+Flying a Plane in Fog

Pilots early in their careers learn to be cautious in fog. When landing it’s possible to fly over an airport and see the runway and buildings very clearly and then in the last part of the landing, as the plane descends into the fog layer, for the visibility to be so reduced that it’s impossible to see to land. So we learn about fog and visibility very early on, and it comes under the heading of slant visibility if a fog layer is 50 feet thick it would be more than likely you could see through it from directly above.

Aviation law will state what equipment has to be available, how it is to be checked and calibrated and which aircraft can use it. Some of the requirements are listed here.

Correct runway markings and lighting as laid down
There have to be marker boards showing stop and holding points before the runway
Protected areas around transmitters of landing guidance.

Pre flight preparation Taxiing Taking off Landing

On reporting for duty a crew will be given or will get a weather briefing. According to the runway visual range (RVR) certain things will apply. First, the minima which are parts of the pilots operating procedure. Nothing can happen unless the RVR is above a certain amount and this is called the operating minima (or limits). These apply to take off, landing and making an approach to land. The RVR value is part of the minima laid down.

When taxi-ing to the runway the aircraft will be given, as always, the precise route to take. For instance ” Safeair 123 Proceed to runway 27 via Alpha Charlie and hold at block 22, over”. This means that the pilot should make his/her way to the westerly runway via the taxiways called Alpha and Charlie but not proceed further than block number 22. Extensive ground radar shows ATC where aircraft are even in the most limiting visibility.

When at block 22 the pilot will send a message saying where he is. at that point the aircraft will stop and await further clearance. Without ATC permission aircraft are not allowed to deviate from previous instructions. Some aircraft are fitted with a device called a PVD a para visual display which is a rotating ‘barbers pole’ indicator that rolls in one direction or the other, to show if the pilot is deviating from the centre of the runway. It’s very easy to use and is instinctive. As the plane becomes airborne the pilot will disregard anything outside and fly the plane by referring only to the flight instruments.

Before you can land … you have to make an approach and as you’d expect there are minima to be applied. It will surprise you … to be for the landing! This is because in foggy conditions the visibility can change from minute to minute so it would be silly to stop an aircraft approaching the airport when there’s a possibility that the visibility may have improved as the aircraft gets closer to the runway. In effect the rules allow an approach to a certain height but no lower, and if a landing is not allowed then the plane would climb away and either divert or go back into the holding procedure. If the weather is at or above minima the plane can continue to a landing IF at the decision point the captain can see the runway at whatever his landing minima require. However, nowadays many aircraft can fly right down to the touchdown and the pilot doesn’t need to see anything at all. This is a perfectly normal operation and totally safe.

Best wishes, Captain Keith

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