• Aircraft altimeter: What is it and how it works

There is a well-known Spanish saying in aviation that goes: “Velocidad y altura mantienen la dentadura” (literally, it is essential to control speed and altitude so as not to crash); which means that these two variables are fundamental to manage a compromised situation in flight.

Well, having already dealt with speed of aircraft, this saying serves to introduce today’s post, in which we talk about the aircraft altimeter and how this absolutely fundamental instrument for pilots works. Welcome aboard!

What is an altimeter and what are its components

The barometric altimeter is a tool which measures the altitude of an aeroplane above a specific pressure level. Because of its importance, it is essential that pilots understand and know how the altimeter works.

Each altimeter is formed by aneroid capsules that expand or compress when there’s variation in static pressure. Higher static pressure makes it compress whereas lower pressure makes it expand.

Inside the altimeter, through a series of gears and needles, pilots are given indications brought about by the changes in static pressure of the aneroid capsules.

But before we continue, we’d like to review certain concepts such as static pressure, dynamics and total pressure:

  • Static pressure is what we experience independently of the speed at which we move; it acts in all directions and is usually expressed as Ps.
  • Dynamic pressure is what is experienced when we move; the higher the speed, the higher dynamic pressure is. It’s represented as Pv.
  • The total pressure is the sum of static and dynamic pressure; it’s represented as Pt.

So, how do altimeters work?

Firstly, altimeters need a static pressure measurement to work. In Diamond DA20, for example, we can find them in the lower part of the pitot tube; however, in Diamond DA40, it is located to both sides of the fuselage.

At sea level, we have a pressure of 1013 millibars which decreases as aeroplanes ascend because the air column above the aircraft becomes gradually less. In Standard International Atmosphere, variation in altitude changes static pressure by 1 millibar, every 27 feet.

So a plane at 6000 feet is under a static pressure of 791 millibars (1013 – (6000/27) = 790,7).

Altimeters, thanks to the aneroid capsules and internal gears, measure this variation in static pressure and show the calculation of the aeroplane’s flying altitude.

Another essential factor for altimeters to work correctly are pressure levels and adjustments to Kollsman’s window; we will cover this topic in future posts, though.

Effects of temperature in the use of an altimeter

In ISA, or International Standard Atmosphere, temperature at sea level is 15º. As we ascend, temperature decreases 2ºC every 1000 feet. This means that at 10,000 feet, the temperature would be -5ºC, right? Well, not quite.

As meteorological conditions rarely correspond with ISA, the effects different thermal variations have on altimeters needs to be known.

When it’s hot, air density decreases, making distance between isobars increase. In contrast, when it’s cold, distance decreases.

This means that on days when temperatures are below those given by ISA, caution is needed when reading altitude as the altimeter may indicate 5000 feet but in actual fact, it might be 4,500. To help you remember this, we have another saying that goes: “From high to low, look out below!”.

Altimeters are calibrated according to the type of aircraft

As different aeroplanes fly at different heights, there are altimeters calibrated for the different altitudes they fly at.

Light aircraft have altimeters calibrated to 30,000 feet with a precision of ± 60 feet; while commercial aeroplanes use altimeters calibrated to 50,000 feet, with a precision of ± 80 feet.

This difference is due to the maths calculation taking pressure to decrease constantly at 1 millibar every 27 feet despite the real atmosphere not behaving in this manner. Pressure variation in altitude is much higher the nearer we are to the ground: at 50,000 feet, pressure changes 1 millibar every 50 feet.

The radio altimeter is more precise at low altitudes

The radio altimeter is an instrument that sends signals towards Earth’s surface below a specific altitude, usually around 2,500 feet. It measures how long rebound signals take to reach the aeroplane and determines the exact altitude it is at.

To put it in other words, if barometric altimeters show altitude with a reference at a specific pressure level, radio altimeters give measurements with reference to the terrain beneath the aeroplane.

For example, when an aeroplane flies above Teide (12,000 feet altitude) the barometric altimeter will show 13,000 feet whereas the radio altimeter will indicate 1000 feet. The former uses sea level as reference and the latter, the distance to the ground.

Also, the radio altimeter can give precisions of ± 2 feet from heights of 500 feet until landing.

Barometric altimeter vs radio altimeter: Which one is better?

It’s impossible to say which is better, as each of them carries out a specific task.

At great heights, radio altimeters don’t work because they don’t go far enough; whilst in reduced altitudes, altimeters lose accuracy therefore radio altimeters are needed.

However, it’s a good idea to keep in mind that radio altimeters are tools that are used exclusively in commercial aeroplanes because of their high cost and maintenance.

Additionally, in the lighter aeroplanes used in general aviation, operations are VFR, i.e. with visual reference, so during approaches, runways and airports are always within sight.

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