ALTIMETRY
How many different altimeter settings confront you when operating internationally? Not only that, how many different ways are there to report an altimeter setting? If not operating internationally on a regular basis it can get confusing as to what altimeter setting did you receive and what measurement was used in providing the current station pressure. This can be an insidious problem, which can lead to an improperly set altimeter. As we all know, flying with an incorrect altimeter setting may bring us into contact with the ground before we are ready. We will explore the various altimeter settings used around the world and look at the ways of reporting station pressure. There are three altimeter settings that are used in aviation QNH, QFE, and QNE. Flying in most of the world you use QNH and QNE. Only a small number of countries use QFE and QNE. Lets start by defining each of these altimeter setting (see Figure 1): QNH: With this setting set on your altimeter you will be reading altitude above mean sea level based on the local station pressure. QFE: With this setting set on your altimeter you will be reading height above the airport or runway threshold elevation based on local station pressure. QNE: With this setting you will be reading the flight level you are at referencing a standard datum plane(29.92 HG, 1013.2 hpa or 760 mm). Figure 1 QNH and QFE are used in the vicinity of airports and are used prior to climbing through the transition altitude/height and then set on descent when descending through the transition level. Both altimeter settings provide information based on the local station pressure. The only difference being one references mean sea level and the other references the airport.
Meaning that if you are sitting on the ground at an airport with QNH set on your altimeter you will be reading the field elevation. If using a QFE altimeter setting you will read zero (0) on your altimeter. Therefore the only difference between the two settings is field elevation. There is nothing magical, mystical, or otherwise confusing between the two altimeter settings it is quite simply the reference used by the altimeter setting to determine vertical position. The Commonwealth of Independent States (CIS) are the only countries in the world that still use QFE as the primary altimeter setting. The People s Republic of China has adopted the use of QNH at its international airports, however if operating at domestic or military airfields you may still receive a QFE altimeter setting. At locations where QFE is the standard altimeter setting, if possible, ATI recommends using the QFE altimeter setting. If the situation arises where you are given a height to climb or descend to, In a QFE environment (where QFE is the altimeter setting used ATC expects you to position yourself vertically relative to the airport. If you have chosen to use QNH and climb or descend to the assigned height without making any adjustments you are not going to be where you are supposed to be in the vertical plane (see figure 2). If you are using the correct QNH altimeter setting, which is usually available on request, you must remember to add the field elevation to the assigned height so you will be at the correct position in the vertical plane. Depending on your equipment it may not be possible to set the QFE altimeter setting. The older altimeters have a stop installed, which won t allow a setting below 28.00 inches or 950 hpa. This means when the field elevation reaches approximately 2000 feet you won t be able to set QFE and are forced to fly QNH settings at these locations. Remember when using QNH in a QFE environment, if assigned a height to fly, you must add the field elevation to the assigned height and fly to that value on your altimeter to achieve the correct vertical position. Figure 2
While defining the various altimeter settings it was pointed out that QNH gives you an altitude and QFE gives you a height. Remember this distinction QNH = altitude and QFE = height. This carries through when reading Jeppesen Instrument Approach Procedures (IAPs). All values published on a Jeppesen IAP that are outside of the parenthesis are altitudes and therefore based on QNH while all values published inside of the parenthesis are heights and therefore based on QFE. When looking at a Jeppesen approach plate, no math is involved as far as the published altitudes/heights because both values are available for use on an approach procedure. Just make sure you are using the correct numbers based on the altimeter setting you have decided to use. Not only do you need to be aware of the different altimeter settings that you may encounter, but you must also be sure of the unit of measure that was used to report the altimeter setting. Simply stated the altimeter setting is the pressure being exerted on the earth by the atmosphere, which will displace a column of Mercury. The amount of displacement can be measured in inches, millibars/hectopascals, or millimeters. These three different units of measure are used in various locations around the world. In North America we are accustomed to receiving an altimeter setting in inches of mercury. We have learned through instruction and experience what to expect based on the use of inches and understand that an International Standard Atmosphere (ISA) altimeter setting is 29.92 inches. We have also learned that if the altimeter setting is less than ISA the lowest useable flight level moves up. In other words with an altimeter setting of ISA or greater the lowest useable flight level in the U. S. would be flight level 180. However with an altimeter setting less than ISA the lowest useable flight level will generally be flight level something greater than flight level 180. Altimetry vertical change over points are associated with altitudes, heights and flight levels. They are identified as transition altitudes or heights and transition levels. By definition there must be 1000 feet of separation between the transition altitude/height and the transition level. That is why the lowest useable flight level must move up when a lower than standard altimeter setting exists. If the lowest useable flight level did not move up under this condition there would be less than the required 1000 feet of separation between the transition altitude/height and the lowest useable flight level. Although we change our altimeter setting climbing through 18,000 feet on climb and Flight Level 180 on descent in the U.S., the reality is that the transition altitude is 17,000 feet. In many countries of the world the transition level is determined on a daily basis by ATC. They are unwilling to block useable flight levels by establishing a transition layer. A transition layer is defined as that airspace located between the transition altitude/height and the transition level. In countries that do utilize transition layers no aircraft is allowed to cruise between the
transition altitude/height and the transition level. In those locations that list transition level by ATC, the transition level will be 1000 feet above the published transition altitude/height with a standard day altimeter setting or greater or the lowest useable flight level when the altimeter setting is less than a standard day setting. To determine when cleared below the transition level watch for the change in terminology. You will be cleared to an altitude or height and should be given the local altimeter setting at that point. When climbing out the altimeter setting is always changed when passing the published transition altitude or height. In most of the rest of the world altimeter settings are reported in millibars (mb) or hectopascals (hpa). It should be understood that a millibar and a hectopascal are one in the same, merely different names for the same unit of measure. The relationship between inches and the mb/hpa is 1 inch equals 33.863 mb/hpa. An ISA altimeter setting would be 1013.2 mb/hpa, which is equal to 29.92 inches of mercury. In today s cockpit you can utilize either inches or mb/hpa settings on your altimeter, as there are options for using either setting, thus negating the necessity of using a conversion table. However, if a requirement for conversion presents itself the Jeppesen Airway Manual tables and codes section provides the necessary conversion table to convert inches to mb/hpa or vice versa. The biggest problem which looms over the use of the mb/hpa settings is not getting them set on the altimeter but misinterpreting the altimeter setting received from ATIS or when provided by ATC. Until 1996 when the U.S. adopted the use of the Terminal Aerodrome Forecast (TAF) and the Meteorological Terminal Aviation Report (METAR) pilots in the U.S. were taught to automatically put a 2 in front of any altimeter setting given in a three digit format starting with a 9 and to put a 3 in front of any altimeter setting given in a three digit format starting with a 0. This procedure and habit sets us up for error when dealing with altimeter settings given in mb or hpa, because it can and will be given in a three-digit format beginning with a 9. First, you must be aware of where you are flying and what the altimetry rules are for that location. Secondly, you must listen closely when an altimeter setting is given to be sure of the unit of measure that was used. You should expect to receive the current altimeter setting with the phraseology; Altimeter setting is QNH 991 hectopascals. If you inadvertently put a 2 in front of the 991 and set your altimeter to 29.91 inches you are going to be flying well below where you think you are. Converting 991 hpa to inches results in an altimeter setting of 29.26 inches. Using an altimeter setting of 29.91, which should be 29.26 means you will be 640 feet below where you think you are. This can lead to arriving at the ground before you are ready and well short of the runway. There are documented cases of this happening in the Aviation Safety Reporting System (ASRS). You can include this in the category of those who have and those who will.
If flying in the CIS you will encounter the third unit of measure used to report an altimeter setting, as this part of the world utilizes metrics and therefore uses millimeters to report altimeter settings. The relationship of millimeters to mb/hpa is 1 millimeter of mercury equal 1.3332 mb/hpa. An ISA altimeter setting in millimeters would be 760 mm equaling 1013.2 mb/hpa equaling 29.92 inches of mercury. When an altimeter setting is provided in the CIS you should expect it to be in millimeters and that it is QFE. The setting should be issued with the following phraseology; Altimeter Setting is QFE 775 millimeters. Remember the rule of thumb, if unsure of what unit of measure was used. If the altimeter setting given is in the seven hundreds or low eight hundreds it probably is millimeters. If the altimeter setting is given in the nine hundreds or low thousands it probably is mb/hpa. The only altimeters that would be able to have a direct millimeter setting capability would be those built in Russia. When confronted with the altimeter setting given in millimeters you have two choices for conversion. One, use the tables and codes conversion chart and convert it yourself, or two, have the controller convert it for you. Most approach plates for areas using millimeters indicate that mb or hpa are available on request. The question here is who do you trust, yourself or the controller? Operating internationally poses many challenges that are not common place to domestic flying. Altimetry is only one of them. You must do your research and be aware of the measurement standards used in the location where you are flying. The Federal Aviation Administration (FAA) provides a list of recommended subject areas you should be knowledgeable on to be considered qualified for international operations. One of those subjects specified is ICAO Measurement Standards. ICAO Measurement Standards are published in Annex 5 of the Standards and Recommended Practices (SARPS).