29 CFR 4219.11: Withdrawal liability upon mass withdrawal.

(a) Initial withdrawal liability. The plan sponsor of a multiemployer plan that experiences a mass withdrawal shall determine initial withdrawal liability pursuant to section 4201 of ERISA of every employer that has completely or partially withdrawn from the plan and for whom the liability has not previously been determined and, in accordance with section 4202 of ERISA, notify each employer of the amount of the initial withdrawal liability and collect the amount of the initial withdrawal liability from each employer.

(b) Mass withdrawal liability. The plan sponsor of a multiemployer plan that experiences a mass withdrawal shall also—

(1) Notify withdrawing employers, in accordance with § 4219.16(a), that a mass withdrawal has occurred;

(2) Within 150 days after the mass withdrawal valuation date, determine the liability of withdrawn employers for de minimis amounts and for 20-year-limitation amounts in accordance with §§ 4219.13 and 4219.14;

(3) Within one year after the reallocation record date, determine the reallocation liability of withdrawn employers in accordance with § 4219.15;

(4) Notify each withdrawing employer of the amount of mass withdrawal liability determined pursuant to this subpart and the schedule for payment of such liability, and demand payment of and collect that liability, in accordance with § 4219.16; and

(5) Notify the PBGC of the occurrence of a mass withdrawal and certify, in accordance with § 4219.17, that determinations of mass withdrawal liability have been completed.

(c) Extensions of time. The plan sponsor of a multiemployer plan that experiences a mass withdrawal may apply to the PBGC for an extension of the deadlines contained in paragraph (b) of this section. The PBGC shall approve such a request only if it finds that failure to grant the extension will create an unreasonable risk of loss to plan participants or the PBGC.