Overview
An air data system usually consists of a primary system that includes three redundant channels and has, in addition, a separate stand-by channel.
Traditional air data standby channels are composed of pitot tubes and pressure ports, which deliver parameters such as airspeed and pressure altitude. The standby channel has generally neither a temperature probe nor an angle of attack probe. The standby static pressure probe location on the fuselage is selected so as to limit the influence of sideslip.
Air data standby channels are therefore composed of equipment very similar to that encountered on the primary channels: pitot probes, static probes and pneumatic tubing.
The main reason for aircraft accidents to be linked to air data systems is probe obstruction due to icing problems, volcano ash or bugs, although some accidents have occurred due to the failure of pneumatic connection after maintenance operations.
Even though efforts are made by manufacturers to design dissimilar air data channels, there is no reason that external aggressions such as ice, ash or bugs will independently affect the primary air data system probe and the standby air data system probe.
The purpose of NESLIE was to demonstrate that a LIDAR-based (light detection and radar) air data standby channel will help to suppress the major drawbacks of existing pneumatic systems whilst maintaining the performance required by the related standards.
The aim of the NESLIE project was to develop a 3-axis laser function for air data standby channel for implementation on civil aircraft on the horizon 2010. Actual air data standby channel is composed of individual probes and pressure sensors. Air data standby channels information delivers vital parameters for the safety of the aircraft's flight such as air speed, angle of attack and altitude. But since primary and standby channels have similar failure modes, the use of laser based standby architecture with drastically different failure modes must increase aircraft's safety by reducing the probability of common failures.
LIDAR allows the implementation of a measurement principle that is very different from existing systems. The sensitivity of LIDAR to wear and pollution differs from traditional pneumatic systems because there is no part of it outside the fuselage (the LIDAR window is mounted flush with the fuselage); in fact, the presence of air pollution-like droplets or ash improves the LIDAR signal.
The LIDAR can be installed in a large range of available locations on the fuselage whereas standby channel probes should be installed on locations where there is minimal sideslip effect. NESLIE should demonstrate that it is possible to design an entire LIDAR-based air data standby channel with few or no pneumatic measurements.
The NESLIE project provided a non-protruding probe to measure air speed, angle of attack and slide slip angle in comparison to traditional probes, which have protruding parts subjected to external aggressions.
The design of a Lidar based air data standby system optimised in term of size and weight, allowing at least true air speed (TAS), angle of attack (AOA) and side-slip angle (SSA) measurements. The laser detection range being quite short, from 10 to 100cm.
- The use of guided optics technology (integration on substrate) so as to guarantee compactness and cost reduction;
- The design and manufacture of needed technological bricks (ie. separator, commutation, pumps hybridisation, low noise detector, low noise power supply) and merge into a minimal number of modules;
- A system analysis and definition of sub-systems to be integrated in a standby instrument;
- The design and optimisation of an adapted signal processing.
Funding
Results
The NESLIE project objectives have been met:
- The Optical Air Data System (OADS) architecture has been specified and designed.
- The technological bricks have been developed and validated (integrated laser, integrated optics, optical head and windows, real time signal processing, air density acquisition theoretical study.
Only the IR detector was not developed, but a back-up solution was defined and used, so XenICs failure had no consequence on the overall project achievements.
- A LIDAR mock-up was integrated and flight tested.
The flight tests results validate the Optical Air Data System concept:
- The mock-up provided measurements in all atmospheric and altitude conditions.
- The analysis of the mock-up measurements show very interesting performances with respect to the specification objectives.
The NESLIE project showed very promising results and represents a large progress with respect to the state of the art in Optical Air Data System.
Technical Implications
The analysis of the flight tests data gave the following results:
- The NESLIE mock-up was operational and provided air speed measurement for the whole flight campaign, in all altitude and atmospheric conditions. In addition, the number of detections complies with the models describing the aerosol content in the atmosphere.
- The 4-measurement axis provided measurements that were in very good agreement with each other in most of the time. This demonstrates the very good performances of the mock up, independently from the local aerodynamic conditions at the mock-up measurement point.
- The comparison with the aircraft system data, after some calibration operation to compensate for the local aerodynamic conditions, demonstrates the very good coherence between the NESLIE mock-up and the aircraft data. This also applies to large dynamic maneuvers, including abrupt variations of the Angle Of Attack and the Side Slip Angle.
The analysis of the flight tests results showed clearly that the technology developed within the frame of the NESLIE project is very promising regarding its potential use for future Air Data Systems applications.
Readiness
In addition to those results, a large signal database is now available for the development of improved signal processing algorithms in the frame of the on-going DANIELA FP7 project.