There are a large number of terms concerning the measurement of altitude within aviation. This chapter will deal with the most usual terms and explain in what situations you use them.
3.2 Pressure [S]
The static pressure is defined as the pressure that the atmosphere is producing. You’ve probably heard of high and low pressure, but in aviation you have to be more accurate than that and measure the pressure with digits.
In aviation air pressure is measured in hectopascal, hPa. ICAO has defined a standard atmosphere* in which the pressure at sea level is 1013.25hPa. This is also called ‘standard setting’ or QNE.
Since the air gets thinner with increased altitude, the pressure is reduced. More precisely with 30hPa / 1000feet*. An aircraft altimeter uses this fact by measuring the pressure around the aircraft and translating it to an altitude.
The pressure at sea level is however not static and an altimeter has to compensate for this in order to show the correct altitude. This compensation is done manually by the pilot by setting a reference that is calculated from the present air pressure. There are two commonly used methods for calculating this reference and they are abbreviated; QNH and QFE.
QNH is the actual air pressure reduced to sea level in standard atmosphere*. When setting the correct QNH, the altimeter will show the airfield’s elevation over MSL (see below), providing that the aircraft is standing on the airfield. QNH is the most common setting in civil and private aircrafts.
QFE is the actual air pressure, not reduced to sea level i.e. the air pressure at the airport. When setting the correct QFE, the altimeter will show zero, if the aircraft is standing on the airfield. QFE is rarely used in commercial civil aviation. VFR-traffic sometimes uses QFE and it is common that military aircraft uses QFE instead of QNH.
To be correct, not only the pressure, but also the temperature has to be taken in consideration in order to measure the true altitude. OAT combined with QNH is used to calculate true altitude.
* In standard atmosphere – as defined by ICAO
Mean sea level (MSL) is the average height of the sea, with reference to a suitable reference surface.
Altitude is defined as the vertical distance between mean sea level (MSL) and an aircraft. When the pilot has set the correct local QNH he will fly on an altitude.
Height is defined as the vertical distance between the ground (GND) or an airport and an aircraft. Height may be expressed in charts as number of feet above ground level (AGL). If an aircraft is flying on altitude 1000ft and the airports elevation (which means height above MSL) is 300 ft, the aircraft height is 700ft.
Flight levels are expressed in hundreds of feet of vertical distance from the pressure reference 1013.25. Hence, when flying on flight levels, the pilot has to set the altimeter to the standard pressure 1013.25hPa.
If the QNH is 1013.25 FL 80 would be 8000 ft above MSL (in a standard atmosphere).
If the QNH is 1003 hPa (i.e. lower then 1013.25) FL 80 would be less then 8000 ft above MSL. (It would be 1013-1003 = 10 x 30ft =300 ft less = 8000-300ft = 7700ft).
Flight levels are used above a certain altitude called the transition altitude.
This is to avoid that pilots flying en-route have to set their altimeters to local QNH all the time as they fly though areas with different pressure.
If two aircraft are at the same location, one at FL160 and the other at FL170 they are still 1000ft apart, as their altimeters deviate by the same amount from the true altitude.
Transition altitude (TA) is always the same at an airport, but differs from airport to airport. In Europe 5000 feet is the most common. When an aircraft is climbing trough the transition altitude the pilot will set the altimeter to standard setting (QNE) 1013.25hPa.
When an aircraft is above the transition altitude it will fly on flight levels. Upon descent the pilot should set the altimeter to local QNH (or QFE if appropriate) when passing the transition level (TL) at the latest. This means that below the transition level, altitude will be used.
Transition layer is defined as the airspace between the TA and the TL. In some countries the transition layer has to be 1000 feet or more, but in some it can be thinner.
Whatever the rules, in a specific country are, the TL will thus vary with the air pressure in order to maintain a certain minimum thickness. (This will change, since new rules are soon implemented) You have to look up the local rules in your vACCs operating manual.
Rule of thumb is that the TL is lower when the pressure is above standard 1013.25 Hpa, and higher when the pressure is below standard 1013.25 Hpa.
|Country||TA*||* This is the most common setting, variations may occur on some airports within the country; please refer to the AD for specific settings.|
Aircraft that are flying at Transition Altitude (TA) or above, set their altimeter to a standard pressure of 1013 Hpa. Because the pressure changes en route, they are actually flying at different altitudes as they travel through areas with differing atmospheric pressure. This works fine for all aircraft at TA or above, because regardless of their actual altitude above mean sea level, they are still separated from each other by 2000 feet.
3.10 Minimal altitudes MOCA, MRVA and MSA [C+]
MOCA = Minimum Obstacle Clearance Altitude
This is the lowest altitude that an aircraft can fly in IMC (Instrument Meteorological Conditions) and still keep safe clearance from terrain and obstacles. MOCA is often lower then MEA (se below). It is only used in emergencies, especially to get below icing.
MRVA = Minimum Radar Vectoring Altitude
MRVAs are established where ATC vectors are initiated or requested frequently. They are often lower than other applicable minimum IFR altitudes because they can be more focused in specific areas rather than general like a 25 NM safe altitude around a final approach fix, for example. As a result, these are not published on any instrument approach procedure, but may coincide with published values.
Since ATC is responsible for obstacle clearance of an aircraft on radar vectors, the MRVA will, by definition, provide a number of issues like cold weather altimeter errors, terrain and obstructions, restricted airspace, etc. The MANOPS Definition for an MRVA also includes radio coverage, but oddly enough, not radar coverage. It is also supposed to consider the base of controlled airspace in a given area, so the radar vectoring altitudes are often lower in terminal areas where the airspace classification is better designated to include lower transition areas. On this note, ATC may provide vectors to an aircraft in uncontrolled airspace (class G) if requested by the pilot, or if ATC suggests it and the pilot accept it.
MSA = Minimum Safe/Sector Altitude
Minimum Sector Altitude is the minimum altitude that may be used under emergency conditions which will provide a minimum clearance of 1000ft above obstacles and terrain contained within a sector of 25 NM radius centred on a radio navigational aid. MSA can be given as areas between radials from a VOR at the airport.
3.11 Minimum En route altitudes (MEA) [C+]
MEA's takes into account both navigational aid limitations and obstruction clearance. This is important to you for two reasons.
First, if you clear an aircraft below the MEA in your area, and the pilot discovers rocks in the clouds ahead, he may be cranky.
Second, in case of radio failure, be aware that the pilot will climb to the MEA if he has not received a new altitude clearance prior to the radio failure.
You need to become familiar with the MEA's in your VACC area to the extent that you will not inadvertently clear an aircraft below them.
The MEA's can be found on the low altitude en route charts along airways. The number starting with an asterisk is the MOCA, which can be assigned under certain circumstances, but will not be useful in our environment (see above).