Flight Testing DELICAT - a Promise for Medium-range Clear Air Turbulence Protection

Henk Veerman, Senior Scientist, NLR, The Netherlands
P. Vrancken, DLR, Germany

Abstract

Atmospheric turbulence encounters are a major cause of injuries to passengers and flight crew in non-fatal airline accidents. A whole class of turbulence, representing 40% of turbulence accidents and designated as Clear Air Turbulence, cannot be detected by any existing airborne equipment, including state-of-the-art weather radar. Also the number of turbulence accidents has been growing since 1980, 3 times faster than the increase of air traffic.

Flight operational concepts for protection against turbulence hazards include:

• Short range (50 m to 300 m) measurement of air speed ahead of the aircraft and action on the aircraft flight controls to mitigate the effect of turbulence
• Medium range (10 km to 30 km) detection of turbulence and securing of passengers and crew members by seat belts fasten

Both concepts could be supported by UV LIDAR technology. The objective of DELICAT was to validate the concept of LIDAR-based medium-range turbulence detection.

Within the EC FP7 project DELICAT a UV LIDAR system was designed and manufactured for application in airborne environment. First the LIDAR was laboratory tested and ground tested scanning the atmosphere. Subsequently, it was installed in NLR’s Cessna Citation research aircraft and flight tested in atmospheric conditions from non-turbulence up to medium-turbulent level. During the flight tests the atmosphere was analyzed by the UV LIDAR in combination with aircraft on-board sensors. The collected data from aircraft sensors vs. LIDAR were compared after the flight. The correspondence between LIDAR backscattered energy fluctuations and turbulence experienced by the aircraft, for a given atmosphere volume was evaluated. The paper will discuss the flight test techniques needed for the project, i.e. a description of the instrument evaluated, installation of the instrumentation in the aircraft, the flight test plan, the execution of the flight test campaign. the measurement results and the flight test lessons learnt.

During the project challenges of various kinds were met. A heavy, powerful laser had to be installed into the aircraft cabin without compromising cabin and airspace safety. Also aircraft external modifications were made, such as vanes at a nose boom measuring airflow, a fairing enabling guidance of the laser beam from cabin into airspace ahead of the aircraft, exchange of standard cabin windows with dummy windows with inserts, mounting of a fast temperature probe, etc. The aircraft modifications required approval by the Dutch CAA leading to a Supplemental Type Certificate. Other challenges were design and aircraft integration of a beam steering system enabling the laser beam to direct into the flight direction on the aircraft whatever its attitude. Key to success was finally finding enough turbulent airspace encounters. This was realized in cooperation with European meteorological organizations Meteo France and ICM from Poland forecasting promising areas and time slots in European airspace.

In total about 40 hours of flight testing were executed in European airspace in which events of clear air turbulence were encountered. This enabled the team to demonstrate the working principle of the system, confirming that LIDAR technology can indeed detect clear air turbulence 10 kilometres ahead of the aircraft.