Change 1 AC 23-17C 7/16/2012
173 Applicants should expect certificating officials to consider this information when making findings of compliance relevant to new certificate actions. Also, as with all advisory material, this policy statement identifies one means, but not the only means, of compliance.
Amendment 23-7
A proposed revision in NPRM 67-14 explains this change as follows: “This proposal would remove the unnecessary distinction between radio and other equipment that now exists in Sec. 23.1301 (a) (2) and (3), and extend the provisions of Sec. 23.1431 to all equipment, not only radio equipment. There is no basis for distinguishing between radio and other equipment so far as general functioning requirements are concerned. The other changes are intended to bring the sections in closer conformity with the corresponding sections in Part 25 through 29, without substantive change.”
Original Issue through Amendment 23-19
A system/equipment that is neither essential for safe operation nor required by airworthiness or operating rules may be approved if it is not a hazard in normal operation or when it malfunctions/fails. It does not have to perform its intended function.
Section 23.1301 requires that instruments be installed under prescribed limitations.
Therefore, if an instrument manufacturer specifies any allowable installation requirements (that is, panel slope for gyroscopic instruments); the installer should stay within the limitation(s).
Each piece of installed equipment must be either labeled about its identification, function, operating limitations, or any combination. This applies to the manufacturer of the equipment not the installer. The installer is required to verify the intended function and make any placards or flight manual limitations for the installed equipment per Subpart G as necessary.
See ACs 23-8C, “Flight Test Guide for Certification of Part 23 Airplanes; and 20- 41A, “Substitute Technical Standard Order (TSO) Aircraft Equipment.”
Amendment 23-20 and Subsequent All installed systems/equipment should perform their intended functions. For systems/equipment neither essential for safe operation nor required by airworthiness or operating rules, the manufacturer should define the intended functions the FAA will verify as part of the certification project.
Section 23.1301 requires that instruments be installed under prescribed limitations.
Therefore, if an instrument manufacturer specifies any allowable installation
174 requirements (that is, panel slope for gyroscopic instruments); the installer should stay within the limitation. FAA recommends the slope be no more than 15 degrees.
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175 If applicants want a slope greater than 15 degrees, they should show decisively by tests or analyses the instrument will function properly when subjected to all expected airplane maneuvers.
Each piece of installed equipment must be either labeled about its identification, function, operating limitations, or any combination. This applies to the manufacturer of the equipment not the installer. The installer is required to verify the intended function and make any placards or flight manual limitations for the installed equipment per Subpart G as necessary. There has been a trend for applicants to install equipment, mainly navigation related such as moving maps, labeled as nonrequired, “Not approved for primary navigation” or “Situation Awareness Only (SA- Only).”
Intended Function and Associated Flightcrew Tasks:
(1) In order to show compliance with § 23.1301, the intended function of a system and the tasks expected of the flightcrew must be known.
(2) An applicant’s statement of intended function must be sufficiently specific and detailed that the FAA can evaluate whether the system is appropriate for the intended function(s) and the associated flight crew tasks. For example, a statement that a new display system is intended to “enhance situation awareness” must be further explained. A wide variety of different displays enhance situation awareness in different ways. Examples are terrain awareness displays, vertical profile, and even the primary flight displays. The applicant may need more detailed descriptions for designs with greater levels of novelty, complexity or integration.
(3) An applicant should describe intended function(s) and associated task(s) for:
(a) Each item of flight deck equipment.
(b) Flightcrew indications and controls for that equipment.
(c) Individual features or functions of that equipment.
This type of information is of the level typically provided in a pilot handbook or an operations manual. It would describe indications, controls, and flightcrew procedures.
Novel features may require more detail, while previously approved systems and features typically require less. Some are sufficiently novel that additional scrutiny is required. Applicants may evaluate whether statements of intended function(s) and associated task(s) are sufficiently specific and detailed by using the following questions:
1. Does each feature and functionality have a stated intended function?
2. Are flightcrew tasks associated with the function described?
3. What assessments, decisions, and actions , are flight crewmembers expected to accomplish based on information provided by the system? 4. What other information is assumed to be used in combination with the system?
5. Will installation or use of the system interfere with ability of the flightcrew to operate other flight deck systems?
6. Are there any assumptions made about the operational environment in which the equipment will be used?
7. What assumptions are made about flight crew attributes or abilities beyond those required in regulations governing flight operations, training, or qualification?
Amendment 23-62 and Subsequent
In Docket No. FAA-2009-0738, the Discussion of the Regulatory Amendments stated: The Aviation Rulemaking Committee (ARC) did not make specific recommendations regarding § 23.1301. However, the FAA seeks to clarify the intent of this section, which is frequently misinterpreted and misapplied. Clarifying the intent of § 23.1301 would improve standardization for systems and equipment certification, particularly for non-required equipment and non-essential functions embedded within complex avionic systems. The intent is for the applicant to define proper functionality and to propose a means of compliance acceptable to the Administrator. The FAA expects applicants to coordinate or negotiate deviations from established means of compliance with the Administrator as early as possible to minimize delay to program schedules.
The FAA deleted subparagraph § 23.1301(d) which currently states that equipment must “function properly when installed.” The revision limits the scope since it applies only to equipment required for type certification or the operation. FAA made a related change to clarify similar language in § 23.1309 for proper functionality of installed equipment.
The Final Rule addressed the concerns on § 23.1301(d) by revising § 23.1309(a) and added § 23.1309(a)(3). Further explanation of § 23.1309(a)(3) appears later with §§ 23.1309(a)(1) and (a)(2). Section 23.1301 is adopted as proposed.
Accuracy of the Magnetic Gyroscopically Stabilized Heading System
PURPOSE.
The purpose of this guidance is to provide information on accuracy of the magnetic gyroscopically stabilized heading system that can be displayed on a Horizontal Situation Indicator (HSI) installed in part 23 airplanes.
177 The operating rules such as 14 CFR, part 91, and 14 CFR, part 135, specify the minimum required equipment that must be installed based in part 23 airplanes for the type of operation, such as VFR or IFR. 14 CFR, part 91, § 91.205 requires for heading information, under VFR operation, a magnetic non-stabilized direction indicator (that is compass) and; in addition under IFR operation, a gyroscopically stabilized heading system.
The general airworthiness requirements in 14 CFR, part 23, § 23.1301 and § 23.1525, determine the flight instrument and equipment accuracy requirements for part 23 airplanes. Part 23 does not prescribe specific accuracy requirements for magnetic gyroscopically stabilized heading systems. Specific accuracy requirements for avionics may be found in TSOs and as acceptable means of compliance to § 23.1301 in ACs, notices, or policy statements/letters.
ACCURACY REQUIREMENTS:
Magnetic Non-Stabilized Direction Indicator. A magnetic non-stabilized direction indicator (compass) is required (reference § 23.1327) to have an accuracy of 10 degrees or have a correction card, placard, or a back-up gyroscopic direction indication provided indicator has an accuracy better than 10 degrees. If the sole purpose of the gyroscopic direction indication is for backing up the magnetic nonstabilized direction indicator, then the accuracy of displayed headings can also be to 10 degrees. However, if a gyroscopic direction indicator is installed to meet the IFR operating rules, then the installation requirements are defined by § 23.1301.
Magnetic Stabilized Gyroscopically Stabilized Direction Indicator. An installed final accuracy for a magnetic stabilized gyroscopically direction indicator of ±4 degrees on the ground or ±6 degrees in normal level flight on any heading would meet the requirements of 14 CFR, part 23, § 23.1301. This accuracy applies after compensation and should include cumulative errors in all combinations because of:
1. the equipment itself,
2. the current flow in any item of electrical equipment and its associated wiring,
3. the movement of any component, (for example controls or undercarriage), and
4. the proximity of any item of equipment containing magnetic material direction indicators.
COMPARATOR MONITOR
Some systems installations include two magnetic gyroscopically stabilized heading systems and a comparator that monitors the differences between the headings of the two systems. The comparator trip point set as follows would meet the requirements of 14 CFR, part 23, § 23.1301:
Change 1 AC 23-17C 7/16/2012
1. six degrees in stabilized level flight.
2. six degrees plus ½ of the bank angle; or
3. 12 degrees with a bank angle greater than six degrees;
4. The alert function can be disabled at a bank angle greater than 20 degrees.
5. An alert is provided if the condition exceeds 60 seconds, but allow two minutes for a turn error as stated in the TSO.
The six-degree trip point during level flight allows a heading error of as much as 12 degrees. This comprises one system at the six degree in-flight tolerance limit while the other system, presumably with some malfunction, has an error of 12 degrees in the same direction before the comparator monitor alert is tripped.
BACKGROUND INFORMATION. The following background information is provided for additional information.
EQUIPMENT REQUIREMENTS.
Instruments and equipment installed to meet the requirements of § 91.205 should meet either the TSO or equivalent requirements. Either the TSO or equivalent requirements are an acceptable means of compliance for either the instrument or equipment standards for installation in small airplanes. The relevant requirements in 14 CFR, part 23, for the basis of this statement are as follows:
Section 23.1301, Function and installation.
Section 23.1525, Kinds of operation.
VFR REQUIREMENTS.
A magnetic non-stabilized direction indicator is required equipment by § 23.1303 and for VFR operation by § 91.205. The required accuracy for magnetic non-stabilized direction indicator is prescribed in § 23.1327 as ±10 degrees unless a correction card, placard, or gyroscopic direction indicator is used. If the sole purpose of the gyroscopic direction indication is for backing up the magnetic non-stabilized direction indicator, then the accuracy of displayed headings can also be to ±10 degrees. This should not be interpreted as the required accuracy for the gyroscopic direction indicator in general.
IFR REQUIREMENTS.
For IFR, a gyroscopic direction indication is required by § 91.205. The gyroscopic direction indicator should meet either the TSO-C5f or TSO-C6e, or equivalent. A
Change 1 AC 23-17C 7/16/2012
179 direction instrument, magnetic (gyroscopically stabilized) should meet the minimum performance standard of TSO-C6e. As with most TSOs, TSO-C6e refers to SAE documents AS 8013A for the minimum performance standards. In this document, the standard in paragraph 4.4, Turn Error, is “Two minutes after resumption of straight and level flight the scale error resulting from coordinated turn of 180 degrees in one minute at a true airspeed of 180 Miles Per Hour (MPH) (289 km/h) shall be within two degrees.” During stabilized and under environmental conditions, the test requirement for the error tolerance is two degrees.
The TSO is an equipment minimum performance standard and not an installation standard. For the TSO qualification, the equipment is tested under extreme environmental conditions and they should be compatible for this airplane installation.
See AC 23-8C, “Flight Test Guide for Certification of Part 23 Airplanes" and AC 23.1311-1C, “Installation of Electronic Displays in Part 23 Airplanes,” for additional guidance.
23.1303 Flight and navigation instruments
The corresponding rule in CAR 3 is CAR 3.655.
The corresponding airworthiness criteria is section 6.2, Airship Design Criteria, FAA-P-8110-2, Change 2.
Original Issue and Subsequent Altimeters A servo-corrected altimeter may be installed as the required altimeter provided an electrical failure is apparent to the pilot and the altimeter meets the accuracy requirements of the standard pneumatic altimeter without electrical power. Or, a servo-driven or servo-corrected altimeter with insufficient accuracy may be installed with at least one pneumatic altimeter installed for use by the pilot. On aircraft requiring two pilots, instruments should be located in front of each pilot. Therefore, either the pneumatic or the electrical altimeter can be installed in either location. The desired level of safety could be achieved without a pneumatic altimeter if the electrical supply is ensured. Providing a pneumatic altimeter is usually more practical than the design and installation of a suitably reliable electrical supply system.
Altimeters that employ a “Smiths Law” correction are acceptable provided they are:
1) identified by an appropriate part number, 2) marked clearly for use only on the airplane on which they are calibrated, and 3) give available information to the pilot to enable manual correction computations at airspeeds other than those used in designing the instrument correction.
Magnetic Compasses
Under § 23.1327 of this AC, the FAA has an ELOS policy on installation and accuracy of magnetic compasses.
Amendment 23-17 and Subsequent A proposed revision in NPRM 75-10 explains this amendment as follows: “The proposal would update Sec. 23.1303 to require a free air temperature indicator for turbine engine powered airplanes. The proposal would also update the section to provide for a speed warning device for turbine engine powered airplanes and certain other airplanes for which VMO/MMO and VD/MD have been established. The speed warning device proposal is similar to the requirement contained in Part 25 to the extent practicable for Part 23 airplanes. It should be noted that under the proposal turbopropeller powered airplanes would be required to have installed both a free air temperature indicator and speed warning device since service experience does not
181 indicate any need to differentiate those airplanes from other turbine engine powered airplanes with respect to the need for such equipment.”
See AC 23-8C, “Flight Test Guide for Certification of Part 23 Airplanes,” for information on free air temperature instruments and speed warning devices required for turbine engine powered airplanes.
Altimeters For installation of electronically powered altimeters, when the regulations were issued for the requirements of altimeter systems, only pneumatic altimeters were envisaged. The minimum level of safety established by the regulation was based on the reliability and failure modes of pneumatic altimeters.
a. Service history has shown many instances of complete loss of primary electrical power for both single-engine and multiengine airplanes. The complete loss of altimeter information from a failure of primary power could adversely affect the safe operation of the airplane and is considered an unsafe feature. An electrically powered altimeter installation should have a level of safety equivalent to a pneumatic altimeter installation, and it may be found acceptable if there is no unsafe feature or characteristic.
b. In assessing an electrically powered altimeter with pneumatic reversion capability, the means of providing continuous and usable altitude information should be considered on a failure of the primary electrical power. An electrical powered altimeter may be acceptable under one of the following types of installation:
(1) An electrical powered altimeter with pneumatic reversion that provides a power failure warning as an integral part of the instruments display, and appropriate correction information is provided for the reversionary pneumatic mode.
(2) An electrical powered altimeter that is provided with an alternate power source independent of the electrical generating system. Satisfactory information should be provided to the pilot on the operating limitations and procedures when operating on the alternate power source.
(3) An electrical powered altimeter without a pneumatic reversionary mode may be installed at any pilot’s position provided a pneumatic altimeter is found on the instrument panel to be usable from any pilots' flight position.
Radar Altimeters
Radar altitude is not required for part 23, so it could be installed as no-hazard if it is a stand-alone installation of a TSO-C67, “Radar Altimeter,” with structural implications of the antenna considered, on a nonessential bus, with circuit protection,
182 and separate annunciation from other aircraft warning systems. In this case, a field approval could be acceptable.
If it is tied in to an approved warning system, or if it is part of a required TAWS, or if it is part of a precision landing system (Cat II or Cat III), then a STC is required.
For TAWS, use AC 23-18 and TSO-C151b.
For precision landing, use 14 CFR, part 91, Appendix A.
Guidance On Latent Failures of Altimeters
Though “pilot error” seems likely as a primary cause in a Controlled Flight Into Terrain (CFIT) accident, FAA recognizes that CFIT accidents are unfortunately common and a misleading, latent failure of an altimeter could impact other airplanes.
Rulemaking to require monitoring of an encoding altimeter to activate a warning means for a failure is possible. Whether this is economically justified will have to be determined.
The following options would be useful in a single pilot, high workload situation such as a descent in instrument meteorological conditions. The Small Airplane Directorate recommends that applicants do one of the following when installing an altitudeencoding altimeter:
1. Install a monitor between another al titude source and the primary, encoded altimeter. If installed, a GPS source would be best. Another altimeter would also be acceptable. A warning would indicate a discrepancy and permit troubleshooting to determine which altitude source is failed, or
2. Install a non-encoding altimeter as pr imary with the encoding altimeter as secondary. This would allow for warnings from air traffic controllers about terrain clearance when primary system failures were undetected by cross checks, or
3. Install a TAWS.
TAWS Class A and B are required for turbine-powered airplanes configured with six or more passenger seats operating under parts 91 and 135 and all turbinepowered airplanes operating under part 121. (See AC 23-18 for more details).
FAA plans to initiate a rulemaking project for item 1.
Long-Range Navigation-C System (LORAN-C) See AC 20-121A, “Airworthiness Approval of Loran-C Navigation Systems for use in the U.S. National & Airspace System (NAS) and Alaska,” for information on LORAN-C installations.
183 Global Positioning System (GPS) See AC 20-138B, “Airworthiness Approval of Global Navigation Satellite System (GNSS) Equipment”, September 27, 2010.
For GPS based navigation systems, the maximum allowable time to reestablish a valid navigation position is five seconds. This reacquisition period is considered normal operation; the navigation failure flag(s) or annunciation(s), therefore, should not be displayed and the equipment must not present misleading information to the flight crew.
Questions have arisen on the intent of paragraph (a)(3)(x) of TSO-C129 as it applies to approach qualified (Class A1) GPS receivers. The paragraph states in part that the databases would contain all Standard Instrument Departures (SIDs) and all Standard Terminal Arrival Routes (STARs). The initial intent was that these procedures be selectable by name and not require manual selection of all waypoints. The workload associated with manual selection of individual waypoints has been determined to be unacceptable for single pilot operation. There are two potential deviations to the original intent of the TSO that must be requested formally by the TSOA holder:
1. If the SIDs are not stored and selectable by name, then a limitation must be placed in the AFMS stating that SIDs must be entered before departure.
2. If all named waypoints of the STARs are st ored individually in the database (not selectable as a procedure), then a limitation must be placed in the AFMS stating that GPS equipment is not approved for conducting STARs. This limitation may be removed as part of an installation approval for aircraft requiring more that one pilot crewmember.
Terrain Awareness Warning System (TAWS) This guidance is in response to a request for clarification on follow-on installation field approval by the FSDO for the TAWS, Class B, for part 23 airplanes. The questions regarded field approvals by the Airworthiness ASI approving an AFMS, or supplemental AFM. FAA has coordinated this guidance with the Continuous Airworthiness Maintenance Division, Flight Standards Service (AFS-300).
Flight Standards Information Bulletin for Airworthiness (FSAW) 02-03A, “Follow- On Approval of Class B Terrain Awareness and Warning Systems (TAWS) (Amended)” was issued April 16, 2002. This bulletin explains the standards a Class B Terrain Awareness and Warning System (TAWS), also called an Enhanced Ground Proximity Warning System (EGPWS), and must meet to qualify for a follow-on field approval.
184 Amendment 23-43 and Subsequent
A proposed revision in NPRM 90-23 requires the direction indicator be a nonstabilized magnetic compass based on the following explanation: “This proposal would clarify the intent of the regulations for a magnetic nonstabilized direction indicator. The nonstablized magnetic direction indicator, which does not require power from the airplane's electrical systems, provides directional information to the pilot when all other directional navigation systems have failed due to loss of power.
There have been contentions that a magnetic direction indicator with a remote magnetic sensor should be acceptable because only one magnetic direction indicator is required and it is more accurate for navigation.
When the requirement for a magnetic direction indicator was promulgated, only a nonstabilized magnetic direction indicator (magnetic compass) was envisioned.
Requirements in the operating rules for various kinds of operations are in addition to these basic requirements for type certification. The minimum level of safety established by these regulations was based on the reliability and failure modes of the nonstabilized magnetic compass and its ability to provide continuous heading information due to its functional independency.”
Amendment 23-49 and Subsequent
A proposed revision in NPRM 94-21 explains this amendment as follows: “The lead in for Sec. 23.1303(a) would be revised to clarify that the instruments required by this section are the minimum ones required. Also, Sec. 23.1303(d) would add a requirement for those airplanes whose performance must be based on weight, altitude, and temperature to be equipped with a free air temperature indicator. A new sentence added to Sec. 23.1303(e)(2) would state that nuisance overspeed warnings should not occur at lower speeds where pilots might ignore the warning. A new paragraph (f) would propose requirements for attitude instruments that include a means for flightcrew members to adjust the reference symbol. Finally, it would add a new paragraph (g) to define certain specific instruments required for a commuter category airplane.
The proposal for Sec. 23.1303(e)(2) was developed following a Joint Aviation Authority recommendation that the warning should not occur below the maximum operating limit speed (VMO/MMO)/. To determine the effect that this recommended VMO/MMO limit would have on the design of overspeed warning devices, the FAA contacted several equipment manufactures. These manufacturers responded that it would be possible to establish a lower limit at VMO/MMO, but that the design changes needed to ensure that the warning occurred between the presently required upper limit and the recommended lower limit would be very expensive.
The FAA notes that no known safety problem justifies that cost of these design changes. However, the FAA is also aware that if warnings of any type occur when the pilots know that no particular problem exists, such warnings may become a