Basic Questions regarding DH concept for CAT I/II/III Ops
I have a question regarding the decision height concept during ILS approaches. I am only an enthusiast not holding any pilot license, so I am sure that I am missing some important points. However I am trying to understand the concept behind DH and what allows lower DHs (from a system point of view) for higher ILS Approach Categories (I/II/IIIa/IIIb).
What follows is a short summary of what I have read up to this point. Please correct me where I am wrong. My questions are integrated in this summary, where they fit. Assumption for my question is that the aircraft is not equipped with HUD.
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Fail/Passive Autoland System
The system is made up of 2 redundant units that guarantee the integrity of the whole system. As long as integrity is still given, the system is able to perform an autoland perfectly in the center of the touchdown zone. A violation of integrity (component in any of the 2 units failing) is detected via monitoring of unit inputs/outputs/internal data. In this case the system as a whole becomes passive. The aircraft is left in a situation where it is not significantly out of trim, but the autoland function is lost.
Fail/Op Autoland System
The system is made up of at least 3 redundant units that guarantee the integrity of the whole system. The 3rd redundant unit guarantees a higher availability of the autoland function. The first failure, causes the system to loose redundancy. Only after a second failure, integrity cannot be guaranteed anymore, which results in the system as a whole becoming passive. Autoland function is lost thereafter.
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CAT 1 Operations
No Fail/Op System required. In CAT 1 operations autoland should not be conducted as the beam quality may not sufficient (LVO not in force). The DH assures that the pilot can judge the point where he has to initiate the flare.
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CAT 2 Operations
No Fail/Op System required. In CAT 2 operations, autoland as well as manual landing would be allowed.
Q1: What is the reason for the DH being lower than in CAT 1 operation from a system point of view (is it due to the absence of beam fluctuations (LVO in force), and more stringent requirements for the ground equipment, allowing the pilot to rely on ILS data down to a lower height).
In CAT 2 operations the DH has to guarantee that the pilot is able to judge the flare point.
Q2: This seems to be clear when conducting a manual landing, but what’s the reason when performing an autoland. If a failure in the Fail/Passive autoland system would occur below DH, aircraft would not be out of trim, the failure would be announced to the pilot, after which he would be able to perform a G/A. Is he required to be visual when performing the go around from a low altitude, or what is the reason for a DH in CAT 2 autoland?
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CAT 3a Operations
No Fail/Op System required as long as DH>50ft. In CAT 3 operations, autoland would be mandatory, as visual references do not allow the pilot to continue visually. DH only has to guarantee that Pilot is able to judge if landing is performed within the touchdown zone.
Q3: Again same question as for CAT 2 autoland, why is DH necessary and why is it lower than for CAT 2 autoland?
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CAT 3b Operations
Fail Operational System with rollout system mandatory. Autoland would be mandatory, as visual references do not allow the pilot to continue visually. No DH necessary. Alert Height is chosen to meet the “robustness” of the system (bigger Alert Height, more robust system). Fail Operational System has a risk time within which it is guaranteed that no dual failure will occur. So probability of a loss of autoland function below Alert Height is sufficiently small.
Q4: Is this necessary to prevent the pilot from encountering a situation where he just touched down, selected the reverser (no G/A anymore) and the autoland system failing – leaving him in a situation where he has to continue landing manually without visual reference?