Travelers and flight crew members all want a safe and smooth flight that departs and arrives on schedule. However, certain weather conditions, like thick fog, cause low visibility conditions that present significant challenges. These issues are especially impactful on taxiing, takeoff, and landing operations. Let's explore the tools and procedures used in the aviation industry to ensure safe flying in fog.

How does fog wreak havoc on the ground?

According to the World Meteorological Organization, fog, as a recognized weather phenomenon, is the suspension of microscopic droplets of water in the air. It may cover a large area of a given airport, or form disjointed patches across the airfield. In the aviation industry, if such conditions result in horizontal visibility of less than about 3,281 feet (1,000 meters), then fog is contributing to low visibility.

Taxiing any aircraft, but especially the large, modern commercial airliners that we have come to know over the years, in dense fog can be immensely challenging. This is because fog can cause difficulty in assessing the aircraft's position on the airfield. Pilots may not be able to see all the runway lighting, and the tower controller experiences the same low visibility challenges as the pilots.

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In this scenario, the taxiing speed of the aircraft that are navigating the airport in such foggy conditions is reduced. This helps to ensure that the aircraft is staying on the right path. Additionally, when pilots are uncertain regarding their position at the airport, they may need to stop the aircraft and check in with ground control.

Given that, as detailed in the video above, fog played a role in the Tenerife Disaster, ensuring the safety of aircraft on the ground in such conditions is a high priority. Interestingly enough, however, foggy weather conditions do not present as much difficulty when an airplane is flying at altitude. Air traffic control is able to monitor radar signals that detect moving aircraft and provide direction.

Takeoff minima

In order to ensure that departing aircraft are taking off in suitably safe weather conditions, airports provide data points on the minimum visibility required to depart. This data is known as the takeoff minima. Airlines may also have their own minima, which may be influenced by the type of aircraft flown and available equipment.

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Takeoff minima are determined by the measurements at the touchdown, mid-point, and stop-end points on the runway, where sensors measure visibility distance levels. This set of three data points is referred to as the Runway Visual Range, or "RVR." If all three figures meet the takeoff minima, the aircraft may take off.

Approach minima follow similar rules

Like takeoff minima, there are also minimum standards for approach. One category of precision approach uses the Instrument Landing System (ILS), in which the aircraft detects signals projected from the runway and displays them on the flight deck screen. These signals are highly accurate and, therefore, reliably used in thick fog when flying the aircraft visually would be a much greater challenge.

Precision approach operations are defined according to the applicable Decision Height (which is measured at ground level), Decision Altitude (which is measured above mean sea level), and RVR data. In a Category I approach, the normal ILS approach, either a Decision Altitude or Decision Height may be used. The vertical minima are measured by reference to a barometric altimeter.

Airbus A320 Cockpit
Photo: Airbus

For Category II and III approaches, a greater level of precision is required. Decision Height with reference to a radio altimeter, which measures the height of an aircraft above terrain immediately below, is used to measure the vertical minima.

In the cockpit, flight crew personnel make sure that they always keep an eye on the weather conditions at the destination. As such, if the available information indicates that there will be foggy conditions awaiting the flight, ideally, there is enough time to plan. As a last resort, flights may be diverted to another airport for landing if there are adverse weather circumstances at the destination airport.

The world's foggiest airport

In a recent article, Simple Flying took a look at the world's foggiest airport. This title falls to Arcata-Eureka Airport (ACV) in northern California, which experiences dense fog or rain for an incredible average of 97 days per year.

Source: World Meteorological Organization