BA PEOPLE • March 2016
Fog can be a bewildering problem for frequent flyers. Surely today’s hi-tech flying machines can land without relying on such crudities as good visibility? British Airways’ Senior First Officerand Manager of Air Traffic Control (ATC), David Wood explains why it is that low visibility can cause delays
How does an aircraft land in fog?
As pilots, we would always prefer to land the aircraft manually. However, when visibility drops below approximately 0.5km, we have to rely on the autopilot to land it. The aircraft receives information from something called an Instrument Landing System (ILS), which is based on the ground and transmits a signal that the aircraft locks on to for the approach and landing. The aircraft can land in zero visibility, but the pilots still have to be able to taxi the plane off the runway and to the stand. Conversely, we do not rely on the autopilot for departure because all take-offs are flown manually, even in fog.
Why does fog cause such disruption?
Firstly, I think it is important to say that there are only a few days each year where Heathrow or Gatwick has an issue with fog, but it’s often widely reported in the media. In low visibility, part of the procedure is to increase the distance between aircraft coming in to land because this ensures the ILS signal is not interrupted by other aircraft that have just arrived. In foggy conditions, aircraft move around the airport more slowly, and given that we have to make sure the area is clear before others can land, it takes a bit longer to taxi off the runway, in a similar way to driving a car in fog. At any major airport – particularly at Heathrow – the flight arrival and departure slots are quite tightly packed. This can have a knock-on effect and cause delays even after the fog has dispersed.
How do you decide what the visibility is?
Airports use an instrument called a transmissometer. It fires a laser to give a precise measurement of visibility. Safety is our first priority, and we don’t want to get caught out, so ATC will often initiate low-visibility procedures even if the Met Office predicts that there is only a low probability of fog. Unfortunately, sometimes the weather man is wrong and that means customers can experience disruption when, clearly, there is no fog.
How do you decide how far apart to space the aircraft coming in to land?
The space needed between each aircraft is different. In many cases we follow ‘wake vortex separation’ requirements. This is how much space there normally needs to be between aircraft to ensure the air is clear for the following plane. When a small plane like an Airbus A320 follows a larger plane, such as an A380, too closely, it can fly into the ‘wake vortex’ and experience turbulence. At Heathrow, the distance between the planes can be as low as three nautical miles, which roughly translates to one landing every 75 seconds. In periods of low visibility, the distance is extended to at least six nautical miles to give everyone enough time to land and taxi clear, and ensure that the ILS signal is free from interference during the latter stages of the approach and landing. Fog halves the number of landings we can have in an hour. There’s no increase in the wake vortex between planes, but it shows how much disruption something like fog can cause.
Can anything be done to improve the disruption levels?
The airline is working all the time to lessen the impact of low visibility. One of the most recent advances is the use of an Enhanced ILS, which is in the process of being installed at Heathrow and means we can reduce the distance between arriving aircraft in fog. BA is also working with Heathrow Airport and the Met Office to improve the quality of the fog forecast, because forecast ‘misses’ create disruption; if fog is accurately forecast (both the onset and the clearance), we can have a robust plan that best looks after our customers.
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