185 nuisance. If warnings become a nuisance, a pilot may disregard a warning when the airplane is approaching a flight speed where an unsafe flight condition may occur.
Regulatory action is therefore needed to ensure that the warning will occur within appropriate speed limits. Proposed Sec. 23.1303(e)(2) would require manufacturers to establish a lower speed limit so that nuisance overspeed warnings will not occur.
The manufacturer would be required to show that this limit is appropriate for the airplane design but would not be required to set this lower limit at one specific speed, such as VMO/MMO, which would be costly to achieve.
A new Sec. 23.1303(f) is proposed because attitude instruments are available that provide a means accessible to the flightcrew members, for adjusting the reference symbol through ranges that could result in unsafe pitch angles in small airplanes.
These instruments were developed for airplanes that use high pitch angles for approved climb or descent gradients. By permitting these airplanes to use instruments that can be adjusted for these higher pitch angles, pilots are able to maintain the design gradients using an instrument that provides a normal indication at that pitch.
If each attitude instruments are installed in small airplanes, pilots could adjust the reference symbol to ranges that could result in unsafe pitch angles. The recommendation showed that some instruments can be adjusted to result in pitch angles that are nearly the same as the pitch angle that many small airplanes achieve before stalling. To preclude potential cases of unwanted pitch adjustments of attitude instruments installed in small airplanes, Sec. 23.1303(f) proposes to limit the adjustment range to that limit that is needed for parallax correction.
Proposed new Sec. 23.1303(g) would identify specific instruments, and limits of those instruments, required for commuter category airplanes. When the JAA initiated their consideration of commuter category airplanes, one of the proposals they received recommended adding the instrument requirements of Sec. 25.1303 to part 23 for commuter category airplanes. In considering this recommendation, a review of the requirements showed that many instruments required under Sec. 25.1303 are presently required by the operating rules. In addition, Sec. 23.1583(h) requires a list of the equipment that must be installed for the kinds of operation for which the airplane is approved. Based on the review, it was determined that many of the requirements in Sec. 25.1303 would be redundant, and the recommendation was not accepted.
In considering a portion of the recommendation to require a third attitude instrument, the FAA noted that Sec. 91.531(a)(3) requires a commuter category airplane of ten or more passengers to be operated with a second-in-command and that Sec. 23.1321 requires flight and navigation instruments for such required pilot. Accordingly, two attitude instruments are required for a ten passenger, IFR approved commuter category airplane. Service experience has shown that failures of an attitude instrument system can occur where there will be a time period in which the indicator appears to be working but is providing incorrect information. During such a failure of one instrument in an airplane equipped with only two instruments, the pilots may
186 have difficulty determining which instrument to follow, and hazardous flight attitudes may result. A third attitude instrument would allow the crew to retain reliable attitude information at all times, and thus the proposed rule would require a third attitude instrument for commuter airplanes operated by two pilots.”
Guidance on third attitude indicators with dedicated batteries
For an acceptable compliance to section 23.1303 (g)(3)(iv), when the standby attitude indicator has a built-in dedicated battery with a timing circuit to save the battery charge upon shut down of the airplane, the timing circuit should wait at least 5 minutes before automatic shut-down. The unit should provide a flashing amber annunciation during that 5 minute or longer interval before the unit would power down when the indication would change to a red warning flag in view. To override the automatic timing circuit, the pilot can acknowledge the flashing amber annunciator by pushing the STBY PWR button on the unit within the first five minutes or longer the flashing amber annunciator would go out and the unit would continue to operate normally on dedicated battery power for the remainder of the battery life, up to 30 to 60 minutes. The purpose of this feature is to save the charge in the dedicated battery when the pilot is shutting down the airplane and forgets to turn off the attitude indicator.
Amendment 23-62 and Subsequent
In Docket No. FAA-2009-0738, the Discussion of the Regulatory Amendments stated: The ARC did not make a specific recommendation for § 23.1303. However, the FAA sought to clarify the intent of this rule to accommodate new technology and eliminate the need to issue an ELOS for part 23 airplanes. FAA amended § 23.1303(c) by changing the current requirement from “A direction indicator (nonstabilized magnetic compass)” to “A magnetic direction indicator.” Section 23.1303 does not include a direction indicator, other than the typical non-stabilized compass for part 23 airplanes. As new technology becomes more affordable for part 23 airplanes, many electronic flight instrument systems will use magnetically stabilized direction indicators (or electric compass systems) to measure and indicate the airplane heading to provide better performance.
This amendment removes the need for an equivalent level of safety finding when not installing a magnetic compass.
The Final Rule revised § 23.1303(c) to accommodate new technology and eliminate the need to issue an ELOS for part 23 airplanes by changing from a non-stabilized magnetic compass to a magnetic direction indicator. Section 23.1303(c) is adopted as proposed.
23.1305 Powerplant instruments
The corresponding rules in CAR 3 are CAR 3.655 and 3.675.
The corresponding airworthiness criteria is section 6.3, Airship Design Criteria, FAA-P-8110-2, Change 2.
Original Issue and Subsequent
a. Fuel Pressure Indication. Section 23.1305(b)(4)(ii) requires fuel pressure indicators for pump-fed engines. An ELOS finding can be made for a warning (red per § 23.1322) light set to operate when the primary pump fails and the emergency pump must be manually activated. A caution (amber per § 23.1322) light is acceptable for an automatic switchover to the emergency fuel pressure pump. Also, a fuel flow indicator can be used to indicate the primary pump is operating normally if there is either a placard or AFM to advise the pilot on how to determine primary pump condition from fuel flow information.
b. Powerplant Instrument Marking. See AC 20-88A, “Guidelines on the Marking of Aircraft.” In consideration of the policy in item 6d of AC 20-88A, where the rate of change is small or nearly steady state (that is, cylinder head temperature, exhaust gas temperature, or turbocharger inlet temperature), reciprocating engine parameter instruments may use direct reading digital (alphanumeric) instrument displays with ancillary displays such as warning lights. These ancillary light displays should include amber lights for takeoff/cautionary ranges and red lights for appropriate limits. Placards containing operating range and limitation information should also be included.
c. Fuel Flowmeters. This guidance is applicable to installing fuel flowmeters in small airplanes with continuous-flow, fuel injection, and reciprocating engines.
1. RELATED REGULATIONS These acceptable means of compliance refer to certain provisions of part 23 and the corresponding provisions of the former part 3 of the CAR in airplanes for which those regulations are applicable. Listed below are the applicable part 23 sections with the related CAR sections shown in parentheses:
Part 23 Sections § 23.773 (3.382) § 23.955 (3.433) § 23.961 (3.438) § 23.991 (3.449) § 23.993 (3.550) § 23.1183 (3.638) § 23.1191 (3.624)
188 § 23.1305 (3.655) § 23.1337 (3.673) § 23.1529 § 23.1541 (3.755) § 23.1543 (3.756) § 23.1549 (3.759)
2. BACKGROUND
a. Recently there has been a trend toward replacing fuel pressure indicators and analog reading fuel flowmeters with digital fuel flowmeters/fuel totalizers.
Developments in microprocessor technology have resulted in digital fuel flow computer systems that are economical, accurate, and that provide data for improved fuel management. These digital fuel flow computer systems also have features for displaying total fuel consumed, total fuel remaining, and time remaining. The accuracy of these readings is dependent on the initial fuel supply entered into the fuel computer. The precise digital readings that are displayed to the nearest tenth of a gallon could give a pilot a false sense of accuracy and security, especially the readings for total fuel remaining and time remaining.
b. Digital fuel flowmeters are not a requ ired powerplant instrument except for turbine engine airplanes with an Amendment 23-43 certification basis. They are optional equipment and should not be considered replacements for fuel quantity or fuel pressure indicators. Different interpretations of the regulations have caused conflict and lack of national standardization on installation of fuel pressure indicators and fuel flowmeters/fuel totalizers in small airplanes that have continuous-flow, fuel-injection systems in reciprocating engines. Inquiries from members of the aviation community and manufacturers have indicated a need for information on approval and installation of digital fuel flowmeters/fuel totalizers.
The location of the fuel flow transducer in the fuel system is critical for measuring the total fuel flow consumed by the engine and maintaining engine performance.
Each type of installation has an impact on the operation of the fuel system and needs to be evaluated and approved.
3. DISCUSSION
a. Fuel Pressure and Fuel Quantity Indicator
(1) A fuel pressure indicator is requir ed for pump-fed engines according to § 23.1305(g). It is intended to monitor metered fuel pressure at the inlet to the injector and to advise the pilot of a fuel pressure deficiency. Many small airplanes with reciprocating, continuous-flow, fuel-injection engines are equipped with fuel pressure indicators that measure metered fuel pressure.
Metered fuel pressure in a fuel-injection system also relates to fuel flow, and can provide a satisfactory method for displaying fuel flow. However, replacing the metered fuel pressure indicators with fuel flowmeters could cause an unsafe condition by failing to provide critical fuel pressure
189 information to the pilot that is especially important during the takeoff phase of flight. Fuel flowmeters are not required powerplant instruments for reciprocating engines to meet airworthiness standards of part 3 of the CAR or part 23.
(2) Digital fuel flow computer systems have a fuel flow transducer that directly measures the fuel being fed to the engine. The fuel flow transducer may be a small paddle wheel, an impeller, or spring-loaded movable vanes. Digital displays with a fuel computer also allow these instruments to display total fuel consumed, total fuel remaining, and time remaining at the present fuel flow rate for fuel management. Overall accuracy for fuel remaining and time remaining readings depends on the transducer processing unit and display.
The largest possible error is the initial fuel supply, which is entered by the pilot at the start of each flight. Errors in the initial fuel supply may be caused by an uneven ramp, unusual loading, volume changes of the fuel because of temperature variations, malfunctions in the fuel system such as leaks, siphoning actions, collapsed bladders, and other factors. So, total fuel remaining should be verified with the fuel quantity indicator. According to § 23.1337(b)(1), fuel quantity indicators are required to be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply. Therefore, fuel quantity indicators should be used as the primary fuel-remaining instruments. Fuel quantity indicators that are inaccurate should be periodically calibrated, repaired, or replaced, as necessary, to ensure reliable readings.
b. Fuel-Injection Systems Fuel-injection systems have been designed for many types of reciprocating engines, and they vary in details of construction, arrangement, and operation.
Only continuous-flow, fuel injection systems for reciprocating engines will be discussed in either the speed-sensing pressure pump or constant-pressure pump categories.
(1) Fuel Injection System with In tegral Speed-Sensing Pressure Pump
(a) A fuel injection system with an integral speed-sensing pressure pump delivers fuel at a pressure proportional to engine speed, and the pump is approved as part of the engine type design during the engine certification process. The fuel-injection system has fuel lift capability that enables the system to function with a negative inlet pressure within specific limits as indicated by the engine type data sheet. An emergency fuel pump is not required when the fuel injection pump is approved as part of the engine according to § 23.991(b). The airframe manufacturers may provide an auxiliary fuel pump located upstream of the fuel-injector pump for priming the engine and suppressing fuel vapors. This auxiliary fuel pump can provide some fuel during emergency operations but may not sustain engine operation at full power if the engine-driven, fuel-
190 injector pump fails; therefore, it is not considered an emergency fuel pump.
(b) If the fuel system in the airplane can meet the fuel flow requirements of § 23.955(c) at the minimum allowable inlet pressure limits without the need of an external pump, a fuel pressure indicator is not required.
Nonetheless, some manufacturers have installed a fuel pressure indicator that senses metered fuel pressure at the fuel distribution valve. Since metered fuel pressure is related to fuel flow, it can provide a means for displaying fuel flow. A pressure indicator that is measuring metered fuel pressure may have the scale marked for fuel pressure, fuel flow, or percentage of engine power. With these fuel flow markings, the indicator sometimes is referred to as an analog pressure-type flowmeter.
If an analog pressure-type flowmeter is installed as part of the airplane manufacturer's TC, a replacement digital or analog fuel flowmeter/fuel totalizer is acceptable, provided the installation meets the applicable airworthiness requirements mentioned in the acceptable means of compliance.
(2) Fuel Injection System with Constant Pressure Pump
(a) A fuel injection system with cons tant discharge pressure during normal flight-engine-revolutions usually requires that fuel be supplied at a positive pressure within specified limits to the fuel-injector inlet. To provide this inlet pressure, the engine-driven fuel pump and the emergency pump are usually installed by the airplane manufacturer. An emergency fuel pump is required by § 23.991(b), and this pump should meet the fuel flow rate of § 23.955; therefore, it will sustain engine operation if the engine-driven fuel pump fails.
(b) A fuel pressure indicator is requir ed for pump-fed engines according to § 23.1305(g) and is intended for monitoring unmetered fuel pressure at the inlet to the injector. The fuel pressure indicator provides a means for the pilot to determine if the fuel pressure is within safe limits for proper operation.
(c) Several airplanes have been approve d with a fuel pressure indicator connected to the fuel distribution valve where the fuel flow is a function of metered fuel pressure to the discharge nozzle. Metered fuel pressure is related to fuel flow and engine power output. In some applications, metered fuel pressure has been found acceptable for monitoring fuel pressure and controlling engine performance. The scale on the pressure indicator is to be marked in fuel pressure; in addition, it may be marked in either fuel flow or percentage of engine power output. A fuel pressure indicator at the inlet to the injector provides a more positive means of monitoring the operation of the engine-driven fuel pump and the emergency fuel pump.
(d) An airplane that has both a separate unmetered fuel pressure indicator and an analog pressure-type flowmeter may have the analog pressuretype fuel flowmeter replaced with a digital fuel flowmeter/fuel totalizer.
If only an analog pressure-type fuel flowmeter is installed that operates from metered fuel pressure, the analog pressure-type fuel flowmeter may not be replaced with a digital fuel flowmeter/fuel totalizer unless another fuel pressure indicator is installed to sense the fuel pressure at the fuelinjector inlet. Or, the analog pressure-type fuel flowmeter may be replaced with a digital fuel flowmeter/fuel totalizer if an ELOS for the airplane shows that replacing the fuel pressure indicator with a flowmeter will still meet the applicable airworthiness requirement. A finding of ELOS should confirm the instrumentation provided by the fuel flowmeter is satisfactory, reliable, and safe under all reasonably foreseeable operating conditions.
4. Acceptable Means of Compliance An acceptable method of compliance with the airworthiness standards for installation of fuel flowmeters in small airplanes with continuous-flow, fuel-injection system, reciprocating engine is described below.
a. FAA Approval of Technical Data/Installation
Installation of the fuel flowmeter/fuel totalizer may be approved through TC or STC for either the airframe or the engine. FAA approval is obtained after the applicant shows the fuel flowmeter/fuel totalizer will perform its intended functions and ensures that no unsafe features are incorporated. The need for certification approval for the engine will be determined for each particular installation. Certification approval for the engine is not required when the applicant provides FAA approved data that shows an alternate configuration that allows a digital flowmeter with specific instructions. An improper installation not only will risk the safety of the present designs, but could also increase the likelihood of system failure. Installations should comply with the airworthiness regulations and with the manufacturers installation criteria.
b. Airworthiness Considerations
(1) Fuel Injection System with In tegral Speed Sensing Pressure Pump Installation of a digital or analog fuel flowmeter may replace the analog pressure-type flowmeter.
(2) Fuel Injection System with a Constant Pressure Pump Installation of a digital or analog fuel flowmeter may replace the analog pressure-type flowmeter, provided an unmetered fuel pressure indicator is
192 installed or it has been determined that replacing the fuel pressure indicator with a fuel flowmeter constitutes an ELOS.
(3) General Considerations Changes to the fuel systems should be evaluated for: 1) fuel flow rates,
2) maximum allowable pressure drop, 3) hot weather operations, 4) vibration and loads on lines and fittings, 5) fire protection, and 6) powerplant instruments, including effects of glare and reflections on instruments in the pilot compartment. An engineering analysis should be made to assure: 1) good engineering practices are incorporated in the design and 2) the installation is in accordance with airworthiness standards of §§ 23.773,
23.955, 23.961, 23.993, 23.1183, 23.1191, 23.1301, 23.1309, 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 on each particular installation.
c. Evaluation Modification of the approved fuel system may have major effects; therefore, an evaluation should be conducted to confirm 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; therefore, the fuel flow transducer should be installed upstream of the alternate fuel flow path.
(2) Fuel flow transducer should be inst alled downstream of any bypasses or vent returns to the fuel system.
(3) Maximum fuel pressure drop across th e 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 discover their capability to supply the required fuel flow rate at the higher pressure.
Flight tests for turbocharged engines may be required to determine the minimum fuel injector inlet pressure meets the engine TCDS 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 provided with the safe operating limits. A placard should be 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. Aircraft Flight Manual (AFM) Either an AFM or AFM Supplement, 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 in clude information on the operation and function of the equipment. Included in this section should be information 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 on the initial fuel supply programmed into the computer before the start of each flight. Doubts about initial fuel supply and total fuel remaining can be because of an uneven ramp, unusual loading, volume changes of the fuel because of 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 the pilot determine 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 re quire an AFM with the airplane, the applicant may provide a supplemental AFM or provide the necessary information to the pilot with placards.
Amendment 23-7 and Subsequent A proposed revision in NPRM 67-14 explains this amendment that added turbine engine requirements as follows: “The powerplant instruments in present Sec.
23.1305 are not appropriate for turbine engine powered airplanes. The instruments
194 in this proposal are those that experience has shown to be necessary for the safe operation of those airplanes.”
Changes to the NPRM explain Final Rule, Docket 8083 as follows: “The notice proposed to amend Sec. 23.1305 to require specified powerplant instruments for turbine engine powered aircraft. One comment objected to the requirement for a propeller blade angle sensor. However, the FAA considers that the reliability of direct sensing is essential, considering the critical nature of an unwanted travel of the blade below the flight low pitch position. In response to another comment, the FAA has withdrawn the proposed requirement for a reverse-blade angle sensor. The FAA is aware that such a sensor is not currently practical. The proposed requirement for firewarning indicator is also withdrawn consistent with the withdrawal of proposed new Sec. 23.1203. The notice also proposed to add a new Sec. 23.1307(c) to require, for turbosupercharger installations, specific instruments to allow monitoring of these installations. In response to a comment received, the proposed amendment has been changed to require these instruments only if limitations are established for either carburetor air inlet temperature or exhaust gas temperature and those limitations can be exceeded in operation, and the amendment has been adopted as Sec. 23.1305(p).
In addition, the cylinder head temperature indicator requirements of present Sec. 23.1337(e) are transferred to new Sec. 23.1305(f) inasmuch as it is a required instrument and should properly be included with the other required powerplant instruments. No substantive change is intended. In addition, the proposal concerning thrust indicators has been changed to specify the equivalent means, and the reference to an equivalent to a torque indicator has been deleted since there is no equivalent, and the purpose of the tachometer for turbine engines has been specified.
Finally, the format of present Sec. 23.1305 has been changed to simply list the required instruments inasmuch as the requirement for each instrument is now set forth as to each instrument.”
1. Digital (Alphanumeric) Instruments See AC 23.1311-1C 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.
Change 1 AC 23-17C 7/16/2012
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 ELOS.
4. Fuel Strainer or Filter Indicators Acceptable means of compliance for fuel strainers or filter indicators for turbineengine 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 an airframe-mounted filter is not installed, care should be taken to assure there are no undrainable low spots between the fuel tank outlet and the inlet to the engine.
b. A fuel strainer approved under § 33.6 7, 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 so it can be readily inspected for operation before 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), because the rule requires an indicator, which is a cockpit-mounted indication.
For reciprocating engines, the fuel strainer should comply with all the requirements of § 23.997.
See AC 23.1311-1C for guidance on electronic displays of propulsion parameters.
Diesel engines were not considered when this rule was written. For policy on diesel engine installations in small airplanes, see PS-ACE100-2002-004, Final Policy Statement; “Diesel Engine Installation.”
196 Amendment 23-14 and Subsequent
A proposed revision in NPRM 71-13 added requirements for: 1) an oil pressure warning means for each turbine engine and 2) an induction system air temperature indicator for each engine equipped with a preheater and having induction air temperature limitations which can be exceeded with preheat. No explanation was given. Amendment 23-14 is explained by Final Rule, Docket 11011, as follows:
“Proposed Sec. 23.1305 (a), now Sec. 23.1305(q), has been revised to clarify that the only oil pressure warning means required is for low oil pressure.”
Amendment 23-15 and Subsequent
This amendment added these instruments:
1. For each turbine engine, an indicato r to indicate the functioning of the powerplant ice protection system.
2. For each turbine engine, an indicator for each fuel strainer or filter to indicate the degree of contamination of the strainer or filter to the degree established as an operating limitation for the engine.
3. For each turbine engine, a warning means fo r each oil strainer or filter that has no bypass, to warn the pilot of the occurrence of contamination of the strainer or filter before attainment of contamination of the degree established as an operating limitation for the engine.
4. An indicator to indicate the functioni ng of any heater used to prevent ice clogging of fuel system components.
A proposed revision in NPRM 71-12 explains Amendment 23-15 as follows: “The provisions of paragraph (q) would be transferred from present Sec. 23.1093. The indicator and the means proposed in paragraphs (r) and (s) are necessary to implement the requirements of proposed Secs. 23.997 and 23.1019, respectively. The indicator proposed in paragraph (t) is necessary in the event that a mechanical means, such as a fuel heater, is used in the airplane to heat the fuel as it passes through a filter or screen in the fuel system. Such an indicator could be used in meeting the requirements of proposed Sec. 23.951.”
Amendment 23-15 is further explained by Final Rule, Docket 11010 as follows: “As a result of the issuance of Amendment 23-14 (38 FR 31816), which contains new paragraphs, Sec. 23.1305(q) and (r), the proposed paragraphs (q), (r), (s), and (t) are redesignated (s), (t), (u), and (v), respectively. The following discussion is keyed to the new designations.
One commentator questioned whether the fuel strainer or filter indicator referred to in Sec. 23.1305(t) were required on all filters, even "last chance" filters. Consistent