Altitude
Lower altitude is defined as the measured vertical height of an aircraft above a certain reference surface. Depending on your situation there are different reference surfaces into consideration. The accurate measurement of altitude is on aviation security is of great importance in order to ensure a sufficient safety distance from other aircraft and ground obstacles can.
Measurement of altitude
The altitude is generally measured by the barometric altimeter working (English altimeter ), where one makes use of the fact that the air pressure decreases with increasing altitude. However, since, first, different reference areas are used for altitude measurement and second, the air pressure varies not only with the height, but also with the weather, altimeter always have an adjustment for the reference pressure ( the pressure at which they would show the level zero).
- It would be natural to refer to the ground as a reference surface in order to meet such as security heights above the terrain and ground obstacles can. The altitude above ground (GND, ground) is called the height ( HGT). In the aeronautical practice, however, this height plays a role only in special cases. During a haul flight, the height of the underlying terrain changes too fast for such a measurement would be useful. The reference pressure (QFE ) would have to be adjusted several times per minute, the ground level and would be on top of that from just flown by weather conditions.
- Therefore, in the navigation altitude is generally based on height above sea level ( MSL, mean sea level ). Also obstacle heights on air tickets are given in feet above MSL, so that safety levels can be maintained. The height relative to MSL is called altitude (ALT). The reference pressure for it, so the converted at sea level current air pressure value set in each case is called QNH and set before starting. The altimeter setting must be adjusted to weather-related changes in air pressure at distance flights at low altitude.
- Above the so-called transition height, the actual height is not more interesting, as distances to the ground obstacles are no longer relevant. It is much more important now is to use an independent also from the weather patterns of reference to ensure that all aircraft measure their height by the same reference pressure and vertical distances can be each other reliably maintained. At standard pressure ( 1013.25 hPa) set - Thus from transition altitude, the altimeter - regardless of the actually existing air pressure. The measured value of the flying height is not more than the amount, but ( divided by 100 ) as flight level ( FL, flight level ) refers to: FL 120 corresponds to an altimeter indication of 12,000 feet above the theoretical normal printing surface.
It is common international practice altitudes in feet ( ft) must be indicated; in Germany only measure gliders, airships and parachutists, the height in meters. 100 ft correspond to 30.48 m. Sufficiently accurate rule of thumb: Height in feet · 3/10 ≈ height in meters.
The measurements of a barometric altimeter always depend on the standard atmosphere. However, the relationship between pressure and height also depends ( slightly) on the temperature and water vapor content of the air, so the next flight altitudes correspond seldom exactly to the actual values. However, since the display error is insignificant and also fails the same for all aircraft, this is not critical.
A direct ( and accurate ) radio altitude measurement is performed only under certain circumstances. During the landing approach with commercial aircraft direct altitude determination by radio altimeter (radar altimeter) for example, is common.
In addition, the altitude can also be observed from the ground by means of radar and transmitted to the pilot by radio.
Typical altitudes
The following table, in which altitudes are aircraft which does not constitute rules or fixed rules, but only serves that the layman can make a rough idea. For a better overview the heights in meters above the ground (GND) and are valid only conditionally at higher altitudes.
The minimum safe altitude for aircraft, which may be below only off or landing, is in Germany:
- Generally 500 ft (150 m ) GND
- About towns or large gatherings of people 1000 ft (300 m) above the highest obstacle within 600 m
Cross-country flights are when weather and airspace structure to allow it to perform at an altitude of at least 2000 ft GND.
Health aspects
The human body is adapted to life on the ground. The air pressure conditions encountered in flight can therefore be problematic:
- The pressure of the middle ear has to be constantly adapted to the external pressure to keep the eardrum relaxed. For this, the Eustachian tube is responsible that opens in swallows. Is this tube about as a result of disease, swollen shut, then the ear internal pressure changes in the external pressure when flying, especially the rapid pressure increase as it descends, not follow, after which the higher external pressure, the eardrum stretches inward and causes severe pain. Can be remedied by the Valsalva maneuver. Chewing and sucking also help the Eustachian tube to open as often as possible.
- From altitudes of 8,000-10,000 ft MSL, the oxygen content of the breathing air is so low that a reliable supply of ( untrained ) human organism can no longer be expected, and either oxygen equipment or pressurized cabins must be used. In particularly sensitive individuals may be oxygen deficiency symptoms occur even at 5,000 ft MSL (corresponding approximately to the height of the Feldberg in the Black Forest ).