Icing Risk in Aviation Fuel Systems
Aviation turbine fuels (Jet-A, Jet-A1, JP-8) contain dissolved water, up to approximately 80 ppm at saturation at 20°C. As an aircraft climbs to altitude and fuel temperature drops (often to −40°C or below), this dissolved water precipitates as ice crystals. These can block fuel filters, restrict heat exchanger flow, and, critically, plug the fuel control unit, potentially causing engine power loss at altitude.
FSII additives dissolve into the fuel and depress the freezing point of any water present, preventing ice crystal formation throughout the operating temperature range.
Dual function: At the treat rate approved for icing inhibition (0.10–0.15% v/v in fuel), FSII also provides effective antimicrobial action against Hormoconis resinae (the kerosene fungus), delivering both ice protection and biocidal function in a single additive. This dual benefit is recognized in aviation fuel specifications including DEF STAN 91-091.
Specification Requirements
| Fuel Specification | FSII Requirement |
|---|---|
| DEF STAN 91-091 (Jet-A1) | Required for military supply; optional for commercial where approved filter monitors are fitted |
| ASTM D1655 (Jet-A) | Optional; required when specified by airline or operator procedures |
| NATO JP-8 (MIL-DTL-83133) | Mandatory, 0.10–0.15% v/v FSII |
| Marine Diesel / MGO | Not a standard requirement; used where tank condensation and microbial risk are concerns |
Handling and Dosing Considerations
- FSII must be blended into the fuel. It does not distribute correctly if added to free water. Thorough mixing is required to ensure uniform distribution.
- Over-treatment must be avoided: excess FSII can cause turbidity in cold fuel and may interfere with fuel gauging systems. Maximum treat rates are defined in each fuel standard.
- FSII partitions preferentially to any free water phase. Bottom water sampling is an unreliable method for checking FSII concentration. Measurement requires sampling from the fuel bulk.
- Compatibility with other fuel additives (corrosion inhibitors, static dissipators) should be confirmed before use in a complete additive package.