Friday, October 20, 2017
Transport Canada - Aviation Safety Letter Flight in the Vicinity of Convective Weather

Flight in the Vicinity of Convective Weather

by Thomas Smyth, Civil Aviation Safety Inspector, Airline Standards, Standards, Civil Aviation, Transport Canada

Since the earliest days of flight, convective weather has posed many risks to commercial and general aviation. Whereas some convective weather systems present little risk to aviation activities, others are extremely hazardous. The focus of this article will be on the extreme events such as thunderstorms, and specifically during the approach and landing phases of flight.

The summer season in Canada can offer some of the most enjoyable flying conditions of the year. However, the heat of the summer also provides the energy for the formation of powerful air-mass and frontal thunderstorms. The chances of encountering a thunderstorm will largely depend on where you are flying within Canada. The Yukon Territory, Nunavut Territory, Northwest Territories, and British Columbia have the fewest number of thunderstorms each year. Alberta through to Nova Scotia, on the other hand, average 20 to 30 days of thunderstorms each year with some localized areas getting more, such as southern Saskatchewan, which may experience daily thunderstorms during the month of July.  Thunderstorms are evolving phenomena as they have several stages to their life cycle, which generally last one to two hours. The first stage, called the developing stage, is characterized by updrafts that can be seen as bubbling cumulus cloud with rapid vertical growth. The mature stage follows and is the most likely time for hail, heavy rain, and frequent lightning. The dissipating stage signals the end of the life cycle, but by no means the end of the hazards generated by the thunderstorm since strong winds and lightning can still be present. Pilots should be aware of the characteristics of each stage—along with their associated hazards—and learn to recognize them while flying.

During their initial training, pilots are taught about the power and dangers of thunderstorms. They are also taught that aircraft of any size are no match for a thunderstorm, as many accidents have proven. Windshear, microbursts, hail, heavy rain, lightning, and reduced visibility are some of the hazards of thunderstorms that may be encountered during the approach and landing phase of flight. The consequences of flying through such conditions can range from experiencing uncomfortable turbulence to crashing short of the runway. We can reduce the risks of thunderstorms to an acceptable level by giving ourselves the tools to make informed decisions so that we can manage the threat appropriately. These tools include weather information gathered prior to flight and especially en route, onboard weather detection, alternative plans that are devised prior to the flight, and of course, common sense. The objective is to prepare for the flight so that we are not forced into a situation where we need to make a critical decision under a great deal of stress because, as we all know, such decisions are not usually our best.



Whether you are flying using instrument flight rules (IFR) or visual flight rules (VFR), the hazards from thunderstorms during the approach and landing are the same. Let’s look at an example: An approaching thunderstorm will be preceded by a gust front as cold, dense air descends from the expanding storm and strikes the ground, moving outward at speeds that can approach 50 kt. The gust front creates medium-to-severe turbulence, which can cause small, general aviation aircraft to momentarily depart controlled flight and larger aircraft to experience some very uncomfortable turbulence. As the thunderstorm and the aircraft move closer to the airport, the level of risk will increase, especially as the aircraft descends for a landing. If caught in a microburst, the aircraft would have great difficulty recovering given its close proximity to the ground. In the recent past, this phenomenon has contributed to several fatal accidents involving large aircraft.

With thunderstorms nearby, even being on the landing roll can bring significant challenges. Rapid changes in wind velocity and direction can change a 20-kt headwind to a 20-kt tailwind very quickly, leading to the aircraft touching down well past the normal touch-down point. The problems of a long landing can be compounded by deteriorating runway conditions because a runway that was dry only minutes before can not only become wet from a heavy downpour, but can reach the point where the runway is considered contaminated (more than 3 mm of water). Hydroplaning and loss of directional control, particularly if there is a crosswind, can be expected in these conditions. There have been numerous accidents in which pilots had the good judgment to navigate around storm cells while approaching the airport, but then landed in heavy rain and overran the runway due to contamination of the runway surface by water.

Pilots should also be aware that airports currently do not have the technology to detect if runways are contaminated by water, but can only report that the runway is wet.

Now for the human factors side of this issue: for all the positive characteristics we pilots have, our focus on the final objective can sometimes override common sense. It would be difficult to find a pilot who thought that landing at an airport during a thunderstorm was a good idea, but even with all we know about this topic, it still happens, and often with tragic consequences. Poor decision making can be caused by many factors, such as fatigue, lack of information, and inexperience. To a pilot, a thunderstorm is an obstacle in the way of the ultimate goal of the flight, which is arriving at the destination. When a thunderstorm is encountered en route to the destination, there is usually a way around it. However, when the thunderstorm is positioned close to an airport, and the aircraft is on final approach, it is easy to say “The aircraft ahead of us landed safely; we should be fine as well,” because the goal of the flight is so close. Still, the weather conditions can quickly change for the worse, making a successful landing all but impossible. Each situation is unique, and just because you were able to make the approach and landing in similar conditions last time does not mean that luck will be on your side this time.

Windshear- and weather-detection technology that is available for airports and on board aircraft is helping to reduce the risks associated with convective weather by giving pilots and air traffic controllers the information they need to make informed decisions and avoid the potentially catastrophic results of flying into convective activity. Technology, though, is not the final solution to the risks presented by thunderstorms, but along with education and awareness, the risks can be greatly reduced, and flight operations in the vicinity of thunderstorms made as safe as possible.

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