This Aerospace Recommended Practice (ARP) covers a brief discussion of the icing problem in aircraft fuel systems and different means that have been used to test for icing. Fuel preparation procedures and icing tests for aircraft fuel systems and components are proposed herein as a recommended practice to be used in the aircraft industry for fixed wing aircraft and their operational environment only. In the context of this ARP, the engine (and APU) is not considered to be a component of the aircraft fuel system, for the engine fuel system is subjected to icing tests by the engine/APU manufacturer for commercial and specific military applications. This ARP is written mostly to address fuel system level testing. It also provides a means to address the requirements of 14 CFR 23.951(c) and 25.951(c). Some of the methods described in this document can be applied to engine and APU level testing or components of those application domains.

This revision does not completely address new developments in ice accretion resulting from internal flow in tubing. This will be addressed in a future revision when more experimental data is available. Some background information on the topic is planned to be available in AIR790.

Purpose

The cold temperatures to which jet aircraft are exposed can have a detrimental effect upon the fuel system and its components by causing water that is entrained in the fuel to form ice and clog filter and screens or cause component or system malfunction. As a result, fuel flow to engines can be reduced, restricted or stopped altogether causing flameouts or loss of power which pose a flight safety hazard. Even though many aircraft normally operate with fuels treated with an anti-ice additive, (Reference 2.1.2), fuel without this additive is routinely encountered. Consequently, fuel systems should be tested with fuel that does not contain the anti-icing additive unless the aircraft is specifically restricted from operating without the additive. Because there is great disparity in the requirements and procedures used to perform these icing tests as evidenced in military specifications and aircraft industry reports, it is difficult to determine which testing procedure is the best for a particular fuel system or component.

Therefore, this ARP recommends practices for icing tests for aircraft fuel systems and fuel system components and helps avoid expensive overdesign and overtesting of a component or system while insuring the reliability of a fuel system in icing conditions. In general, system tests are used to assess the overall airframe fuel system tolerance to icing using a representative test setup and test duration. Although system testing has been used in lieu of component testing and viceversa, in general component level tests are more specific and more severe than system level testing.

Three icing test procedures are presented herein:

a. Continuous Operation

b. Emergency Operation (Component or System)

c. Filter with Bypass Function Operation

These three procedures fullfill different purposes. The test paradigm used in this ARP is to create a representative ice accumulation test that would simulate a typical aircraft mission. The representative scenario includes a take-off, climb and cruise segment followed by a descent, approach and go around before landing segment.

The Continuous Operation procedure simulates the "cruise" portion of the flight, when the aircraft fuel system is subjected to cold temperatures with water saturated fuel and no excess water. The take-off and climb time segments are typically rolled in the Continuous Operation regime given their relatively short duration when compared to the cruise portion of a typical mission.

The Emergency Operation procedure simulates the effect of excess water being introduced in the system by condensation from lower altitude air and the verification that the fuel system can still supply an adequate amount of fuel in case of engines being powered up for an "emergency", such as a go around maneuver. This testing can also be used to simulate conditions in which large amounts of water could suddenly be mixed with fuel. For the purpose of the test, this test regime can include the descent, approach and go around segments of the mission.

Although designed with system test in mind, the abovementioned procedures can be applied to individual component testing to verify susceptibility to ice formation during specific aicraft operating conditions.

The Filter Bypass Function Operation test is very specific in that it aims at demonstrating the functional capability of the bypass feature of a filter element (or, alternatively, specific component) when subjected to ice blockage.

If aircraft level flight testing is performed, the recommended practices herein should be applied in the preparation of the flight test demonstration plan. This would include the discussion as applicable on water saturation levels, fuel conditioning, critical icing temperatures and durations of simulated mission profiles.