Thread: QNH or QFE ?
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Old 9th Aug 2018, 09:14
  #42 (permalink)  
Join Date: Feb 2007
Location: Amsterdam
Posts: 4,592
RPS is totally different from a QNH in that it's the lowest FORECAST pressure setting in a particular region rather than an actual measured and reported pressure setting at an airfield or other defined point.
Originally Posted by eckhard View Post
And useless for most pilots as a result.
I don't agree. We measure altitude (or height) in an aircraft not just so that we can impress our passengers about how high we actually are - although from the PA announcements in a commercial airliner you might think that that is still the case.

We measure altitude or height for at least three specific purposes:
1. To measure how high we are above an airfield or runway. This is important for circuit altitude/height, and for IFR procedures to determine when to go around or land (DA, MDA).
2. To measure how high we are related to surrounding terrain, and to achieve whatever obstacle clearance we think is necessary.
3. To have a common datum when aircraft are reporting altitude, so that two aircraft that are reported to be in the same position, but at a different altitude, are indeed a certain amount of feet apart. In other words, for collision avoidance.

RPS is useless for 1, as it is not sufficiently accurate. But it is perfectly usable for 2 and 3.
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