The barometric altimeter uses the atmospheric pressure to provide the current altitude of the aircraft. The pressure calibration value can be set manually, usually by operating a small knob on the altimeter itself. Such value must be set to compensate for the pressure variations induced by the weather.
Units of Measurement
Depending on factors such as the manufacturer, the country of origin and even the age, the altimeter can use a different unit of measurement:
- Millibar/Hectopascal (mb / hPa): standard according to the Système international (SI). 1mb = 1hPa;
- Inches of Mercury (inHg): common in American aircraft, such as the F-14;
- Millimetres of Mercury (mmHg): used in Russian/CIS aircraft (e.g. Ka-50).
Note: In the Kneeboard pack, Ka-50 folder, you can find a simple table I made years ago to quickly convert the pressure through different UM. Back then our ATIS used to provide only inHg.
Pressure conditions can be expressed in different ways, each resulting in a different altitude reading.
A subset of the Q-Codes is used to indicate notable settings:
- QFE: atmospheric pressure above ground level. For example, by setting the QFE of an airport, then the altimeter reads zero when the aircraft is WoW (Weight-on-Wheels), and it shows the altitude above the airfield when in the air. To some extents, it reminds a simple radar altimeter (it can work as a backup if necessary in case of damages or malfunctions).
QFE is not used often in modern times, especially by NATO countries.
- QNH: the most used value in DCS, it allows the altimeter to indicate the current local altitude above sea level. This means that the altitude of airfields and geographical features from a map or FLIP are immediately applicable as the QNH is set correctly.
For example, if the runway of an airfield is located at an elevation of 300m msl, then the altimeter will mark such altitude when the aircraft is holding prior taking-off.
- QNE: atmospheric pressure value at mean sea level using ISA (International Standard Atmosphere). It is used to provide a common means of measuring altitude above the Transition Altitude by setting the altimeter to the Standard Pressure value (1013.2mb/hPa, 760mmHg, 29.92inHg).
Transition Altitude and Transition Level
Above a certain altitude the altimeter is set to the Standard Pressure (QNE), in order to unsure a standard measurement method without the differences induced by local variation of pressure. Such threshold altitude is called Transition Altitude. Above TA the altitude is stated in Flight Levels. Flight Levels are represented as “FL” plus the hundreds of feet of the altitude reading. For example, “FL200” indicates an altitude of 20,000ft (QNE).
When an aircraft is descending from high altitude, as it crosses a specific FL called Transition Level, it changes its altimeter settings to the local QNH.
Between TA and TL there is a buffer zone called Transition Layer.
It is immediately understandable how FL improves the safety of a mission or scenario as air assets do not have to rely on constant updates of the altimeters to fly using a common standard.
Note: remember to use “Altitude” when below the TL and “Flight Level” above TA (similarly to how “Angels” is used for the altitude of friendlies and Thousands of feet are used for other contacts).
What happens if the altimeter is not set correctly?
These are a couple of very simple examples to show why setting the correct altimeter is important:
- Imagine flying between two waypoints in low visibility conditions in a fairly mountainous area. You have a map, and aware that the highest peak is 3100ft. For safety reason, you planned to fly above 3500ft. But you forgot to set the QNH, the altimeter reads 3500ft but your actual altitude at the correct local QNH is 2800ft. You can immediately imagine how dangerous this situation is.
- Another example is a stack or the simple flow of air traffic over the same area: if you QNH is wrong there is a serious risk of colliding with another aircraft. The correct QNH/QNE instead should ensure the respect of the safety buffer between the allocated altitudes.
Unfortunately, a simple query on Google returns multiple accidents, some with fatalities, caused by either faulty altimeters or straight human error.
Thanks [DR]BlueRaven for suggesting this topic. He flies with the Italian group “The Drunken Reapers” and he is also the coordinator of a brilliant project called JATF (Joint Air Task Force), of which I may talk about in the future.