The 90 Second Rule

As part of the testing of a new aircraft, the manufacturer must demonstrate that all passengers can be evacuated in a short amount of time when an emergency arises – within 90 seconds to be precise. If it doesn’t comply, the aviation authorities won’t approve its safety certification. But where does this 90-second time limit come from?

Post-Crash Fires

In the early 1960’s, research was conducted into the cause and development of fires after an aircraft crash. In April 1964, the Federal Aviation Administration (FAA) crash-tested a Douglas DC-7 transport plane, and in September the same year, followed up with the crash test of a Lockheed L1649. The tests aimed to simulate real crashes that had happened with resulting fatalities.

During these tests, high-speed cameras were set up inside the aircraft and around the crash site to record the effects of the impacts on the planes and the dummy pilots and passengers. The wrecked fuselage of the Lockheed was later used in evacuation tests to examine post-crash survivability.

At the same time, the FAA initiated research into the prevention of two types of post-crash fires. The first was the “fireball,” where escaped fuel creates a mist that ignites and overcomes the plane when it comes to rest. The other was a “flash over” in which the inside of the plane reaches a high enough temperature to ignite instantly. For lives to be saved after a crash, a fireball must be prevented, and the cabin must remain habitable long enough to evacuate passengers.

During the research, it was found that a structurally sound cabin engulfed in flames remained habitable for around two minutes. Beyond that, the heat inside becomes so intense that a flashover condition develops. The flashover point was deemed to be the time available for evacuation. As a result of the findings, the FAA initially proposed an evacuation time of two minutes. However, further analysis of the tests and additional research led the FAA to reduce the time to 90 seconds.