Aviation

How planes land safely in thick fog

Flying blind is easy. Fog on the ground is another matter

Not being able to see where you’re flying is – believe it or not – perfectly safe. Aircraft have radars to ‘see’, while movements are digitally monitored by air traffic controllers to keep aircraft at a safe height and distance from each other. But that’s at altitude.

‘On the ground it’s less straightforward,’ says Captain James Toye, Line Operations Manager at Cathay Pacific and Boeing 777 pilot. ‘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.’

Fog slows everything down, including the arrival (or ‘flow’) rate of aircraft. ‘At Hong Kong on a clear day – or night – we can turn off the runway at 50 knots (90 kilometres an hour),’ adds Toye.

‘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 20-30 metre intervals. Looking for one reference point can be disorientating, and it has to be done slowly. We normally taxi at up to 30 knots (55 kilometres an hour), in low visibility that’s reduced to 10 knots or slower (20 kilometres an hour).’

Company policy dictates that low visibility landings must use the aircraft’s automated systems that interact with the airport’s instrument landing system. This is the array of metal poles at the end of a runway that generate a radio beam for aircraft to follow. The airport will also transmit readings from transmissometers, which measure light every 15 seconds, to help pilots decide if they have enough visual references (ie, they can see the runway and runway lights) to land.

While most airports are equipped to deal with fog safely – some are more efficient than others. Taipei’s Taoyuan International Airport has dedicated low-visibility taxiways with fewer turnings. But even for an airport like London Heathrow, which is well used to fog, the volume of landings means it doesn’t take long for a reduced arrival rate to cause disruption.

Hong Kong International Airport is not often affected by fog, but can witness thick, localised patches causing flights to be diverted to Macao, 40 kilometres away. But it could be worse. ‘There’s no low visibility support at Adelaide, for instance,’ says Toye. ‘Get fog there, and you could be diverting to Melbourne…’