Elevating the senses

Sensors are becoming more commonplace every year. Each home has an array of sensors unthinkable 50 years ago and the rate of progress is mirrored in most industrial sectors. The cockpit of a Second World War aircraft now looks spartan compared with the banks of dials and indicators on modern aircraft.

The drive to reduce crew workload and enhance aircraft performance has been enabled by advances in electronics and computers. Sensors that used to drive directly analogue gauges monitored by pilots, or a flight engineer, are now interfaced to an automatic control or monitoring system, eliminating the need for a flight engineer.

The same type of sensor can be used in different areas. But according to Paul Capener of Smiths Aerospace: 'The pressure sensor associated with measuring barometric pressure, so that the aircraft's altitude can be established to within one or two feet, is completely different to those used for measuring hydraulic pressure (typically 200 to 350 bar) for flight control.'

Aircraft sensors fall into three categories: they detect where the aircraft is relative to its environment, gather information about the state of the aircraft, or sense data external to the aircraft.

It is critical that a pilot knows how high the aircraft is. Altimeters commonly contain pressure transducers — for example, Ametek Aerospace makes models incorporating Honeywell transducers — from which atmospheric pressure can be compared to a sea-level reference to calculate the aircraft's height.

Pressure sensors are also used to determine an aircraft's 'angle of attack' (a term applied to commercial as well as military aircraft) and sideslip. Commonly an array of pitot tubes and capacitive, fixed-pressure sensors are arranged around the plane. Different pressure measurements between static and pitot pressures can be used to calculate the angle of attack and sideslip — for example, the RAF has previously used arrays of Rosemount sensors. Ametek and Space- Age Controls also produce angle-of-attack transducers as individual instruments.

Sensors are also used to determine the position of various parts of the aircraft. These are commonly linear, rotary or angular and are typically used in areas such as landing gear, ailerons, brake systems and doors.

Precision transducers (such as those made by Penny and Giles/Curtiss Wright) are often used to send a signal, depending on the component's position, to the pilot or control system.

Proximity sensors are also regularly used in aircraft. These operate by detecting some electrical property such as inductance — Honeywell includes reluctance and Hall-effect devices in its range — and are often used for sensing the position of doors and landing gear.

At the fore of safety-related issues is the detection of ice on aircraft — a common phenomenon at altitude and a factor capable of bringing down aircraft in certain conditions. This happened in 1985 when a McDonnell Douglas DC-8 crashed just after taking off from Gander in Canada.

One solution to this problem, developed by Cambridge Optical Sciences, is a fibre-optic sensor that can detect ice films as thin as 0.1mm. The sensor works by looking at infrared light scattering and reflection from the surface. It is also small enough to be placed in critical zones, for example in the part of an aerofoil most prone to icing.

Other probes, such as those developed by New Avionics Corporation, claim to be able to detect the first 0.001 inches of airframe icing. Such probes can inform the pilot of ice problems and take the appropriate remedial action, for example dropping altitude by a few hundred feet. They also relieve the pilots of the pressure of having to constantly look for ice, as was once the case.

Another important measurement for aircraft is how much fuel is on board. A common way of measuring this is to insert a capacitance probe into the aircraft's fuel tank, which detects changes in the permittivity of the fuel/air and vapour mix in a tank so that, with electronic manipulation, a reasonably accurate estimate of the amount of fuel left is obtained. These type of sensors are made by companies such as Smiths, Westach and Insco.

The Eurofighter uses Druck pressure sensors to determine fuel distribution to tanks contained in both wings. The aircraft also uses pressure sensors for engine diagnostics. Gems sensors form part of the engine management system and measure pressure at different stages of engine compression.

The state of mechanical components is often determined by attaching strain gauges to aircraft parts, such as engine mountings and control linkages. For example, Variohm produces a range of pins containing inbuilt strain gauges used on the aircraft and on test rigs.

External data is collected in civil aircraft for safety purposes such as identifying the runway, but military aircraft have a different set of needs.

One example comes from Sensors Unlimited, which has developed a short-wave infrared (Swir) camera to enhance aircraft vision. The reflected signature of Swir is similar to that of the visible spectrum, with the advantage that it can be seen through fog, making it useful for landing in foggy conditions, because Sensors Unlimited's camera allows pilots to 'see' the Swir emitted by runway lights.

Martin Ettenberg of Sensors Unlimited said: 'As far as aircraft are concerned, the beauty of [the Swir camera] is that it can see through glass, so can be placed anywhere. Public demand for the on-time arrival of commercial aircraft will make Swir vision common in future: it allows aircraft to land in fog, avoiding having to 'stack' circling aircraft, so will save large sums in fuel alone.'

Military aircraft, especially unmanned craft, may also carry acoustic sensors, which can be useful for identifying targets made 'stealthy' to most electromagnetic radiation sensors. The acoustic signature of, for example, anti-aircraft guns can be pinpointed once the background noise generated by the carrier aircraft is filtered out.

As the sensors used by aircraft are supplied by many manufacturers, there are going to be integration problems.

Smiths overcomes these problems by supplying to a number of aircraft a set of remote interface units for the simpler sub-system sensors. These units take inputs from different sensors and, via a central database table, interpret outputs according to type and position. Sets of these units are being used on the Lockheed Martin joint strike fighter, the Airbus A380 and Boeing 787, among others.

The trend for greater use of sensors is set to continue. Rodney Bogue of NASA's flight instrumentation branch identified a number of trends in sensor use. 'Sensors are being developed that remotely detect hazardous weather conditions, which eventually should allow aircraft to avoid encounters to improve safety,' Bogue said.

'Also, units that readily interface with bus structures allow sensors to be added with minimal effort, thus providing more information for flight applications.'