Travelers and crew members all want a safe, smooth flight that departs and arrives on time. However, certain weather conditions, such as heavy fog, cause low visibility conditions that present significant challenges. These problems have a particular impact on taxi, take-off and landing operations. Let’s explore the tools and procedures used in the aviation industry to ensure safe flight in foggy weather.
How does fog wreak havoc on the ground?
According to the World Meteorological Organization, fog is the suspension of microscopic water droplets. It can cover a large area or form disjointed patches on the airfield. In the aviation industry, if such conditions result in horizontal visibility below approximately 3,281 feet (1,000 meters), then fog contributes to low visibility.
Taxiing in dense fog is difficult as it can be difficult to accurately assess the aircraft‘s position on the airfield. Pilots may not be able to see all of the runway lighting, and the tower controller faces the same low visibility issues as pilots. In this scenario, the taxi speed is reduced to ensure the aircraft stays on the correct course. Additionally, when pilots are unsure of their position at the airport, they may need to stop the aircraft and check in with ground control.
Fog challenges on takeoff and landing
Interestingly, fog doesn’t 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.
Airports provide data points on the minimum visibility required for take-off, called take-off minima. Airlines may also have their own minima, which may be influenced by the type of aircraft used and the equipment available.
Take-off minima are determined by measurements at the touchdown, mid-course and stopping points on the runway, where sensors measure visibility. This set of three data points is called the runway visual range, or “RVR.” If all three numbers meet the take-off minima, the aircraft can take off.
Like take-off minima, there are also minimum approach standards. A 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 cockpit display. These signals are very accurate and therefore reliably used in heavy fog.
Precision approach operations are defined based on the applicable decision height, measured at ground level, decision altitude, measured above mean sea level, and RVR data. In a Category I approach, the normal ILS approach, either a decision altitude or a decision height, may be used. Vertical minima are measured by reference to a barometric altimeter.
Pilots have instruments that facilitate landing in low visibility. Photo: Airbus
For Category II and III approaches, a higher level of precision is required. Decision height in reference to a radio altimeter, which measures an aircraft’s height above the terrain immediately below, is used to measure vertical minima.
Flight crew personnel keep an eye on the weather conditions at destination, so if available information indicates that there will be foggy conditions awaiting the flight, ideally there is ample time to plan. As a last resort, flights may be diverted to another airport to land in the event of adverse weather conditions at the destination airport.
Source: World Meteorological Organization