Originally Posted by
Clinton McKenzie
(Chronic Snoozer nailed it by nominating the RADALT as the most accurate gizmo to measure the distance between a point on an aircraft’s airframe and the ground or water. These days I think the margin of error in RADALTs is specified in centimetres. But why do some aircraft have RADALTs? Precisely because the other gizmos aren’t precise enough when an aircraft is getting close to the ground in zero or bad viz.)
RADALT can also read the height above the terrain below the water, not always the height above water, you can see that sometimes coming in over water to land.
Originally Posted by
Clinton McKenzie
As the CASA person who made the statement in front of the Senate Committee wasn’t in the aircraft with the pilot, the CASA person can’t be referring to the aircraft’s altimeter or the display on the aircraft’s transponder. As the pilot involved denies the allegation, I can only assume - reasonably I suggest – that the pilot doesn’t reckon the altimeter was "saying" 125’. I can only assume – reasonably I suggest – that the CASA person was referring to some gizmo on the ground relying on Mode C transponder / ADS-B data.
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The ADS-B data packet can contain the encoded pressure altitude which is either in a 25 or 100 ft resolution in ft, or the GNSS height above the WGS-84 geoid in meters. If they are basing the 125' off GNSS, if the aircraft is in south west Australia it could under read by around 330 ft, and NE Australia over read by 330 ft. A line through around Hobart, Adelaide, Alice Springs, Port Headland would have close to zero difference between the WGS-84 geoid and MSL.