Safe and effective handling of aircraft requires feedback on a wide range of flight conditions as well as the states of various flight equipment and systems. A diverse assortment of sensors continuously monitor these conditions, feeding information to flight computers for processing before being displayed to the pilot.
Many types of sensors are used on aircraft. Flow sensors sense the amount of lubricating oil and liquid coolant in motion as well as fluid moving in fuel transfer and bleed air systems. Liquid level sensors monitor oil, fuel and coolant levels, as well as fluid levels in potable and gray (waste) water reservoirs, collection sumps and hydraulic reservoirs. Pressure sensors monitor pressure in hydraulic systems, including those used for moving control surfaces, braking and raising and lowering landing gear.
Position sensors such as linear variable differential transformers (LVDT) and rotary variable differential transformers (RVDT) sense the displacement of various aircraft components, including, for example, the deployment status of thrust reversers. Force and vibration sensors are also used on aircraft to measure the torque and force in braking and actuation systems as well as in flight controls.
Temperature sensors play a key role in monitoring the conditions of hydraulic oils, fuels and refrigerants, as well as temperatures in environmental cooling systems. Types of temperature sensors include bimetallic temperature gauges; electrical resistance thermometers, such as Wheatstone bridge indicators and ratiometer indicators; and thermocouple temperature indicators. Thermocouples consist of two different metals contacting at separate junctions. The temperature difference in the metals produces a voltage proportional to the temperature.
The pitot-static pressure system provides the source pressure for a variety of aircraft instruments, including airspeed indicators, vertical speed indicators and altimeters. Pitot-static systems gather two air pressures from separate ports. The pitot tube collects impact air pressure measuring the full force of air as the aircraft moves forward through the atmosphere, while the static port gathers static air pressure representing atmospheric pressure outside the airplane in still conditions.
The pitot-static system is not the only pressure measurement system on board aircraft. Pressure sensors of various types are also used to measure pressure in engine oil, pneumatic deice boots, oxygen tanks, hydraulic systems and heating and coolant fluids. Types of pressure sensors include the bourdon tube, diaphragm, aneroid and bellows mechanisms, as well as solid-state sensors. In solid-state pressure sensors such as crystalline piezoelectric, piezoresistive or semiconductor chip sensors, a change in pressure causes a deflection in the material resulting in a current or change in resistance that is proportional to the pressure change.
Sensors are integral to the instrument systems on board aircraft, including flight, engine and navigation instruments. Flight instruments include altimeters, airspeed indicators and vertical speed indicators. Altimeters measure changes in static air pressure to determine the altitude of the aircraft. Airspeed indicators calculate true air speed based on pitot and static pressure and temperature data. In vertical speed indicators, a solid-state pressure sensor known as an aneroid measures static air pressure changes. Combined with the time signal from a digital clock, the rate-of-climb is calculated.
Other flight instruments include direction indicators, artificial horizons and attitude indicators. Compasses and magnetometers indicate aircraft heading by measuring the Earth’s magnetic field. Gyroscopes can also be used for heading indication as well as controlling common flight instruments like turn indicators and attitude indicators. In addition to mechanical versions, gyroscopes are available in high reliability solid-state form, including ring laser gyros and microelectromechanical systems (MEMS) gyroscopes. Ring laser gyros function by measuring the frequency difference between two laser beams traveling around a ring in opposite directions. MEMS gyroscopes detect changes in the capacitance or voltage of a piezoelectric material as it oscillates or vibrates.
Attitude heading and reference systems (AHRS) have replaced gyroscopes and other instruments on modern aircraft. They receive data from MEMS devices, GPS, solid-state magnetometers and solid-state accelerometers and display attitude information such as roll, pitch and yaw in addition to aircraft heading.
Engine instruments include tachometers, engine temperature gauges, fuel and oil quantity and pressure gauges. Tachometers, which indicate engine rpm, are available in a variety of technology types. One type, the tachometer probe, is used in turbine engines. It senses changes in magnetic field flux density as a rotating gear wheel moving at the same speed as the compressor shaft travels through the probe’s magnetic field. The resulting voltage signals are directly proportional to engine speed.
Air Data Computers
Aircraft with digital instrument systems (e.g., glass cockpits) receive data from various sensors located remotely around the aircraft, including, for example, the total air temperature probe, angle of attack probe and the pitot-static pressure system. Air data computers process the inputs from these sensors, apply compensating factors and present information on flight displays to the pilot. The electrical output from analog sensors must be preprocessed by analog-to-digital converters, while the digital outputs of solid-state sensors can be handled directly by the computer. Air data computers compensate for factors such as very low ambient air temperature and air density variations due to the compressibility of air at high speeds. They also output data to a variety of aircraft systems, including the transponder, fuel temperature indicator, flight director system, flight management computer, autopilot system, inertial reference units and flight recorder.
Flight control systems in modern aircraft are fly-by-computer systems. Signals from sensors monitoring the thrust levers, pedals and flight stick are received by a flight management system that controls engine power and actuates control surfaces such as ailerons and flaps. Sensors play a critical role in providing the data necessary for safe and effective aircraft operation by the pilot and automated control systems.