Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: altimetry



Hugh Haskell wrote:

FAA requires every aircraft to have a
"sensitive" altimeter ..... By sensitive, IIRC, they mean capable of detecting altitude
changes of +/- 10 ft.

A "sensitive altimeter" is only required for IFR.
Otherwise a generic "altimeter" will do.
http://www.risingup.com/fars/info/part91-205-FAR.shtml

But I've never seen an altimeter that wasn't designed
to IFR specs.

They must also be adjustable to the local sea
level barometric pressure, and when so adjusted they read height
above sea level, to be certified for use, I think they have to be
within +/- 20 ft. when within about 2,000 ft of the ground

Oooh, it's !!much!! worse than that. It could be off by 30 feet
out of 2000 during a bench test, and the test might have not
have been done very recently, and then there could be hysteresis
plus 70 feet of friction if you've been descending at 750 feet
per minute, plus installation error, .....
http://www.risingup.com/fars/info/part43-E-APPX.shtml

Pilots are commonly trained to complain if the altimeter
appears to be off by 75 feet, but I have never found a
regulatory basis for this.

relative to their neighbors... is vitally important when
the outside conditions are lousy--what the Brits call "instrument
meteorological conditions"--that is, when you can't see anything
outside and have to rely on the aircraft's instruments for flying and
navigating.

Aircraft equipped with a fully functional radar transponder (I doubt
you can buy otherwise today, but you used to be able to)

You still can. You can't fly such a critter near big
airports (class C and class B airspace) but there's still
lots of places you can fly.

have a pick-off from the pressure altimeter

You can have a transponder without having an encoding
altimeter.

that sends the altitude
information to the radar control center to a precision of +/- 500
feet.

I read that number a lot, I find no basis for it.
Every transponder I've ever seen reports 100-foot
increments. In my experience ATC will be happy to
tell you your Mode-C reading to the nearest 100 feet.
http://www.airsport-corp.com/modecascii.txt

This is a good example of the use of a Gray code
http://mathworld.wolfram.com/GrayCode.html

The new Mode-S stuff will optionally do 25-foot increments.
http://www.ibac.org/Bulletins/ibac_b02-2.htm

Unless this precision has been changed recently, and it well
might have been, the recent FAA proposal to reduce the altitude
separation between aircraft under their control bothers me.

Recent proposal? The RVSM (Reduced Vertical Separation
Minimums) has been active for years over the atlantic. The
only "recent" thing is to do the same in domestic airspace.
It's a gradual roll-out, starting at the highest altitudes
and working down. It reduces the high-altitude spacings from
2000 feet to 1000 feet. The low-altitude spacings have been
at 1000 feet all along.

Aircraft
whose altimeters may be off by a few hundred feet (not too uncommon)

A couple hundred, maybe.

can easily end up at the same altitude and not know of each other's
presence, unless the altitude separation used by the controllers is
such that such errors would still keep them vertically separated.

It's not a problem (for separating aircraft from
aircraft, that is).

The interesting case arises on an
instrument approach into an obstructed field
at really cold temperatures. The air shrinks
when it gets cold, but the obstruction doesn't
shrink. And the intended vertical clearance is
only a few hundred feet. So if you consider
worst-case instrument errors and worst-case
weather, you've got a problem. Canadians and
other intelligent people require the pilot to
do the math and apply correction terms, but US
pilots are not required or even trained to do
this.

Radar altimeters have more restricted use, and are primarily used by
military aircraft, although commercial airlines are commonly equipped
with radar altimeters to help them with ground clearance. Radar
altimeters, by their nature, give elevation above the ground and are
primarily to keep aircraft from running into the ground when they
don't want to.

Nowadays they use GPS + terrain models for that. A
radar altimeter still comes in handy for the final
moments of a Category-III fully automatic landing, to
measure the height of the wheel above the runway.