Key outcomes:

  • Terrain induced wind shear and turbulence predicted
  • Operational aviation safety risks understood

St Helena’s £285,500,000 airport has received much attention since construction was completed in early 2016. The first flights to the island encountered high levels of wind shear and turbulence on final approach to the new runway. In the five months from May to September 2016, only eight flights braved the hazardous wind conditions at the airport and commercial airline services are yet to commence. In this study, Synergetics used computational fluid dynamics (CFD) to understand the complex nature of the wind environment over St Helena Airport and to provide insight that will help prevent a reoccurrence of similar problems at other airports located near complex terrain or large buildings. Similar studies can also be applied to optimise the location of wind turbines in complex terrain.

Figure 1: Computational fluid dynamics (CFD) was used to simulate unsteady turbulence and wind shear over St Helena Airport. In this video, regions of high turbulence intensity across the northern approach trajectory are highlighted in dark red.

The simulated results show that headwind approaches from the north experience the most severe wind shear effects, with mean wind speed changes of up to 20 knots occurring over a short distance along the approach trajectory. The high wind shear coincides with large turbulent wind speed fluctuations with a standard deviation of up to 6.2 knots.

Simulated atmospheric boundary layer over St Helena Airport

Figure 2: As wind flows over the island’s steep terrain, the crosswind speed profile undergoes a transition that is marked by winds slowing, reversing and recirculating at low altitudes.

With the benefit of hindsight, it is clear to see that the wind shear and turbulence hazards at St Helena airport could have been readily predicted by using CFD or wind tunnel testing at an early stage in the project. Such analysis could have assisted with the selection of a more suitable site for the runway and would have given stakeholders a more complete understanding of operational and commercial risks. Indeed, this analysis has been employed at many airports, including Amsterdam Airport Schiphol and Hong Kong Airport, and guidelines for identifying and mitigating wind shear risks are published by the International Civil Aviation Organization (ICAO).

A more detailed description of this study has been published in the March 2017 issue of Airports International magazine.