I was reading some military fiction and saw a comment that army airborne operations were required to repack all parachutes every ninety days, allowing parachutes that were due to be repacked soon to be cheaply used for training. This got me thinking: do parachutes need to be repacked on a schedule? Is this uniform across all types of parachutes and use cases? (are there differences between military and civilian requirements?) How long is the repack cycle? Why is scheduled repacking needed?
Yes, parachutes need to be repacked regularly. The length of the interval depends on the material of the parachute and is between 60 and 180 days. Every parachute should have a small pocket with a piece of paper which lists the most recent repack date and the name of the packer (who needs to comply with FAR part 65, subpart F).
FAA part 91.307 says:
(a) No pilot of a civil aircraft may allow a parachute that is available for emergency use to be carried in that aircraft unless it is an approved type and has been packed by a certificated and appropriately rated parachute rigger—
(1) Within the preceding 180 days, if its canopy, shrouds, and harness are composed exclusively of nylon, rayon, or other similar synthetic fiber or materials that are substantially resistant to damage from mold, mildew, or other fungi and other rotting agents propagated in a moist environment; or
(2) Within the preceding 60 days, if any part of the parachute is composed of silk, pongee, or other natural fiber or materials not specified in paragraph (a)(1) of this section.
The reason should be clear from the regulation: If moisture is present, the growth of mold may prevent the parachute from unfolding easily.
For the T-10D parachute, which is the standard US Army parachute for airborne assault operations, the repack period is 120 days.
I'm adding this as an answer, because it is too long to be a comment, but it is meant as a supplement to the already accepted answer
As a skydiver of 13 years, and having been a member of two different national teams and three different national skydiving associations (US, NO, DK), I will add that repack cycles are in most countries/states defined for reserve canopies. Both main and reserve canopies have main control inspection intervals as well, where a repack also occurs, ofcourse. The intervals I've come across differs. Student gear have in most cases more frequent cycles, due to the fact that they land in trees and similar, more often than experienced skydivers. Reserve canopies seems to have cycles that somewhat depends on climate, more specifically humidity. Florida for instance has the most frequent reserve repack cycle I've come across. I think it was three months. In Norway however, it is 12 months. I am not a rigger (an educated skydive-gear god), and I haven't checked this specifically, but my impression is that the opening functionality of a canopy might be compromised if it stays compressed for too long, and that humidity speeds up this process. We like the opening sequence to be as predictable as possible, and even though it would probably work anyway, it is better to be on the safe side. It is generally recommended to keep your main canopy unpacked if you are not going to use it for a while, as long as it is not exposed things that might compromise fabric or lines.
Military gear is sometimes round canopies, instead of the standard square sports-canopy. I have no idea how the round ones needs to be packed, but from what I've seen of the military square-jumpers, I wouldn't be surprised if they have the same repack/control cycles as student canopies, and for the same reasons (trees).
I normally repack my main canopy before jumping, if it has been more than 3 months in my container.
When I started flying hang-gliders, there were a number of incidents (some fatal) where reserve chutes were thrown but did not deploy properly. Competence at chute-packing was the main cause, but some came from chutes where the fabric was simply too stuck together. Clubs started running chute repacking days in local halls or sports centres, sometimes with a zip-wire from the roof which would let you practise throwing your chute under realistic conditions.
For our reserves, we tended to reckon on repacking being a yearly activity, but then many people would go their whole flying career without ever throwing their reserve. More frequent repacking was not justifiable given the odds of needing it. If you're skydiving or doing military airdrops, using the chute is non-negotiable, and reducing risks by more regular repacking is very sensible.
If you go into a glider hangar, you'll find a bunch of club chutes, all of which have been repacked before the beginning of the season, and last 180 days. Sometimes the chute has a pocket for the log, other times it's under one of the flaps.
I always wonder how much risk there would be flying in a glider with a slightly outdated chute at the end of the season.