Altitude is life, so being able to tell it with accuracy is of utmost importance. Altitude indicators, commonly called altimeters, are a family of instruments that can rely on various data sources to feed the pilot reliable altitude data throughout the flight.
Altitude Indicators: What Are They?
Altitude meters, most known by their portmanteau altimeter, are part of The Six Pack – the six most important instruments for a flight crew, present in every aircraft. In short, the altimeter provides the pilot with the aircraft’s current altitude, that is, the aircraft’s height relative to the mean sea level.
The Altimeter’s History
The altimeter used today is based on a patent by a young German-American inventor called Paul Kollsman. Having emigrated to the United States in 1923, Paul and his brother Otto Kollsman founded Kollsman Instruments Co. in 1928. A year later, they crossed paths with James H.
Doolittle, back then a young Lieutenant heavily invested in developing equipment and techniques for safe instrument flying. A submission by Kollsman Instruments Co. had caught the eye of Doolittle, and a meeting between him and Paul was arranged to test the new altimeter proposal in practice.
An experimental flight was performed aboard a US Navy Vought O2U observation biplane, with Doolittle at the controls while Kollsman rode in the observer’s seat with his new altimeter on his lap to verify its precision.
The results were stellar – Harry Guggenheim, heavily involved with the Daniel Guggenheim Fund for the Promotion of Aeronautics and chief sponsor of the new program, reported an accuracy of 10 feet.
Following its early success, the Kollsman altimeter was installed on the dashboard of a Consolidated NY biplane, the Navy’s version of the PT-1 trainer, to prove its mettle: Lt. James Doolittle had set out to establish a record as the first pilot to take off, fly and land an aircraft relying exclusively on instruments, and he required an accurate altimeter to do so safely.
The attempts were successful: Doolittle performed two ‘blind’ flights at Mitchel Field, and used them as basis for creating other instruments that would eventually lead to safe, regular airline travel regardless of visibility conditions.
Since the Kollsman altimeter became the new standard, it has undergone evolutionary changes, but none too radical.
The analog steam gauge has been replaced by a digital altitude indicator and the raw barometric data is processed by a central air data computer before being displayed to the pilot in order to correct for induced inaccuracies, but the operational principle remains the same.
How Altimeters Work
Standard altimeters are barometric instruments that use the pressure differential to determine the aircraft’s altitude. Sealed aneroid wafers, which resemble round capsules, expand or contract based on changes to static pressure around them. The needles on altimeters move in relation to these wafers.
Static pressure is a function of a fluid’s density, the force of gravity and how deeply immersed the object is. As altitude decreases, the aircraft becomes more ‘immersed’ in the ‘fluid’ surrounding it, in this case atmospheric air. The walls around the wafers compress them and make the altimeter’s needles spin anti-clockwise.
The opposite is true during climbs, with the pressure around the wafers decreasing and allowing them to expand, spinning the needle clockwise.
While standard atmosphere conditions exist and have a sea level pressure of 29.92 inHg, 1013.25 hPa or 760 mmHg depending on the unit system used, most of the time meteorological phenomena make the local pressure above or below that value.
If an altitude indicator is calibrated for standard values, it will show an incorrect readout in those cases. To prevent that, altimeters have a small window called the “Kollsman window”, linked with a small dial.
The Kollsman window displays what is the sea level altitude the altimeter is set for – in other words, it sets at what pressure it will give a readout of 0m.
Which Pressure to Use?
While the Kollsman altimeter was designed to indicate an aircraft’s height above sea level, this is not always the most convenient reference for a pilot or an air traffic controller. There are three types of pressure standard in use in aviation, and most pilots end up using all of them at some point.
QFE – Field Elevation Pressure
The QFE standard is the simplest of them all, but its usage has become increasingly rare in the air. The pressure readout at an airfield at any given time is the QFE.
When a pilot enters this value in the altimeter Kollsman window, the instrument will show an altitude of 0m – that is, there is no difference between the altitude indicator’s pressure setting and the pressure at the airplane’s position.
QFE has been largely replaced by QNH and QNE, but it is still the standard at airfields in certain countries in Eastern Europe and Asia.
QNH – Sea Level Pressure
Since the 1980s, the push to standardize aviation procedures across the world has set QNH as the standard for most situations and countries. The QNH value is defined by the equivalent sea level pressure at a certain location.
This is obtained by using the QFE value and applying the hydrostatic pressure equation to convert it to what the pressure would be at 0 meters.
An important detail about this conversion is that, while air pressure is directly affected by temperature, a factor ignored by those in charge of issuing the QNH on the weather report. The reason for that is the altimeter design: for standardization’s sake, all altimeters are calibrated for ICAO standard atmosphere conditions.
These mean an altitude of zero, a temperature of 15 ºC, a pressure of 1013.25 hPa and a lapse rate (i.e. decrease in temperature relative to altitude) of -1.98 ºC/1000ft.
By getting a QFE (local pressure) reading on a barometer set to ICAO standard atmosphere conditions and converting it to a QNH value entered on an altimeter calibrated by the same rules, it provides the pilot with a consistent altitude readout.
This is perfectly adequate for short flights, particularly those starting and ending at the same airfield, but can become troublesome for longer legs which involve different meteorological conditions during departure, cruise and arrival, as pilots rely on their altimeters to stay within their assigned altitude blocks to avoid interfering with other aircraft in the same area.
This problem is remedied in two ways. In the first one, the controller responsible for that airspace can provide the pilot with an updated altimeter setting when entering or exiting an area. In the second, a universal standard is used to guarantee that all aircraft in that vicinity are operating with the same setting. That is what we call QNE.
QNE – ICAO Standard Atmosphere
The QFE and QNH methods described above rely on actual altitude readouts to provide a pressure setting, with the goal of accurately describing the height between the aircraft and a chosen reference point.
QNE switches things around – it has no actual physical referential attached to it, and instead just means calibrating the altitude indicator to the pressure described by the ICAO Standard Atmosphere of 1013.25hPa (or 29.92 inHg and 1013.25 hPa, depending on what units the aircraft being flown accepts).
The goal of QNE is not to tell a pilot how high they are, but to allow for safe travel in busy airspaces. As it has no referential on the ground, QNE is only used above a certain altitude to prevent inadvertent flight into terrain. This altitude is called transition altitude and is defined by local regulations.
QNE is used to define flight levels (FL), altitude blocks assigned by controllers to aircraft to guarantee all aircraft will be far apart from each other in case they cross paths.
Broadly speaking, aircraft flying above the transition altitude take-off with their altimeters set to QNH, change it to QNE at said altitude and establish themselves at their assigned flight level. When authorised to descend, they change from QNE to QNH as they pass the transition altitude and proceed to land.
Altitude Indicators: Special Cases
Barometric altimeters with readouts in feet are the main altitude indicators used in aviation, but they are not the only ones. There are a few other ways of measuring altitude that, while not universally adopted, can be very useful.
This instrument, often called a radar altimeter, is a device used to accurately measure the height between the aircraft and terrain immediately beneath it. The operational principle is simple: an antenna sends a signal directly downwards and then analyses its characteristics once it bounces back to it.
The most common methods are time between emission and return, and changes of phase between the initial and the received signal.
Because it works relative to terrain immediately below the aircraft, it has become a particularly useful tool in instrument flying. Its most common usage is by setting an altitude alert when the pilot reaches the allowed minimums for the approach type being flown.
Once the alert rings, the crew decides if conditions for landing are met or not and proceed accordingly.
As they rely on radio transmissions, radio altimeters are only used at relatively low altitudes relative to the ground, as readings become increasingly inaccurate as the waves must travel more.
Aircraft fitted with a modern avionics’ suite will have GPS-based navigation available, which greatly increases positional accuracy and pilot comfort. On top of its benefits on those fields, most aviation GPS units are also able to provide the pilot with an altitude readout.
This is based on triangulation data by comparing the time between signals take between the multiple satellites at different altitudes in orbit and the aircraft’s systems.
This is an excellent source of mostly accurate altitude data relative to the sea level and is a good back-up in case of altimeter problems.
The reason why GPS altitude readouts are not relied on as a primary is because all other aircraft are expected to be using barometric settings, so a pilot using GPS-provided altitude in an aerospace using barometric altitude would create deconfliction issues.
Metric Altitude Indicators
Most of the world has adopted knots for airspeed and feet for altitude in aviation, but certain aviation branches in parts of the world have stuck to other conventions for various reasons – most pilots have probably come across a Cessna with an airspeed indicator in miles per hour, for example.
In terms of altimeter, the special cases here are some using meters instead of feet. This is most found in certain older European and Chinese aircraft that have made their way to the civilian market, but modern Russian and Chinese aircraft used by their militaries still use the metric system across the board.
Question: What are altitude indicators?
Answer: They are instruments that tell you the height of an aircraft relative to a certain referential, usually mean sea level or field elevation.
Question: How Do Barometric Altimeters Work?
Answer: Barometric altimeters use the pressure-induced expansion or compression of aneroid wafers to accurately indicate the current altitude based on the local static pressure, while radio altimeters bounce waves off the terrain below the aircraft and analyze the returns to determine the height.
Question: Are There Other Types of Altitude Indicators?
Answer: Yes, the two most common are radio altimeters and GPS altitude indicators, both of which rely on analysis of radio waves to accurately determine the aircraft’s position relative to a ground referential.
Question: What Pressure Standard Is Used By Altimeters To Define Their Zero?
Answer: The pressure settings used can be QFE (field elevation), QNH (mean sea level) or QNE (standard atmosphere), depending on the situation.
Question: Is Temperature Accounted For When Setting an Altimeter?
Answer: No. As the altimeter is calibrated for standard atmosphere conditions, the pressure settings used follow those instead of local temperature at the time of flight.
Question: What Measurements Are Usually Displayed By Altitude Indicators?
Answer: Most altimeters are displayed in feet, but it is not uncommon to find aircraft with altitude expressed in meters in some parts of the world.