Pressure Instruments
General
All pressure instruments suffer from the standard errors. These can be
remembered by the pneumonic PITHBLOT.
Pressure: Incorrect altimeter subscale settings give rise to
this error.
Instrument: Errors in calibration.
Temperature: Changes in temperature affect the instrument and
also air density which gives rise to the difference between pressure altitude
and true altitude.
Hysterisis: Commonly called "metal memory". This is caused by
the metal in the instrument which is bent as part of its workings trying
to straighten or vica versa.
Blockage: Any blockage in the system will cause problems from
errors to total failure.
Lag: The very nature of the system with piping and mechanical
instruments mean that a certain amount of time must elapse before a change
is seen on the instruments.
Orographic: Air masses that strike terrain will be compressed
and will therefore change the pressure which will translate to inaccuracies
in instruments.
Transonic Jump: Approaching the speed of sound shock waves and
compression of the air ahead of the aircraft change the local pressure
and therefore cause errors in the instruments.
Altimeter
The altimeter is essentially a barometer. It measures the static pressure
and displays this as altitude.
When cruising at high altitudes then changes in pressure are very small
and therefore amplification is required. A servo altimeter is used to overcome
this problem, it uses an electrical amplifying circuit.
A sub scale setting is used to correct errors which would occur due
to local area pressure variations. These can be quite large as an error
of 30ft is seen for every millibar that the sub scale differs from actual
sea level pressure.
If there is a static port blockage then the altimeter will freeze in
its current position.
Vertical Speed Indicators
The VSI has two small aneroid capsules connected by a small hole. The instrument
needle is connected to the wall between the two capsules and measures the
distortion. A change in static pressure will slowly filter through to the
second capsule. While there is a difference a climb or descent will be
displayed on the instrument.
If there is a static blockage then the VSI will always indicate level
flight.
Airspeed Indicators
The ASI has pitot pressure, which is equal to static plus dynamic, and
pipes that to an aneroid capsule which is surrounded by static pressure.
This results in the capsule being distorted by an amount equal to the dynamic
pressure. This distortion is amplified and then presented to the pilot
as IAS.
If the aircraft suffers a blocked pitot tube then as the aircraft climbs
it will appear to increase speed and vica versa.
If the aircraft suffers a blocked static port then as the aircraft
climbs it will appear to slow down.
Back
to Main Menu