The dominant approach is bleed-air-fed air separation using a hollow fiber membrane, which strips oxygen from engine bleed air to produce a nitrogen-enriched stream that is ducted into the fuel tank ullage. This architecture underlies the most-cited records in the field, including the earliest foundational patents from the 1980s and 1990s, and it corresponds to the B01D (separation processes) classification that covers 35.5% of the 1,593 records in scope. Most current filing activity, including the B64D aircraft-equipment class covering 65.9% of records, builds on refinements to this same core system rather than replacing it.

Filing is concentrated: the leading assignee alone accounts for 540 of the 1,593 records in scope, and the five leading assignees combined account for 58.6% of all records. The ten leading assignees together reach 70.7% of records, leaving a long tail of smaller filers with only a handful of records each. This concentration reflects the fact that aerospace primes and their systems suppliers, rather than a broad base of independent inventors, have driven most of the filing in this field.

Filings peaked in 2019 at 189 records and have declined since, with the clearest complete-year comparison showing a drop from 45 filings in 2021 to 28 in 2024, a -38% change. That said, 2025 and 2026 figures should not be read as confirming further decline, because patent publication typically lags actual filing by around 18 months, so the most recent one to two years in any such dataset are always undercounted at the time of publication. The pattern is best read as core architecture claim space maturing rather than the underlying engineering activity necessarily shrinking.

The classification data shows several branches with much lower filing density than the core B64D/B01D combination: A62B (rescue and life-saving, 3.6% of records), B65D (containers and packaging, 2.8%) and C01B (non-metallic elements and inorganic compounds, 3.6%) all trail well behind the dominant classes. These thinner branches suggest claim space around emergency-use applications of nitrogen-enriched air systems, fuel-tank-adjacent container and fitting designs, and chemical inerting agents is comparatively under-claimed relative to the densely occupied membrane-and-ducting core. Anyone evaluating a new filing should check these branches before assuming the whole field is saturated.

US20160311551A1, filed by Hamilton Sundstrand Corporation and published in 2016, describes a nitrogen generation system that uses a bleed air source feeding an air separation module with a hollow fiber membrane, producing separate nitrogen-enriched and oxygen-enriched output streams, with the module positioned in a defined location relative to the engine. It matters because it is a representative example of the engine-proximate placement variant of the standard architecture, and its specific combination of module placement and dual-outlet routing is a detail that later filers and competitors need to check their own designs against. It is one data point among the 1,593 records in scope, not the sole blocking patent in the field, but it illustrates how placement and routing details, not just the separation principle itself, are what gets claimed.