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15 May 2009

The Most Bird-prone: Frontier, United, Hawaiian

In late April, the FAA released the long-awaited bird strike data. It shows every recorded bird strike since 2000.

Since then, we've had a whole host of stories bemoaning the doubling in bird strikes since 2000, complete with worrisome bar graphs and explanations from experts.

But as far as I can tell, the stories seem to have forgotten about the denominator: the number of flights, which has been increasing just as rapidly since 2000.

To test this, I went and pulled commercial flight totals from a public BTS data set from 2003 on. Then I limited the bird data to the same airports, months, years and carriers as appear in the BTS flight data. Then I divided bird strikes by flights, and presto: the strike rate has been flat since 2003.

But the most interesting part of this mashup was breaking down the figures by carrier. Do some airlines strike birds more than others? The answer appears to be "Yes."

At the top of the pile are Frontier, United and JetBlue. Frontier Airlines has a staggering 9.4 strikes per 10,000 flights, compared to the industry average of 4.0. Now, a good statistician (or a Frontier executive) would wonder about confounders. Frontier's Denver hub is the most bird-prone major airport in the U.S., with 7.8 strikes per 10,000 flights. Here's the breakdown by airport for the top 34 airports in the U.S..

To try to account for all the possible confounders, I fit a Poisson regression modeling strike rate using the following covariates:

• Year
• Month
• Airport
• Operator

Since the data have 100,000 rows and hundreds of columns after expanding all the categorical covariates, I used bigglm in R to fit the model. The operator coefficients (actually, exp(coefficient)) are shown below. 1.0 refers to the "base" rate -- the strike rate you would expect given the airline's flight history of airports, years and months. The "winners" -- Hawaiian, United and Frontier -- all have values above 2, which means their strike rates are more than double those of any other airline with their flying schedule.

So why do some airlines strike more birds than others, even after accounting for airport and month? Possibilities include differing planes, pilots or maintenance crews.

Posted by Kevin Bartz at 3:06 PM