Questions from the Cockpit: Bizarre expiration dates

Kristi, a student pilot from Nevada, asks: What’s up with the expiration dates on Sectional Charts? I bought several for planning cross countries and I found that they are all different in how many days they are good for. The AIM says that Sectionals are printed twice per year, except for Alaska, so I figured they should be good for either 182 or 183 days, but one I have here is only good for 168 days. How on earth do they get the math to work? The months also seem completely random, too. And do you know why they are always valid from 0901 Zulu? How did that time get picked?

I dug, and dug, and dug, and dug to find the answer to this question for you, and finally I gave up and just wrote to the FAA for answers. We have Tammy L. Jones, of the FAA’s Office of Communications, to thank for giving us the skinny on the bizarre schedule of Sectional charts.

Here’s a summary of what I learned from her:

Except for Alaska, as you noted, each Sectional chart is currently printed twice per year, alternating between 168-day cycles and 196-day cycles.

Uhhh… But that’s only 364 days, doesn’t that get us back to a bit of a math problem?

Well, yes and no. Charts always become valid and expire on a Thursday. What happens is that the 364-day “year” creates a regular printing schedule, that while differing by one day each year, allows the system to stay on track when it comes to days of the week.

The problem, of course, is that the transit of the earth around the sun doesn’t take an even number of days each year, a fact that calendarologists have been struggling with since Ancient Rome.

Now, I mentioned that the Sectional charts always become valid and expire on a Thursday. Why a Thursday? Apparently, that day was chosen “early last century” by member countries of the International Civil Aviation Organization (ICAO), the United Nations agency that creates universal, universally agreed-upon rules for the inherently borderless global aviation industry.

As to the 0901 Zulu time you asked me about, Jones tells me that the FAA chose that time (while the days are assigned by the UN, member countries choose their own times), as it’s a period of “minimal aviation activity.” That makes sense, as 0901 Zulu is basically 4 a.m. in the nation’s capital, 1 a.m. on the West Coast, midnight in the land of the midnight sun, and 11 p.m. out in Hawaii.

As a side note, it amuses me that for one minute twice a year, both the expiring and replacement charts are simultaneously valid. Someday I’ll figure out how to build that bizarre factoid into the cornerstone of a New York Times best selling thriller!

Now, the issue dates of the 38 Sectional charts in the USA are spread out throughout the year, with at least some of them expiring and renewing every eight weeks.

And, to complicate things further, I have to introduce you to the AIRAC cycle, or Aeronautical Information Regulation and Control cycle. These are 28-day intervals during which charts can be updated — the goal being to try to maximize the freshness of the information. These 28-day blocks, in turn, are organized to occur in the eight-week intervals we just talked about. Again, we have the ICAO to thank for this, and we’ve been using the system since 1964.

Just for giggles, I pulled out a couple of my antique charts from before 1964. Sectional charts were first published in 1930s, not that I have any that old (if you find one in your attic during pandemic spring cleaning, feel free to send it my way).

But in looking at charts from the 1950s, some were labeled, “Next edition is scheduled in approximately one year;” while others say six months. None of them have the type of valid to-and-from dates we’re used to on modern charts.

But leaping back into your question…Oh! Wait! What about Leap Year? Jones tells me that, “Leap year has little effect on the chart schedule other than leap year charts will be available two calendar days earlier than the previous year, but still adhere to the 168/196/364 availability schedule.”

Okay… I confess that I’m having a bit of a hard time wrapping my head around that, but I’m not going to waste too much time on it, as all of this changes next year, in February 2021, when Sectionals will join enroute charts, terminal charts, and supplemental charts. These are on a 56-day publication cycle (sometimes called a Double AIRAC).

The reason for the change on the Sectionals? To reduce the mountains of chart-related NOTAMs that pile up in our modern world between chart publications on the long-established twice-a-year schedule.

But won’t this make staying current, you know, rather expensive? You’d have to buy seven charts a year for each section you fly, rather than two.

Apparently, not so for most pilots, according to the Aircraft Owners and Pilots Association‘s Rune Duke, who says that surveys of the organization’s members show that 90% of pilots are already using electronic charts, rather than dead tree charts.

So, as long as your automatic updates are turned on, the electronic charts stay magically fresh, current, and legal. Regardless of the number of days and weeks between. 

Or leap year.

William E. Dubois is a two-time National Champion Air Racer, NAFI Master ground instructor, and collects flight charts from before 1964.