On 14.12.18 03:35, Martin Gregorie wrote:
>
>> What is the typical +/- for air-pressure airspeed and altitude readings?
>>
> ASIs are calibrated (and annually checked) to be within +/- 2 knots of TAS
> (True Airspeed).

It is actually IAS (indicated airspeed), which matches the true airspeed
at standard pressure and temperature.

>> How much does ground-level air pressure vary from time to time
>>
> Base is 1013 millibars. The normal range in the UK is about 990 to 1035
> millibars but storms may have lower pressures.

The scale on altimeters is usually 950 to 1050 millibars. I have
once (midsummer day 1987) been sitting in the cockpit and wondering
how to set the altimeter to 1053 mb.

>> to place, and does it drop as altitude increases by a consistent
>> formula?

This is what is called the barometric formula. It is not strictly
the correct geometric altitude at different conditions, but it
is accurate enough to guarantee altitude separation between
aircraft flying on similar altimeter indications.

> Fairly much. Otherwise a pressure altimeter would be telling lies.
>
>> Are such errors in altitude readings smaller than you'd get
>> with consumer or professional GPS?
>>
> Difficult to say because GPS altitudes are generally within +/- 5m for an
> uncorrected  GPS receiver but can be considerably more accurate if its a
> DGPS receiver and reasonbly close to a DGPS transmitter (see above).
>
>> I presume both airspeed and altitude gauges include a fairly long
>> time-constant in the needle damping to avoid gusts of wind causing
>> apparent fluctuations in speed and altitude.
>>
> No. Normally there's no damping. Its not needed in normal conditions. An
> ASI is an undamped pressure sensor that shows the pressure difference
> between the static vent and the pitot but is calibrated in knots rather
> than pressure units.
>
> An altimeter shows the undamped pressure at the static vent but is
> calibrated in feet or metres of altitude rather than pressure units .
>
> Pilots of aircraft that take traffic services set their altimeters to the
> base pressure supplied by the controller they're taking a service from.
> As a UK glider pilot I never use traffic services. I zero my altimeter
> (to airfield height) before launch but, if I'm talking to air traffic
> control and they want to know my altitude I reset my altimeter to the
> base pressure they supply - this means my pressure altitude is directly
> comparable with that of other aircraft in the area: if ATC has radar (not
> all do) it may be just a 2D system rather than 3D.

The ATC secondary radar (SSR) using the aircraft transponders has
a mode to interrogate the altimeter in the aircraft. The altitude
encoder is set for standard pressure (QNE), 1013 millibars,
regardless of the real air pressure on ground.

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-TV

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