Heatwave: the Science behind London Luton’s Distorted Runway

[In The News]

DALLAS – The recent intense heat in the UK has caused the mercury to reach previously unseen highs and has also crippled key elements of the transportation infrastructure.

Yesterday, operations at London Luton Airport (LTN) were temporarily halted due to unexpected repair work on the airport’s runway. At 16:20 UK time, the airport operator tweeted that a “surface defect was identified” and that “repair works are currently in progress.” Later, at 18:05 UK time, a tweet confirmed that full operational capability had been restored and the runway was open again.

LCY London City Airport runway. Photo: Iain Marshall/Airways

Pavement Classification Number

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Rather than being an event that could happen at any airport, different types of runway structures and strengths are more vulnerable to adverse climatic effects. Airport operators are required to publish a Pavement Classification Number (PCN) for each runway at their facility, and this data is made available through documents published by each country’s aviation authority. Simply put, the higher the PCN, the greater the load bearing capability.

Each aircraft type is assigned an Aircraft Classification Number (ACN) to ensure that it is not operated on runways that are not capable to handle larger aircraft on a regular basis. The higher the ACN value, the more load stress the landing gear will exert on the pavement surface. The weight of the aircraft, as well as the number of wheels that comprise its landing gear, are factors that influence the ACN.

Spreading the weight of the aircraft across more wheels reduces the stress on the runway surface. Simply put, the ACN of the aircraft must not exceed the PCN of the airport’s runway, but this is not a hard and fast rule. Small exceedances are sometimes permitted with limits placed on the number of movements allowed.

ACA B38M at YVR | Boeing 737-8 MAX. Photo: Michal Mendyk/Airways

Runway Structure Classification

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The structure of a runway is classified as ‘Rigid’ or ‘Flexible,’ and more information on some of the materials that are commonly used can be found here. Simply put, flexible pavement is made up of multiple layers of surfacing that allow for more flexing than a rigid surface. As a result, in hot weather, a flexible runway surface is better suited to dealing with the expansion that extreme heat can cause.

In addition to the numerical PCN value, the classification includes a string of letters, which also mention if a runway is rigid or flexible. The strength of what lies beneath the runway surface, known as the subgrade, is also of interest in this scenario. The classifications span four categories that  range from ‘High Strength’ to ‘Ultra Low Strength.’ After reviewing the PCN data for Runway 08/26 at Luton, the runway surface is declared rigid and the subgrade is ultra-low strength.

For comparison, LTN is the only one of the four largest airports serving London (Luton, Gatwick, Heathrow, and Stansted) that has a rigid runway. Perhaps this is why the runway at this airport is more susceptible to heat damage, despite the fact that other airports around London appeared to be able to continue operations during the current spell of extreme heat.

If the UK continues to experience such high temperatures in the future, such disruption at Luton or any other airport with similar runway construction characteristics, certainly cannot be ruled out.

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Featured Image: Aerial view of London Luton Airport. Photo: London Luton Airport

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