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Writer's pictureG. Rhodes

A Look at Fog and Aviation Operations


Fog reduces visibility and poses challenges for aircraft takeoff, landing and ground movement.

Fog is a cloud of water droplets suspended in the atmosphere at or near the Earth’s surface. It reduces visibility and can become a true hinderance to aircraft operations. And for airports, the reduced visibility can result in disruptions and may even cause cancellations to flight operations. According to the BBC Science Focus Magazine, fog consists of water vapor molecules, suspended in the air as tiny water droplets, which linger close to the surface. Essentially, fog is just a cloud that touches Earth’s surface and forms in the same manner as clouds themselves. High humidity is a major contributing factor to the formation of fog and, according to the periodical, a sudden cooling of warm. humid air means that water vapor clumps together into clouds that reflect light. This is helped along by dust or other forms of air pollution. This fog creates a visibility problem for everyone, but for pilots it’s more serious. Fog means poor visibility low to the ground, making it hard to visually judge location, altitude, and particularly a proper landing approach.


Pilots skillfully navigate a complex landing in heavy fog, relying on illuminated cockpit instruments.

According to the US National Weather Service, fog triggers different flight rules in different conditions. Instrument Flight Rules (IFR) or Instrument Meteorological Conditions (IMC) applies when ceilings are below 1,000 feet above ground level (AGL) and/or visibility is less than three miles. Marginal Visual Flight Rules (MVFR) are applicable with ceilings from1,000 to 3,000 feet AGL and/or visibility of three to five miles. Pilots may operate under Visual Flight Rules (VFR) with ceilings greater than 3,000 feet AGL and visibility greater than five miles. To be an airline transport pilot in the United States, one is required to have an IFR rating, so fog does not mean aircraft cannot fly. However, more spacing is required between airplanes, therefore takeoff and landing capacity can be impacted at airports, causing delays and potential cancelations at the busiest hubs.


Illuminated runway lighting systems augment the ILS by guiding aircraft to the touchdown zone.

Major airports employ Instrument Landing Systems (ILS) that are designed to safely guide aircraft down from altitude through the clouds and fog to a safe landing.The ILS is a precision radio navigation system that provides short-range guidance to aircraft, allowing them to approach the runway at night or in bad weather. The minimum altitude requirement for both Category I and Category II ILS systems is 200 feet above the ground at which point the runway should be visible to the pilot. But for airports equipped with Category IIII-A and Category  III-B ILS systems, the altitude required to visually see the runway goes down to 100 feet or 50 feet, respectively. However, not every airport is equipped with the Category III systems and special aircrew certification is required in order to use them. 


Ice forming on the wings often leads to potential hazards affecting aerodynamics and flight safety.

Can fog itself be physically harmful? The answer is yes when it's ice fog. Ice fog occurs when water droplets form fog and the temperature is below freezing. At that point, the fog itself becomes crystals of ice which can freeze onto the airplane and disrupt aircraft aerodynamics by adding drag and weight. Icing is caused by the presence of Supercooled Water Droplets (SWD). It is these droplets that are most dangerous to aircraft - when supercooled water droplets make contact with an aircraft, they freeze instantly because the surface of the aircraft becomes the freezing nuclei. When ice builds up on the leading edges of the wings, it disrupts the smooth airflow that is required to generate proper lift. This results in a loss of lift and an increase in drag, causing the aircraft's performance to become highly inefficient. This also increases the stall speed of the aircraft by reducing the stall angle of attack.


Cathay Pacific's policy dictates that low visibility landings must use the aircraft’s automated systems.  

According to Captain James Toye, Line Operations Manager for Cathay Pacific Airways and a Boeing 777 Captain, the most significant phase of flight that fog affects is taxiing. “A lot of the ground control from the tower is visual – and it’s hard to direct aircraft taxiing around if you can’t see them,” he explained. “In low visibility it’s not just about leaving the runway quickly, it’s also about finding the turns on the taxiways. I’ve been in conditions where you can only see a few lights ahead of you, and these are spaced at 60 to 100 foot intervals. Looking for one reference point can be disorientating, and it has to be done slowly." Toye also explained that Cathay Pacific’s policy is to use the aircraft’s autopilot that links up to the airport’s ILS for a safe landing. There is still a decision height however for a pilot to make visual contact with the runway.


Ramp tower controllers oversee aircraft movement aided by multiple screens displaying flight data.

In addition to the main air traffic control tower, ramp towers also play an important role, particularly during fog. The purpose of a ramp tower is to manage ground operations, including directing aircraft from the gate to the taxiway, ensuring efficient taxiing, and coordinating ground traffic within the airport's ramp area. This helps to optimize aircraft flow and minimize congestion on the ground. At busier airports, it’s sometimes necessary to have separate ramp towers because of the increased demands on ground communications with taxiing aircraft. In foggy conditions, ensuring safe operations on the ground certainly adds to the already stressful job of working in air traffic control. When visiting a busy airport, it's worth keeping an eye out for a black and yellow non-movement area boundary line. This line serves as an essential marker, indicating where the ramp tower's jurisdiction ends, and air traffic control's responsibility begins.


United's Virtual Ramp Control Tower in Houston is the first such tower used by a commercial airline.

Besides traditional ramp towers, some airports and airlines are exploring the use of Virtual Ramp Control Towers (VRCT), using cutting-edge technology to enhance efficiency and coordination. For example, United Airlines is using virtual ramp control at some airports to streamline aircraft movement on the ground. VCRT is a system that uses technology to provide a view of an airport's gates and aprons without the need for a physical tower. VRCTs are becoming more popular at US airports because they can address safety challenges and limited ramp space. VRCTs use technology such as: high-resolution digital cameras, aircraft tracking systems, air-to-ground radios, surveillance and meteorological sensors, and microphones. VRCTs can be set up in a room anywhere on the airport, and the camera systems can be built into existing terminals or new structures. They can also integrate with airport surface detection equipment to provide controllers with information about aircraft flight numbers and destinations.


New technologies such as VRCT will enhance safety for airlines and their passengers and will help to limit the visibility dangers presented by foggy conditions.


Until next time...safe travels.








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