23.1303 Flight and navigation instruments 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 servocorrected 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. The provision of 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 identified by an appropriate part number, marked clearly for use only on the airplane on which they are calibrated, and information is available to the pilot to enable manual correction computations at airspeeds other than those used in designing the instrument correction.
Amendment 23-17 and Subsequent See AC 23-8A, 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 promulgated for the requirements of altimeter systems, only pneumatic altimeters were envisioned. 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 numerous occurrences 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 upon 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 instrument’s 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. Adequate 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 located on the instrument panel so that it is found to be usable from any pilot’s flight position.
Guidance Regarding 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 altitude encoding altimeter:
1. Install a monitor between another altitude source and the primary, encoded altimeter. If installed, a Global Positioning System (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 Terrain Awareness Warning System (TAWS).
TAWS Class A & B are required for turbine-powered airplanes configured with six or more passenger seats operating under parts 91 and 135 and all turbine-powered airplanes operating under part 121. (See AC 23-18 for more details). We plan to initiate a rulemaking project for Item 1.
LORAN-C See AC 20-121A, Airworthiness Approval of Airborne Loran-C Navigation Systems for Use in the U.S. National Airspace System (NAS), for information on Loran-C installations.
Global Positioning System (GPS) See AC 20-138, Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR Supplemental Navigation System.
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 regarding the intent of paragraph (a)(3)(x) of TSO-C129 as it is applies to approach qualified (Class A1) GPS receivers. The paragraph states in part that the databases would contain all Standard Instrument Departures (SID's) and all Standard Terminal Arrival Routes (STAR's). 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 SID's are not stored and selectable by name, then a limitation must be placed in the airplane flight manual supplement stating that SID's must be entered prior to departure.
- If all named waypoints of the STAR's are stored individually in the database (not selectable as a procedure), then a limitation must be placed in the airplane flight manual supplement stating that GPS equipment is not approved for conducting STAR's. This limitation may be removed as part of an installation approval for aircraft requiring more that one pilot crewmember.
148 GPS as a Primary Means of Navigation See Action Notice N8110.60, GPS as a Primary Means of Navigation for Oceanic/Remote Operations, and the following clarifications:
Paragraph 6.d states that follow-on approvals in a different make/model aircraft require an STC. An engineering review is necessary to establish compliance with the requirement that the loss of navigation is demonstrated to be improbable (paragraph 4f of N8110.60).
This may require dual GPS antennas.
As a supplemental system, GPS may be used as one of two long-range navigators where two are required for oceanic/remote operations and the other is a sole-means navigation system (e.g., INS, Omega). Supplemental GPS may also be used as the only long-range navigation system for oceanic/remote operations that require one long-range and one short-range navigation system (Reference AC 90-94, Section 1, paragraph 3b); these operations, therefore, do not require GPS equipment that complies with N8110.60, and fault detection and exclusion (FDE) is not required. For these operations, GPS equipment should satisfy the requirements described in TSO-C129a and be installed in accordance with AC 20-138 or AC 20-130A.
As stated in paragraph 6b of the Notice, no additional tests above those specified in AC 20-138 and AC 20-130A are required. New installations of primary means GPS equipment can be treated like supplemental equipment except that the loss of the longrange navigation function must be demonstrated to be improbable (paragraph 4f of the Notice).
Some GPS manufacturers may wish to demonstrate that their previously approved articles comply with N8110.60. They must obtain a letter of design approval as described in paragraph 6a of N8110.60 and send a copy to all their customers who own the compliant equipment. Since the equipment was previously installed and approved and there are no modifications, none of the qualification procedures in AC 20-138 or AC 20-130A are needed. However, the installation will still have to be certified for compliance with the requirement that loss of the long-range navigation function is improbable. The Airplane Flight Manual Supplement (AFMS) should be updated per paragraph 6c. If the second GPS unit was not originally installed and approved, the second unit should be evaluated in accordance with AC 20-138 or AC 20-130A, as applicable.
Some GPS manufacturers may develop an upgrade to a TSO Authorized GPS unit. This upgrade is considered a major design change as described in 14 CFR part 21, § 21.611(b). They must obtain a letter of design approval as described in paragraph 6a of N8110.60 and include a copy with each upgraded unit. Provided there are no other changes to the article's design, none of the qualification procedures in AC 20-138 or AC 20-130A are needed. However, the installation must still be certified as the upgrade is a
149 modification to the approved type design. The certification will evaluate compliance with the requirement that the loss of the long-range navigation function is improbable. The AFMS should be updated per paragraph 6c. If the second GPS unit was not originally installed and approved, the second unit should be evaluated in accordance with AC 20- 138 or AC 20-130A, as applicable.
Paragraph 4g of N8110.60 requires an FDE prediction capability, and the notice defines an algorithm for calculating the acceptable duration of an exclusion outage. This capability should allow the user to specify the maximum outage duration for the assessment of whether the operation can be conducted, since additional criteria may be specified for specific operations (e.g., RNP operations).
Paragraph 5a recommends that the GPS equipment be capable of using GPS satellites down to a mask angle of zero degrees. Installed, achieved mask angle will frequently be higher due to installed antenna gain characteristics, signal blockage, and aircraft pitch during level flight. For these reasons, a mask angle of less than two degrees should not be used for prediction capability. Installations previously approved using a lower mask angle do not need to be re-evaluated.
Terrain Awareness Warning System (TAWS) This guidance is in response to a request for clarification on follow-on installation field approval by the Flight Standards District Office (FSDO) for the Terrain Awareness and Warning System (TAWS), Class B, for part 23 airplanes. The questions regarded field approvals by the Airworthiness Aviation Safety Inspector (ASI) approving an Airplane Flight Manual Supplement (AFMS), or Supplemental Airplane Flight (AFM) Manual. 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) (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), 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 informed that some Aircraft Certification Offices (ACO's) have stopped certification projects of Terrain Awareness Warning Systems (TAWS) with electronic displays. The Small Airplane Directorate did not issue any policy, etc., to stop any installations of TAWS B in part 23 airplanes. This policy is intended to permit current and future installations of TAWS equipment, which is not only required for some part 23 airplanes, but also proven to enhance safety.
150 The Small Airplane Directorate’s position is that 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 enhancements: 1) improved situational awareness, and 2) fewer altimeter errors (e.g., due to ground-pilot communication/interpretation errors, pilot setting errors, static source errors, static system failures, icing effects, pressure and temperature changes, non-standard pressure gradients, etc.) However, as the current universal standard, pilots must use their primary barometric based altimeter instrument for navigation within the National Airspace System. Thus, the MSL label on the TAWS altitude display, without clarification of its geometrically calculated multisensor source, could be misleading due to customary pilot expectations of a barometric based altimeter source. Nonetheless, at this time 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 for previous installations. Therefore, because the need or nature of a future labeling change is still being debated within the FAA, and until such time that 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 as per the following paragraphs.
1. The Airplane Flight Manual (AFM) or AFM Supplement (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 upon the use of the Terrain Awareness Display. The Terrain Awareness Display is intended to serve as a situational awareness tool only and may not provide the accuracy and/or fidelity on which to solely base terrain or obstacle avoidance maneuvering decisions.
2. The Airplane Flight Manual (AFM) or AFM Supplement (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
151 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 the above information to all installations until or unless they develop a software change that will re-label geometric altitude something else such as “GPSA.”
23.1305 Powerplant instruments Original Issue and Subsequent
a. Fuel Pressure Indication Paragraph (b)(4)(ii) of § 23.1305 requires fuel pressure indicators for pump-fed engines.
An equivalent level of safety 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 a placard or Airplane Flight Manual (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 Powerplant Instruments (Displays). In consideration of the policy in Item 6d of AC 20-88A, where the rate of change is small or nearly steady state (i.e., 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 the installation of 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 the case of 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)
153 § 23.991 (3.449) § 23.993 (3.550) § 23.1183 (3.638) § 23.1191 (3.624) § 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. New 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; however, the accuracy of these readings is dependent upon 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 required powerplant instrument except for turbine engine airplanes with an Amdt. 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 concerning 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 required for pump-fed engines in accordance with § 23.1305(g). It is intended to monitor metered fuel pressure at the inlet to the
154 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 actually 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 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 amount of 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 permit 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 due to temperature variations, malfunctions in the fuel system such as leaks, siphoning actions, collapsed bladders, and other factors.
Consequently, total fuel remaining should be verified with the fuel quantity indicator. In accordance with § 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. For this reason, 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 Integral 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
155 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 in accordance with § 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 in the event the engine-driven, fuel-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 in terms of 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 type certificate, 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 constant discharge pressure during normal flightengine-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 required for pump-fed engines in accordance with § 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 approved 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
156 and also relates to 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 pressure-type fuel flowmeter replaced with a digital fuel flowmeter/fuel totalizer. If only an analog pressure-type fuel flowmeter is installed that actually 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 fuel-injector inlet. Or, the analog pressure-type fuel flowmeter may be replaced with a digital fuel flowmeter/fuel totalizer if an equivalent level of safety for the airplane shows that replacing the fuel pressure indicator with a flowmeter will still meet the applicable airworthiness requirement. A finding of equivalent level of safety should substantiate that 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 Type Certification (TC) or Supplemental Type Certification (STC) for either the airframe or the engine. FAA approval is obtained after the applicant shows that 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 permits a digital flowmeter with specific instructions. An improper installation not only will jeopardize the safety of the present designs, but could also increase the probability of system failure. Installations should comply with the airworthiness regulations and with the manufacturer's installation criteria.