Radar altimeters are generally not used to clear mountains. (You'd be too
close by the time it went off.) Enroute terrain clearance is achieved under
instrument (controlled) flight rules by altitude assignment; under visual
flight rules by minimum altitude indications on each sector of the sectional
map.
Radar altimeters ARE useful on landing approach because they provide a
direct indication of height above ground. A regular pressure altimeter
shows your height above sea level (roughly), and you have to subtract field
elevation (the airport where you're landing) from that in order to know
(again, roughly) how high above the ground you are.
Altimeter settings in the USA are used below Flight Level 180. (FL 180 is
determined by setting your altimeter to standard pressure (101.32 kPa or
29.92 in Hg) regardless of what the actual barometric reading is; since all
airplanes are setting to standard pressure at and above FL 180, they all
have the same altitude reading relative to each other. (The altitude
reading is only reasonably accurate if the actual air pressure where they
are happens to be the standard pressure.) Below FL 180, the controller
gives the pilot the actual pressure of a nearby airport (or the pilot
listens to an recorded radio reading of it). It's also part of the hourly
weather report for an airport. In the USA unfortunately it's still given in
inches of mercury.
Current weather for San Francisco International Airport:
KSFO 240156Z 26016G22KT 9SM BKN012 13/08 A3006 AO2 PK WND 26027/0109 SLP179
T01280083
KSFO - San Francisco International Airport
240156 - April 24, 0156 UTC ("Zulu") - 1856 local time April 23 (PDT)
26016G22KT - Wind from 260 degrees (west-northwest), 16 knots, gusting to 22
knots
9SM - visibility 9 statute miles
BKN012 - cloud cover broken, ceiling 1200 ft (broken means clouds with a few
holes - that is, the famous San Francisco fog is in)
13/08 - dunno
A3006 - altimeter setting/barometric picture is 30.06 inches of mercury
A3006 - dunno
A02 - dunno
PK WND 26027 - peak wind 260 degrees, 27 knots, observed at 0109 UTC
The rest - dunno (this report has changed since I was flying regularly)
More:
http://www.wrh.noaa.gov/mesowest/getobext.php?wfo=mtr&sid=KSFO&num=48&raw=0&;
dbn=m
It's still an inch-pound world out there in US aviation.
Carleton
-----Original Message-----
From: owner-u...@colostate.edu [mailto:owner-u...@colostate.edu] On Behalf
Of mech...@illinois.edu
Sent: Friday, April 23, 2010 14:33
To: U.S. Metric Association
Subject: [USMA:47247] Re: Air flight altitudes in meters
Michael,
Thanks for your essay on the navigation of large aircraft.
>... You wrote that:
>It is possible to look up the GPS altitude but it's not used for any
purpose. We still use the same pressure level converted to a Flight
Level(1013 hPa) or an altitude MSL based on a local altimeter setting
(QNH)...
When and where do the pilots key local altimeter data into the flight
computers, and in what units (hPa, or km in Eastern countries)?
What are the units of instrument display(s)?
Is there a separate *radar* altimeter for clearance of mountain peaks? (in
addition to the "local" altimeter?)
Gene Mechtly.
