ABC News
The Odds of Airborne Terror

Not going to do any editorializing here; just going to do some non-fancy math. James Joyner asks:

There have been precisely three attempts over the last eight years to commit acts of terrorism aboard commercial aircraft. All of them clownishly inept and easily thwarted by the passengers. How many tens of thousands of flights have been incident free?

Let’s expand Joyner’s scope out to the past decade. Over the past decade, there have been, by my count, six attempted terrorist incidents on board a commercial airliner than landed in or departed from the United States: the four planes that were hijacked on 9/11, the shoe bomber incident in December 2001, and the NWA flight 253 incident on Christmas.

The Bureau of Transportation Statistics provides a wealth of statistical information on air traffic. For this exercise, I will look at both domestic flights within the US, and international flights whose origin or destination was within the United States. I will not look at flights that transported cargo and crew only. I will look at flights spanning the decade from October 1999 through September 2009 inclusive (the BTS does not yet have data available for the past couple of months).

Over the past decade, according to BTS, there have been 99,320,309 commercial airline departures that either originated or landed within the United States. Dividing by six, we get one terrorist incident per 16,553,385 departures.

These departures flew a collective 69,415,786,000 miles. That means there has been one terrorist incident per 11,569,297,667 mles flown. This distance is equivalent to 1,459,664 trips around the diameter of the Earth, 24,218 round trips to the Moon, or two round trips to Neptune.

Assuming an average airborne speed of 425 miles per hour, these airplanes were aloft for a total of 163,331,261 hours. Therefore, there has been one terrorist incident per 27,221,877 hours airborne. This can also be expressed as one incident per 1,134,245 days airborne, or one incident per 3,105 years airborne.

There were a total of 674 passengers, not counting crew or the terrorists themselves, on the flights on which these incidents occurred. By contrast, there have been 7,015,630,000 passenger enplanements over the past decade. Therefore, the odds of being on given departure which is the subject of a terrorist incident have been 1 in 10,408,947 over the past decade. By contrast, the odds of being struck by lightning in a given year are about 1 in 500,000. This means that you could board 20 flights per year and still be less likely to be the subject of an attempted terrorist attack than to be struck by lightning.

Again, no editorializing (for now). These are just the numbers.

Nate Silver is the founder and editor in chief of FiveThirtyEight.