The higher you go in altitude, the lower the ambient air pressure. This principle is used by various aircraft instruments and equipment, such as:
- Altimeters,
- Vertical Speed Indicators (VSI)
- Airspeed indicators
- Mach indicators
- Air data computers
- Transponder altitude encoders.
- Pressurization systems
The static port collects the outside ambient air pressure and feeds it to the instruments.
Altimeters
An altimeter indicates the aircraft’s barometric altitude.
Inside the altimeter there is a sealed box, called an aneroid. The aneroid is built from stacks of metal sheets which can expand and contract similar to an accordion. The static air is fed to the case of the altimeter and surrounds the aneroid. When ambient pressure increases and decreases, the aneroid contracts and expands, accordingly.
A mechanical link between the aneroid and the instrument’s display moves the hands of the dial when this happens.
When the aircraft goes up in altitude, the outside pressure decreases and the aneroid expands. When the aircraft goes down in altitude, the pressure increases and the aneroid collapses.
The altimeter is calibrated based on a pressure model defined by the International Standard Atmosphere (ISA).
The standard pressure at sea level is 29.92″ hg (or 1013.25 mbar). One problem is that, on a given day and location, the pressure may be (and usually is) higher or lower than standard. To compensate for this, modern altimeters have a knob that lets the pilot adjust the reference sea level datum for that area and time. This is called an altimeter setting, or QNH, and is indicated on the Kollsman window of the altimeter.
A little off-topic, but important to mention for distinction, is the radio (radar) altimeter, which doesn’t use the static port at all. This instrument indicates actual height above ground by measuring the time it takes for a radio signal sent downwards to bounce back to the aircraft. It only operates at lower altitudes (typically below 2500ft), and used mainly for instrument approaches and ground awareness.
Vertical Speed Indicator (VSI)
The Vertical Speed Indicator shows rate of climb or descent, usually in feet per minute. It does that by measuring the rate of change in ambient pressure from the static port.
It works in a similar way to the barometric altimeter, except that the static port is connected to a diaphragm, instead of a sealed aneroid. The case of the instrument has a calibrated leak, that allows air to move in and out at a slow rate. When the aircraft climbs, the ambient pressure measured by the static port decreases. This causes an immediate decrease in pressure inside the diaphragm, which expands. The mechanical link to the dial moves to indicate a climb.
Once the climb stops, the pressure eventually evens out through the calibrated leak and and the instrument shows 0 rate of climb.
During a descent, the opposite happens.
The key is that the pressure outside the diaphragm always lags behind the pressure inside of it, thanks to the calibrated leak. This leads to a lag error, which is fixed by accelerators in a more advanced form of this instrument, called an instantaneous VSI.
Airspeed Indicator
As the aircraft moves forward, the airflow exerts pressure in the direction opposite to the flight path. This pressure is the combination of the static pressure (ambient, caused by the weight of the air column above the aircraft) + dynamic (pressure caused by the air molecules hitting the aircraft as its moving forward). The dynamic pressure gives us a pretty good (but not perfect) representation of airspeed.
To obtain dynamic pressure, we have to substract the ambient pressure from the total pressure.
How is this done?
The airspeed indicator uses an additional port, called the pitot tube , which measures total air pressure. We already talked about the static port, which measures ambient pressure. The airspeed indicator mechanically subtracts the ambient pressure from the total pressure.
I said that the airspeed indicator only gives us a pretty good indication of airspeed, not an actual airspeed. This is due to several errors. Airspeeds are classified by the level of their occuracy:
- Indicated Air Speed (IAS) – The speed as its read straight from the dial.
- Calibrated Air Speed (CAS) – IAS corrected for calibration errors, such as the position of the static port and pitot tube at different angle of attacks. Obtained from aircraft charts.
- Equivalent Air Speed (EAS) – CAS corrected for compressibility errors at high speeds and altitudes. Obtained from charts.
- True Air Speed (TAS) – The actual speed of the aircraft through the air. In no-wind conditions, it is equal to ground speed. It is also EAS corrected for density (usually obtained as a function of temperature and altitude).
The static and pitot systems are usually referred to as the pitot-static system.
Machmeter
A machmeter uses the pitot-static system to display the ratio between true airspeed and the local speed of sound.
It works like a combination of an airspeed indicator and an altimeter. The altimeter part adjusts the ratio arm, which correlates to the local speed of sound at that pressure altitude.
Air Data Computers (ADC)
Forget everything you just learned! (well, not really, the principles are important…)
Everything I described above is a little archaic, it is how older, mechanical instruments work. Modern airplanes are usually equipped with Air Data Computers that calculate the parameters electronically, instead of mechanically.
Still, the ADC needs to get physical inputs, just like mechanical instruments do:
- Static air from the static ports
- Total air from the pitot port
- Temperature from the Total Air Temperature (TAT) or Outside Air Temperature (OAT) ports.
Some of its outputs:
- Airspeed (CAS or EAS)
- True airspeed
- Vertical Speed
- Pressure Altitude (based on standard 29.92″ hg sea level datum)
- Baro-corrected altitude
- Mach number
- Total Air Temperature (TAT)
- Static Air Temperature (SAT)
The information is usually presented on electronic, “glass cockpit”, Primary Flight displays (PFDs):
On the left bar you can see the indicated airspeed (250 kts), under it you’ll see the mach number (.795).
The right bar shows the altimeter (38,000ft) and to its right the Vertical Speed (0 rate of climb).
Pressurization systems
Pressurization systems also use static ports for cabin differential pressure (the difference between inside cabin pressure and outside ambient pressure) calculation and regulation.
The pressurization system may have its own, dedicated, static ports.
Combined Probes
Some aircraft combine the static, pitot, and other probes into a single replaceable unit. They provide better accuracy, easier maintenance, lighter, and are quicker to replace.
Here is a Air Data System’s SmartProbe that combines pitot, static and angle of attack probes into a single unit:
Author – Amir Fleminger
