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 per Subpart G the installed equipment makes necessary.
See ACs 23-8B, “Flight Test Guide for Certification of Part 23 Airplanes; 20-67B, Airborne VHF Communications Equipment Installations;” 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 requirements (that is, panel slope for gyroscopic instruments), the installer should stay within the limitation. We recommend the slope be no more than 15 degrees. 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 per Subpart G the installed equipment makes necessary.
There has been a trend to install equipment mainly navigation related such as moving maps, as non-required, “Not approved for primary navigation” or “Situation Awareness Only (SA-Only).” The basis for certification has been to perform its intended function and not present a hazard by this section. Instruments that aid situational awareness should be certified per the requirements in § 23.1309, functional hazard assessment. It is not acceptable to label an instrument as “SA- Only” and assume its failure in normal operation is acceptable. It is also unacceptable to place such an instrument in the primary field of view of the pilot.
Based on history where pilots have flown to known failed gyroscopic instruments, they cannot be expected to ignore an instrument in this position. Last, it is not acceptable to install an instrument as non-required and use its outputs as input data to required instruments.
Guidance for Required Instruments
The purpose of this guidance is to provide clarification on instrument and equipment requirements that are required by 14 CFR part 91, § 91.205 for part 23 airplanes.
Instruments and equipment installed to meet the requirements of § 91.205 should meet the TSO or equivalent requirements. TSO or equivalent requirements are an acceptable means of compliance for the instrument or equipment standards for installation in part 23 airplanes. The pertinent requirements in 14 CFR, part 23, for the basis of the above statement is as follows:
(1) Section 23.1301 Function and installation.
Each item of installed equipment must--
(a) Be of a kind and design appropriate to its intended function.
(b) Be labeled as to its identification, function, or operating limitations, or any applicable combination of these factors,
(c) Be installed according to limitations specified for that equipment, and
(d) Function properly when installed.
and
(2) Section 23.1525 Kinds of operation.
The kinds of operation authorized (for example, VFR, Instrument Flight Rules (IFR), day or night) and the meteorological conditions (for example, icing) to which operating the airplane is limited, or from which it is restricted, must be established appropriate to the installed equipment.
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.
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) is required and; in addition under IFR operation, a gyroscopically stabilized heading system is required.
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:
• the equipment itself,
• the current flow in any item of electrical equipment and its associated wiring,
• the movement of any component, (for example controls or undercarriage), and 172
• 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:
• six degrees in stabilized level flight.
• six degrees plus ½ of the bank angle; or
• 12 degrees with a bank angle greater than six degrees;
• The alert function can be disabled at a bank angle greater than 20 degrees.
• 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-degrees trip point during level flight allows a heading error of as much as 12 degrees. This comprises one system at the six degrees 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 is as follows:
Section 23.1301, Function and installation.
Each item of installed equipment must--
(a) Be of a kind and design appropriate to its intended function.
(b) Be labeled to its identification, function, or operating limitations, or any applicable combination of these factors;
(c) Be installed according to limitations specified for that equipment; and
(d) Function properly when installed.
Section 23.1525, Kinds of operation.
The kinds of operation authorized (for example, VFR, IFR, day or night) and the meteorological conditions (e.g., icing) to which the operation of the airplane is limited, or from which it is prohibited, must be established appropriate to the installed equipment.
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-C5d or TSO-C6c, or equivalent. A direction instrument, magnetic (gyroscopically stabilized) should meet the minimum performance standard of TSO-C6d. As with most TSOs, TSO-C6d refers to SAE documents AS 8013 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 23-8B, “Flight Test Guide for Certification of Part 23 Airplanes," for additional guidance.
23.1303 Flight and navigation instruments
The corresponding rule in CAR 3 is CAR 3.655.
The corresponding rule in the Airship Design Criteria, FAA-P-8110-2, Change 2, is section 6.2.
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.
Amendment 23-17 and Subsequent A proposed revision to 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 indicate any need to differentiate those airplanes from other turbine engine powered airplanes with respect to the need for such equipment.”
See AC 23-8B, “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 pilots 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, 176
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-C151.
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, we recognize 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 altitude 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 primary 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).
We plan 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.
Global Positioning System (GPS) See AC 20-138A, “Airworthiness Approval of Global Navigation Satellite System (GNSS) Equipment”, December 22, 2003.
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:
- 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.
- If all named waypoints of the STARs are stored 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. We have 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) 178
(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.
Policy for TAWS B Displays of Geometric Altitude Labeled Mean Sea Level (MSL)
The Small Airplane Directorate was recently told that some ACOs have stopped certification projects of TAWS with electronic displays. The Small Airplane Directorate did not issue any policy to stop any installations of TAWS B in part 23 airplanes. This policy is intended to allow current and future installations of TAWS equipment, which is not only required for some part 23 airplanes, but also proven to increase safety.
The Small Airplane Directorates position is the TAWS B system with a display is a safety improvement over a warning only system in reducing CFIT accidents/incidents. The display provides two significant improvements: 1) improved situational awareness, and 2) fewer altimeter errors (for example, because of groundpilot communication/interpretation errors, pilot setting errors, static source errors, static system failures, icing effects, pressure and temperature changes, and nonstandard pressure gradients). However, as the current universal standard, pilots must use their primary barometric based altimeter instrument for navigation within the NAS. Thus, the MSL label on the TAWS altitude display, without clarification of its geometrically calculated multi-sensor source, could be misleading because of customary pilot expectations of a barometric based altimeter source. Nonetheless, now there is no history of accidents or incidents from use of the MSL label on this geometric altitude.
Because this label has not been shown to be a safety hazard, the FAA cannot mandate a corrective Airworthiness Directive (AD) for previous installations. The need or nature of a future labeling change is still being debated within the FAA. Until a change may be required, current and future installations in part 23 aircraft may continue to be approved. These installations will require agreed-to additions to airplane flight manuals and TAWS user guides according to the following paragraphs.
1. The AFM or AFMS will describe the following limitations of the TAWS altitude displayed:
The indication of MSL altitude on the upper left-hand corner of the terrain awareness display must not be used for navigation, especially for maintaining an ATC assigned altitude.
Navigation must not be predicated on the use of the terrain awareness display. The terrain awareness display is intended to serve as a situational awareness tool only and 179
may not provide the accuracy and/or fidelity on which to solely base terrain or obstacle avoidance maneuvering decisions.
2. The AFM or AFMS will describe the following normal procedures of the TAWS altitude displayed.
“The indication of MSL altitude is shown on the upper left hand comer of the Terrain Display. This altitude is the reference altitude for the display and the terrain awareness algorithm. This reference altitude is based on internally calculated Geometric Altitude and NOT corrected barometric altitude that must be used when navigating within the National Airspace System. Geometric Altitude is the height above mean sea level (MSL) derived from the GPS receiver, filtered by the vertical figure of merits from the same GPS and complemented by short-term variations in barometric altitude. It represents the aircraft's calculated true height above MSL and serves as the reference altitude for color-coding of the terrain display and the altitude input to the look-ahead algorithm. Because it is primarily comprised of GPS altitude, this reference altitude will often differ from cockpit displayed corrected barometric altitude. The geometric altitude is not to be used for navigation. It is presented to provide the crew with additional situational awareness of true height above sea level upon which terrain alerting and display is based. GPS altitude is an altitude above mean-sea-level and it is the geodetic height above the WGS-84 ellipsoid corrected by the geoid height in the GPS receiver itself. With Selective Availability turned off as currently, the accuracy is usually better than 75 feet and with Selective Availability turned on, short term accuracy is in the order of 400 feet, but the geometric altitude should be within 100 feet.”
3. The TAWS manufacturers will provide information to all installations until or unless they develop a software change that will re-label geometric altitude something else such as “GPSA.”
Amendment 23-43 and Subsequent
A proposed revision to 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 180
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 to 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 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 181