Abstract
Wind shear at low altitudes represents a potential hazard to landing aircraft. Based on two wind lidar data sets of one year, the occurrence of low-level jets (LLJs), the vertical wind shear and the rotation of the wind direction were analysed. The lidar system was located at the sites of Braunschweig in the North German Plain, Germany, and Clausthal-Zellerfeld in the low mountain range Harz, Germany. The observed wind shear gradients between the altitude of 40 m and the altitude of the maximum wind speed was in the range of −0.23 s−1 to +0.20 s−1. The rotation of the wind direction with altitude occurred both in clockwise and anticlockwise direction. The ratio of clockwise versus anticlockwise occurrence of directional shear was 4:1 for Braunschweig and 3:1 for Clausthal-Zellerfeld. The observed wind shear gradients were compared to values for hazard potential of different levels for a typical aircraft. Although the LLJ was not hazardous for manned aircraft in any observed case, the awareness of LLJ helps to reduce the pilot’s workload and possible pilot-introduced oscillations caused as a result of the wind shear and aircraft characteristics. In contrast to manned aviation, the value of changes in wind speed and direction during LLJ conditions can cause significant risks for unmanned aerial system operations with less than 25 kg of take-off weight. This is a result of the lower airspeed-wind-speed ratio and the flight control and flight planning.
Highlights
Strong vertical wind shear can have two main negative and potentially hazardous effects on landing or departing aircraft: the disturbance of the flight path, such that the actual flight path deviates from the nominal flight path, and the deviation of the instantaneous approach or take-off speed from the nominal value
The goal of this study is to estimate the occurrence of hazardous conditions due to vertical wind shear associated with level jets (LLJs) for commercial aircraft and unmanned aerial systems (UAS), based on observational 1-year data sets at two different sites
Around one-fourth of the LLJ events observed at Braunschweig were associated with a rotation of the wind direction to the left
Summary
Strong vertical wind shear can have two main negative and potentially hazardous effects on landing or departing aircraft: the disturbance of the flight path, such that the actual flight path deviates from the nominal flight path, and the deviation of the instantaneous approach or take-off speed from the nominal value. Both cases assume a 3◦ glidepath as typically guided by the instrument landing system, and an initial landing speed of v LD. In the first case (Figure 1, left) the aircraft is subject to wind shear with a decreasing headwind The further it approaches the ground, the more the airspeed (speed of the aircraft relative to the surrounding air flow) is reduced and lift is reduced, subsequently leading to a larger descent angle as a result of the transient force imbalance
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