Why Do Planes Fly So High?

The higher they go, the thinner the air is. The thinner the air is, the lower the drag. The lower the drag, the faster the plane can go for a given amount of fuel.

An aeroplane’s wings can generate enough lift to balance the weight either by flying low and slow or high and fast. This is because the air is less dense at altitude. If the lift stays the same so does the drag (roughly!). Better still a jet engine’s efficiency improves with speed and altitude. Thus flying high gives roughly the same drag at a higher speed and the engine is more efficient.

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Altitude or height (sometimes known as depth) is defined based on the context in which it is used (aviation, geometry, geographical survey, sport, and many more). As a general definition, altitude is a distance measurement, usually in the vertical or “up” direction, between a reference datum and a point or object. The reference datum also often varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage

In aviation, the term altitude can have several meanings, and is always qualified by either explicitly adding a modifier (e.g. “true altitude”), or implicitly through the context of the communication. Parties exchanging altitude information must be clear which definition is being used.

Aviation altitude is measured using either mean sea level (MSL) or local ground level (above ground level, or AGL) as the reference datum.

Pressure altitude divided by 100 feet (30 m) as the flight level, and is used above the transition altitude (18,000 feet (5,500 m) in the US, but may be as low as 3,000 feet (910 m) in other jurisdictions); so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at “Flight level 180”. When flying at a flight level, the altimeter is always set to standard pressure (29.92 inHg or 1013.25 hPa).

On the flight deck, the definitive instrument for measuring altitude is the pressure altimeter, which is an aneroid barometerwith a front face indicating distance (feet or metres) instead of atmospheric pressure.

There are several types of aviation altitude:

  • Indicated altitude is the reading on the altimeter when it is set to the local barometric pressure at mean sea level. In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from mean sea level; this is referred to over the radio as altitude.(see QNH)
  • Absolute altitude is the height of the aircraft above the terrain over which it is flying. It can be measured using a radar altimeter (or “absolute altimeter”). Also referred to as “radar height” or feet/metres above ground level (AGL).
  • True altitude is the actual elevation above mean sea level. It is indicated altitude corrected for non-standard temperature and pressure.
  • Height is the elevation above a ground reference point, commonly the terrain elevation. In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from a specified datum; this is referred to over the radio as height, where the specified datum is the airfield elevation
  • Pressure altitude is the elevation above a standard datum air-pressure plane (typically, 1013.25 millibars or 29.92″ Hg). Pressure altitude is used to indicate “flight level” which is the standard for altitude reporting in the U.S. in Class A airspace (above roughly 18,000 feet). Pressure altitude and indicated altitude are the same when the altimeter setting is 29.92″ Hg or 1013.25 millibars.
  • Density altitude is the altitude corrected for non-ISA International Standard Atmosphere atmospheric conditions. Aircraft performance depends on density altitude, which is affected by barometric pressure, humidity and temperature. On a very hot day, density altitude at an airport (especially one at a high elevation) may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.

These types of altitude can be explained more simply as various ways of measuring the altitude:

  • Indicated altitude – the altitude shown on the altimeter.
  • Absolute altitude – altitude in terms of the distance above the ground directly below
  • True altitude – altitude in terms of elevation above sea level
  • Height – altitude in terms of the distance above a certain point
  • Pressure altitude – the air pressure in terms of altitude in the International Standard Atmosphere
  • Density altitude – the density of the air in terms of altitude in the International Standard Atmosphere

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