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I was watching The Flight Channel videos and have noticed that the accidents seem to happen more often in winter months (for the Northern Hemisphere). Are there some statistics to back this claim?

It seems logical: Winter months means icing on wings, slippery runways, etc. Are there any not so obvious reasons for the increased accident rate during winter?

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    $\begingroup$ You can have icing in the middle of the summer, too, even without going to extremes. At about 2 deg C colder per 1000 ft altitude, if the air temperature at ground level is +20 C (not unreasonable in the summer months at latitudes where you get below freezing temperatures in the winter), at 10k ft the outside air temperature is around freezing. Add a bit of moisture to that and you get icing pretty quick. $\endgroup$ – a CVn Apr 1 at 11:07
  • $\begingroup$ @aCVn Indeed. And for aircraft that fly at higher altitudes, sub-freezing temperatures happen on virtually every flight. It's interesting watching the OAT on the in-flight entertainment displays on an airliner in the summer. It may be 100+ degrees F on the ramp and still be well below 0 F at cruising altitude. $\endgroup$ – reirab Apr 1 at 15:54
  • $\begingroup$ You can review dates of when accidents happened here ntsb.gov/_layouts/ntsb.aviation/index.aspx and determine which time of year more events occurred in. I am not going to compile that for you. $\endgroup$ – CrossRoads Apr 1 at 17:25
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I'd argue fall and spring bring more unpredictable weather, and slippery runways are year-round in the tropics.

That being said, for all jet (commercial) accidents, the weather-related contributing threats are:

  • Meteorology (e.g., failure to identify threats before a flight) 30%
    • Windy conditions 16%
    • Poor visibility 10%
    • Thunderstorms 9%
    • Icing 1%
  • Poor braking action (e.g., slippery runways) 10% (falls under airport facilities, not meteorology)

All figures are from PDF page 172 of the IATA 2017 Safety Report.

Given the opening statement, [near ground] icing is more attributable to cold winters than the rest. The rest are not limited to Dec–Mar in the Northern Hemisphere. The report does not attribute the seasons to the accidents.

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  • $\begingroup$ It's true that rain can cause runways to be slippery year-round... but not typically the same degree of slippery that snow/ice causes them to be. The coefficient of friction of tires on smooth ice is much lower than on a wet road in most cases. And while poor braking action is certainly a concern with contaminated runways, ice also significantly increases the odds of loss of directional control leading to excursions off the side of the runway. $\endgroup$ – reirab Apr 1 at 15:59
  • $\begingroup$ Too bad that if you want IATA's data broken down by month or day of year, even ten year old historical data costs $3000. Their annual safety reports aggregate data at no finer than per year. $\endgroup$ – Camille Goudeseune 2 days ago
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Accidents are rarest in winter.

In https://www.ntsb.gov/_layouts/ntsb.aviation/index.aspx, click "Download all (text)", to get a file AviationData.txt containing 80,000 accidents and 4,000 incidents.

Keep only the Accidents, split the |-delimited fields to get mm/dd/yyyy, then split by / to get the month, then count how often each month appears. From a Linux shell:
grep Accident AviationData.txt | awk '{split($0,a,"|"); print a[4]}' | awk '{split($0,a,"/"); print a[1]}' | sort | uniq -c

Result:

   4439  01
   4697  02
   5989  03
   6632  04
   7838  05
   8758  06
   9806  07
   9223  08
   7607  09
   6267  10
   4922  11
   4485  12

Accidents are rarest in January, become smoothly more frequent until July, and then become smoothly rarer until December, i.e., the subsequent January.

But is this trend just because summer has more flights?

Let's compare apples to apples, instead of 747s to everything including powered parachutes. This table shows that airline passengers have the same trend, about 26% to 31% more in July than January. Numbers that big argue that airline flights have the same distribution. When filtering AviationData.txt to include only Boeing, Airbus, etc, July is about 90% more than January.

So summer has 30% more flights than winter, but 90% more accidents.

Even after adjusting for this, an airliner flight in winter is only a third as likely to have an accident as a flight in summer.

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Ill second Camille's answer and add the missing factor of GA vs. Commercial Aviation. The NTSB makes the 2012-2016 data available in spreadsheet format here. If you download that and combine all the accident data you will see that the overall accident data for those years, plotted by month looks something like this:

enter image description here

This data set for even the limited years is in line with the other answers larger data sets. But if you limit the data set to only part 121 accidents the trend disappears:

enter image description here

For those years April and September were the Safest months to fly but a trend really cant not be observed here.

So why does GA cause so many summer accidents? Well its simple, *most GA planes dont have ani/deice equipment so winter flying in the northern parts of the country is out of the question for many GA pilots. The simple fact is there are substantially more GA flights in the summer.

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