This article was first published in The Skeptical Inquirer.
Flying in an airplane is incredibly safe despite what our anxieties and fears might tell us. According to the International Civil Aviation Organization (ICAO), aviation has become the first ultra-safe transportation system in history. That means that for every ten million cycles (one cycle involves both a takeoff and landing), there is less than one catastrophic failure.
It may not feel intuitively true, but you’re much safer traveling in an airplane than in a motor vehicle. In the United States, there are around 1.13 fatalities per every 100 million vehicle miles traveled, compared to just 0.035 fatalities per every 100 million airplane miles traveled. Put another way, your chances of dying in a U.S. car crash are around one in 114. Your chances of dying in a U.S. plane crash are around one in 9,821.
And yet, aviation accidents and incidents do still happen. I recently became deeply interested in aviation safety and got to wondering: Are there monthly or seasonal trends in when aviation accidents occur? Essentially, is there a statistically safer time to fly?
To answer that, we need to define the difference between an accident and an incident. It’s a subtle but important differentiation, because incidents happen all the time, while accidents are quite rare.
The ICAO defines an accident as “an occurrence associated with the operation of an aircraft which takes place between the time any person boards the aircraft with the intention of flight until such time as all such persons have disembarked, in which a person is fatally or seriously injured” and/or “the aircraft sustains damage or structural failure … or the aircraft is missing or is completely inaccessible.” On the other hand, an incident is defined as “an occurrence, other than an accident, associated with the operation of an aircraft which affects or could affect the safety of operation.” In car terms, an accident would be something like a fender bender or crash, whereas an incident would be something like your check engine light coming on or your headlight burning out.
At the time of writing this article in late January 2023, globally, there have been seven accidents in 2023, only one involving fatalities (seventy-two people presumed dead after Yeti AT72 crashed in Pokhara, Nepal). Compare this with incidents, of which there are usually around three or four every single day. If that seems like a lot, remember that the strict reporting of nearly any deviation from perfect plane operation and function is a big part of what has made aviation “ultra-safe.” No piece of machinery as complex as planes will function perfectly 100 percent of the time. By strictly cataloging all incidents, we can continuously identify trends, issues, and ways to improve aviation safety even further.
If there are temporal trends in aviation safety, there are a few reasons those could exist. One potential would be due to weather. There are definite seasonal trends in weather considered hazardous. For example, winter in Canada and the northern United States sees more ice and snow. But the question is whether these weather trends translate into accident trends.
A 2018 study examined all reported worldwide weather-related aircraft accidents from 1967 until 2010. The absolute number of weather-related accidents has increased over that period but so has the annual number of flights, so that is expected. More interesting is the percentage of accidents that are weather-related, which has also increased from about 40 percent to about 50 percent.
This rise could be due to changing weather patterns. The potential effects of climate change on airline safety are rarely discussed, but as incidences of severe weather continue increasing, presumably, so will weather-related incidents and accidents.
The authors of that study, however, believe that this increase is primarily due to “the aviation safety improvements conducted between 1967 and 2010 hav[ing] had a smaller effect on weather-caused aircraft accidents compared with other accidents.” Essentially, while improvements in areas such as crew resource management, training, and maintenance have had positive effects on aviation safety, weather-related accidents have been less sensitive to these improvements.
To look for seasonal trends, the authors of the study divided the globe into four symmetric zones according to latitudes: Zone 1: Within 12 degrees of the equator; Zone 2: between 12 and 38 degrees (which is roughly the middle of the United States); Zone 3: between 38 and 64 degrees (which encompasses most of Canada) and Zone 4: the polar regions in the far north and south.
Image source: https://www.timeanddate.com/geography/longitude-latitude.html
While each zone experiences different weather and climate trends in all but the polar regions, “weather-caused accidents can be considered as uniformly distributed in the various meteorological seasons.”
The U.S. Federal Aviation Administration (FAA) agreed with the study’s conclusions, telling me that they “have not identified any other broad, seasonal or monthly incident trends.” So basically, no, there are not seasonal trends in weather-related aviation accidents, even though there are definite seasonal trends in weather considered severe.
There are three other very interesting takeaways from this study. First, the two zones nearest the equator show a much larger proportion of weather-related accidents, but that isn’t necessarily due to experiencing more severe or dangerous weather. Instead, the authors state that this is due to these zones containing a greater proportion of developing countries that, while adherent to the ICAO safety standards, tend to operate with older planes and equipment.
Second, weather is much less relevant in accidents in developed nations. While the global percentage of weather-related accidents is approaching 50 percent, in the United States and the United Kingdom, it was only 23 percent (in 2012 and between 1977 and 1986, respectively).
Third, despite snow being a widespread occurrence in Zone 4, it has never been reported as the primary cause of any accident. On the other hand, snow accounts for 7 percent of accidents in Zone 2 despite being far less common. This highlights both the disparities in safety between “developed” and “developing” nations and the increased danger associated with unusual weather. It is far safer to land in a snowstorm at an airport that frequently experiences snowstorms because it has systems in place to handle it. Unfortunately, climate change will likely only increase the incidences of unusual weather.
What about non-weather-related temporal trends in airline safety?
Dr. Daniel Bubb, former airline pilot and currently an associate professor at the University of Nevada, Las Vegas, explained to me that we tend to see more accidents in the months of June to September simply because a lot more people are flying. A 2020 analysis of airplane crash data echoed this, as did the National Transportation Safety Board: “the more risk exposure tends to track closely with the actual number of accidents,” which makes a lot of sense.
Another potential trend in aviation safety could come from something analogous to the “July Effect,” as it’s called in North America, or “Black Wednesday,” as it’s known in the United Kingdom—the idea that the day/week/month when new student doctors and nurses start at hospitals is associated with a rise in mortality or morbidity.
Luckily, the aviation industries have safeguards in place to avoid an influx of new workers. For example, both the FAA and NAV CANADA told me they specifically stagger the starts of their new air traffic controllers. A representative of Republic Airways (a regional U.S. airline) told me the same for new pilots and other employees.
An important thing to remember is just how frequently pilots have their soundness evaluated. Dr. Bubb writes that pilots “undergo recurrent training each year” and “undergo physicals each year to maintain their licenses.” With so much oversight, intense training, and staggered starts, the potential for a “July Effect” in aviation is vanishingly small.
In fact, evidence is mounting against the existence of the July Effect in medicine. A 2022 comprehensive meta-analysis of 113 studies published between 1989 and 2019 demonstrates “no evidence of a July Effect on mortality, major morbidity, or readmission.” Studies comparing teaching versus nonteaching hospitals have found teaching hospitals safer year-round!
So, is there a time of the year you should avoid flying? No, not in terms of safety. And you likewise should not avoid heading to the hospital if you feel you need to. However, if you want to decrease how much you drive, that could help with both your safety and the environment.
