As the driver of a car it is easy to recognise the impact of fog. When it is really thick, it is probably best to avoid driving rather than face the danger and delays caused by the adverse weather. But in surprisingly dense fog aircraft manage to fly. So, how does that work? 

It is all down to modern technology and the intense training and skill of the pilots. Pilots train for all eventualities and even when qualified for low visibility operations (fog), pilots must still undergo specific training and testing every year to be allowed to continue to operate in these conditions and to maintain high standards.

During training and testing, pilots are given various different emergency scenarios to deal with in fog, including aircraft problems (for example an engine malfunction) as well as the airport system suddenly failing. They also have to be able to answer various technical questions on the procedures during these yearly tests. Not all pilots are qualified for low visibility operations, although most European airlines do qualify their pilots as Europe suffers from foggy conditions quite often in the winter.

But for foggy weather technology is crucial. Without special equipment and facilities on the aircraft and at the airport, it is impossible to be able to fly in fog and not every airport has these facilities. 

Landing: 

A modern aircraft may be able to technically land with no visibility at all, but there is actually a minimum limit that is specified (usually 75metres visibility) so that after landing the pilot can taxi the aircraft to the terminal safely.

The auto-flight systems in an aircraft that are used in foggy landings are simply a whole load of computers that talk to each other, and like any computer, if you put rubbish in you will get rubbish out. So, preparing these computers for a landing in foggy weather is comprehensive and time consuming – unlike on a clear day when the pilots can simply look out of the window and take control if something goes wrong. If it’s foggy, we rely on the computers to do what they are told to, and if they don’t then the only option is to abandon the approach and climb away from the ground – this is known as a go-around.

During a foggy weather approach the pilots constantly need to ‘instruct’ the aircraft what to do through the flight management system (another computer) without visual reference to the outside world, and they must always be ready to take control if anything doesn’t look right inside the cockpit. At 1,000 feet (about three miles or one minute before touch down) the pilots cross check everything and decide whether it is safe to continue the approach. If the decision is to continue, they will both monitor the flight path and engine parameters on the instruments and one of them will have his or her hands resting lightly on the controls ready to take over if required. In the last few seconds certain things have to happen (for example the auto pilot needs to reduce the rate of descent and the system has to tell the pilots that it is capable of keeping in a straight line once on the runway) and if they don’t, then the pilots will elect to go around, after which they will diagnose what went wrong and, if possible, rectify the fault for a further approach.

On the ground:

And it is not over once the aircraft is on the ground… the difficult task of locating the taxiway and finding their way to the terminal building – sometimes this needs the assistance of a ‘follow me’ truck which is a brightly coloured vehicle with flashing light that the pilots can follow, very slowly, to ensure they go the right way.

Take-off: 

Aircraft cannot take-off using the autopilot, so there is a minimal visibility for take-off (usually 125 metres) so that the pilot can see to keep the aircraft straight in the centre of the runway as it accelerates but also has be able to bring the aircraft to a stop safely should an emergency arise.

Delays and Disruption

One of the extra limitations for take-off and landings in fog, is that there has to be a bigger separation (time) between aircraft so that means at busy airports, such as Heathrow, not as many aircraft can take-off or land each hour compared to normal conditions which is why fog brings delays.