On February 25, a Southwest Airlines Boeing 737 executed a go-around after a Flexjet Bombardier Challenger 300 crossed an active runway at Chicago Midway International Airport in which the Boeing 737 was cleared for landing. The decision of the flight crew to execute the go-around averted what could have been a catastrophic accident.

Video footage of the serious accident was captured by a camera at the airport.

The International Civil Aviation Organisation (ICAO) defines a runway incursion as "any occurrence at an aerodrome involving the incorrect presence of an aircraft, vehicle or person on the protected area of a surface designated for the landing and takeoff of aircraft."

In the US, the FAA classifies runway incursions into three types, with five levels of severity.

The three main types of runway incursions are an operational incident, a pilot deviation and a vehicle/pedestrian deviation.

An operational incident is when the action of an air traffic controller results in less than required minimum separation between two or more aircraft, or between an aircraft and obstacles such as vehicles, equipment and personnel on runways or clearing an aircraft to takeoff or land on a closed runway.

A pilot deviation is when a pilot violates any rules of the air regulation such as crossing a runway without a clearance while en route to an airport gate or the takeoff point.

A vehicle/pedestrian deviation is when pedestrians and/or vehicles enter any portion of the airport movement areas such as runways and taxiways without approval from air traffic control.

The five levels of severity are listed in descending order:

– Accident: An incursion that resulted in a collision.

– Category A: Serious incident in which a collision was narrowly avoided.

– Category B: An incident in which separation decreases and there is a significant potential for collision, which may result in a time-critical corrective/evasive response to avoid a collision.

– Category C: An incident characterised by ample time and/or distance to avoid a collision.

– Category D: An incident that meets the definition of runway incursion such as incorrect presence of a single vehicle/person/aircraft on the protected area of a surface designated for the landing and takeoff of aircraft but with no immediate safety consequences.

[caption id="attachment_1142561" align="alignnone" width="498"] Diagram showing two types of runway-holding position markings. -[/caption]

The worst runway incursion accident known as the Tenerife airport disaster involved the collision of two Boeing 747s on a runway in Tenerife, Spain which occurred on March 27, 1977, when a KLM Boeing 747 collided with a Pan Am Boeing 747 on the runway at Los Rodeos Airport. This resulted in the deaths of 583 people, making it the deadliest aviation accident in history.

The investigation concluded that the fundamental cause of the accident was the KLM captain's attempted take-off without clearance.

The investigators suggested the reason for this was a desire to leave as soon a