What is an Aircraft Altimeter?

The determination of an airplane's altitude by a pilot is paramount for several operational necessities, such as maintaining a safe distance from other in-flight aircraft, landing estimation, and terrain clearance. There are six types of altitude that are of navigational importance in this context, those of which include true altitude (TA), calibrated altitude (CA), density altitude (DA), pressure altitude (PA), indicated altitude, and absolute altitude.

An altimeter is a navigation device used in aircraft to measure and maintain a safe distance above mean sea level (MSL). Most modern altimeters used today are barometric, implying that they use the ambient atmospheric pressure to measure the aircraft's location. With an increase in altitude, the air pressure intercepted by the static port decreases by about one millibar every 10 meters. Based on this principle, the functions of different altimeter types vary, a few of which will be described throughout this blog.

As previously discussed, most altimeters rely on barometric pressure to calculate a location above sea level, that of which is then displayed to the pilot as a visual indicator referencing the vehicle’s distance from the MSL in feet. Modern altimeters use a stack of sealed aneroid wafers (at 29.92" Hg internal standard pressure), which expand (implying increasing altitude) or contract (denoting decreasing altitude) in response to the static pressure within the encasing. On the other hand, skydivers and mountaineers use wrist-mounted versions of such altimeters to suit their needs. Such altimeters utilize a  graduated scale with equal intervals where a hinged pointer can freely move towards the right to indicate an increase in pressure.

Types of Altimeters

Although conventional altimeters are designed to use atmospheric pressure, other advanced altimeters use more sophisticated technologies. For instance, laser and radar altimeters work similarly to sonars for measuring the seafloor. Such altimeters send a radio or laser beam to an object, and the time taken for the beam to return (a phenomenon referred to as an echo) is used to calculate the distance between the object and source. Such equipment like the TOPEX/Poseidon radar altimeter by CNES and NASA have been used to construct topographical maps of ocean floors and rugged terrains.

Another prominent type of altimeter is the encoding altimeter which functions in the same way as a standard altimeter. However, encoding altimeters work by sending a digitized output to the transponder within the radar beacon system of air traffic control (ATC) in 100-feet intervals. On the other hand, blind encoders are cost-effective alternatives to encoding altimeters that transmit information to ATC transponders, and their pressure is always set to 29.92 mm of Hg. Altimeters are manufactured with a variety of displays to suit different navigational needs. Some of the most common types are:

Analog Three Pointer Altimeters

A typical three-pointer altimeter consists of three-pointers with different sizes to indicate different readings. For instance, the longest of the three displays readings at intervals of 10,000 feet, while the shorter one counts at 1,000 feet intervals. The median, thinner pointer moves at intervals of 100 feet.

Drum Type Altimeters

Drum-type altimeters contain a drum that makes a revolution per 1,000 feet. At the same time, these altimeter displays also include gradations that indicate intervals of 100 feet with further divisions of 20 feet intervals between them. The drum is directly connected to the components which move the pointer, showing altitude in thousands of feet, with the pointers moving simultaneously in hundreds of feet.

Digital Altimeters

Regular analog altimeters are susceptible to changes in atmospheric conditions at different altitudes. This problem is solved by using digital altimeters which enable a pilot to achieve a smooth landing on the runway with accurate altitude readings. Furthermore, the regular vibrational motion produced by the aircraft engine can make inaccurate readings in a mechanical altimeter, which is another problem corrected by digital altimeters that display precise altitude measurements on LCD screens. Furthermore, some digital altimeters come equipped with LCD screens and a direct drive so that projected altitude readings are visible to the pilot at all angles in conditions of bright sunlight. Meanwhile, they also often contain night or dusk lighting to enhance the readability of the display during low-light conditions.

Counter-Pointer Altimeters

A counter-pointer altimeter has two digital counter displays, one of which is situated at the nine o'clock position, while the other is a pointer that moves on a fixed circular scale with gradations indicating intervals of 100 feet. One pointer displays altitude changes every 1,000 feet in the range of 0 to 80,000 feet, while the other completes a revolution  in 1,000 foot increments.

Counter-Drum-Pointer Altimeters

Similar in display to the counter-pointer altimeters, counter-drum-pointer altimeters differ in their presence of a drum window, apart from the usual counter display and a pointer. While the counter windows display increments per 1,000 and 10,000 feet, the drum window corresponds to the pointer's movement and shows increments per 100 feet. A pilot could read all values simultaneously from the counter and drum displays or read the two counter displays while ignoring the drum window to get a precise reading from the pointer itself. Counter-drum-pointer altimeters use a transducer which converts electrical signals generated from the differential pressure measured by the altimeter and sends them to an ATC transponder.

In Conclusion

Now that we know how crucial altimeters are in aircraft operations, pilots and ATC crew should not just use any altimeter available, but only trusted ones sourced from reliable names in the aviation industry, such as NSN Target. NSN Target is the leading supplier of top-grade altimeters and quality NSN, NIIN, and other aircraft parts. In addition, we pride ourselves on our expedited shipping process, streamlined parts procurement, and 24/7x365 customer assistance service. Therefore, we invite you to start your parts procurement journey with us through the Instant RFQ service on our website, and a dedicated accounts manager will respond to your quote within 15 minutes or less.


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