25 normal and failure conditions should be determined. Systems, such as over-temperature protective devices, should be evaluated to determine if periodic health checks are necessary to assure that latent failures do not exist.
11. INSTRUCTIONS FOR CONTINUED AIRWORTHINESS OF THE FUEL TANK SYSTEM--§ 25.1529.
a. Based upon the evaluations required by § 25.981(a), Amdt. 25-102, also added a new requirement to § 25.981(b) to require that critical design configuration control limitations, inspections, or other procedures be established, as necessary, to prevent development of ignition sources within the fuel tank system, and that they be included in the Airworthiness Limitations section of the Instructions for Continued Airworthiness (ICA) required by § 25.1529. This requirement is similar to that contained in § 25.571 for airplane structure. Amendment 25-102 also added a new requirement to Appendix H to part 25, Instructions for Continued Airworthiness (ICA), to provide any mandatory fuel tank system inspections or maintenance actions in the limitations section of the ICA.
b. Critical design configuration control limitations include any information necessary to maintain those design features that have been determined by analysis of the fuel tank system as needed to preclude development of ignition sources. They may include any maintenance procedure that could result in a failure, malfunction, or defect endangering the safe operation of the airplane, if not performed properly or if improper parts or materials are used. This information is essential to ensure that maintenance, repairs, or alterations do not unintentionally violate the integrity of the original type design of the fuel tank system.
c. Any fuel tank system components that are determined to require periodic maintenance, inspection, or overhaul to maintain the integrity of the system or maintain protective features incorporated to preclude a catastrophic fuel tank ignition event must be defined and included in the Limitations section of the Instructions for Continued Airworthiness. The inspection interval should be established based upon the standard practices defined in AC 25.1309 for evaluation of component failures. Examples of such items include:
(1) Aging fuel line couplings seals/ o-rings. In certain instances, materials used in fuel line couplings may lose flexibility and harden with age. During suction feed or pressurized operation, the seal may allow air to enter the fuel line or leak, allowing spraying of fuel in the tanks or other areas of the airplane where spraying fuel could create a fire hazard. Repetitive inspections, functional checks, or mandatory replacement intervals may be required to prevent leakage.
(2) Wear of pump bushings, bearings, and seals may significantly affect the performance of fuel pumps and degradation of features necessary to maintain the explosive proof qualification. In most cases these failures conditions are latent, therefore incorporation of other fail-safe features, as discussed earlier in this AC, would be required. If fail-safe features are incorporated, such as installation of feeder tanks that are filled with ejector pumps, functioning of the ejector pumps would need to be assured by indications or periodic functional tests.
26 Installation of flow indicators in the motive flow line of the pump that can be viewed by maintenance personnel and a mandatory inspection of this function is one example of a method of mandatory maintenance action.
(3) Fuel pump protective features. If failure of an arc/ground fault protective feature and/or a thermal fuse (closed) is latent and this feature is needed to maintain fail-safe features, periodic checks may be needed.
(4) Transient suppression/energy limiting devices. If failure of the device is latent and this feature is needed to maintain fail-safe features, periodic checks will be needed.
(5) Wire shield grounding.
(6) Component grounds and wires will likely require inspections and measurements to determine proper grounding.
(7) Fuel tank access panel/door seals leakage, resistance checks.
(8) Fuel pump connectors, corrosion, wear.
(9) Fuel pump electrical supply conduit structural, sealing integrity.
d. Maintainability, both in the design and procedures (i.e., MMEL, Airplane Maintenance Manual, etc.), must be verified. This should include, as a minimum, verification that the system and procedures support the safety analysis assumptions and are tolerant to anticipated human errors, and that any critical procedures are highlighted for consideration as required inspection items. (See 14 CFR 121.369(b) for considerations of a "required inspection item.")
e. Visible Identification of Critical Design Configuration Limitations.
(1) Section 25.981(b) states that “…visible means must be placed in areas of the airplane where maintenance, repairs, or alterations may violate the critical design configuration limitations.” The design approval holder should define a method of ensuring that this essential information will be communicated by statements in appropriate manuals, such as Wiring Diagram Manuals, and be evident to those that may perform and approve such repairs and alterations.
(2) An example of a critical design configuration control limitation that would result in a requirement for visible means would be maintaining wire separation between FQIS wiring and other high power electrical circuits where separation of the wiring was determined to be a critical design configuration control limitation. Acceptable means of providing visible means would
27 include color coding of the wiring or, for retrofit, placement of identification tabs at specific intervals along the wiring.
Donald L. Riggin Acting Manager, Transport Airplane Directorate Aircraft Certification Service