By Heathrow

Published 9th July 2018

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Nobody enjoys sitting in an airport or on an aircraft waiting for a weather delay to pass, but the question is what causes these delays? Weather is the one thing airports can’t control. Whether it’s snow, fog, rain, lightning or ash – the effects on air travel can range from long delays to deadly accidents.

Here at yourHeathrow we thought we’d explore some of the key points surrounding whether or not, weather affects your flight.

How does airport capacity play a part in weather delays?

As an example, Heathrow Airport operates at between 98 and 99 per cent capacity across its two runways. In perspective, that’s a flight taking off or landing approximately every 45 seconds across the airport, meaning a weather caused delay of only two minutes can affect or cancel up to three flights.

It should be noted, Heathrow Airport is an extreme example as most other major airports operate at a much lower level of their capacity due to a greater number of runways or less international traffic.

Why does snow cause delays?

Snowmen, snowball fights and snow angels are the fun side of frosty weather, while the flight delays caused are the opposite. A mere 10cm of snow is enough to dump 60,000 tonnes across Heathrow Airport and cause widespread delays as runways are cleared.

While £36 million has been spent by Heathrow Airport since 2010 on ways to reduce delays in such weather, that amount of snow would fill over 4,000 lorry trucks.

Over 90 snow clearing vehicles, millions of litres of de-icer liquid and in excess of 500 extra staff are used during such events to reduce the impact on flights. Clearing the runway itself takes up to 30 minutes during which time there is a need to keep it clear of aircraft taking off or landing.

While Heathrow is responsible for ensuring the runways and taxiways are open and operational, it is the airlines themselves who are responsible for de-icing their aircraft.

How does fog affect flights?

Whether you’re driving a car or piloting an aircraft you need to be able to see where you’re going. As fog increases, pilots need to increase the distance between each aircraft landing and taking-off due to the reduced visibility, which can mean delays and cancellations.

IMAGE: Due to Heathrow operating at 98% capacity, foggy weather can quickly create delays due to the extra spacing required between aircraft.

Spacing between aircraft in the air approaching the runway depends on the size of the plane (due to the vortex left in the air), however, during fog conditions the average distance of three miles apart is extended to six miles.

Modern navigation technology means there are few issues for aircraft in the air during foggy conditions, with the majority of delays caused by landing and take-off processes at each airport.

Who decides which flights to cancel and why?

Each airport and airline has different procedures meaning that decisions on weather related flight cancellations can be extremely difficult to enact.

At Heathrow, when severe disruptions are expected a decision is made by a group of representatives from the airport (known as HADACAB), the National Air Traffic Control Service (NATS) and the airlines themselves to reduce the flight schedule in order to introduce slack into the system. The airlines themselves will then decide which of their flights to cancel. This process involves reallocating passengers onto flights with spare seats so as to minimise the number of people affected.

For more information please see the linked factsheets on HADACAB, the effects of snow on Heathrow, and the processes for dealing with fog conditions.

Our advice to passenger

Before travelling to the airport during adverse weather conditions you should always check the status of your flight with your airline. Please also take care when travelling to the airport during such conditions.

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By Heathrow

Published 9th July 2018