Bad weather of any type can create flight delays for arriving and departing aircraft at any airport around the world. Due to Heathrow operating at 98% capacity, the effects of fog on flights can be more noticeable than most airports so we've taken a look at why and what happens as a result.
Why does fog cause delays?
When there is reduced visibility, drivers have to leave more space between them and the car in front. The situation is no different with aircraft. And if there is more space between planes, not as many planes can land or take-off each hour – which can mean delays and cancellations.
Why can other airports cope with fog but Heathrow can't?
The vast majority of airports have spare runway capacity so aircraft can be spaced out more during fog without causing delays and cancellations. Heathrow is unusual in that it operates at 98% capacity, with a plane landing or taking off every 45 seconds. Without taking preventative action, spacing flights out more during fog inevitably causes delays and cancellations because there is simply no room to accommodate the delayed flights.
What is the normal gap between planes and how does that change in fog?
It depends on aircraft type – bigger planes have to have bigger gaps behind them than smaller planes due to the vortex left in the air. Generally speaking, arriving aircraft are spaced around 3miles apart. In low visibility that increases to 6 miles.
Why does the spacing have to be increased in low visibility when there is so much advanced technology available?
Although aircraft are equipped with technology that enables them to fly through fog, the issue rests with safe manoeuvring on the ground. In low visibility protocols (LVPs) the preceding aircraft has to be allowed to land and clear the runway before the following aircraft is given landing clearance. Aircraft are also more widely spaced when manoeuvring or taxiing at the airport.
When LVPs are in use at Heathrow the spacing on approach is increased from a minimum of three miles to 6 miles to ensure that aircraft have cleared the runway before the following aircraft is given landing clearance.
Did you know? Heathrow has almost 1,300 flights (arrivals and departures combined) a day. Find out more about how Heathrow operations work via the links at at the bottom of this article.
Why cancel flights in advance?
Who decides which flights to cancel?
When severe disruption at Heathrow is expected, a decision may be made to reduce the flight schedule in advance. This decision is made by a group comprising representatives from Heathrow, NATS and the airlines and they will collectively agree how much the schedule will be reduced by in order to introduce the necessary slack into the system. The airlines will then decide which of their flights to cancel.