Fuel Tank Inerting System (FTIS)

What Is a Fuel Tank Inerting System?

A Fuel Tank Inerting System (FTIS) is a safety system that continuously displaces oxygen in the empty space (ullage) above the fuel in aircraft fuel tanks with nitrogen-enriched air (NEA), reducing oxygen concentration below the 12% threshold required to support combustion. The system emerged from accident investigations, most notably the TWA Flight 800 disaster on July 17, 1996, in which a center wing tank fuel vapor explosion caused by an ignition source killed all 230 people aboard.

How It Works

The FTIS uses Air Separation Modules (ASMs) — hollow fiber membrane assemblies — to separate pressurized air (typically from engine bleed or electric compressors) into an oxygen-rich exhaust stream vented overboard and a nitrogen-enriched air (NEA) product stream routed into the fuel tank ullage. The ASMs are similar in principle to those used in medical oxygen concentrators but optimized for high purity nitrogen production.

NEA with approximately 12% oxygen is continuously supplied to the center fuel tank — the most vulnerable to ignition due to heat from air conditioning packs below — and wing tanks. Flow rates are managed by ullage oxygen sensors or calculated from fuel quantity and temperature data. Ground mode inerting prevents heat-soak ignition during post-flight sit periods when fuel tanks are warm and partially empty — the most dangerous condition for fuel vapor accumulation.

The FAA mandated FTIS installation on new U.S. transport aircraft designs after 2008 (14 CFR Part 26) and on existing fleets via retrofit. The system adds approximately 45–90 kg (100–200 lb) of weight but provides a substantial reduction in fuel tank explosion risk, estimated at a factor of 100 compared to unprotected tanks.

Key Components

• Air Separation Modules (ASMs): Hollow fiber membranes producing 95%+ pure nitrogen; typical flow rate 1–5 kg/hr (2–11 lb/hr) NEA.
• Oxygen Sensors: Tank ullage oxygen monitors verifying adequate inerting levels (target: below 12% O₂).
• Inerting Control Unit (ICU): Manages NEA flow rate based on flight phase, fuel quantity, and sensed oxygen levels.
• Distribution Plumbing: Dedicated tubing routing NEA to center and wing tank ullage spaces.
• NGNM / Ground Inerting Mode: Continues NEA injection after engine shutdown during post-flight heat-soak periods.

Aircraft Examples

• Boeing 737 MAX: FAA-mandated FTIS installed on center wing tank; ASMs fed by engine bleed air.
• Boeing 787-9: Electric FTIS using electrically driven compressors consistent with no-bleed architecture; covers center and main tanks.
• Airbus A320neo: FTIS mandated per EASA CS-25 and FAA 14 CFR Part 26 retrofit requirements, center wing tank priority.
• Boeing 747-8: Comprehensive FTIS covering four main tanks, two reserve tanks, and center wing tank — largest commercial FTIS installation by tank count.