01. The Protection Areas
Even when an airport authority maintains the ILS as recommended by ICAO, it may very well occur, that large deflections do not trigger the monitoring equipment. These deflections are caused by vehicular movement. Especially airplanes in the vicinity of the Localizer or Glide path antenna cause interference of the respective beams. For this reason all ground movement is banned from the critical areas during actual CAT II or III weather conditions. These areas are called the protection areas. These protection areas are shown in the figure below.
The protection areas must be clear to perform an actual CAT II or III approach. Should this not be the case, CAT II limits apply. Because of this need to keep the protection areas clear, taxiing aircraft will be kept at some distance from the runway and holding short of the runway will have to be done on designated CAT II holding positions outside these critical areas. ATC must also ensure that in actual CAT II and III conditions aircraft do not take-off from the landing runway, while an aircraft is on final approach. For the same reason additional longitudinal separation is required between successive landing aircraft. If for any reason it appears to be impossible to keep the protection areas clear, ATC will inform you accordingly. Actual CAT II or III conditions can cause the capacity of an aerodrome to decrease considerably.
02. Radio Altimeter
The radio altimeter measures the altitude of the main gear above the terrain. This is straight forward. Operational problems are usually caused by the terrain in the approach area. Some of the problems encountered in the past are:
- Too late activation of the gain program, caused by a dip in the terrain. The result is an unstable approach.
- Terrain warnings when flying with a relatively high speed over upsloping terrain.
- Problems to establish a DH over uneven terrain or over tidal waters, where high and low tide can give substantially different read-outs or fluctuating ones.
When problems like these cannot be solved, CAT II and III limits will not be published.
03. RVR Reporting
RVR gives the pilot an indication of visual reference at the descent limit. The importance of RVR calls for systems with a high degree of reliability and integrity. ICAO has set the requirements in that respect. One of the ICAO requirements is, the period within which the RVR read-out must be updated for any significant change in visibility. In order to meet these requirements states have installed equipment for RVR assessment at suitable locations with automated display and back-up.
Some notes on this are necessary. Modern equipment with digital read-outs as available at e.g. Schiphol Tower is updated every 30 seconds. However, will ATC continuously transmit the latest read-out to the flight crew?
Within Europe anyway, the respective authorities have agreed upon not to burden the pilot with updated RVR values in the final stage of the approach.
The effect of this is, that in principle once past the Outer Marker no RVR values will be volunteered by ATC. So a reduction in RVR could remain unknown to the pilot.
The RVR value presented to ATS is not the actual RVR value at that moment, but an average over a certain period, which is updated at regular intervals. This is done to dim large fluctuations. As a consequence the read-out at the time the aircraft reaches the descent limit is influenced by the measurements over a period before.
At Schiphol for instance, the RVR available at the Tower is the average over the last minute and it is updated every 30 seconds. The RVR read-out at ‘Schiphol Arrival’ is the average over the last 5 minutes updated every 2.5 minutes.
More important probably than the above are other phenomena, which cause the reported RVR to differ from the actual flight visibility. Patchy fog is one. If the RVR is measured in an area of relatively good visibility the crew could become preconditioned by too optimistic reports.
Runway lights can have a blinding effect and can affect flight visibility. However, dimming the lights may cause the RVR’s to drop below limits.
Transmissometer read-outs are the most accurate available indications of visibility for the area they serve. However, there can be limitations such as those discussed above, which require the crew to judge RVR reports with care. A RVR report above minimum cannot automatically mean ‘landing’ and vice versa.
04. Flight Visibility and Cockpit Cut-Off Angle
How much of the terrain below a pilot can actually see from the cockpit depends on a number of factors; weather, the pitch attitude of the aircraft and the so-called pilots eye height are the most important ones.
The position of the eye should be in the eye reference position which gives the optimum combination of good inside and outside visibility. All calculations are based on this eye position. On the DC-10 for example 1 inch below eye reference point will reduce the terrain, visible from the cockpit, by 50 metres. The part of the terrain visible from the cockpit is called visual segment. It is limited by the actual flight visibility and the aircraft’s nose. How limiting the aircraft’s nose is, depends on the geometry of the aircraft and the pitch attitude of the aircraft. With a high pitch-up attitude you will see less than with a smaller pitch angle. Anticipate this, when approaching with a reduced flap setting. (Large) drift angles during approach also influence flight visibility as the down-vision angle is different.
Cockpit horizontal line of sight showing the longitudinal axis, horizontal reference and cockpit cut-off.
Reported RVR and Actual Flight Visibility
Quite some investigation has been done into the actual relation between reported RVR and actual flight visibility. The research has led to a method to calculate RVR minima.
We will not go into the details of this calculation; factors included in the calculation are aircraft geometry, SVR/RVR ratio based on 95% probability and of course DH.
The calculated result must ensure, that if the reported RVR really equals the calculated minimum RVR, normally the actual flight visibility will be sufficient to continue the approach and land. In other words, the RVR minimum is calculated to guarantee a high approach success rate. What is considered sufficient visibility to continue the approach?
Visual Segment During CAT II/III Approaches
On CAT II approaches sufficient flight visibility will offer you a visual segment of at least 90 m at DH. Below DH the length of the visual segment will normally increase rapidly. On CAT IIIA approaches you may expect a visual segment of about 120 m at DH if no roll-out guidance available or 90 m if fail passive roll-out is available and even 60 m with fail operational roll-out. In the following 2 examples you can see how this visual segment will expand during the approach with RVR reported as minimum.
Visual Segment Example 1
A rather schematic view from the cockpit at DH is given in this figure:
The perhaps more realistic glow is shown in this photograph:
Visual Segment Example 2
With DH and RVR at their lowest values the same schematic drawings may be used for any aircraft type, because DH and RVR are calculated so as to give the already mentioned visual segment at DH.
As a general rule, on a CAT II approach with 100 ft DH and reported RVR equal to the minimum for the aircraft type (refer to AOM), the visual segment at DH will be 90 m long.
On a CAT IIIA approach the visual segment at DH will be 120 m (no roll-out guidance) or 90 m (fail passive roll-out guidance) or 60 m (fail operational roll-out guidance).
05. Alert Height (ILS CAT III)
With the introduction of ILS CAT III operation, the FAA required the establishment of an Alert Height.
Definition:
Alert Height is a height (100 ft or less above the highest elevation in the touch down zone), established, based on the characteristics of an aircraft and its particular fail-operational airborne CAT IIIA system, above which a CAT IIIA approach would be discontinued and a missed approach executed if a failure occurred in one of the required redundant operational systems in the aircraft or in the ground equipment.
The reliability and performance of each of the required redundant operational systems must be such, that below the alert height continued safe operation to a successful landing can be effected with a high level of confidence after a failure occurs in one of the redundant operational systems.
The alert height is 100 ft RA.