How high are we?

When flying, there is always the question of how high you are. The answer seems simple: look at the altimeter and read the height from there. But are we actually at that height?

2024-04-28 15:12:34 - Dave Lewis

Whenever we think about height, we naturally assume it is measured in one of the standard distance scales—centimetres, inches, feet, meters, miles, kilometres, etc. However, we can't simply fly along and dangle a tape measure out of the back of the aircraft, so we traditionally measure the aircraft's height by the pressure of the air around us. However, atmospheric pressure doesn’t change uniformly with altitude—this change is influenced by temperature, humidity, and weather patterns. Known as "altimeter temperature error"

If we flew over the sea and could drop a tape measure down, it would tell us the true vertical distance from sea level to the aircraft. However, the 'pressure altimeter altitude' could differ from the tape measure's reading due to non-standard atmospheric conditions. For example, in colder air, atmospheric pressure decreases more quickly with altitude than in warmer air, causing the pressure altimeter to show a higher altitude than the actual geometric altitude measured by the tape. Additionally, the change in atmospheric pressure with altitude is not linear. The relationship between pressure and altitude is logarithmic, meaning that the change in pressure is greater at lower altitudes and diminishes as altitude increases. This is described by the barometric formula, which models how atmospheric pressure decreases with a rise in altitude. The standard atmosphere is based on an average temperature lapse rate, but any deviation from this average, such as a warmer or colder day, will skew the pressure readings, affecting altimeter accuracy."Here's a rough breakdown of how pressure typically changes with altitude up to 15,000 feet. Pressure is shown as Hectopascals (typically Europe) and inches of mercury (typically US)

These values are based on the International Standard Atmosphere (ISA), which assumes a constant temperature lapse rate and other standard conditions.


The altimeter we use is, in fact, a pressure instrument. If you were flying along and asked how high we are, being told '840 hectopascals' or '24.81 inches of mercury' wouldn't mean much. So, the pressure instrument is designed with a dial that shows numbers representing height; it's still the same instrument, meaning you would be told '5,000 ft' not '840 hPa', although today's 5,000 ft may be higher or lower than yesterday's 5,000 ft due to temperature changes.


About now, you might start to find this confusing and perhaps worrying, but the system works quite well for two reasons. First, you don’t have a 5,000 ft tape measure, so you can't prove it wrong, and second, all aircraft use the same system, so the majority rules.

Therefore, if I'm heading towards another aircraft and they report they are at the same height as me, say 5,000 ft, then we know we have to change altitude to avoid a collision. It doesn't matter if it's an actual 5,000 ft or not. It just matters that we are both at the same level and we need to do something about it.


However, it's not entirely accurate either, as their 5,000 ft may be measured from a different starting pressure.

The lines on the pressure chart connect all points where the pressure is the same. It's like a vast dot-to-dot drawing, although even cloud gazer might struggle to see a rabbit in it.

We know that air pressure at ground level is always changing, and of course, the height of the ground level itself is not uniform—there are hills, mountains, and valleys. For instance, the airfield from which you departed may be at sea level, but another aircraft's airfield might be at 500 ft above sea level or higher. Now, if you are flying at 5,000 ft, having set your altimeter to zero at sea level, then you are 5,000 ft above the sea. If an aircraft flying towards you took off from an airfield that is 500 ft above sea level, its altimeter would show 5,000 ft above their airfield, which is actually 5,500 ft above sea level. This means they are actually 500 ft higher than you, but both aircraft report the same height. If the aircraft at 5,000 ft above sea level decides to climb 500 ft to avoid the other aircraft, then both would be at 5,500 ft above sea level. However, the one from the higher elevation airfield will still show 5,000 ft on their altimeter.

To alleviate this issue, pilots typically only set zero on the altimeter at the airfield, known as the QFE, if they plan to stay very local - and that is more of a UK thing. Pilots adjust the altimeter via the subscale to a local mean sea level pressure for longer journeys, which can be obtained from various sources. This is often done by contacting a Lower Airspace Radar Service (LARS) on the radio, which will provide the regional pressure setting. Each part of the country has a regional pressure setting, denoted by an area referred to as the QNH.

There is another pressure reading known as QFF. This pressure reading is adjusted to sea level, accounting for temperature and gravity variations to provide a theoretical pressure at sea level. QFF is primarily used by meteorologists for weather forecasting purposes. This adjusted reading is crucial for their predictions and is the start of where weather forecasts go wrong.

What is with all the Q’s, you ask?

The letters in Q codes don't actually stand for anything; they are simply a coding system created around the 1900s. The use of 'Q' without a following 'U' indicates that it's a code and not a standard word. This convention stems from the ease of transmitting short, distinct signals in Morse code, where three-letter codes beginning with 'Q' can be quickly and clearly communicated. For example, 'QRB' translates to 'How far are you from my station?' and might be answered with 'QRB 2 miles'. When voice transmissions became common, Q codes continued to be used because they could be more reliably understood through poor transmissions. The letter combinations from QAA to QNZ are assigned to aviation, while other segments are designated for maritime and telecommunications uses. Many of these Q codes are still in use today."



Now that we understand the basics, we can navigate around the country without flying into each other. However, for the big airlines travelling at speeds of 500 mph, frequently adjusting the altimeter across different regional zones would be a hectic job. For this reason, the concept of Flight Level is used, where the altimeter’s sub-scale is set to a standard 1013.2 hPa (29.92 inHg), irrespective of the local QNH, QFE, geographical location or anything those weather people have to say about it. This setting means that three different aeroplanes might all show 5,000 ft on their altimeters but be at different heights. Normally, an aircraft using Flight Levels is in Controlled Airspace, where transition levels are broadcast, indicating when to switch from the regional QNH to the QNE for Flight Level.



Some aircraft are also fitted with radio altimeters. These send out radio waves below and in front of the aircraft, measuring the time it takes for the wave to reflect back. Radio waves travelling at the speed of light divided by time allow for the distance to be calculated. These are normally considered effective up to a range of 2,500ft/760 meters. Spacecraft use these for landing on the moon and other celestial bodies due to the total lack of an atmosphere. In aircraft, they are typically tied to terrain awareness equipment, “TERRAIN, PULL UP”, and other messages of encouragement can be played into the pilot's headphones.

Over the past 25+ years, GPS has become vital for navigation, providing altitude readings as height above the Earth’s ellipsoid, a model representing the Earth's shape. It’s important to note that this 'GPS altitude' differs from 'true altitude' above sea level and 'absolute altitude' above ground level.

Various countries have developed their satellite navigation systems:


GPS is the name we generally use in the Western world for Satellite Navigation, but as you can see, this is the name of the original satellite navigation system. Typically, handheld tablets, iPads, Androids, etc., use the GPS system, but you can get devices that will use a selection of the other satellite systems and create an average of the data.

Modern aircraft instruments integrate with the pitot-static systems, gyroscopic instruments, and radio navigation aids like VORs and ADFs, significantly enhancing navigation and safety. Specifically designed aircraft satellite navigation equipment often refreshes data in less than a second, allowing for accurate approaches in poor visibility. In contrast, hand-held tablets, typically with slower refresh rates, are considered less accurate and should not be used for approaches.


So to address the initial question of 'How high are we?' – We're flying at 5,000 ft, so there!

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