Aircraft Wake Vortices Affecting Airport Wind Measurements

Jan Krummen; Lena Noelke; Raphael Monstein;
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Abstract

The influence of wake vortices on other aircraft has been extensively studied and is well understood. However, to date, it has not been investigated how wake vortices can affect wind velocity measurements at airports. This study investigates this previously overlooked issue, focusing on departures from runway 34 at Zurich Airport. These departures are suspected to affect a wind sensor situated at the runway’s end. Through a combination of visual identification and application of a wake vortex model, instances where wakes affected the said anemometer were identified for a fifteen-month period. Analysis of the resulting data shows that approximately 5% of all departures generated such occurrences. In addition, specific wind conditions and aircraft types were identified as being necessary for such events to occur. The observed cases of wake hits have a significant effect on the wind measurements, altering even averaged values used by air traffic control for clearance by several knots and up to 50 degrees. That, in turn, can result in cases where aircraft performance does not allow take-off based on altered wind readings, even though the actual wind conditions would not prevent the departure. Such cases have the potential to cause significant disruption to flight operations. Even though this paper focuses on this phenomenon at Zurich Airport, similar issues are likely to occur at other airports as well.

Introduction

Aircraft wake vortices, also known as wake turbulence, are counter-rotating columns of air that form downstream of the wingtips as a direct result of lift generation [Gerz et al. 2002]. These vortices dissipate relatively slowly and can pose a significant hazard to other aircraft that encounter them, even minutes after their initial formation [Holzäpfel 2017]. In terminal airspace, the hazard of encounters is thereby considered to be particularly high due to the proximity to the ground during the take-off and landing phases, resulting in little time for reaction and recovery [Sammonds et al. 1976]. To mitigate the risk of such encounters, wake turbulence separation standards [Authority 2016] have been established. They specify minimum separation distances between successive approaching and departing aircraft based on their respective wake turbulence categories. These separation standards govern current operations in the airspace around aerodromes and are one of the main factors limiting airport capacity [Gerz et al. 2002]. To address this limitation, the International Civil Aviation Organisation (ICAO) initiated the wake turbulence re-categorisation (RECAT) to optimise the pairwise separation standards [Cheng et al.; Rooseleer and Treve 2015]. In addition to RECAT, other current research includes work on plate lines, also with the ultimate goal of increasing capacity. Plate lines are an array of vertical plates placed in front of a runway to accelerate the decay of wake vortices and, thus, to reduce the required wake separation [Vechtel et al.]. Overall, the physics of wake vortices is well researched and understood and the field is still active. However, while many aspects of how wakes impact airport operations are well-known and studied, the effect of wake vortices on airport wind velocity measurements has been overlooked.

When issuing a take-off or landing clearance, air traffic control provides the flight crew with information about the prevailing wind conditions at the relevant runway. Accurate wind information is considered safety-critical as aircraft are restricted to operations within specific wind limits, mainly dictated by the performance of the aircraft. Given the importance of accurate wind measurements, large airports usually host multiple anemometers that can be used as the source for up-to-date wind data representative for the relevant runway. ICAO provides guidance on meteorological measurement systems in Document 9837 [International Civil Aviation Organization 2011]. This document recommends installing anemometers at a height of 10m above ground and outside of obstacle clearance areas. The document states that "whilst wind sensors should be located close to the runway(s) to achieve representative wind measurement, every effort should be made to site the sensors to minimise the effect from artificial gusts, e.g. due to jet efflux or wake vortices" [International Civil Aviation Organization 2011:3.6].

Nevertheless, one of the authors recently learned that at least one of the anemometers at Zurich International Airport, Switzerland, is suspected to be likely affected by wake vortices from departing aircraft on runway 34. The concerned anemometer is located in proximity to the thresholds of runways 14 and 16, approximately 215m and 170m away from their respective centrelines, as illustrated in Fig. 3. The sensor logs wind speed and direction measurements at 3-second intervals, which serve various applications. Amongst them is the computation of a 2-minute moving average that is displayed to air traffic controllers and is used in take-off and landing clearances.