICAO recommendation represents in fact the minimum safe flight path.

Flying a non-precision approach can be difficult

Flying a non-precision approach can be difficult. Trying to establish on a 3° descent profile from point ‘D’ requires much attention to additional aids.

Bear the following essential advices in mind:

Minimum crossing altitudes

Leveling off is required when otherwise a fix would be passed below the minimum crossing altitude. Experience has shown minimum crossing altitudes were disregarded by crews due to fixation of the attention to fly a continuous final approach descent profile. Minimum crossing altitudes may be dictated by obstacles.

When reaching MDA

To continue the approach below the MDA, you must be convinced that a safe landing and roll-out on the intended runway can be made, using normal flight techniques and a normal rate of descent, all the way to the touchdown within the touchdown zone. This is possible if you manage to arrive at a point ‘D’ on the MDA located on a suitable approach angle to the runway and if sufficient visual reference is available.

If the MDA is reached too late

If the MDA is reached too late (above the ideal ‘glide path’), a go-around must be initiated immediately, even if the required visual reference is available. Landing in such a situation cannot be accomplished within range of normal flight techniques.

If the MDA is reached too early

If the MDA is reached too early, level flight to regain the optimum ‘glide path’ is allowed. Although obstacle wise safe, the company however considers level flight in a vulnerable configuration (low and slow) without visual reference unsafe.

Therefore, for a straight-in approach, the company decided to apply the following restriction on level flight at a low altitude:

1. At or above basic circling MDA (600 ft HAA for CAT C and 1000 ft HAA for CAT D) without visual reference with the runway.
2. At or above straight-in MDA, but below the basic circling MDA, only if visual reference with the ground can be established.

Note that in this case visual (ground) reference is defined here as the requirement to see objects identifiable with the runway. The runway itself, or its approach lights need not to be in sight.

Presentation of the non-precision approach procedure

Company charts help to establish on a continuous final descent profile by giving a point ‘D’, normally located on a 3 degrees approach angle, to the runway. (The approach angle is indicated on the approach chart).

Whenever possible, DME readings on other suitable fixes will be published to establish a point ‘D’. If fixing a point ‘D’ is not possible (e.g. a single NDB approach), outbound timing as published on the approach chart are calculated to (more or less accurately) establish on the final approach track at point ‘D’.

To monitor the ‘smooth out’ final approach descent profile, whenever possible, DME readings with corresponding altitudes will be published.

The ICAO recommended flight path is depicted on the company approach chart as a dotted line and serves the following purposes:

1. To indicate the discrepancies between the company’s preferred profile and the procedure as published in the AIP (and used by ATC).
2. To indicate the margin of protected airspace as provided by the state when the approach is flown using the preferred company Standard Operating Procedure.
3. To facilitate interpretation of NOTAM’s or the use of the approach charts from another source (e.g. FAA charts).

The dotted lines are NOT published when:

1. A step down fix is unsafe or otherwise prohibited; or,
2. The publication of dots serves no purpose (e.g. The State procedure provides for a descent profile according company’s wishes, or point ‘D’ is defined by a DME and the same DME is officially required for other step-down fixes, refer to Fig. 5).

Some examples of the presentation

05. CIRCLING APPROACH

Note: The diagram shows the instrument approach, circling area, published missed approach procedure, maximum indicated airspeeds, and the requirement to remain within the circling area.

06. DH/DA/MDA AFFECTED BY AMENDED OCH AS PUBLISHED BY NOTAM

OCH = Obstacle Clearance Height, OCA = Obstacle Clearance Altitude.

In your routine operation you will not be bothered by this OCH/OCA business. However, there are always these few nasty cases, when a changed OCH or OCA is printed in the NOTAM. How to decide on the correct course of action?

ILS CAT I

Compare notam OCH with published HAT. If OCH is higher, add threshold elevation to OCH, to find DA. You may round-off to next 10 ft. Anticipate the possibility of a reduced visual segment with a higher descent limit.

Compare notam OCA with published DA. If OCA is higher, raise DA to OCA value. You may round-off to the next 10 ft. Anticipate the possibility of a reduced visual segment with a higher descent limit.

This may seem easy, but unfortunately there is a complication. You could have to face the situation where the notam supplies you with two different OCH’s or OCA’s, one with the indication pressure altimeter, the other with the indication Radio Alt. Just, as an example see the copy of such a notam below.

Why, you may ask, and which one should I choose? ICAO recommends to establish two different OCH’s or OCA’s. The Radio Alt one is for operations where a DH is used rather than a DA. In other words, the descent limit is established using the radio altimeter, the indicated value is not needed to be the RA setting for CAT III. The Pressure Alt one (the same case) applies to those operations where pressure altimeter is used to establish the descent limit.

ILS CAT II

The OCA/A based on pressure altimeter is used by the company to establish the descent limit.

The OCA/A based on radio altimeter is usually lower, but never higher than the one based on pressure altimeter. (Pressure altimeter induces more errors).

Note that an indication of OCH is not meant to be an indication of ‘based on radio altimeter’ as OCA does not necessarily mean based on pressure altimeter. A value between brackets normally indicates OCH.

An OCH is always a height above the threshold. As long as this OCH is below our minimum DH or 100 ft above threshold, CAT II approaches using published RA setting and RVR are permitted. This is demonstrated in the figure above.

Should an amended ILS CAT II OCA have been published in the Notam, then subtract the threshold elevation to find the applicable value to be compared with our 100 ft; if the OCH is higher than 100 ft, CAT II approaches are no longer permitted until an adjusted RA setting has been published.

Nothing to do really with OCH, but everything with obstacles is the feature Obstacle Free Zones (OFZ). For CAT II and III approaches the principle of see-to-avoid-obstacles is considered to be unacceptable. ICAO has therefore established so-called Obstacles Free Zones. They consist of a few mathematically defined planes. No obstacles are permitted to penetrate these obstacle free zones. If an obstacle does penetrate, CAT II and CAT III operation is no longer permitted. It is not very likely to happen, but if so, it will be indicated as ‘OFZ penetrated’. In that case CAT II and III are no longer permitted.

NOTE: Amended ILS CAT I OCH/OCA does not have to affect the OCA/A of the ILS CAT II approach. Amended ILS CAT II OCH/OCA does not affect the ILS CAT III operations.

ILS CAT III

The OCH for an ILS CAT III approach is always zero. In other words, there is no ILS CAT III OCH or OCA. The only requirement for CAT III approaches is that the obstacle free zones are not penetrated (refer to ILS CAT II).

NON-PRECISION APPROACHES

Compare Notam OCH with published HAT. If OCH is higher, raise HAT to OCH value. To find MDA add threshold elevation. May be rounded off to the next 10 ft. Anticipate the possibility of a reduced visual segment with a higher descent limit.

Compare Notam OCA with published MDA value. If OCA is higher, raise MDA to OCA value. May be rounded off to the next 10 ft. Anticipate the possibility of a reduced visual segment with a higher descent limit.

7. PIREP / STAREP

At some airports experience shows that reported weather conditions are not always conform the actual situation. Meteorological Visibility or RVR values could be reported well below the actual ones. This could preclude continuation of the approach and result in an unnecessary diversion.

PIREP (Pilot’s weather report) or STAREP (Station’s weather report) may be used instead of the ATC reported conditions to determine whether a continuation of the approach is permitted.

Use of PIREPs and STAREPs is restricted to those airports where we have reason to doubt weather reports. It will be indicated in the Aerodrome Information if PIREPs and/or STAREPs may be used. STAREPs are only allowed at those airports where no State minima are applicable. Publication in the route documentation takes this restriction into account.

PIREP’s can be obtained from crew or previous landed aircraft.

Only FFSS personnel is authorized to measure and pass to the crew an observed visibility report. The measurement can be based on an assessment using the runway edge lights or known objects in the surrounding of the airport. This human observation will preferably be reported to the crew by means of the company channel. Company staff is advised to make observations in case of doubt about the reliability/accuracy of the official reported observation.

If no company channel is available, STAREP will be passed through ATC channels, preferably the Approach Control Frequency. This will be done as a company message, using a fixed RT format: “company (flight number) Company message: STAREP (Meteorological Vis in meters)”. If applicable, a trend could be added e.g. ‘increasing’, ‘decreasing’ or ‘fluctuating’.

The crew is authorized to use this report instead of the official report.