David Learmount/MALVERN

analysis of an aircraft wake-vortex study has encouraged the UK Civil Aviation Authority to consider sponsoring more detailed work on the subject. A sample analysis of extensive, but complex, data gathered by the UK Defence Evaluation and Research Agency (DERA), using laser-radar technology, has shown that wake vortices do not simply trail in a line behind aircraft, but can dip and rise in ways peculiar to specific aircraft types, the CAA says.

Analysis of a larger sample of the data obtained by the DERA will need to be commissioned, says the CAA, which sponsored the test programme and analysis completed so far. "The tests have added to our knowledge [of wake-vortex behaviour]. We hope to be able to take this forward from here," the CAA says.

The DERA-developed light-detection-and-ranging (LIDAR) system used for examining wake vortices in a real operational environment is, in principle, like primary radar, but uses low-power laser beams instead of high-intensity radio waves.

Radar and sonic technologies were tested also, but the LIDAR (also known as laser radar) was found to be by far the most effective, the CAA says.

The laser light hits particulates and vapour in the atmosphere and is variously scattered, reflected, absorbed, frequency-shifted or depolarised, says the DERA. A receiver collects and measures the Doppler shift in some of the returning light, to derive velocity information and build a picture of the airflow.

The DERA programme provided 3,000 recordings, using a portable LIDAR unit stationed at London Heathrow Airport. The subsequent sample analysis demonstrated that wake vortices show differences in their formation and in the way they dissipate, with strong variations between different aircraft types.

For example, Boeing 747-produced wake vortices were observed to dip markedly, then rise again before dissipating (see diagram).

There was no operational agenda in the wake-vortex study, the CAA says. The intention was simply "-to build up our knowledge" of how wake vortices behave in the atmosphere rather than in wind-tunnels. The CAA admits that, when more is known, operational use may be made of the wake-vortex information, but, at this stage, no action has been taken as a result of the sample analysis.

The DERA's Malvern unit continues to work with the LIDAR and is already offering a range of other potential uses, including cloudbase measurement, upper-wind tracking, windshear and turbulence warning and pollution detection. The LIDAR also shows potential for onboard aircraft operation as a true-airspeed detector, and for obstacle/terrain detection, and from space for global windfield measurement.

Source: Flight International