b. Airworthiness Considerations
(1) Fuel-Injection System with Integral Speed Sensing Pressure Pump Installation of a digital or analog fuel flowmeter may replace the analog pressuretype flowmeter.
(2) Fuel-Injection System with a Constant Pressure Pump Installation of a digital or analog fuel flowmeter may replace the analog pressuretype flowmeter, provided an unmetered fuel pressure indicator is installed or it has been determined that replacing the fuel pressure indicator with a fuel flowmeter constitutes an equivalent level of safety.
(3) General Considerations Changes to the fuel systems should be evaluated for fuel flow rates, maximum allowable pressure drop, hot weather operations, vibration and loads on lines and fittings, fire protection, and powerplant instruments, including effects of glare and reflections on instruments in the pilot compartment. An engineering analysis should be made to ensure good engineering practices are incorporated in the design and that the installation is in accordance with airworthiness standards of the following §§ 23.773, 23.955, 23.961, 23.993, 23.1183, 23.1191, 23.1337 and 23.1529 of part 23. The fire-resistant capability of fuel system components in the engine compartment should be evaluated. The extent and nature of ground and flight evaluations depend upon each particular installation.
c. Evaluation Modification of the approved fuel system may have major effects; therefore, an evaluation should be conducted to substantiate continued compliance of the fuel system with airworthiness requirements. FAA approval is issued when all airworthiness requirements are met. The following items should be considered:
(1) Fuel flow transducer should measure the total fuel flow under all operating conditions with either the engine-driven or the emergency fuel pumps. Some fuel systems provide an alternate fuel flow path under different operating conditions; for this reason, the fuel flow transducer should be installed upstream of the alternate fuel flow path.
(2) Fuel flow transducer should be installed downstream of any bypasses or vent returns to the fuel system.
(3) Maximum fuel pressure drop across the fuel flow transducer (normal and blocked conditions) should be within manufacturer's specifications and airworthiness requirements. Fuel pressure drop may affect the minimum fuel injector inlet pressure. The minimum fuel injector fuel inlet pressure may require redefinition, and the instrument range markings on the fuel pressure indicator may need to be revised. An engine-driven pump and emergency or boost pump may require adjustment to a higher pressure to account for the added restriction of the transducer. The pumps should be tested to ascertain their capability to supply the required fuel flow rate at the higher pressure. Flight tests for turbocharged engines may be required to determine that the minimum fuel injector inlet pressure meets the engine type certificate data sheet at the maximum approved altitude.
d. Markings and Placards for Powerplant Instruments AC 20-88A provides guidelines on markings of airplane powerplant instruments.
Sections 23.1541, 23.1543 and 23.1549 of part 23 provide the airworthiness requirements for instrument markings and placards. Either the required range marking or placards, or both, should be furnished with the safe operating limits. A placard should be located near the fuel flowmeter/fuel totalizer display with the following statement: “Original equipment fuel quantity indicator is the primary reading of fuel on board the airplane.”
e. Airplane Flight Manual (AFM) A flight manual supplement or supplemental AFM or placards, if appropriate, should be prepared by the applicant. The information should be presented for FAA approval in the following sections:
(1) Limitation section should include placard information and instrument markings.
(2) Normal procedure section should include information on the operation and function of the equipment. Included in this section should be information that the fuel totalizer does not sense the quantity of fuel in the tank and it should not be used as a fuel quantity indicator. The accuracy of total fuel remaining displayed on the fuel flowmeter/fuel totalizer is dependent upon the initial fuel supply programmed into the computer before the start of each flight. Uncertainties about initial fuel supply and total fuel remaining can be due to an uneven ramp, unusual loading, volume changes of the fuel due to temperature variations, malfunctions such as leaks, siphoning action, collapsed bladder, and other factors; therefore, the total fuel remaining should be verified with the fuel quantity indicator. Before flight, it is essential that the pilot determine that the fuel programmed into the computer is the same as the usable fuel on board the airplane.
(3) The emergency procedure section should include any system malfunction that may occur due to electrical power failure and the procedures for verifying proper operation after power outages.
(4) If the certification basis does not require an AFM with the airplane, the applicant may provide a supplemental AFM or provide the necessary information to the pilot by means of placards.
Amendment 23-7 and Subsequent
1. Digital (Alphanumeric) Instruments See AC 23.1311-1A for guidance on this topic.
2. Torque Meter Markings Markings on torque meters should be as follows:
a. The maximum safe operating torque should be indicated by a red radial.
b. The green arc should extend across the complete normal operating range.
c. Takeoff torque can be indicated by the word “Takeoff” or the letters “T.O.” arranged as a radial with an explanation of their significance in the AFM.
3. Warning Means Instead of Indicators Warning means for § 23.1305: oil quantity measuring device, powerplant ice protection indicating means, fuel system anti-ice indicating means, thrust reverser indicating means, and propeller blade angle indicating means, can be acceptable as an equivalent level of safety.
4. Fuel Strainer or Filter Indicators Acceptable means of compliance for fuel strainers or filter indicators for turbine-engine airplanes are as follows:
a. A fuel filter approved under 14 CFR part 33, § 33.67, Amendment 33-6, installed within the engine upstream of the high-pressure engine-driven positive displacement pump or the fuel metering device will comply with the provisions of § 23.997 without an airframe supplied filter. The fuel filter should be capable of sustained operation while operating with water in the fuel as specified in §§ 23.991(c) or 33.67(b)(4). An engine-driven, low-pressure fuel pump may be installed upstream of the fuel filter. If
160 an airframe-mounted filter is not installed, care should be taken to ensure there are no undrainable low spots between the fuel tank outlet and the inlet to the engine.
b. A fuel strainer approved under § 33.67, Amendment 33-6, would not require an indicator in the cockpit to indicate the occurrence of contamination before it reaches the capacity of the fuel strainer, as required by § 23.1305(c)(8). However, an indicator on the engine should be installed such that it can be readily inspected for operation prior to flight. Instructions for this inspection should be included in the Preflight Check Procedures in the AFM.
c. Turbine engine installations that do not have a fuel filter per § 33.67 should have an airframe mounted fuel strainer to comply with § 23.997. Also, an indicator for contamination before it reaches the capacity of the fuel strainer, as required by § 23.1305(c)(8), should be provided. A pop-out button on the filter is not acceptable for compliance to § 23.1305(c)(8).
For reciprocating engines, the fuel strainer should comply with all the requirements of § 23.997.
See AC 23.1311-1A for guidance on electronic displays of propulsion parameters.
23.1307 Miscellaneous equipment No policy available as of June 30, 2001.
23.1309 Equipment, systems, and installations Amendment 23-14 and Subsequent The FAA has reviewed the part 1 definition of the word “instrument,” and other data and has concluded as follows:
a. Where a light is sufficient, the instrument requirement should be changed to a warning means.
b. Where trend information is needed, the word “indicator” should be retained.
c. Where point information or steps in a sequence need to be shown, the words should be changed to “indicating means” (i.e., the functioning of the ice protection system).
Software Notice N8110.89 provided Guidelines for the Approval of Software Changes in Legacy Systems Using RTCA DO-178B. Theoretically, RTCA/DO-178A and RTCA/DO-178B are not very different in terms of objectives. DO-178A states software life-cycle objectives implicitly, while DO-178B states them explicitly. The applicant should determine which DO- 178B objectives are currently not met. Considering this data, the FAA should determine if the missing objectives would affect airworthiness or safety and may require those objectives to be met.
Changes that affect airworthiness per 14 CFR part 21, § 21.93 should be considered. The original System Safety Assessment determined the required design assurances for each aircraft function. Software changes to these functions should be further analyzed to determine if the design assurance of those functions is maintained after the software changes are made.
Another approach would be to use service experience as defined in DO-178B, Section
12.3.5, when the software is not being changed for the application. Service experience can be used for commercial-off-the-shelf (COTS) software provided the COTS is not embedded within the application.
Unless the applicant has complied with the provisions of DO-178B, Section 12.1.4, the accomplishment summary and any box markings shall not indicate approval of the software to DO-178B.
Software development and certification for components with multiple functions and interfaces such as electronic engine controls requires significant effort. Even though the software
163 functional requirements are included in DO-178, the verification of software functions as installed in the airplane is necessary to establish that the software and associated functions are correct and certifiable.
See AC 23.1309-1C, Equipment, Systems, and Installations in Part 23 Airplanes, for additional guidance.
Airplane Parachute Recovery Systems Use of a parachute recovery system is not covered by part 23. Applications for the installation of this system will require special conditions.
Amendment 23-41 and Subsequent Lightning Protection See AC 23.1309-1C, AC 20-136, Protection of Aircraft Electrical/Electronic Systems Against the Indirect Effects of Lightning, and RTCA DO-160D, Environmental Test Conditions and Test Procedures for Airborne Equipment, Section 22, for guidance on lightning certification of IFR airplanes. As part of the ongoing review of natural lightning by the SAE Lightning Committee AE2 and EUROCAE WG-31, the multiple stroke and burst environmental criteria was revised from the AC 20-136 requirement as defined in SAE ARP 5412, Aircraft Lightning Environment and Related Test Waveforms.
Several SAE documents have been issued and the FAA finds the following SAE documents acceptable:
SAE ARP 5412, Aircraft Lightning Environment and Related Test Waveforms.
SAE ARP 5413, Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lightning, (being converted to an AC, AC 20-136A).
SAE ARP 5414, Aircraft Lightning Zoning.
SAE ARP 5415, User’s Manual for Certification of Aircraft Electrical/Electronic Systems for the Indirect Effects of Lightning.
High Intensity Radiated Fields (HIRF) Special conditions will still be required for critical systems for High Intensity Radiated Fields (HIRF), since the words “radio frequency energy” in this rule are not intended to include HIRF. RTCA DO-160D, Section 20, is applicable for bench level testing for HIRF.
Electronic Engine Control (EEC) Systems Section 23.1309 does not apply to powerplant systems provided as part of the certificated engines per paragraph (f). This rule did not envision digital engine controls. Special
164 conditions will be required for electronic engine controls without a hydromechanical back-up system that has the reliability and performance of a traditional recip engine without electronic engine controls. The special conditions will require compliance to HIRF and § 23.1309(a) through (e), per the latest amendment, for these digital engine controls. Appropriate engine certification data may be used for airplane certification. Even though an engine control may be certificated as part of the engine, the installation aspects of the installation require certification to part 23. Experience has shown that changes required for aircraft certification may require changes to the engine software with the associated engine certification of those changes.
Acceptance of engine mounted and engine certificated FADEC or EEC's does not mean that approval at the airplane level is automatic. Flight-testing is still required to assure all part 23 requirements are met.
Software changes initiated by the engine manufacturer (or FADEC or EEC manufacturer) should be validated on each software change as to their effect on part 23 certification requirements. Any and all engine control software changes must be coordinated with the appropriate ACO to assess the impact on part 23 certification requirements.
165 INSTRUMENTS: INSTALLATION
23.1311 Electronic display instrument systems Amendment 23-41 through Amendment 23-48 Attitude and Heading For part 23, there is not a specific requirement that instruments at each pilot station be independent unless two pilots are required by the airworthiness or operational rules.
Therefore, both electronic attitude and heading instruments can utilize the same attitude and heading reference source. For a single Attitude Heading Reference System (AHRS), the Airplane Flight Manual should include equipment operating limitations to alert the pilot(s) that a failure of the AHRS could simultaneously affect both attitude and heading instruments.
However, a single AHRS may not be acceptable if its affect on an autopilot system is a possible catastrophic failure caused by the AHRS, such as an unannunciated slowover (softover) failure.
Amendment 23-41 and Subsequent See AC 23.1311-1A, Installation of Electronic Displays in Part 23 Airplanes, for further guidance.
Paragraph (e) in § 23.1311 has definitions specific to this rule for “instruments” and “primary.”
The definition of “indicators” is not in the regulations. In a search for the word “indicator,” the text does not disclose a precise definition or the intended use of the word.
For § 23.1311, an indicator is a means for displaying information on a parameter (and is not the same as an instrument). The preambles for § 23.1311 were intended to allow advanced and new electronic display technologies on part 23 airplanes and to permit the collection of indicators on a single electronic display. More than one indicator could be depicted on one display. For example, a primary flight display may have indicators for attitude, altitude, airspeed, heading, and navigation.
Altitude Digital-Only (alphanumeric) displays for barometric altitude should not be approved.
23.1321 Arrangement and visibility Original Issue and Subsequent When applying this rule to powerplant instruments in multiengine airplanes, ensure there is no confusion as to the engine/instrument relationship. For instance, powerplant instruments for the right engine in a twin-engine airplane may not be placed over, under, or to the left of the left-engine instruments.
Amendment 23-14 and Subsequent For all installations, the evaluation should consider the different environmental conditions under which the airplane may be operated as defined by § 23.1559.
Basic “T” This rule applied the Basic “T” to standardize flight instrument locations. This was not intended to require a “perfect T.”
Also, for all installations, the FAA has always intended that § 23.1321(d) apply to each pilot’s station for both type certification and for any operations for which the airplane is approved. Therefore, when an airplane is type certificated with the “basic T” instrumentation at only one pilot’s station, that airplane is limited to operations where only one pilot is required in accordance with §§ 23.1525 and 23.1583(h).
23.1322 Warning, caution, and advisory lights Policy is available in AC 23.1311-1A.
23.1323 Airspeed indicating system Original Issue and Subsequent Part 135, Operating Requirements: Commuter and On-Demand Operations, requires that IFR airplanes have a heated pitot tube for each airspeed system. In consideration of a fourpound bird strike, the minimum distance between pitot tubes that can be accepted is 14 inches, measured in a straight line.
Amendment 23-34 and Subsequent See AC 23-8A, Flight Test Guide for Certification of Part 23 Airplanes, for additional guidance.