What made me current? What will it take to regain currency?

Two questions that often arise regarding currency (particularly instrument currency) are:

1. I'm not current.  What's the least experience I need to regain my currency?
2. I am current.  What set of flights are the reason (contain the relevant flight experience) that I'm current?

MyFlightbook does already tell you what you need to do to get current (question #1 above), but it can't promise that it's the minimal path ("least experience") to get there.  This is because the “minimum” criteria is actually impossible to determine, as there may be multiple ways to get current, and there is often a tradeoff between optimizing for cost and optimizing for amount of currency gained.

For example, given the current instrument currency rules in 61.57(c) - which I should note will (thankfully) change in November - suppose that it is now July 31 and you did 5 approaches + hold in an FTD in January (short on 61.57(c)(2) by one approach), and you had also done 5 approaches + hold in an airplane, also in January (short on 61.57(c)(1) by one approach), and that was it (and you don't - yet - need an IPC).

Here are some options:

• You can do a single approach in an FTD (relatively cheap).  You'll be current per 61.57(c)(2) until...midnight tonight (July 31).
• You can do a single approach in an airplane (more expensive). You'll be current per 61.57(c)(1), also until midnight tonight.
• You could do 6 approaches + hold + some maneuvers in an ATD (cheap) and be good through Sept 30 per 61.57(c)(3).
• Or you could do an IPC (more expensive) and be good until Jan 31 2019 (longer duration).

Any of these options is OK.  Which counts as “minimal criteria?"  Depends on whether you are optimizing time (the first two are the quickest), cost (the ATD or the FTD probably win on this), or  how long you want your currency to extend.

So what MyFlightbook does here is it picks the simplest path towards currency that involves actual flight experience.  In the case of instrument currency, for example, it will tell you how many more approaches or holding you need in an aircraft, or if it doesn't matter because you need an IPC, it will tell you that.  If you've got 2 night takeoffs/landings in the past 90 days, it will tell you that you need one night takeoff/landing to get night current, etc.

In a similar vein, determining the specific flights that made you current is non-deterministic.

A simple example is that you did a lot of approaches/holding earlier in the month in preparation for your instrument checkride, took and passed the checkride, and then did a bunch more approaches/holding.  Are you current because of the checkride?  Because of the approaches at the start of the month?  The ones at the end of the month?  What if you did the 2-month ATD thing in July (good through the end of September), but you also did 6 approaches + hold in a real airplane in March – you’re current by both.

Even night currency is surprisingly complicated.  It’s not simply “3 takeoffs/landings to full stop at night”, because if you’re a turbine pilot and have the right number of hours and the right other experience, that can extend your currency (see 61.57(e)(4)).

When I compute currency, I actually compute all the ways you could be current and then OR them together, picking up the latest expiration date.  It’s deterministic in that direction, but not in reverse.

The result is that you could search for flights containing "instrument" experience or "night landing" experience, etc., but inevitably it will pick up a superset of flights; like the game Jenga, you can remove multiple distinct subsets of those flights without disturbing your ultimate currency computation.

This is also why all currencies on MyFlightbook are “clickable” except for the regulatory ones; the regulatory ones, unfortunately, have so many exceptions and alternatives that you can’t determine which flights contributed.

Update on April 2, 2020: 
I have gone ahead and made many of the regulatory currencies clickable.  I'm using the simplest test, so it's important to note that the results you see may NOT tie to the expiration date that is reported.

To use an analogy: computing a currency is like taking two numbers and multiplying them; it's deterministic and yields one answer.  4x6=24.  Deciding which flights made you current, though, is the reverse operation - it's like looking at 24 and asking "which two numbers were multiplied?"  Could be 4x6, could be 2x12, could be 3x8, could be 1x24. 

So, for example, night currency by 61.57(b) simply looks for 3 night takeoffs/landings within the preceding 90 days, and most of the time, that's the test.  But if you meet the requirements of 61.57(e), you may actually not have any night landings within 5 and a half months and yet still be night current.

Also, where currency is expired, I do not modify the query to show you the flights that contributed to that old currency.  That's a historical artifact, but I think it's much more useful to know why you are not current right now.  So, for example, if you're no longer night current, clicking on the expired night currency will show you flights with night landings within the past 90 days (in which, presumably, you will fail to see 3 night takeoffs/full-stop night landings).