1. Introduction
According to international legislation, an operator is obliged to establish ’Aerodrome Operating Minima’ for each aerodrome of intended landing.
This requirement is laid down in ICAO documents (Annex 6) and in several local regulations such as FAA legislation (FAR) and European legislation (JAR).
Based on ICAO Doc. 8168 PANS-OPS, States are obliged to provide for instrument approach procedures, free of obstacles. The State-published Obstacle Clearance Height (OCH) is one of the factors to be considered by the operator for the establishment of Aerodrome Operating Minima.
There is not yet a common method for establishment of these Minima. ICAO All Weather Operations Manual, published in 1982, was the first ICAO publication to provide for some guidelines.
These guidelines came very late as every airline had already ’their own policy’ at that time.
Specifically for ILS CAT II and III operations the European Civil Aviation Conference (ECAC) produced ECAC Doc 17. Although this document was produced as recommendation material, most European States applied the rules as Standard material to be adhered to.
The European Joint Aviation Authorities (JAA) has produced new rules laid down in Joint Aviation Regulations (JAR). JAR-OPS subparts D and E are dedicated to the establishment of Aerodrome Operating Minima. The company changed the Minima policy in 1997 so as to comply with this new regulation.
Despite the ICAO requirement that the Operator is responsible for establishing Aerodrome Operating Minima, some States not only publish OCHs, but also MDAs and visibility minima. Of course the Operator is obliged to take these minima into account.
2. Factors to Be Considered
For the establishment of Aerodrome Operating Minima the following factors play an important role:
1. Man 2. Machine 3. Environment.
Let us look more in detail into each of these factors.
2.1 Man
Training is the most important factor to be considered. The pilot is trained to operate the aircraft so as to cater for the worst case situation (e.g. extreme weather conditions, critical engine failures etc.).
2.2 Machine
Three main areas can be distinguished:
a. On board navigation equipment such as ILS, VOR, ADF, DME, pressure and radio altimeter. Still in development stage and not yet certified for approach and landing: RNAV (with DME or GPS sensor) and MLS.
b. Aircraft capability: autocoupling gives a better performance of track keeping compared to manual flying. Also system redundancy for ILS CAT III operations and aircraft performance (climb gradient) play an important role.
c. The manoeuvrability in the air of the aircraft determines the protected airspace required for the approach (and missed approach) procedure. ICAO categorized aircraft from A to E, where ’A’ requires less and ’E’ most airspace (see chapter 6.1.1). This airspace requirement is the most important factor for the calculation of protected airspace to avoid obstacles, in those phases of flight where manoeuvring is required (initial approach and missed approach).
The manoeuvrability on the ground determines the minimum visibility required for take-off.
2.3 Environment
Here also three main areas can be distinguished:
a. Navigational aid providing track guidance: ILS, VOR, NDB, DME and MLS determine the accuracy of the aircraft position relative to the threshold and relative to obstacles.
b. Fixed obstacles, determine, together with the accuracy of the navigational aid, the required protected airspace and thus the OCH.
c. Visual aids provide for the required visual cues. These are required for the decision whether or not the approach can safely be continued to land.
3. Calculation of Aerodrome Operating Minima
3.1 Take-off Minima
Basic criterion for take-off minima is the aircraft ground manoeuvrability during a (rejected) take-off roll.
Legislation requires that the minimum visibility for take- off is based on the assumption that the aircraft remains controllable during take-off ’..in the event of both a discontinued take-off in adverse circumstances and a continued take-off after failure of the critical power unit’ (JAR).
Flight simulators are not very helpful in testing the minimum values for this phase of ’flight’, as ground manoeuvring is not very well simulated due to technical limitations. Several operators’ long term experience of training in low visibility conditions demonstrated that the presently published values are safe.
Normally no ceiling is required for take-off, unless an obstacle has to be avoided visually.
3.2 Landing Minima
The type of approach aid (ILS, VOR etc.), the OCH (calculated by the State and checked by the company), aircraft category and the visual aids on and near the runway, are the ’ingredients’ required to establish landing minima.
Let us, for the sake of clarity, divide the landing minima in a vertical component (DH / DA / MDA) and a horizontal component (VIS / RVR).
3.2.1 Vertical Component (DH / DA / MDA)
The vertical component is determined by the OCH and the system descent limit.
The OCH is obstacle related, calculated by the State in accordance with ICAO or FAA guidelines and checked by the company in accordance with ICAO guidelines.
The system descent limit is the lowest allowable descent height, based on the capability of the navigational aid. For example an ILS CAT I system descent limit is 200 ft above the threshold.
ICAO Annex 10 specifies the accuracy and integrity level of an ILS CAT I system that must be complied with till that height.
For each navigational aid such a system descent limit has been specified.
The operator may add an extra margin to this limit, based on factors such as training and exposure to the system.
On the approach chart the descent limit is published as a Minimum Descent Altitude (MDA) for a non-precision approach, as a Decision Altitude (DA) for an ILS CAT I approach or as a Decision Height (DH) in case of an ILS CAT II/III approach. This minimum value is the highest of the ’obstacle determined’ (OCH) and the ’system determined’ minimum altitude.
Of course, the lowest descent limit is the ILS CAT IIIB ’no DH’ limit.
3.2.2 Horizontal Component (VIS / RVR)
A horizontal component is required in order to comply with the ’see to land’ concept. This concept specifies that the pilot must verify the aircraft position relative to the threshold in order to continue the approach and to land safely.
Therefore visual ground contact is required from the point at which the decision has to be made.
Of course not only ’ground contact’ is required, visual elements (cues) must be seen to help the pilot in this decision process. As sometimes there is not much time for this process, the quality of the visual aids must be high. ICAO Annex 14 and an ICAO Visual Aids panel guarantee the high Standard.
Visual aids are:
- Runway markings.
- Runway edge, centre line, touchdown zone and threshold lights.
- Approach lights.
- PAPI / VASIS.
3.3 Non-precision Approach
With the adoption of the European guidelines regarding the establishment of non-precision Aerodrome Operating Minima by the company in 1997 (precision landing minima and take-off were already adopted in 1993) some principles changes in the calculation method took place.
The former method was based on a geometric calculation of the assumed position of the aircraft relative to the threshold, upon the moment for the decision to land. A correction for visual segment and lighting facilities was taken into account.
The JAA non-precision landing minima, although basically based on the same philosophy, are much simpler in set-up. As was seen in the field of ILS CAT II RVR determination method, which was initially based on sophisticated calculations, result of practical experience was taken into account and finally replaced the calculation method.
The same occurred in the field of non-precision landing minima derivation. During the process of developing the values, the practical experience of many State/Operator calculation methods were taken into account. The result of this process is reflected in a set of tables with the aircraft category, the descent limit and the lighting facilities as input-variables. The results, compared to the former company minima, are: lower values.
This result, however, was intentional. It is based on a hard application of the approach ban policy (point on final approach where the minima must be at or above the prescribed values). Until that time, this policy was not a requirement in legal documents such as the ICAO All weather Operations Manual. Not new, however, for the company.
The application of the approach ban policy is to be seen as a safety net in a system tending towards a ’look-see’ method. This trend from a fully protected system where an approach could not be initiated without having the highest possible change to a successful landing, towards a more liberal system of ’look-see’ was started in the mid 70-ies when the ceiling requirement for approach initiation was deleted from the policy.
The effect of the lower minima will be a higher chance to go beyond the approach ban point (OM/FAF) but places a higher load on the pilot’s shoulder to make the right decision when reaching the MDA.
This, however, is not seen as an unsafe manoeuvre. Since the application of the revised ICAO PANS-OPS guidelines, the missed approach procedure is well protected against obstacles. Much better than the former guidelines did.
All these factors where taken into account when establishing the new landing minima tables.
3.4 Precision Approach ILS CAT I / PAR
The vertical component (DH) is the higher of the system descent limit for ILS CAT I (200 ft) and the OCH.
The changes in the derivation of the landing minima for ILS CAT I are similar to those prescribed for the non-precision approach (refer to the chapter before). Most significant change compared to the former company minima policy is the cut-off value. In the old company method, the minima values were balanced with the HAT, without limitation. JAA guidelines are based on the assumption that values above the transmissometer range tend to be too inaccurate.
3.5 Precision Approach ILS CAT II
The vertical component (DH) is the higher of the system descent limit for ILS CAT II (100 ft) and the OCH.
There has been an evolutionary process of establishing the horizontal component of the ILS CAT II landing minima.
With an earlier method, still used as a guideline for new aircraft, the horizontal component is calculated based on the cockpit cut-off angle, an average aircraft pitch, a correction factor for increasing thickness of mist with increasing height and the length of the visual segment. The length of this segment depends on aircraft capabilities such as autocoupling, system redundancy etc.
A second development was that the method of calculating was computerized. UK CAA developed a computer model, based on the same principles. The effect of runway and approach lights was also taken into account. This model however has not been used as an international guideline.
In 1993, new JAR became in effect. Values of the horizontal components are now based on the results of calculation as mentioned before but takes into account years of experience with ILS CAT II approaches, specifically in the European countries.
3.6 Precision Approach ILS CAT IIIA
For CAT IIIA approaches the vertical component has been established as that point where the aircraft has demonstrated to be capable of executing a go-around without touching the runway. For most aircraft a value of 15 or 20 ft is sufficient.
The horizontal component is limited to the ICAO definition of an ILS CAT IIIA: minimum RVR 200 m.
An option that the company adopted in 1993 was, apart from the 737, the capability to apply a Fail Passive operation. This is an option for those cases where the aircraft is fail passive certified and/or is temporarily limited to a fail passive operation. In a fail passive situation, the aircraft has insufficient redundant systems to continue the operation coupled to the autopilot. A manual intervention must be catered for.
Therefore a higher DH is required (50 ft). For the horizontal component, legislation provides for two options:
1. To land after a failure : minimum RVR is 300 m 2. To go around after a failure : minimum RVR is 200 m.
For the (fail passive) 737 the company applies the second option, for the 747-400 and the 767 (certified to operate in a fail passive mode) the first option is applied. Other aircraft types make no use of it.
Training requirements are part of this choice.
3.7 Precision Approach ILS CAT IIIB with DH
The required DH is the lowest allowable as for ILS CAT IIIA: 15 or 20 ft.
Again for this type of approach the minimum horizontal component is established via a calculation method, based on the same principles as explained for ILS CAT II.
Although legislation allows 100 m RVR, local authorities may require higher values.
For the company the authority authorized the larger aircraft to a minimum of 125 m.
3.8 Precision Approach ILS CAT IIIB without DH
This type of approach requires no outside visual reference for the approach to land phase.
A minimum RVR of 75 m has been established by the JAA, in order to allow visual reference during taxi.
It follows that, in theory, no RVR measurements are required along the ’runway of intended landing’.
However, an ILS CAT IIIB operation on a runway without functioning transmissometers is higly unlikely.
Note that an ILS CAT IIIB ’no DH’ landing can be made followed by a manually controlled roll-out.
In these cases the Mid point (B) and Roll-out (C) RVR reports must be at or above 125 m.