Brief Summary
The King's Cross fire of 1987, which killed 31 people, was initially baffling to investigators. Standard fire investigation techniques were insufficient to explain the fire's unusual behavior. The investigation eventually uncovered a previously unknown phenomenon called the "trench effect," where fire and hot gases travel horizontally along an inclined trench, defying normal upward movement. This discovery led to significant safety improvements in the London Underground.
- Fire investigation techniques involve analyzing burn patterns, char depth, and potential accelerants to determine a fire's origin.
- The King's Cross fire defied typical fire behavior, with flames traveling horizontally along the escalator.
- Computer simulations and scale model tests revealed the "trench effect," explaining the fire's rapid spread and intensity.
The King’s Cross Fire
On November 18, 1987, a fire erupted on an old wooden escalator at King's Cross station on the London Underground. What started as a small, seemingly manageable fire quickly escalated, sending a jet of flames into the ticket hall above. This disaster resulted in the deaths of 31 people. The subsequent investigation was complex, leading investigators on a seven-month search to understand the fire's unusual behavior and identify its cause. Ultimately, they uncovered a unique physical phenomenon that had been previously unknown.
Fire Investigations Explained
Fire investigations typically involve analyzing the scene for clues to determine the fire's origin. Investigators look for V-shaped burn patterns on walls, which indicate the fire's upward and outward spread from a source. U-shaped patterns suggest the fire source was further from the wall. Multiple blended patterns can complicate the analysis, as can the fire's tendency to be drawn towards openings with oxygen. Investigators may also look for liquid stains indicating the use of accelerants like gasoline, using sniffer dogs to confirm their presence. Char depth analysis, which measures the carbon layer on burnt materials, can help determine which areas burned first.
The Mystery of the London Underground Fire
Eyewitnesses of the London Underground fire reported unusual fire behavior, noting that the flames moved horizontally along the escalator rather than upwards. This defied conventional fire dynamics and puzzled investigators. To understand this phenomenon, investigators turned to computer simulations, utilizing the Cray 2 supercomputer to model the fire's spread and behavior.
What Caused the London Underground Fire?
Escalator fires were common in the London Underground due to passengers dropping lit cigarettes and matches into the gaps, igniting the wooden steps. While these fires usually extinguished themselves, the King's Cross fire rapidly grew into a major inferno. Investigators discovered a large accumulation of grease mixed with dust and debris beneath the escalators, which had not been cleaned for years. This mixture was easily ignited, but initial tests still showed the fire burning vertically. The ceiling paint was also considered as a possible accelerant, but this theory was inconsistent with witness accounts.
The Trench Effect Explained
The computer simulation revealed that the flames and hot gases traveled along the escalator steps due to the trench-like shape and 30° angle of the escalator shaft. This phenomenon, named the "trench effect," prevented the flames from spreading outwards, forcing the heat to flow along the trench. Hot gases accelerated up the escalator, creating a pressure difference that pushed the fire towards the underside of the steps. This heated the wooden escalator to over 500°C, leading to a flashover where everything ignited rapidly, sending a fireball into the ticket hall. This discovery led to the replacement of wooden escalators, the installation of heat detectors, and automatic sprinkler systems, significantly improving safety in the London Underground.
