4. phase of flight, and so forth.
In the SAE Standard ARP4761, “Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne Systems and Equipment,” exposure time is defined as "The period of time between when an item was last known to be operating properly and when it will be known to be operating properly again."
In this guidance, risk is also being considered. Risk assessment is the composite of:
1. the outcome or effect of the failure condition;
2. the average probability of the failu re condition per flight hour; and
3. exposure time.
For risk assessments, a credit should be factored into the analysis that is proportional to the probability that a given failure condition will occur on the aircraft. This credit is related to the system level of the failure condition, the operational and environmental conditions, and the phase of flight.
To determine exposure time, the time the pilot would only be using heading information to navigate should be considered. Only a percentage of a typical IFR flight is flown navigating solely on the heading instrument. The critical times for heading are when the pilot is not navigating with a navigation instrument but is only using the heading instrument.
The details on how to calculate the “Average Probability Per Flight Hour” for a Failure Condition are given in Appendix C of AC 23.1309-1E. A common assumption for 14 CFR, part 23 airplanes is the average flight duration is one hour.
The FAA’s General Aviation and Air Taxi Activity Survey gives estimated flight hours for IMC operations for specific airplanes. When researching the operations of specific airplane models, the result indicates that Class I airplanes are operated in IMC conditions about 20 percent of their total flight hours and Class II airplanes about 30 percent. Also, the FAA believes that only portions of most IFR flights are flown by navigating solely on the heading instruments. Using the percentages for “phases of flight” from a Boeing accident study, the FAA estimates that on any given IFR flight, navigation solely by the heading instrument would only occur about 20 percent of the time. For this analysis, Class III and Class IV airplanes are assumed to be operating IFR nearly 100 percent of the time.
222 Acceptable Configurations for IFR Use
Some acceptable configurations for IFR approval include:
Configuration A:
One dependable magnetic compass.
a. A reliable magnetic gyroscopically stabilized heading system that together meets the AC 23.1309-1E reliability levels, such as 10-4 per flight hour for Class I airplane.
Configuration B:
A magnetic compass that meets the airworthiness requirements but cannot be given credit for AC 23.1309-1E purposes for the reasons under Magnetic Compass or Independent Heading Instrument paragraphs in the Safety Assessment Process.
b. One magnetic gyroscopically stabilized heading system that meets the AC 23.1309-1E reliability levels, such as 10-4 per flight hour for Class I airplane, alone or in combination with one or more of the mitigating factors addressed such as moving map or TAWS with a display.
Configuration C:
(1) A magnetic compass that meets the airworthiness requirements but cannot be given credit for AC 23.1309-1E purposes for the reasons stated under Magnetic Compass or Independent Heading Instrument paragraphs in the Safety Assessment Process.
(2) Two magnetic gyroscopically stabilized heading systems that together meet the AC 23.1309-1E reliability levels, such as 10-4 per flight hour for a Class I airplane.
Configuration D:
A magnetic compass that meets the airworthiness requirements but cannot be given credit for AC 23.1309-1E purposes for the reasons stated under Magnetic Compass or Independent Heading Instrument paragraphs in the Safety Assessment Process.
(1) Two magnetic gyroscopically stabilized heading systems with each being less reliable than required by AC 23.1309-1E (such as 10-4 or 10-5).
(2) A comparator monitor. In this case, the warning from the comparator monitor would alleviate a hazardous misleading heading. However, if the system still has a reliability problem, many nuisance alarms would cause the pilot to ignore the warnings.
Conclusion
There are many factors that affect the criticality/severity of hazardously misleading heading indication, so a safety assessment process using AC 23.1309-1E should be used to classify this failure condition.
The safety assessment process should consider appropriate mitigating factors that might alleviate failure conditions, including:
1. availability and reliability of magnetic compass;
2. independent heading instrument;
3. navigation information;
4. moving map;
5. terrain awareness warning system;
6. air traffic control monito ring of the aircraft; and
7. exposure time.
Amendment 23-49
A proposed revision in NPRM 94-21 explains this amendment as follows:
“Proposed new Sec. 23.1309(a)(4) would correct an inadvertent omission that occurred when the FAA issued Amendment No. 23-41 (55 FR 43306, October 26, 1990). The omitted requirement was adopted by Amendment No. 23-34 as a portion of Sec. 23.1309(d) and read: "In addition, for commuter category airplanes, system and installations must be designed to safeguard against hazards to the airplane in the event of their malfunction or failure." (52 FR 1833, January 15, 1987.) To correct this oversight, and to continue the single fault provision of this paragraph, Sec. 23.1309(a)(4) is being proposed.”
Amendment 23-62 and Subsequent
The NPRM proposed many changes that were not adopted in the final rule. See AC
23.1309-1E.
23.1310 Power source capacity and distribution
Amendment 23-62 and Subsequent
In Docket No. FAA-2009-0738, the Discussion of the Regulatory Amendments stated: Previous § 23.1309(c) and (d) do not directly relate to the other safety and analysis requirements of § 23.1309, therefore the ARC considered it appropriate to state the requirements separately for clarity. FAA agrees with this suggested change, and FAA proposes to add a new § 23.1310 rule to accommodate the change. The requirements as originally stated in current § 23.1309, would not change, except for a new section number. Therefore, adding proposed § 23.1310 would impose no significant change to the current requirements.
The Final Rule is modified by non-substantial clarification.
225 Section 2. Instruments: Installation
23.1311 Electronic display instrument systems
There is no corresponding rule in CAR 3.
The corresponding airworthiness criteria is section 6.13, Airship Design Criteria, FAA-P-8110-2, Change 2.
Synthetic Vision installations should consider AC 23-26, Synthetic Vision and Pathway Depictions on the Primary Flight Display.
Amendment 23-41 through Amendment 23-48 Attitude and Heading For part 23, there is not a specific requirement that instruments at each pilot station be independent unless two pilots are required by the airworthiness or operational rules, except that independent secondary attitude and rate-of-turn instruments that comply with § 23.1321(a) if the primary electronic display instrument system for a pilot presents this information are required. Therefore, both electronic attitude and heading instruments can use the same attitude and heading reference source. An applicant for a single AHRS can present their rationale, including a hazard analysis, explaining how their single AHRS system can meet reliability requirements for a possible catastrophic failure. For a single AHRS, the AFM should include equipment-operating limitations to alert the pilot(s) that a failure of the AHRS could simultaneously affect both attitude and heading instruments. However, a single AHRS may not be acceptable if its affect on an autopilot system is a possible catastrophic failure caused by the AHRS, such as an unannunciated slowover (softover) failure.
Amendment 23-41 and Subsequent A proposed revision in NPRM 89-6 has the following explanation: “A significant number of electronic display systems have become available for installation in small airplanes. These systems include display of all parameters that are typically displayed on a small airplane instrument panel. Approval of these systems in small airplanes was addressed by several conference proposals that proposed to amend Sections 23.1303, 23.1305, 23.1321, 23.1323, and 23.1337.
Conference proposal 420 from the General Aviation Manufacturers' Association (GAMA) recommended removing the words "instrument" and "indicator" from Section 23.1303 and require specific "data" be displayed rather than require specific "instruments". Conference proposal 428 recommended amending Section 23.1305 by changing "powerplant instruments" to "powerplant displays" and "indicators" and "indicating" to displays". Conference proposal 436 recommended amending Section
23.1321 by changing "instrument" to "display" or "data on the flight displays".
226 Conference proposal 439 recommended amending Section 23.1323 by changing "instrument" and "indicator instrument" to "display".
Conference proposal 450 recommended amending Section 23.1337 by adding a new paragraph (a) to state "Displays other than individual indicators may be used if it is shown that adequate isolation is provided between engines and engine parameters."
The basic justification given for all of the recommended changes is that the current requirements of Part 23 were written when the required data could only be supplied using individual instruments. New technology now allows this same data to be displayed in a different manner, possibly with all the data on a common display.
It is desirable to take advantage of available new technology. The benefits include safer (less prone to misreading) displays with less cockpit space and of equal or lower cost than the cluster of typical individual instruments. As technology advances, the amount of energy used for these displays and the cost can be further reduced.
The FAA agrees that when current requirements of Part 23 were written, only mechanical or electromechanical instruments that functioned independently for each parameter displayed were envisioned. The engine instruments and systems envisioned were isolated and independent. A single failure of any engine or any of its systems could not affect the operation of any other engine. The requirements were based on "single fault" or "fail safe" concepts and, when these current requirements were promulgated, the FAA did not envision use of complex, safety-critical systems in small airplanes. All envisioned instruments were single function; i.e., a failure would cause loss of only one instrument function, although several instrument functions may have been housed in a common indicator case.
Since the conference, the FAA has further studied the problems associated with installation of current technology indicators in small airplanes. As is discussed relative to amending Section 23.1309, these current technology systems have potential for being critical for continued safe flight of the airplane. The potential for increased clarity in data display and the concentration of data displays in a single indicator increases the potential criticality of failures. It is anticipated that pilots using these new instrument systems will become increasingly dependent on the use of them because of the tasks they perform for the pilot. After a period of time, where these electronic indicators are located in the primary instrument panel locations, it is anticipated that pilots will find it more difficult to transition to back-up or secondary indicators when failure occurs, such as reverting to use of needle-ball and airspeed for airplane attitude control when the artificial horizon instrument system fails.
The electronic indicators are expected to have significantly different modes of failure where they go from performing perfectly to total failure, whereas the mechanical and electromechanical indicators typically deteriorated in performance over a period of time such that they were replaced before a total failure that prevented them from providing useful information to the pilot.
227 Current technology instrument systems with electronic indicators for small airplanes may very considerably in functional capability, complexity, and cost. Due to the economic considerations, the most expensive, complex, and reliable electronic instrument systems will only be installed in airplanes fitting a like description.
The electronic instrument systems can readily provide digital indication of exact numbers, moving pointer on a scale, and various other formats and combinations of them all. The FAA is especially concerned that pilots be provided adequate sensory cues as to whether numbers displayed are increasing or decreasing and how fast they are changing. Also of concern is that digital indication may not show the normal operating range cues to direction or rate of change or operational limits.
As a result of these concerns and this further review of possible ways to address electronic instrument systems in Part 23, the FAA concluded that a new Section
23.1311 for these systems is better than amending several sections, which may result in an unclear treatment of the issue.
Relative to identifying the indicating means of the electronic instrument system, the FAA has reviewed existing materials and functions to be performed and has concluded the proper identifier is "indicator" rather than "display" as recommended.
Sections 23.1303(a), (b), and (c) require basic flight and navigational instruments for small airplane certification. These mechanical instruments have performed their intended function very well over the years and these basic instruments will remain necessary for safety even when the current technology systems are installed.
Therefore, this proposal will allow displacement of these instruments from the primary location for such instruments for compliance with the requirements of
23.1321, provided their location in a secondary location is such that they are usable and in compliance with Section 23.1321(a) requirements. It is the FAA's intent that this will continue the requirements that airspeed, altitude, and magnetic compass information will remain available to the pilot after total failure of the airplane's electrical power system.
This proposal will allow electronic display indicators for engine parameters without isolation and independence of engine instruments as is now required. In developing this proposal the FAA considered the operational characteristics of airplane engines, the proposed amendment to Section 23.1309 in this notice for assessing failures and their consequences, and the cues available to the pilots for assuring an engine instrumentation failure would not create a condition where the pilot would encounter significant difficult in operating the engines.
Due to the dependence pilots are expected to place on use of electronic indicators when the indicators include information essential to airplane attitude control, the FAA is proposing secondary attitude and rate-of-turn instrument systems that comply with Section 23.1321(a) when the electronic indicators include display of attitude and rate-of-turn.
228 Electronic indicator legibility is expected to change as the cathode ray tubes (CRT) used in the electronic indicators age. Therefore, it is considered necessary that instructions for continued airworthiness relative to the useful life be addressed in compliance with Section 23.1529.
Electronic indicator systems will have great potential for inhibiting information to maximize the effect of other information in various phases of flight. Attitude, airspeed, altitude, and powerplant parameters needed to set power within established limits are information the FAA has concluded must be displayed during all normal modes of operations and, therefore, may not be inhibited during normal modes of operation. Information that is considered essential to continued safe flight must remain available on indicators usable by the pilot after any single failure or combination or probable failures without need for immediate crew action. At a minimum, without considering specific characteristics of an airplane's design, attitude, airspeed, and altitude must remain available without any crew action after such a failure, whereas a failure that would remove other essential information from displays, without resulting in an immediate hazard, would be acceptable provided the essential information could be returned to a usable indicator in a safe elapsed time.”
This amendment is explained by Final Rule, Docket 25812, as follows: “One commenter asks if the wording of proposed Section 23.1311(c), concerning electronic display indicators with features that make isolation and independence between powerplant instrument systems impractical, will be supported by an appropriate amendment to require such isolation. As discussed in Notice No. 89-6, the current requirements of part 23 address powerplant instruments that could provide the required data only by using individual instruments. Accordingly, the isolation and independence referred to in Section 23.1311(c) are currently required in Section
23.903(c). The objective of this regulation is to allow the use of electronic display indicators that will not provide the isolation and independence considered in the current requirements. The FAA is not considering an additional amendment to address this issue.”
See AC 23.1311-1C, “Installation of Electronic Displays in Part 23 Airplanes,” for further guidance.
Amendment 23-49
A proposed revision in NPRM 94-21 explains this amendment as follows: “This proposal would revise Sec. 23.1311 to remove redundant requirements and to clarify which secondary instruments are required and the visibility requirements for these instruments are required and the visibility requirements for these instruments. When Sec. 23.1311 was adopted by Amendment No. 23-41 (55 FR 43306, October 26, 1990), several nonsubstantive changes were made to the proposals in Notice No. 89-6 (54 FR 9345, March 6, 1989) to remove the redundancy included in the notice. In the process certain provisions, such as the one that permitted the installation of mechanical secondary instruments, were inadvertently omitted from the final rule.
229 Since the final rule, discussions with airplane manufacturer representatives have shown that the requirements defining the instrument panel location where secondary instruments may be installed are also not clear. Accordingly, the FAA is proposing to revise this section to correct and clarify these portions.
Current Sec. 23.1311(a), which requires electronic display indicator installations that are independent to each pilot station, would be deleted because it is redundant with Sec. 23.1321(a). Section 23.1321(a) requires that each flight, navigation, and powerplant instrument for use by any required pilot shall be located so that any pilot seated at the controls can monitor the instruments with minimum head and eye movement. As stated in the preamble of Notice No. 89-6 (54 FR 9345, March 6, 1989) regarding the proposed revision to sec. 23.1321, "This revision also clarifies the rule relative to instrumentation that must be provided for each pilot required for type certification or by the applicable operating rules. If a pilot is required by any applicable requirement, then that pilot must be provided all instrumentation required for any operations for which the airplane is approved." Accordingly, the requirements of current Sec. 23.1311(a) would be removed.
In place of current paragraph (a), proposed Sec. 23.1311(a) would be a revision of current paragraph (c) that would clarify what instruments are required and the visibility of those instruments. Proposed new Sec. 23.1311(a)(1) would require electronic display instrument installations to meet the arrangement and visibility requirements of Sec. 23.1321(a).
Proposed Sec. 23.1311(a) (2), (3), and (4) would be redesignated with no changes from current Sec. 23.1311(c)(1), (2), and (3).
Proposed Sec. 23.1311(a)(5) would continue the requirement of Sec. 23.1303(c) for a magnetic direction indicator and, in addition, would require either an independent secondary mechanical altimeter, airspeed indicator, and attitude indicator or individual electronic display indicators for the altimeter, airspeed, and attitude that are independent from the airplane's primary electrical power. These secondary instruments may be installed in panel positions other than the primary location as long as the selected location allows the pilot to properly monitor the instruments and control the airplane.
The substance of proposed paragraph (a)(5) is both a substantive change and a combination of the current Sec. 23.1311(b), which states that certain electronic display indicators must be independent of the airplane's electrical power system, and current Sec. 23.1311(c)(4) which requires independent secondary attitude and rateof-turn instruments and specifies the location of those instruments and specifies the location of those instruments. Proposed Sec. 23.1311(a)(5) would delete the requirement for a rate-of-turn instrument (in current sec. 23.1311(c)(4)) and specify that the required secondary instruments are those that provide altitude, airspeed, magnetic direction, and attitude. The information that would be provided by a secondary rate-of-turn instrument would not appreciably add to the safe operations
230 of the airplane if the pilot has the information provided by the secondary attitude instrument.
Current Sec. 23.1311(b) requires that electronic display indicators required by Sec.
23.1303(a), (b), and (c) be independent of the airplane's electrical power system. The original intent of the requirement for secondary instruments, as stated in Notice No.
89-6, was to require the installation of either mechanical instruments or independent electronic display indicators powered by a source independent of the airplane's electrical system. However, the current rule does not clearly state this and does not address the installation of mechanical instruments. Proposed Sec. 23.1311(a)(5), would allow either secondary electronic display indicators or mechanical instruments to provide a crew with information essential for continued flight and landing in the event of failure in the airplane's electrical power system.
Current Sec. 23.1311(c) (5) and (6) would be redesignated as Sec. 23.1311(a)(6) and
(7) without change.
Proposed new Sec. 23.1311 (b) and (c) would continue the requirements of current Sec. 23.1311 (d) and (e) without change.
This proposal would revise Sec. 23.1311 to remove redundant requirements and to clarify which secondary instruments are required and the visibility requirements for these instruments. No comments were received on the proposal, and it is adopted as proposed.”
This amendment is explained by Final Rule, Docket 27806 as follows: “This proposal would revise Sec. 23.1311 to remove redundant requirements and to clarify which secondary instruments are required and the visibility requirements for these instruments. No comments were received on the proposal, and it is adopted as proposed.”
Amendment 23-62 and Subsequent
See AC 23.1311-1C.
23.1321 Arrangement and visibility
The corresponding rules in CAR 3 are CAR 3.661 and 3.662.
The corresponding airworthiness criteria are sections 6.4 and 6.7, Airship Design Criteria, FAA-P-8110-2, Change 2.
Original Issue and Subsequent When applying this rule to powerplant instruments in multiengine airplanes, assure there is no confusion about the engine/instrument relationship. For instance, powerplant instruments for the right engine in a twin-engine airplane may not be placed over, under, or to the left of the left-engine instruments.
Amendment 23-14 and Subsequent A proposed revision in NPRM 71-13 explains this amendment as follows: “This proposal is considered necessary because of the increasing complexity of the new high performance airplanes and the increasing volume of air traffic. In addition, the proposed new paragraph (d) is in furtherance of the principle of cockpit standardization.”
The NPRM for magnetic direction indicators was revised by Final Rule, Docket 11011 as follows: “Proposed Sec. 23.1321(d)(4) has been revised to provide that the magnetic direction indicator required by Sec. 23.1303(c) need not be mounted in the instrument panel position proposed. The proposed requirement might create difficulties in calibrating and adjusting those indicators in the proposed position.”
For all installations, the evaluation should consider the different environmental conditions under which the airplane may be operated as defined by § 23.1559.
Basic “T” This rule applied the Basic “T” to standardize flight instrument locations. This was not intended to require a “perfect T.”
Also, for all installations, the FAA has always intended that § 23.1321(d) apply to each pilot's station for both type certification and for any operations for which the airplane is approved. Therefore, when an airplane is type certificated with the “basic T” instrumentation at only one pilot's station, that airplane is limited to operations where only one pilot is required according to §§ 23.1525 and 23.1583(h).
Amendment 23-20 and Subsequent
A proposed revision in NPRM 75-23 explains this amendment as follows: “The present visibility requirement in Sec. 23.1321(a) pertains only to flight, navigation, and powerplant instruments. Since other devices may be intended for use in flight
232 and a pilot may develop vertigo by his head movements to see these devices, the location of all visual indicators for use in flight needs to be considered. Of course, flight safety will remain the primary consideration in position priority. The standard of "minimum practicable deviation" will remain, if the rule is adopted as proposed, thus allowing considerations of all factors related to utilization and flight safety. The same proposal is also made for Secs. 25.1321(a), 27.1321(a), and 29.1321(a).
The proposed Sec. 23.1321(e), if adopted, would extend the requirement of the present and proposed Sec. 23.1321(a) to require consideration of all possible cockpit lighting conditions for visibility of required malfunction indicators.
The FAA considered the question whether the proposal should apply only to required malfunction indicators or to all indicators, and believes that if a malfunction indicator has been provided for the crew it should be effective under all cockpit lighting conditions.”
A revision to NPRM by Final Rule Docket, 14625, was in response to comments as follows: “Several commentators questioned proposed Secs. 23.1321(a), 25.1321(a),
27.1321(a) and 29.1321(a) (Proposals 5-3, 5-23, 5-42, and 5-56, respectively) concerning the visibility of instruments with a visual indicator for use in flight by any pilot. It was stated that the proposed standard did not adequately provide for instruments that were monitored infrequently during flight as contrasted with instruments more critical to flight safety. With respect to rotorcraft, one commentator also objected on the ground that the space available for mounting instruments was extremely limited. The FAA agrees that priority of instruments location needs to be more definitive. Therefore, proposed Secs. 23.1321(a),
25.1321(a), 27.1321(a) and 29.1321(a) are withdrawn.
Several commentators objected to the phrase "under all cockpit lighting conditions "in proposed Secs. 23.1321(e), 25.1321(e), 27.1321(d), and 29.1321(g), contending that it includes everything from lightning to total darkness resulting from complete electrical failure at night. In light of these comments and after further FAA review, proposed Secs. 23.1321(e), 25.1321(e), 27.1321(d), and 29.1321(g) are revised to require consideration of all probable cockpit lighting conditions with respect to the visibility of malfunction indicators.”
Amendment 23-41 and Subsequent
A proposed revision in NPRM 89-6 explains this amendment as follows: “This proposal would require those instruments used during certain maneuvers to be located such that minimum eye or head movement is needed to monitor the airplanes flight path and these instruments. Powerplant instruments for which the location requirements apply would be limited to those needed to set power within powerplant limitations. The proposed revision of paragraph (d) would extend the T-arrangement of flight instruments to include all small airplanes certificated for all flight under instrument flight rules. This revision also clarifies the rule relative to