While downloading the last digital data set (sectionals, TAFs, and supplements) from the FAA:
a question occurred to me.
Why 56 days?
It doesn't divide evenly into 365 or 366, so every year the release dates differ. Why not simply start on 01 January and have a 73 day cycle with 5 releases a year and go 74 days on leap years?
Yeah, yeah, I know 56 days is exactly 8 weeks. So what? Why not 10? Or 12? How much can a chart change? And who picked Thursdays?
Inquiring minds want to know.
I used to work in that office, and while I don't remember all the details and specifics, nor did I have any control of the deadlines or knowledge of the history, I can make some educated guesses.
First, yes the 56-day cycle, as you noted, is 8 weeks. This, of course, is about 2 months.
The publication of the paper products (and even the digital products now) is/was very much a production line, much like any kind of publication like a newspaper or magazine. Presumably two months was seen decades ago as a good amount of time for certain parts of the production process to reset.
But making it every 8 weeks allows the milestones to be set consistently - meaning, for any publication date, certain tasks will always be due on the same day of the week in the weeks and months leading up to that date. If you have, for instance, a milestone set about 6 weeks prior to publication (such as "changes are due to such-and-such office by this date"), it is far easier to manage if that due date is always on a Friday (or a Wednesday, or whatever), than having the day of the week change every time - and 2/7 of the time it would be on a weekend, so you need to move the due date to either Friday or Monday anyway.
Understand that the production process from start to finish for any change to an instrument procedure is at a minimum of about 7 months and typically much longer - meaning multiple publication cycles are being worked at once - with an ultimate deadline of 4 weeks prior to the publication date. So keeping a consistent "day of the week" type of schedule is very desirable.
Why Thursdays? No idea. Probably makes as much sense as any other day. I imagine like lots of things in life, they had to make a choice, and so they probably discussed the pros and cons of each day of the week, and Thursday seemed like the best for who knows what reasons. I can make some up, but I could also make up reasons why I'd think other days might be better. Regardless, it's at 0900Z for US charts, which lessens impact to users, although I've been airborne at that time and had to switch databases.
Oh, and as already pointed out, there are tons of changes every cycle. Many are minor and may not affect you directly, but some are. I encourage you to take a quick look at the latest National Flight Data Digest to get an idea of the data changes (these are released daily!):
And the Transmittal Letter to see changes to procedures (released every two weeks):
