EXECUTIVE SUMMARY:

The Turbulence Problem
Turbulence is a major hazard for aviation. Despite the continuous avionics technology progress (including the weather radar), the number of turbulence accidents has increased by a factor of 5 since 1980. Part of this is due to the increase of traffic, but the rate of accident per million flight departures has also increased by a factor of 2 since 1980. For the aviation transportation industry as a whole, the total cost is estimated over 100 M$ per year.

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. This kind of turbulence is linked to large amplitude gravity waves (caused by wind flow over mountains for example) or to strong vertical shear of horizontal wind (Kelvin-Helmholtz instabilities).

Existing standards (MIL-F-8785, MILHDBK1797) describe the behaviour of turbulence, in term of Power Spectral Density or discrete gust. Those standards are widely used and accepted by the aeronautical industry. However, scientific studies, based on both theoretical and experimental data, propose Power Spectral Densities that are slightly different from the standards.

There exist several criteria for turbulence severity classification. Some criteria are aircraft dependant (qualitative description by aircraft behaviour, aircraft acceleration), some are intended to be independent of the aircraft (probability of appearance, Derived Equivalent Gust Velocity, Eddy Dissipation Rate).
The ICAO severity classification is based on EDR.

Clear Air Turbulence Protection
3 operational concepts for turbulence protection have been identified, each corresponding to a given range of action of the equipment. The characteristics of those concepts are presented in the following chart.

RANGE	CONCEPT	REQUIRED DATA	DISTANCE TIME	SAFETY CRITICAL FUNCTION
LONG RANGE	Avoidance of turbulence encounter	- Severity, position and dimension of turbulent area - Short term evolution of turbulent area and severity	>2 minutes > 30 km	NO
MEDIUM RANGE	Protection of passengers and crew by seat belts fasten	Turbulence detection (for a severity threshold) and time to encounter	30 s to 2 minutes 8 km to 30 km	NO
SHORT RANGE	Protection of aircraft and passengers by mitigation of the turbulence effect with Flight Controls	3 axis air speed ahead of the aircraft	0.2 s to 1 s 50 m to 300 m	YES

The long-range application cannot be fulfilled by any detection equipment, given the current technology status and expected mid-term evolution. Possible solutions could be provided by ground predictions such as provided by the CAT WIMS studied in FLYSAFE WP2.2.3.
However, the UV direct detection Rayleigh LIDAR is a good candidate for medium range and short-range operations.

• The medium-range CAT detection is based on air density fluctuations measured through backscattered energy fluctuations of the LIDAR signal
• The short-range operation is based on 3-axis wind velocity measurement ahead of the aircraft, obtained by spectral analysis of the Doppler shift of backscattered LIDAR signal

The analysis of LIDAR technologies, regarding the main components (laser source and detection device), taking into account the requirements of aircraft installation (size, power, environment) indicates that 2 different maturity steps may be considered, the 1st one corresponding to state of the art of industrial development, available within 5 years for airborne applications, the 2nd corresponds to laboratory developments, commercially available within 10 years.

Performance Analysis and validation
The calculation of LIDAR performance for both short range and medium range applications, for both 1st and 2nd steps of technology maturity level, lead to the following conclusions:

Short-range performance with the first technology step may not be sufficient in regard of requirements for coupling the equipment to the aircraft flight control.
The second technology step will allow a large improvement of the measurement performance, making them suitable for an airborne application.

The medium-range performance is interesting even at the first step, allowing a 1-minute before encounter warning
The second technology step will improve the situation, by providing whether a longer warning time (up to 2 minutes) or a lower FAR

In both Medium and Short Range application, the validation of the equipment should include flight tests, first on a research aircraft to perform functional validation (Technology Readiness Level (TRL) 4/5) then in commercial transport aircraft, as passenger in shadow mode for prototype validation (TRL 7).
For the functional validation of medium range operation, flight tests should be performed with a large support from meteorological services in order to maximise the CAT encounter events.
 
For further information please contact:
Mr. Herve Barny: herve.barny@fr.thalesgroup.com