Features

Aviation safety is a fundamental concern for all stakeholders. The traveling public demands the highest safety standards, but also wants convenience and reliability at a low price with minimal environmental impacts. Taking account of these sometimes competing demands can be challenging.

Figure 1. Wake turbulence forms two counter rotating vortices separated by a little less than the wingspan of the generating aircraft.

A common cause of flight delay is the limited capacity of airport runways. One solution is to build more runways, but this understandably generates opposition from local people and often takes a lot of time. Another solution is to increase the capacity of existing runways. This is normally less costly and sometimes generates less local opposition.

To operate safely, aircraft must be separated from each other. The amount of separation required is determined by collision avoidance, wake turbulence, and other issues. Wake turbulence is generated when an aircraft generates lift (Figure 1). In general, heavier aircraft produce stronger wake turbulence and lighter aircraft are more vulnerable. If following aircraft are not sufficiently separated from the wake of the preceding aircraft, the turbulence could be sufficiently strong enough to result in violent aircraft accelerations. This is one reason why, for take-off and landing, all passengers and crew are seated and belted. In the worst case, wake turbulence could cause the encountering aircraft to lose control and crash. Wake separation standards and their associated procedures are designed to minimize the likelihood of this occurring.

Creating Standards

Figure 2. Evolution of Wake Separation Minima.

Historically, wake turbulence separation standards have been defined by a mixture of expert judgement and review of operational experience. This categorization was done by the International Civil Aviation Organization (ICAO) using three aircraft weight categories (Figure 2a). If subsequently the defined separations were found to be too small, perhaps by the review of wake incident reports, then increased separations were defined. A few years after the introduction of the Boeing 757 (a medium aircraft), the wake turbulence standards were modified to provide greater protection for light following aircraft (Figure 2b).

The initial wake strength of any aircraft can be calculated theoretically from aerodynamic principles. Once formed, the wake decays with time and is transported in space. The rate of decay depends mainly on the stability of the atmosphere and on the proximity of the wake to the ground. Wake is also transported laterally in the wind field and tends to sink vertically. However, it is very difficult to calculate absolute transport and decay rates theoretically, especially close to the ground, even with physics-based equations and supercomputing, with sufficient confidence to satisfy safety regulators.

In the mid-2000s, an innovative international project used wake measurements made by LIDAR (Light Detection and Ranging), which measures both the strength and the position of the wake turbulence from flying aircraft, and risk assessment methods to develop wake separation standards for the Airbus A380 within the context of existing separation standards defined by ICAO (Figure 2c). This successful work provided a framework for an objective, repeatable, and rational method of devising wake separation standards, and motivated international experts to consider if a wholesale redesign of the wake separation standards applied to all aircraft could deliver significant increases in runway capacity. The Wake Turbulence Recategorization Program (RECAT) was born.

NextGen

Figure 3. FAA Risk Matrix. Likelihood ranges from an event that is likely to happen once a week (A) to an event that is likely to happen less than once in 30 years (E). Severity ranges from minimal (5) through increasing levels of loss of control (4 to 2) to actual harm (1).

Under the FAA’s NextGen initiative, RECAT is planned in three phases. RECAT I delivered a new six-category system in 2012. The categories are based primarily on weight: Super (A380), Heavy, B757, Large, Small+, and Small. This replaced the “3+1” historical ICAO system of Heavy, Medium, and Light, plus A380 (Figure 2d). This is currently being deployed at airports across the US, with 16 having implemented it since November 2012, including Memphis International Airport and JFK in New York. RECAT II, active today, will initially define static pairwise separations, to the nearest 0.1 nautical miles, for the most common aircraft at major airports. It provides a matrix of 10,000 pairs of aircraft separations, but the aim is to extend this to all aircraft when data becomes available. RECAT II can only be fully implemented with significant enhancement of air traffic controller support tools. However, it can be implemented as a six-category system similar to RECAT I, for example by bespoke local aircraft categorization such that capacity for the local airport traffic mix is maximized. In the future, RECAT III will deploy dynamic (weather dependent) pairwise separation standards between aircraft.

The goal of RECAT is to reduce wake separations while not increasing wake risk beyond that experienced by the most exposed aircraft pairs today. Risk is the combination of how likely an event is, such as a wake encounter, and the severity of the event should it occur. For wake encounters, there is a wide spectrum of event outcomes, from very severe, which might cause an aircraft crash, at very low likelihood, to mild turbulence, possibly sufficient to spill drinks, at much higher likelihoods. This variation needs to be correctly accounted for and balanced in the safety assessment. This can be done using the FAA risk matrix (Figure 3), or by using other risk assessment tools.

Figure 4. Schematic diagram to show wake turbulence risk today and how it changes after RECAT.

Furthermore, under ICAO wake standards today, different aircraft types experience different levels of wake risk. It is more difficult to violently accelerate larger aircraft because of their greater mass, among other reasons. Thus smaller follower aircraft behind larger leader aircraft within a present- day ICAO weight category, which share a common wake separation minimum today, experience higher wake risk than other follower aircraft in the same weight category. RECAT makes use of this by reducing separations between some aircraft pairs to equalize the wake risk experienced by all aircraft pairs, while not increasing the wake risk for the most exposed aircraft pair today. This process is shown schematically in Figure 4. This strategy for demonstrating safety neatly avoids having to answer the difficult question: what level of absolute wake risk can be tolerated? The level of tolerable wake risk is decided by the highest level of wake risk that is tolerated today. Of course, there are many complexities under this relatively simple, high-level methodology, but these have been addressed by the expertise and experience of the wake scientists, aerodynamicists, operational experts, and safety specialists working in the FAA RECAT team.

Feedback from operators has been uniformly positive. One RECAT user, UPS Airlines, commented that they have greatly benefited from the FAA RECAT initiative with increased throughput for both arrivals and departures. Since the implementation of RECAT, UPS has been able to schedule more aircraft in their peak operating hours. This has also led to better volume management with increased volume availability during critical time periods in the arrival and departure sequence.

Following on after RECAT I, it is expected that there will be further operational benefits from RECAT II and RECAT III to be delivered in the next few years. Wake turbulence risk assessment will be fundamental to delivering these programs safely.

This article was written by Dr. Tim Fowler, Senior Principal Consultant, DNV GL (London, UK). For more information, visit https://info.hotims.com/61066-502.