Decision Height maximum range
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May I add some operating context to this thread?
Almost every airline will use QNH when descending below Transition Altitude. As a consequence, all minima (other than Cat II and cat III) are therefore referred to as Decision Altitudes (DA). With an airport perhaps several hundred or even thousand feet above sea level is not uncommon therefore to be using a DA above 1,000 ft. The ‘bugs’ on the main altimeters are used as a reminder of the DA - setting the bug to the neared hundred and tens of feet, any thousand is of feet being committed to memory. The radio altimeters plays no part in DA other than becoming ‘active’ at and below 2,500 f and providing awareness.
Category II and III approach minima use Decision Height and so the Rad Alt is the ONLY reference for minima.
Cat II will have a DH very close to 100 ft Above the Runway Threshold (arte), and be corrected +/-a few feet by the regulating authority designing the approach to take account of the terrain under the aircraft at the position. Thus, 97 ft might be promulgated as a DA for a particular runway.
For the RJ the minimum DA for a Cat III approach is 50 ft - again, referenced by the RA.
The autopilot is always engaged for Cat II and III approaches to DA, and may remain engaged to 50 ft arte as long as the Cat II light remains lit. The autopilot can remain engaged during a Cat III approach, touch down and rollout as long as the Cat III annunciator remains lit.
During Cat II and III ops the main altimeters remain set to QNH.
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Rad alt alert is used to set a DH, NOT a DA. There is a small knob to set the RA DH on each of the RAs. They are location on the left and right EFIS Dimming Panel (DP).
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@Paul-Cairns thank you!
