In addition to the issues of the MGB - this one gets missed regularly or poorly interpreted -
Chapt 14 CFR's Part 29.607 Fasteners -
(a) Each removable bolt, screw, nut, pin, or other fastener whose loss could jeopardize the safe operation of the rotorcraft must incorporate two separate locking devices. The fastener and its locking devices may not be adversely affected by the environmental conditions associated with the particular installation.
(b) No self-locking nut may be used on any bolt subject to rotation in operation unless a non-friction locking device is used in addition to the self-locking device.
I am not saying that this particular issue is relevant to the accident in question but if you apply the intent to many aircraft and realise that the loss of ONE attachment is enough to bring the aircraft down and yet the fastener is not locked this way and sometimes if at all. Additionally there is no way of telling if it is in fact locked. There are numerous aircraft out there where the intent of this part of the FAR is NOT complied with or may be under an AMC - (Acceptable Means of Compliance) which in some cases is proprietary and is difficult to get verification of.
Take a walk around a few aircraft with this in mind. The assumption so far is that ONE stud on the filter housing failed.