Re: absolute pressure

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 14:44 -0700 6/5/02, Herbert H Gottlieb wrote:
> *** Pilots are more likely to use either altimeters, which are
> specially calibrated aneroid barometers, or radar devices.
They use both, when available. FAA requires every aircraft to have a
"sensitive" altimeter (as far as I know, always an aneroid barometer
type). By sensitive, IIRC, they mean capable of detecting altitude
changes of +/- 10 ft. 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 (I suspect
John Denker can give the current regulations--since I haven't flown
in about 20 years, I'm no longer sure of my memory). Over
international waters, above 18,000 feet everywhere, and in some
foreign countries, the altimeter is to be set to the "standard" sea
level pressure of 29.92 inches of mercury. That way we can all be
assured that any two aircraft in the neighborhood of each other will
have altimeters that are reading the altitude to the same standard.
Within the US, one keeps the altimeter set to the sea level pressure
at the nearest reporting station. As long as everybody in the area
has properly set their altimeter, everyone will know their altitude
relative to their neighbors. All of this 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) have a
pick-off from the pressure altimeter that sends the altitude
information to the radar control center to a precision of +/- 500
feet. 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. Aircraft
whose altimeters may be off by a few hundred feet (not too uncommon)
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.

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. Helicopters like them because they can use them to
accurately hover at a given height.

In the old days when air navigation was primarily done by celestial
methods, which of course, don't work when you are in or below the
clouds, large aircraft would carry a precision radar altimeter, which
was primarily used for navigation purposes. Over the ocean, where
there are no obstructions to upset the airflow patterns, the
difference between the radar altitude and the standard pressure
altitude could be used as a quite accurate means of detecting the
drift of the aircraft away from its heading due to the wind. It only
gave motion perpendicular to the aircraft heading, but that was very
important if your position was uncertain. If you could maintain your
desired track, then the only thing that would keep you from getting
home would be running out of fuel. That beats all getout running past
your island destination because you were way off course and didn't
know it.

Although pressure pattern navigation as this technique was known,
saved my skin more than once it is now totally obsolete and has been
replaced by inertial and GPS, both of which are far more accurate and
reliable. I don't think any aircraft even carries a navigator any
more. We used them routinely on all long over-water flights in the
50s and 60s. I don't know when the airlines gave upon them, but it
was probably sometime in the 60s.

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

(919) 467-7610

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