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How does fog affect a landing?

by Steve Landells Flight Safety Specialist

Whilst there are some days when aircraft just can’t land at a particular airport due to the visibility being so poor, most large airports and modern aircraft are equipped, and pilots are trained, so that landings are possible even in the classic pea-souper by using a combination of the auto pilot, ground systems and techniques that pilots learn during regular ‘low-visibility’ training sessions. But despite what you might think pilots and airports do not particularly relish the thought of doing automatic landings. Why?

Well there are a number of reasons.

From an airport point of view, fog slows things down. The number of aircraft they can land each hour reduces because they need to ensure that the systems on the ground are all working and the signals to the landing aircraft aren’t being blocked by those on the ground; none of us like reduced flow rates as this will inevitably lead to delays and late arrivals.

For the pilots the issues are different. ‘Auto Pilot’ is a bit of a misnomer. The auto-flight systems in an aircraft 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 when it comes to something where mistakes simply cannot happen, such as landing an airliner, the preparation 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.

Before commencing an approach in fog, the pilots have to make sure that they are appropriately trained, qualified and up-to-date to undertake automatic landings. Next they have to ensure that all the primary systems are functioning properly as well as the multiple back-up systems and they have to have a thorough brief between themselves as to what they will do if any part of the system fails – their actions will depend on the nature of the failure and at which point it occurs. Once the aircraft is set up, with the auto pilot engaged, then the initial approach is commenced. This process is the same as any other approach in as much as the pilots need to gradually reduce the speed of the aircraft and lower flaps and undercarriage as well as controlling the height and direction that the aircraft is travelling in.

Typically, aircraft will have four or five stages of flaps to lower (you will often hear whirring sounds at different times as you are coming into land and this is probably the flaps extending) but this has to be at specific speeds. If you are too fast when you lower the flaps you risk damaging them, but if you fly too slowly before selecting the next stage then the aircraft might stall. It’s the job of the pilots to control the speed and the flap selection to make the flight as safe and as comfortable as possible. At an appropriate stage the undercarriage will be lowered (you might hear the ‘clunk’ as the wheels lock down) and that is the start of the final approach.

You have to remember that this is all happening in cloud/fog so 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.

It’s not over once the aircraft is on the ground. You will still be travelling at over 100mph so the pilots have to slow the aircraft down (by selecting reverse thrust and braking) and then they have 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.

In modern aircraft it is rare for the systems to fail and, as a passenger, you may not notice the difference between an automatic landing and a manual one but for me personally it didn’t matter how often I did it, sitting there watching the aircraft land itself, with my hands hovering over the controls, instead of doing it myself was always a slightly peculiar experience.