Why can the rad alt bug be set to -20 ft?
A thought; if the GPWS ‘minimums’ call is tied to the rad alt setting, which is normal, then setting a negative value is one method of disabling the call. Many operators disliked the ‘minimums’ terminology, particularly on NPAs where rad alt, hence the rad alt bug, is not normally used for decision making. I don’t know of any aircraft with a ‘minimums’ inhibit switch, so the -20 ft could be a cheat solution and installed as a customer option.
Don’t forget that rad alt on all NPAs is a very good situation awareness instrument and should be monitored at all times. I set the bug at 250 ft because on all NPAs I should be visual by then.
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