29 CFR §4245.3

1. (a)Requirement of notice. The plan sponsor of a plan that determines that the plan is insolvent in the current plan year or is expected to be insolvent in the next plan year must file with PBGC a notice of insolvency containing the information described in § 4245.4(a) and must issue to interested parties a notice of insolvency containing the information described in § 4245.4(b). Once notices of insolvency with respect to a plan have been provided as required, no notices of insolvency need be provided with respect to the plan for any subsequent plan year. A notice of insolvency may be combined with a notice of insolvency benefit level under § 4245.5 for the same plan year.
2. (b)When to provide notice. The plan sponsor must provide the notices of insolvency under paragraph (a) of this section at the time described in § 4281.43(b) of this chapter.