I saw different types of FPLN like ACT FPLN , SEC FPLN, ALTN FPLN, MOD FPLN are quite confusing, and I did search on Google hoping to get some standard answers. Unfortunately, there’s no detail data provided. It is necessary to figure out what exactly they are:
Active flight plan is the primary flight plan in FMS(also know as primary FPLN). Performance uses ACT FPLN to calculate optimized SPD/ALT, and LNAV/VNAV are engaged with ACT FPLN as well.
Alternate flight plan is the route in ACT FPLN, which leads the A/C to go alternate aerodrome. ICAO defines an alternate aerodrome as “an aerodrome to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to or to land at the aerodrome of intended landing where the necessary services and facilities are available, where aircraft performance requirements can be met and which is operational at the expected time of use”. So when facing an emergence case like engine out, bad weather or ATM inadvisable at destination, pilots can easily switch to ALTN by clicking DIVERT and then have a cup of coffee.
Alternate aerodromes may be designated as Take-off , En-route or Destination alternates. ICAO defines each of these as follows:
A Take-off alternate is an aerodrome at which an aircraft would be able to land should this become necessary shortly after take-off and it is not possible to use the aerodrome of departure.
An En-route alternate is an aerodrome at which an aircraft would be able to land in the event that a diversion becomes necessary while en route.
A Destination alternate is an aerodrome at which an aircraft would be able to land should it become either impossible or inadvisable to land at the aerodrome of intended landing.
Almost all flights require Destination Alternates but Take-off and En-route alternates are only required in specific circumstances and the departure aerodrome may also be an en-route or destination alternate for the same flight.
Secondary flight plan is like a temporary flight plan which can be copied from ACT FPLN or created manually. Any changes made to SEC FPLN do not affect current flying status. Pilots can review the changes on MCDU/ND and decide if SEC FPLN is good to go, then activate the SEC FPLN to ACT FPLN. Obviously, SEC FPLN can have SEC ALTN FPLN as well.
MOD FPLN stands for modification flight plan. Old fashion Airbus FMS does not have functions like MOD FPLN, any change made to ACT FPLN takes effect instantly. Commonly, pilot wants to check and review changes before appling them, SEC FPLN are used frequently in this case. In Boeing logic, the solution is MOD FPLN. Any change made to ACT FPLN (pilot changes route, load AOC/ATC FPLN, load WIND data, add SPD/ALT constraints, etc…) goes to MOD FPLN first, and the difference is highlighted. Pilots can review the modifications and decide what to do next, like EXEC or CANCLE. MOD FPLN becomes ACT FPLN once it gets executed.