How to set up your Instrument – Altimeter Settings & Calibration

Published: RB 12/12/2019

Altimeter USE for PARAGLIDING and HANG GLIDING

Many pilots, both old hands and new, find it hard to remember what settings to use on their Altimeter, what height fields to display and how to calibrate their instrument.

When flying, it is very important to know your altitude in order to avoid other aircraft and controlled airspace. You may also need to know what height you are above the ground to avoid obstacles.

This article aims to help you understand the different settings and data fields that you may wish to use and how to calibrate your instrument before you launch.

It also tries to explain some of the “errors” you may encounter in Altimeter readings.

Some of this subject matter is very complicated and you may wish to just jump straight to the summary for a brief overview.

Altimeters – What do they actually tell you?

Due to the historic nature of aviation and the fact that it predates GPS systems, avaition doesn’t use “absolute height” (that which you could measure with a tape measure) but “Barometric Altitude” based on the decreasing atmospheric pressure as you ascend.

Barometric Altitude infers height as you ascend upwards based on a standard model of the atmosphere, but this will vary in absolute height depending on the weather, temperature and pressure conditions for the day.

The ICAO (International Civil Aviation Organization) Standard Atmosphere assumes a surface pressure of 1013.2mb dropping at approximately 1mb per 30′ as you ascend and a surface temperature of 15°C dropping at 2°C per 1000′. If, on a particular day, the atmosphere followed these “ideal” conditions your (barometric) Altimeter would read your true height (the same as your GPS height) as you ascended, however, this is rarely the case in practice.

As the (barometric) Altimeter is reading altitude based on pressure it needs to be calibrated to a reference value. In days gone by, this used to be done by setting (calibrating) the reference pressure on the “subscale” of your Altimeter, the image below shows this set to 1013mb. Once this reference pressure is set, your Altimeter then displays (barometric) height above this pressure surface. There are three reference pressures that are in general use for calibrating Altimeters and these are referred to as QNH, 1013.2mb (Standard Pressure) and QFE.

QNH, Standard Pressure and QFE

These different pressure references are used depending on where and at what height you are flying. When close to the ground your biggest concerns are obstacles near the ground or low level controlled airspace. When high and clear of the ground, you are only concerned about other airbourne traffic in your vicinity and high level airspace.

QNH – This is the local atmospheric pressure adjusted to mean sea level at your current location. When you calibrate your Instrument or Altimeter using the QNH pressure value your instrument reads (barometric) Altitude above mean sea level. If the conditions on the day are close to the ideal ICAO atmosphere, then your (barometric) Altimeter will read very close to your GPS Altitude.  This is the setting you would normally fly with 99% of the time.

1013.2mb (Standard Pressure) – If you calibrate your Altimeter to read relative to standard pressure of 1013.2mb you are displaying what is known as “Flight Level”. This is used when you are high up and have no concerns about surface referenced objects or low level airspace, your only concern now is seperation to other aircraft, some travelling great distances from/to locations where the surface pressure (QNH) may have been very different.  Aircraft travelling from all over the world will all have their altimeters calibrated to the same reference (1013.2mb) and so can maintain seperation relative to each other and to high level airspace defined by FL (Flight Levels) rather than AMSL (height Above Mean Sea Level). As a free flier, you rarely need to use this setting unless you are flying very high, >5500′ in the UK and need to avoid airspace. On high pressure days (>1013.2mb), airspace defined by FL will be higher(i.e. FL55 is above 5500′), so if you use the QNH setting you will have extra clearance to the airspace. You need to be careful flying on low pressure days as the FL airspace will be lower than your QNH calibrated Altimeter reading.

QFE – This is the local atmospheric pressure at your current location/airfield. When you calibrate your Instrument or Altimeter using the QFE pressure value your instrument reads (barometric) Altitude above that location, this was traditionally used for Take Off and Landing at airfields to display your height above the runway. Pilots often remember this using the acronym Query Field Elevation. Some Free Fliers use a second altimeter on their instrument set to read 0′ at take off, this is essentially a QFE setting.

DATA FIELDS TO DISPLAY ON Your instrument

Flying instruments often have several “Altitude data fields” that can be set to display altitudes relative to the various reference pressures (calibration values) at the same time, rather than having to re-calibrate a single altimeter to the different pressure values (QNH, 1013.2mb and QFE).

Here we will use the Flymaster SD Series Instruments as an example, but you should find the same/similar data fields on any Free Flight instrument.

GPS Alti – This displays the Altitude reported by the GPS, so is displaying an absolute height above mean sea level rather than barometric altitude.

Altitude – This readout gives you your (barometric) Height above the reference, when calibrated to local QNH, you will read barometric Altitude AMSL.

Flight Level FL – This data field will display your (barometric) height above the standard pressure surface of 1013.2mb i.e. Flight Level.

Above Take Off – This is automatically reset to “0” at launch and will display your (barometric) height relative to TO, this is essentailly a QFE setting.

AGL – On the Flymaster instruments this gives a readout of height above the ground using the GPS height and the internal database of Ground level at your current location.

Ground – This displays the height of the ground above mean sea level for the current location from the internal database.

HOW TO CALIBRATE Your instrument

Again, using Flymaster Instruments as an example, but you should find the procedure is similar on your particular instrument.

The Flymaster offers three options to calibrate your instrument to read Barometric Altitude, i.e. relative to local QNH pressure. These are all accessed from “Menu” then “Settings” then “Set Altimeter”.

1. Input the QNH directly – you can input the Sea Level Pressure for your location in the QNH field. This can be obtained from synoptic weather charts or weather apps etc. such as XCWeather.

2. Use the GPS to calibrate – This is the option that you would normally use most of the time. When you get to the flying site, turn on your instrument and allow it to get a good GPS fix while you are sorting out your glider etc. If you set the “Get from GPS” option to “automatic” on a Flymaster it will then calibrate the QNH value automatically from the GPS height as the position fix improves, until you launch. If you select “Yes” in the “Get from GPS” field and then “Enter” this setting, it will calibarate the Altimeter once at that point in time from the GPS and then return the setting to “Get from GPS: No” (Note: this one time adjustment can be made during flight, this can be useful on your “final glide to goal” to ensure your altimeter has the best calibration). The “Automatic” setting is the best option to use. When you calibrate from GPS, you will notice that the “QNH” value changes to the calculated local sea level pressure and “Altimeter” value changes to your current height . You should find similar “calibrate from GPS” options on your particular instrument.

3. Input the actual height of Take Off into the “Altimeter” setting field. In this case you will notice the calculated sea level pressure appearing in the “QNH” field.

EFFECT OF TEMPERATURE

When the atmosphere on a given day doesn’t match the ICAO Standard Atomosphere there can be some large errors in the reading of a Barometric Altimiter compared to absolute height, as reported by a GPS.

On a hot day, then the air will be “expanded ” and less dense and you will have to ascend further to reduce the pressure to a certain value. As you can see in the image above, on a hot day of 35°C, the pressure surface (813mb) that would correspond to around 6000′ in the standard atmosphere is at approximately 6400′,this should be backed up by your GPS Alt reading. On cold day the opposite is true and you will actually be flying lower than you think, when it’s -5°C this pressure surface would drop to approximately 5540′. Of course, other air users will all be experiencing the same conditions, so if you think you are 1000′ clear of another aircraft, as both our Altimeters are showing the same absolute error, you’ll still have the seperation you thought you had.

On the warm days when you are higher than you think, this is normally not an issue unless you need to avoid airspace etc defined by an absolute height rather than the normal pressure altitude. This has happened in the past at some major competitions where the organisation have declared an upper ceiling limit defined by a GPS Height and pilots using Barometric Altimeters have been penalised for exceeding the limit. This can also be a problem for final glide to goal calculations, for this reason some instruments use GPS Altitude for the GR to Goal calculation.

GPS Height and the Shape of the Earth

Not all GPS units will display the same height. The Earth is not a pefect sphere it is slightly squashed from pole to pole and is often modelled as an Ellipsoid by GPS units. Even this is not an accurate model of the surface of the Earth, or where sea level would be, if it wasn’t covered by mountains etc. The Earth is more accurately modelled by the Geoid, this looks rather like a knobly potatoe. The Geoid can deviate from the Ellipsoid in height by up to +/- 100m, so it’s important to choose the correct model in your Instrument settings if this is an option. Most modern instruments use the Geoid by default and you can’t set this option.We should not confuse this with the model of the Earth used for horizontal distance calculations. On the Flymaster this setting can be found in “Menu”, “Settings”, “Navigation settings” and then “Datum”; you have the options of “FAI Sphere” (used occasionally in some competitions) or “WGS84” Ellipsoid (more common use).

SUMMARY

• If you are flying in an area which has no low level airspace, very few other airspace users and the obstacles of concern are scenic mountains, you are best off using GPS Alt – Welcome to The Lake District!
• For the majority of your flying, where airspace and other traffic is a concern, you just need to use (barometric) Altitude – calibrate your instrument to local pressure on the day – QNH.
• To calibrate your (barometric) Altimeter to QNH, either auto-calibrate from the GPS at Take Off or input the local pressure from a weather chart etc.
• If you are flying near high level airspace defined by FL (Flight Level) and the pressure is above 1013mb just use your calibrated (barometric) Altimeter set to QNH and you will have “extra” clearance to the airspace.
• If the pressure is lower than 1013mb you need to be carefull as Flight Levels will be lower than your (barometric) Altimeter set at QNH reads, in this case you might want to display FL on your instrument as well.
• On a very cold day your (barometric) Altimeter will over read and you need to be careful of high mountains/obstacles as you are lower than you think.
• On a very hot day your (barometric) Altimeter will under read. If a competition has set a limit based on absolute height/GPS Altitude you need to be carefull not to break the limit.