Acoustic technology for aircraft wake vortex detection

Aircraft in flight generate a pair of counter-rotating vortices in their wake, each vortex forms near a wing tip, and persists in the atmosphere for some time before dissipating. This trailing vortex pair can contain intense velocities that can cause following aircraft to roll, rapidly descend, or suffer structural damage. As a result, the Federal Aviation Administration requires mandatory separation distance/time standards between all aircraft to avoid wake vortex encounters. However, during takeoffs and landings these separation standards are sometimes over conservative, and act to limit airport capacity. If aircraft spacing distances could be reduced, it would be a great financial benefit to the airline industry.

A ground-based wake vortex detection sensor that indicates the presence of absence of wake vortices has been developed and tested. Field measurements have showed that the system can accurately detect the magnitude and direction of wake circulations of a Piper PA-32 aircraft in ideal atmospheric conditions. Current investigations are focused on the operational capabilities of this technology, specifically in various atmospheric conditions.

Recent attempts to reduce FAA mandated aircraft spacing distances have focused on using meteorological categories and the associated vortex decay rates to create various categories of spacing distances. This measurement technology provides an alternative methodology. This technology detects circulation by measuring the transmission time of acoustic pulses propagating in a closed path around the vortex circulation. Since propagation speeds of acoustic pulses are affected by the velocity of the medium in which they travel, this measurement senses the in-line velocity component for the entire perimeter of the designated path. The net circulation contained within a closed path is directly related to the in-line velocity component around the perimeter of the path. By setting up an acoustic path that encloses an aircraft wake vortex, we are able to calculate the magnitude and direction of the vortex from these travel times.

This technology differs greatly from other technologies being utilized for wake vortex detection, and shows promise for possible commercial development, operational use, and further research. The technology is simple and relatively inexpensive. With a working ultrasonic frequency of 57 kHz, it poses no interference to other airport operations and communication equipment. Calculation of instantaneous speed of sound is achieved by transmitting our acoustic pulses in opposite directions around the same path, and thereby eliminating the need for measurements of atmospheric variables such as temperature, pressure or humidity.

Current research is investigating the operational scope of use for the technology, including evaluating in-situ levels of circulations due to convective instability and ambient winds. Our current study compares theoretical values of in-situ circulations for various meteorological conditions to actual data. Vortex decay rates and lateral transport rates are compared for convective and stable environments, and with an ambient cross-wind. Also discussed are methods to account for in-situ variations in atmospheric circulations, and separate from the aircraft circulation measurements.

This sensor technology has numerous application possibilities, including operational use at airports for active management of runway spacing distances, further theoretical study of the behavior of wake vortices in ground effect and their decay rates in varying meteorological conditions, and the study of vortices produced by other vehicles such as helicopters.