Avionics, Electronics, Instrumentation, Mechanics, Sensors

Avionic sensors and instruments (Part 2)

This is the second part of avionic sensors and instruments. In this post, will be shown other instruments and sensors used in aircrafts.

Click in the button below to see the first part.

Avionic sensors and instruments (Part 1)Click here

Measuring the airspeed

The airplane’s speed in relation to air is one of the most important parameters, because airplane’s stability depends on it. The air enters by a pitot tube and has a chamber which does not receive air flux and with static pressure.

The speed is measured by difference of pressure between air chambers. When the aircraft is stopped, the chamber pressure which receives the air flux is equal to static pressure. When the aircraft flies, the difference pressure transducer moves mechanisms which turn the pointer, as shown in this video.

This sensor stays in the front part of airplanes.

Measuring altitude

Altimeters to aircrafts use a aneroid barometer to measure the altitude from air pressure variation. Has aneroid wafers with an internal pressure. When the air is more rarefied, the wafers expand, moving mechanisms to turn the pointer. When the atmospheric pressure increases, the wafers contract.

Indicating vertical speed

The vertical speed indicator (VSI) works in a similar way to the altimeter, serves to indicate if the airplane is climbing, descending or keeps in constant altitude. In this instrument, the wafers are linked to a static pressure tube and the case has a hole to control the air leaking.

When the airplane climbs, the pressure inside the wafer decreases faster than in the case, which decreases the pressure by the hole. When the airplane descends, the pressure inside the wafer increases faster than in the case. When it stays in the same altitude, the pressures inside and outside the wafer are equals. For what serves the static port? Has the function to transmit air to the instruments, to stay calibrated and to measure the pressure difference.

Air data computer (ADC)

In more modern aircrafts, these avionic sensors are integrated in the air data computer. Purely pneumatic instruments lose precision in high altitudes. The computer has pressure sensors which send electric signals to the computer, which has analog-digital converters and error corrector. The ADC offers a better vision of data than the analog pointers.

The purely pneumatic instruments stay in reserve for backup, in case ADC fails. Other avionic sensors and instruments stay for other posts.


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