Editing Trench effect (section)

==Background==
The trench effect became known because of the scientific investigation of the [[King's Cross fire]], which occurred on the [[London Underground]] in 1987. The fire started on an escalator (containing wood and years of built-up grease) between the [[Piccadilly line]] platforms and the ticket hall at [[King's Cross St Pancras tube station]]. Many eyewitnesses indicated that early on, the fire in the escalator was of manageable size: officers from the Fire Brigade and British Transport Police indicated that the fire appeared no larger than a large cardboard box burning. Those present were surprised when it suddenly threw a sustained jet of flame into the ticket hall. The Coandă effect alone could not explain the sudden blast that blew some survivors off their feet.

The relatively inconspicuous flames associated with the Coandă effect, hugging the space on and under the inclined trench of the escalator steps, led many travellers to believe they were not in immediate danger. One survivor reported approaching the escalator and noting thick black smoke that suddenly blew up. The main loss of life came from the sudden flashover, rather than the precipitating flames of the 'Coandă effect' fire itself.  The people in the ticket hall at the top of the escalators, whilst avoiding the visible flames, were caught in the path of the flashover. 

In the early stages of the fire, most of the flames were lying down in the escalator trench: only a few visibly protruded above the [[Baluster#Banisters|balustrade]]. The sides of the trench further enhanced the effect by preventing heat loss by radiation to the side, insulating and concentrating the heat along the narrow trench. Emergency services were not prepared for a major fire, due to the lack of visible flames. When the treads of the escalator flashed over, the fire grew dramatically and ignited most of the ticket hall. The [[Health and Safety Executive]]'s [[Health and Safety Executive#HSL|Health and Safety Laboratory]] in [[Buxton]] lit fires in {{frac|10}}-scale and {{frac|3}}-scale models of the escalator and ticket hall, which demonstrated that the trench effect was the main cause of the intensity and rapid spread of the King's Cross fire. The sudden flashover may be attributed to the [[wood gas]] (mainly [[methane]]) emitted from the [[pyrolysis]] of the wooden escalator itself. When the concentration of gas reaches a critical value, the [[lower flammable limit]], the gas suddenly catches fire in the presence of a flame, resulting in an [[explosion]]. 

An episode of the television series ''[[Forensic Files]]'' entitled "Flashover" detailed the King's Cross fire, along with the computer modelling and other analyses which discovered the trench effect. An episode of the television series ''[[Seconds from Disaster]]'' entitled "London's Subway Inferno" detailed both the King's Cross Fire based on eyewitness accounts and the investigation leading up to the discovery of the trench effect.

The trench effect has also been cited as the cause of rapid spread of [[wildfire]]s up hills, which has led on several notorious occasions to the deaths of multiple firefighters.