Lower Tier Supplier (Sub-Tier)

Aerospace supply chains are stratified into tiers. The OEM sits on top, Tier 1 delivers major sub-assemblies, Tier 2 delivers parts that go into Tier 1 sub-assemblies, and Tier 3 and below are the forgings, castings, machined parts, fasteners, and raw material. The lower the tier, the longer the lead time and the bigger the leverage on the whole program. A Tier 4 fastener supplier going bankrupt can halt 737 production. Real Tier 1s have full-time sub-tier risk teams. Roles where this matters: Supply Chain, Quality, Procurement.