The
Altimeter
IFR – Altimeter
(includes the Kollsman Window)
Required by FARs 91.205 for IFR Flight
VFR – Altimeter (NO Kollsman Window)
Static Pressure changes
inside & contracts/expands
the AW
SP – receives
Outside Air Pressure
which is continuously
transferred into the case
How is the Altimeter able to indicate my Altitude?
Your Altimeter works, as all other Pitot-Static Instruments,
on pressure differential. The Altimeter, as seen on the picture above,
has an Aneroid Wafer inside the Instrument's case.
What is an Aneroid Wafer?
The Aneroid Wafer (Aneroid = Barometer) can be seen as a sealed but flexible metal box or capsule, which is made of alloy and copper reacting sensitively to pressure changes acting upon it from the air within the case (Atmospheric Pressure). Aneroid also means without fluid. Compared to the ASI, air pressure is not transferred into the aneroid wafer.
The wafer apparently has about 5mbar in it, which compensates for the flexibility of the
alloy during temperature changes.
It is connected through Gears and Linkages to the needle.
The inside of the case is connected to the Static-Port only.
What about the Altimeter Setting in the Kollsman Window?
Setting or changing the ALT-Setting to a certain number (for ex: 29.82) with the knob will mechanically turn a scale, which is located just behind the intrument's face. This will also move your pointer(s) needle to adjust your indicated altitude. So, turning the knob has no affect on the Aneroid Wafer.
So, what happens during Pressure changes?
When ambient (outside) air pressure changes during descends (pressure increases) or climbs (pressure decreases) air pressure within the case changes accordingly.
During a climb: Pressure inside the case decreases. As a result, the Aneroid Wafer
is able to expand as less and less pressure is acting upon it. This makes the needle move clockwise, showing an increase in altitude.
During a descend: Pressure increases, which leads to an increase in pressure within the case, resulting in a contraction of the Aneroid Wafer. In other words, air pressure is now pushing on the Wafer, trying to squeeze it together.
This makes the needle move counter-clockwise, showing a decrease in altitude.
Examples of Operation
-
Imagine the following situations:
Your aircraft is parked on the Runway 'Ready for Take-Off'.
The current airport Altimeter Setting (Kollsman-Window) or Field Elevation* is set.
Your Static-Port receives ambient (outside) air pressure → your needle (indication) remains steady at (0) KIAS.
*Field Elevation can be used if ALT-Setting is unavailable at airport.
Lift-Off / Climb
Very soon after you leave the ground, your needle will start to move clockwise, showing
an increase in altitude. Why? As previously explained, your ambient (outside) air pressure will not continuously decrease as you climb higher. The the same now happens
within the Altimeter case. This leads to an expansion of the Aneroid Wafer showing an
increase in altitude as long as you continue to climb.
Leveling – Off
What happens when you Level-Off? When you 'Level-off' at a given altitude, the pressure of that Pressure Level (Altitude) will remain in the case, keeping the Aneroid
Wafer steady. As a result, the needle (indicator) will stop moving.
Descend
When you initiate your descend, the opposite of the climb will happen.
Air pressure increases, which will increase the pressure within the case as well.
The pressure will now contract the Aneroid Wafer, in other words, press it together.
This results in a counter-clockwise rotation of the needle (indicators),
showing a continuous decrease in altitude until the descend is stopped or landing has occurred.
Types of Altimeters
Encoding (Barometric) Altimeter, a computer inside the instrument measures the pressure as referenced from 29.92 in., converts and delivers it as pulses to the transponder. From here, they are transmitted to an ATC ground station.
2-types: Analogue (big airplanes use it as back-up) & Electronic Display
→ ALT adjustments (through the Kollsman Window) are not detected
→ 14 CFR Part 91 requires the Altimeter to be within 125ft of the indicated altitude.
Absolute (Radio) Altimeter, comparable to the operation of a radar (think of reflection), which is the reason why it is also called a Radar/Radio Altimeter. It sends either a 'Frequnecy-Modulated continuous Wave' or 'a pulse' to the ground and converts the time it takes to travel back & forth into distance in feet; Absolute Altimeter = ALT Above Ground Level
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one can also set a DH/DA or an MDA, that will alert you as soon as you reach it.
→ ALT adjustments are detected and transmitted by the transponder to ATC.
Errors of the Altimeter
A Barometric Altimeter is subject to the following Errors...
Mechanical Error
After you have set your local/current ALT-Setting, make sure that your 'indicated altitude' is within 75ft of the airport/field elevation.
Inherent Error
'Inherent means something that can not be fixed.'
This Error relates to Outside (ambient) Air Pressure changes that occur due to weather changes. What does weather have to do with this you may ask?
During a climb your pressure decreases continuously (it gets lower).
while during a descend pressure increases contiuously (it becomes more).
So, if you are flying from a High Pressure (temperature) Area to a Low
Pressure (temperature) Area, we have a similar (pressure) effect. Your Altimeter thinks that you are actually at a higher Altitude, due to lesser pressure within the case acting on the Wafer, indicating a higher than actual (true) Altitude. That's why it's important to reset your ALT-Setting to a Local-Setting or one within 100nm of your position (as stated in FARs 91.121-Altimeter settings). During X-Countries, ATCCs will usually update a pilot on the most current setting. This is where the following reminder comes from...
'High to Low -> Look out' below! ;-)
What about fly from Low to High?
To keep it short and simple, the opposite happens when you a flying from a Low Pressure Area towards an Area of Higher Pressure. Altimeter will indicate a lower than actual altitude.
Also keep in mind, pressure towards the 'Center of a System' will mostly increase/decrease respectively. Thus, your indications will progressively become worse if you do not update your
ALT-Setting.
Alternate Static-Air
A blocked Static-Port will result in no more pressure changes within the case since your Static-Line is blocked. This means, no more contracting or expanding of the Aneroid Wafer. In other words, the Altimeter (indicator) will freeze or stop moving.
What are our options in this case?
If this should happen, you have the option to open the Alternate-Static-Air.
This air comes from inside the cockpit/cabin.
It's a good alternative. But keep in mind, your Cabin Air Pressure will always be a bit lower, due to the Venturi-Effect around the fuselage (cabin area), resulting in slightly erroneous (higher) Altitude indications. If you are aware of that, especially during approaches, where height is always an important and major factor, you should be fine.
What Altitudes do we have?
Indicated
→ what you read off your instrument
Pressure
→ Altitude corrected for non-standard pressure (29.92), the altitude you are flying
Density
→ Pressure Altitude corrected for Non-Standard temperature
Your aircraft's performance depends on the density of the air existing at you cruising (pressure) altitude. At Non-Standard conditions, your Pres Alt & Density Alt will not be equal
True
→ above MSL
Absolute
→ Above Ground Level (AGL)