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23 Sep 2014

Calculating flight altitude


As is well known, flight is impossible without altitude, and without an altimeter, not a single plane can fly. Despite the fact that modern airliners have evolved significantly since the first planes, the principles of flying have not changed, relying on the same principles of altitude and flight level. Altimeters likewise remain practically unchanged, with only a slightly modified appearance.

After the Wright brothers’ historic first flight in 1903, it was another two decades before anything overtook the eyes and the vestibular apparatus as the most essential tools for the pilot, despite the quick development of aviation technology. Naturally, flights in the dark, fog, or cloud cover carried greater risk. Still, pilots flew at night, though they risked falling into a spin while focusing on lit objects on the ground.

Reliable instruments were needed for so-called “blind” flights that did not have to require on the pilot’s senses.

It should be noted that, by that time, many devices for spatial orientation already existed, such as a turn and slip indicator, made of a glass tube with a metal ball inside, as well as the gyroscope and altitude indicator.

Incidentally, the first altitude indicator was created in Russia in the early 1930s at the Ural Instrument Plant, which is now the part of KRET. With this device the history of Russian gyroscope production began.

At the time, aircraft lacked a reliable and, most importantly, accurate altimeter, a device that identifies flight altitude. The term altimeter comes from the Latin word altus, meaning high.

It can be said that the altimeter was already around by 1843, when French scientist Lucien Vidi invented the well-known aneroid barometer, a device that has proven useful since the beginning of the era of aircraft. This barometric method of measurement, based on the natural phenomenon of decreasing atmospheric pressure with increasing altitude, still exists today.

The accuracy of the first barometric altimeter was 30-50 meters, which is not sufficient for blind flights. The American Paul Kollsman found a solution, using a Swiss chronometer, the most perfect mechanical device at the time. A Swiss watch company produced according to his specifications a mechanism that allowed for measurement of altitude with an accuracy of up to 1 meter.

In September 1929, the first truly blind flight took place, with a plane taking off and flying for 15 miles with a curtained cabin. The pilot navigated the plane using only instruments, one of which was Kollsman’s barometric altimeter.

All barometric altimeters, domestic and foreign, have the same structure: a sealed box with a membrane, whose changes are measured and connected to arrows that move around a scale graduated with numbers. The long hand makes one turn for every one thousand meters, and the short one moves with every single unit of change.

The reliability and accuracy of the altimeter determines the safety of the flight, which is why modern aircraft utilize two devices to measure altitude. There is always an altimeter on board, as well as a different method for measuring altitude, a radar altimeter.

The radar altimeter utilizes radar and thus does not depend on the state of the atmosphere. It is also more precise.

The radar altimeter utilizes a set of electronic equipment and antenna, including a microwave radio; directional antenna located in the belly of the aircraft; signal receiver; and signal processing devices. The only thing visible on the dashboard cockpit is an indicator of the altitude, usually alongside a warning system for dangerous altitudes. In this regard the altimeter is part of a set of warning systems to prevent collision with the ground. Additionally, it significantly increases the possibility for an automated landing.

Various devices have different measuring ranges. For example, altimeters, which can measure height of up to 10,000 meters, are mainly used in planes with a relatively low maximum altitude. Altimeters on aircraft such as the Tu-154 and MiG-29 have a much larger range. Some altimeters can even operate in space, such as those used for moon landing vehicles.

Currently, almost every plane utilizes at least one radar altimeter. Often there may even be several, which even work in conjunction with other aircraft systems. Failure of even one altitude measurement instrument is considered a noteworthy event and a precursor to a flight accident.

Thus, altitude is not just a pilot’s dream but also a very important factor in flight safety. The altimeter is a device without which not a single plane can fly.

Based on materials from the official website of KRET