1. Take-off
1.1 Required Visual Segment
For the take-off roll, outside visual reference is required. On company aircraft no use is made of instrumental guidance. Therefore visual aids, lightings and/or markings, are required.
Taking the requirement of the establishment of take-off minima into account, namely the ability to control the aircraft in case of an engine failure at V1, one can understand that in case of multiple RVR reporting along the runway, the published value should not be seen as applicable for the take-off point only: the contrary is the case. The visibility must be met along the runway, but the most critical point is that point where V1 is reached.
You are however allowed to use only the actual required runway length. This means that, if only a part of the runway is required, only that part of the runway must comply with the RVR minimum.
1.2 Relevant RVRs
For the purpose as mentioned above, you can divide the runway in parts that are “covered” by the RVR measuring positions. The location of RVR assessment units is laid down in ICAO Annex 14. The location of the “A” assessment unit is 300 m after the threshold, the “C” unit 300 m before the runway end. The mid point unit (“B”) will be located approximately at the mid runway position.
For this purpose, company policy allows to roughly divide the runway in three equal parts of 1/3 of the runway length. Each part of the runway is “served” by an RVR measuring unit. In this way, although perhaps not very exact, a usable solution has been found.
The required runway length can be found in the AOM and compared with the Take-off-run Available from the TL table.
1.3 Minimum Visual Segment
There are other interesting points to highlight about the application of the take-off minima.
Transmissometers, human observers and television cameras are all substitutes for the “real” RVR. ICAO definition of “RVR” is: The range over which the pilot of an aircraft on the centre line of the runway can see the runway surface markings or the lights delineating the runway or identifying its centre line. All these substitutes have tolerances.
A 20% deviation is allowed according ICAO Guidelines.
As a consequence, it may be possible that, although a reported RVR is at or above the minimum value required for take-off, the observed RVR, standing at the take-off position, is less than required.
For that purpose, European legislation requires a minimum visual segment to be visible from the cockpit. This minimum visual segment is derived from the lowest allowable take-off minimum RVR value and the obscured segment for the aircraft type, segment below the cockpit nose.
The company uses a minimum visual segment of 110 m for CAT C aircraft and 125 m for CAT D aircraft.
If the TDZ RVR is below the required minimum, or out of service, but the subsequent RVRs are above minimum, the reported TDZ RVR can be overruled by the pilot’s observation as this observation is exactly what an RVR is by definition.
2. Approach and Landing
2.1 The Value of the Horizontal Component (VIS/RVR)
If a choice would have to be made which of the two components of the Landing minima is the most important one, the vertical component, the descent limit, or the horizontal, the RVR/VIS value, the vertical one would turn out to be the most important. Why?
Except in case of an approach procedure without a DH, an instrument approach procedure will never bring you on the runway: it will bring you to the descent limit. Obstacle-wise it is safe to descend down to the descent limit and follow the missed approach procedure as published. As long as you do not descend below the descent limit and the missed approach procedure is initiated at or before the missed approach point, you are protected for obstacles. For that purpose, a horizontal component is not required.
If it is allowed to descend only below the descent limit if:
- adequate visual reference has been established; and
- you are convinced that the landing can be completed safely,
there would be no need to specify a minimum horizontal component, VIS or RVR, on the approach chart.
2.2 “Approach Ban” versus “look-see” Policy
JAA impose a so called “approach ban policy”. Such a policy calls for a check-point somewhere on final approach where the reported RVR or Visibility must be at or above the required minimum. This policy on the one hand prevents a possible go-around from a low altitude, if the required visual cues are not present when reaching the descent limit, on the other hand it does not take into account that the RVR or the Visibility report is often inaccurate and is based on conservative presumptions. This conservatism often results in an actual visibility as observed from the cockpit, “Flight Visibility”, upon descent limit to be better than predicted. In such cases a safe landing could have been made.
An operation without an approach ban is called a “look-see” approach.
Company minima policy is based on the approach ban policy.
2.3 RVR/VIS Conversion Table
JAA aerodrome operating minima tables are based on RVR measured values. For those cases where no RVR can be measured, normally a Meteorological Visibility will be reported. Besides the fact that this is a less accurate method, it does not take the effect of lighting facilities into account. High Intensity lighting facilities in combination with a dark background for example has proven to give more than a duplication to the effect of perception. The table is a simplified method to compare the reported VIS value with the value that would have been given if an RVR measuring device would have been available.
When RVR is not available, an RVR may be derived from a reported VIS applying the following conversion table:
2.4 Additional RVRs
If we look at the operational requirements for the visual segment for the landing, we can distinguish two different phases.
The first one is the “see to land” phase. For this purpose the TDZ RVR is required and the minimum value is based on the assumption that visual cues must be visible.
The second phase is the requirement to control the aircraft during roll-out phase. For roll-out, different values are required, based on the question: what is the minimum visual segment required to control the deceleration process? These values must be measured by the mid point and roll-out RVR units.
As for take-off, only those RVRs actually needed for the ground roll must be taken into account. However, if for example the mid-point RVR is unserviceable, and the roll-out RVR would be located in the non-relevant section of the runway, the roll-out RVR becomes relevant. On the other hand if only the TDZ RVR is relevant because only the first 1/3rd of the runway is used for landing and roll-out, long runway CAT B aircraft, both the mid point and roll-out assessment unit may be unserviceable as the second and third part of the runway is not used.
2.5 Reaching the Descent Limit
When past the RVR/VIS “checkpoint”, OM or equivalent position, subsequent reports can be ignored, as there will be a “final check” on the actual visibility condition at the descent limit.
BOM specifies the required visual cues for landing. Note that in the former policy, until March 1993, it was required to see the runway threshold in case of a non-precision approach before descending below the descent limit. Although the calculation method used by the company still caters for this, in actual flight, seeing the runway threshold is not required anymore. Any one of the visual cues listed in BOM 2.3.6 will suffice.
You must be well aware that the protection by instruments terminates when descending below the descent limit. In this phase of flight you are on your own. There is no protection for obstacles by instruments, although there are several safety tolerances built-in in the protection areas.
For more background information an operation below the descent limit, refer to guide 6.1.8.
3. Inoperative Ground Aids
The published landing minima are based on the instrumental and visual aids required for the approach. A temporary unserviceability of these elements may or may not have effect on landing minima. For this purpose the “components-out table” is published.
This table is not a “permit” for Aerodrome Operators to minimize the visual and instrumental aids.
For example the fact that an increase of the spacing of the runway centreline lights to 30 m does not have an effect on a CAT III operation, does not mean that a CAT III runway could be equipped with this spacing.
The same goes for the amount of RVR assessment units: According ICAO Annex 14 a CAT III runway must be provided with three assessment units. A temporary outage of one unit may not affect a CAT III operation. The ICAO Annex 14 rule must however still be adhered to: three units should be installed.
4. ILS Classification
4.1 Introduction
Recently, ICAO added detailed information to Annex 10 regarding the classification of Instrument Landing Systems. It was felt that the performance categorization as used so far, CAT I, II, IIIA, B and C, was insufficiently refined. In order to more exploit the capabilities of the airborne systems, read: automatic landing and roll-out capability, it was necessary to more accurately describe the ILS.
This was achieved by the ILS Classification system, using three designated characters. It provides a description of those performance aspects which are required to be known from an operation’s viewpoint in order to decide the operational use of a specific ILS.
4.2 Brief Description
The classification is based on three characters:
- Facility performance, Category I, II or III, describes the accuracy of the signal and is measured as a displacement relative to the ideal ILS path.
- Limit of course structure i.e. point to which the localizer conforms to the required accuracy:
- A: 4 NM before the threshold.
- B: 1050 m (3500 ft) before the threshold.
- C: Glide path altitude of 100 ft HAT.
- T: Threshold.
- D: 900 m (3000 ft) beyond the threshold.
- E: 600 m (2000 ft) before the runway end.
- Level of integrity and continuity of service: 1, 2, 3 or 4, where 1 is the lowest and 4 is the highest level. This level gives:
- an indication that, with a certain level of probability, a valid signal is emitted; and
- indicates the probability of signal availability in a given period of time, exposure time: per each 15 or 30 seconds.
ICAO Annex 10 specifies the level of accuracy of the Glide path signal and the Localizer signal. At point B it must reach the highest accuracy based on a 95% containment level.
This 95% containment level, equal to 2 Standard Deviations, is not the only requirement, as in very low visibility operations, for this phase of flight the ILS signal is used as primary and sole means of navigation: there is no visual reference required. Therefore a much higher level of integrity and continuity of service is required.
4.3 Examples and Explanation of ILS CAT III Classification
1. ILS CAT III /E/4
In this example the course structure meets the CAT III accuracy until 600 m before the end of the runway, ILS point “E”.
The integrity and continuity level “4”, means that the chance of an undetected invalid signal is 1 out of 2 billion landings. The chance of a ceased signal is 1 out of 5 million landings during a period of 30 seconds, exposure time, in case of a localizer and 1 out of 5 million landings during a period of 15 seconds in case of the Glide Path, GP.
Moreover, the total system must have proven to meet a Mean Time Between Outage, MTBO, of 4000 hours for the LLZ and 2000 hours for the GP.
2. ILS CAT III /E/3
Differences between level 3 and level 4 are:
- There is no increased exposure time for the LLZ, both GP and LLZ exposure times are 15 sec.
- The MTBO is 2000 hours, LLZ and GP meet the same criteria.
In general one can say that a system classified as III / E / 3 meets the same criteria as a III /E/4 system but that the chance of an invalid or cessation of the signal is slightly higher.
3. ILS CAT III / D / 4 or 3
ILS CAT III / D / 4 or 3 meets the same criteria as indicated above III / E / 4 or 3 but only until 900 m beyond the threshold. The signal beyond that point does not meet the required accuracy or has not been measured.
4. ILS CAT III /D/2
Due to the low integrity / continuity level (2), the system cannot be used for CAT III operations. Higher minima, CAT II, will cater for the probability of an invalid or ceased signal.
4.4 Use of Autopilot
Safe use of autopilot is guaranteed until the ILS measuring point. Beyond that point the ILS signal has not been measured in case of an ILS CAT I and II system. However, use of autopilot, if applicable, is allowed, but must be monitored with outside adequate visual reference: the accuracy of the system may be out of tolerance.
In case of an ILS CAT III, the system is in principle measured until point “E”. Indication of another point then “E” means that the ILS could be out of tolerance beyond that point.
In case of a downgrading to e.g. II / T / 3 a manual CAT II may be allowed. The company however requires autoland for CAT II. The downgrading table limits the operation to ILS CAT I only.
After an assessment of the autoland behaviour, the LLZ deviation is often filtered out, CAT II operation may be authorized via a publication in the Route Documentation.
4.5 Operational Application of the Classification
Only systems III / E / 4 and III / D / 4 are sufficiently accurate and reliable for a “no DH” operation: an operation principally without a need for visual verification of the aircraft’s position prior to land.
In case of an ILS CAT III / D / 4 system there is a requirement for visual reference during the roll-out phase.
All other classifications require visual verification by the pilot, prior to land and during roll-out, to allow to take over manually in case of a failure, erroneous signal or no signal at all, in one of the ground components.
A “translation” from this complex classification system to landing minima is required.
E.g. Schiphol ATIS and NOTAM may specify a temporary downgrading. On the ATIS the following information may be broadcasted:
“ILS RW… CAT III only”
“Classification: Three Echo Three”.
Note that no use is made of the “old” categorization e.g. “CAT IIIA only”. The classification Three Echo Three must be translated directly to the allowable landing minima.
Minimum Allowable Landing Minima
The following table indicate the minimum allowable landing minima related to the ILS classification.
Legend: P = Minima of lowest allowed, downgraded, ILS category.
CAT II only authorized when specifically published in the company route documentation.
5. Low Visibility Procedures
According European legislation, an Aircraft Operator is obliged to verify whether an Aerodrome Operator has installed measures for protection of the ground manoeuvring in case of operations below certain values of visibility and/or cloud base. As a rule, the measures are initiated when VIS/RVR drops below 800 m. When the weather conditions deteriorate further, more stringent measures have to be taken.
Examples are:
- use of Surface Movement Radar,
- fire fighting equipment may be required to take position on the aerodrome in order to fulfill the requirement of response time; and
- in case of complex aerodrome layout:
- specific taxi-measurements may have to be taken,
- runway intrusion measurements,
- etc.
Not only temporarily actions are required, also maintenance of visual and non-visual aids form part of the procedure.
All these actions are known as “Low Visibility Procedures” and are laid down in ICAO Annex 14 and Documents, Manual of Surface Movement Guidance and Control, All Weather Operations Manual.
European legislation requires also that the pilot in command verifies that measures have been taken prior to initiation of a low visibility operation.