Wide body aircraft types not previously authorized or currently authorized by FAA to use minima less than RVR1000 based on a fail passive system must meet the airworthiness requirements of Appendix 3 or equivalent for any new authorization of minima less than RVR1000.
New aircraft types or derivative aircraft with new flight control system designs should be demonstrated in accordance with Appendix 3 for Fail Passive systems, or equivalent requirements, if fail passive authorization is sought.
4.3.8. Category IIIb. Category IIIb operations are usually conducted with fail operational systems.
Fail passive landing systems may be used, but are limited to Category IIIb minima not less than touchdown zone RVR600 (175 m). Airborne systems used for Category IIIb authorized for landing below touchdown zone RVR600 (175 m) must include either a manual flight guidance or automatic rollout or control system for lateral steering which provides the means to control the aircraft until the aircraft slows to a safe taxi speed. Category IIIb operations based on fail operational systems require the use of systems which after passing Alert Height, are capable of the safe completion of the approach, touchdown, and rollout, following any failure conditions not shown to be extremely remote. When fail operational systems are used, they do not necessarily require that operating procedures specify that the approach must necessarily be continued after a failure.
Category IIIb operations based on fail passive landing systems meeting provisions of Appendix 3 of this AC, or equivalent, must use a decision height not less than 50 ft. HAT.
For Category IIIb operations based on fail operational systems, the availability of visual reference is not a specific requirement for continuation of an approach to touchdown. The design of flight instrument systems, annunciations, and alerting systems should be adequate to assure that the pilot can verify the aircraft should touch down within the touchdown zone, and rollout.
Category IIIb operations may be conducted to a touchdown zone RVR and relevant mid or rollout RVR not less than 600 ft. with a fail operational or fail passive landing system without a rollout control system.
Category IIIb operations may be conducted to a touchdown zone RVR of not less than 600 ft. and an relevant mid or rollout RVR not less than RVR400 with a fail operational or fail passive landing system and with any FAA approved rollout control system.
Category IIIb operations may be conducted to a touchdown zone, and relevant mid, and rollout RVR minima not lower than RVR400 (125 m) when using a fail operational landing system and a rollout control system shown to meet Fail Passive criteria of Appendix 3 (or earlier FAA criteria applicable to a rollout system).
Category IIIb operations may be conducted to a touchdown zone RVR, and relevant mid, and rollout RVR minima not less than RVR150 (50 meters) with a fail operational landing system and a rollout control systems shown to meet the Fail Operational criteria of Appendix 3, or earlier equivalent criteria.
See NOTE below for criteria, and Section 4.3.2 above for examples of various aircraft types, systems, and minima which may be authorized.
NOTE: Since amended criteria is specified for fail operational rollout control systems in Appendix 3 of this circular, certain systems previously certificated using earlier criteria of AC120-28C or AC120-28B that at the time of certification were not considered fail operational due to certain conditions that were noted in the AFM as exceptions, but now are considered to meet fail operational criteria, may now be considered as fail operational for the purpose of authorizations under this paragraph.
Accordingly, systems considered to be eligible for this credit are those systems which otherwise previously met all provisions for a fail operational rollout control system as described in Appendix 3 of AC120-28C (or earlier AC 120-28A or B equivalent) except for provisions related to inadvertent control wheel autopilot disconnect and level of redundancy required for nosewheel steering function. Each of these provisions are now revised in Appendix 3.
Operators seeking additional allowable credit for these newly re-classified fail-operational rollout control systems may apply for corresponding minima adjustment through amended operations specifications. "Relevant" or "applicable" mid or rollout RVR is considered to be any transmissiometer report (or equivalent instrumentally derived RVR measurement) considered to be covering a portion of the runway where the aircraft is operating at a speed above a safe taxi speed.
A summary of the minima that may now be authorized based on the above criteria are shown below in table
4.3.8-1.
Landing System Type Rollout System Type TDZ RVR (ft)** Mid RVR (ft)** Rollout RVR (ft)** Fail Op or Fail Passive* No Rollout system 600 600 600 Fail Op or Fail Passive* Any FAA Approved Rollout System 600 400 400 Fail Op* Fail Passive* 400 400 400 Fail Op* Fail Op* 150 150 150 Lowest Minima Authorized for Fail Operational or Fail Passive Landing Systems, or Landing and Rollout Systems Table 4.3.8 -1 Table 4.3.8-1 Notes:
* Note: Fail Op denotes a Fail Operational Flight Guidance System Fail Passive denotes Fail Passive Flight Guidance System ** Note: RVR values are shown in U.S. facility units of feet and as Touchdown zone, Mid, and Rollout RVR values, but corresponding metric values or other designated RVR reporting locations may be applied internationally, if acceptable to the State of the Aerodrome.
Equivalent minima may be specified for systems demonstrated to meet earlier airworthiness criteria of AC 120-28B or AC 120-28C. Credit for systems demonstrated prior to AC 120-28B will be as designated in approved operations specifications or as designated by AFS-400 for new Category III applications using such aircraft.
4.3.9. Runway Field-Length.
a. The Runway Field-Length for Category III is as specified by section 121.195 for a wet runway, if each of the following conditions are met:
1. Anti-skid systems are operative (if installed for the aircraft type).
2. The runway surface braking action is expected to be at least "fair" or better (or equivalent Runway Condition Reading, James Brake Devise, or Tapley reading).
In the event that either of the above conditions are not met, the factor to be applied to the section 121.195b distance is 1.3, unless otherwise demonstrated to the FAA that a factor less than 1.3 is acceptable (e.g., due to other factors, such as the required use of an auto brake system).
b. Once airborne, additional consideration of Category III landing field length by the flightcrew is not required for normal operations. In the event of un-forecast adverse weather or if failures occur, the crew and aircraft dispatcher should consider any adverse consequences that may result from a decision to make a Category III landing (e.g., braking action reports). Category III operations should not normally be conducted with braking action less than "fair".
c. When auto brake systems are used for Category III, information must be available to the flightcrew to assist in making the proper selection of a suitable auto brake setting consistent with the field length available for landing and the runway condition, including braking action.
4.3.10. Landing System Sensors (NAVAID’s) and Aircraft Position Determination. Various landing system sensors (NAVAID’s) or combinations of sensors may be used to provide the necessary position fixing capability to support authorization of Category III landing weather minima. While certain navigation sensors (NAVAID’s) are installed and classified primarily based on landing operations, the sensors described in this section may also be used for takeoff, missed approach, or other operations
(e.g., RNAV position determination). Regardless of the sensors, NAVAID’s, or combination of
NAVAID’s used, the NAVAID’s and sensors must provide coverage for the intended flight path and for anticipated displacements from that flight path for normal operations, rare normal operations (e.g., winds and wind gradients), and for specified non-normal operations where applicable (e.g., "engine-out go-around" flight path). In addition, Category III authorizations should be consistent with the provisions or characteristics for specific sensors listed below in 4.3.10.1 through 4.3.10.3 unless otherwise accepted or approved by FAA.
4.3.10.1. Instrument Landing System (ILS). The ILS provides a reference signal aligned with the runway centerline and deviation signals when the airplane is displaced left or right of the extended runway centerline. The linear coverage area for this signal is approximately 3 degrees either side of the extended runway centerline from a point emanating at the far end of the runway. The ILS also provides a vertical flight path (nominally 3 degree descent angle) to a point in the landing zone of the runway. The vertical coverage is approximately 0.7 degrees on either side of the vertical reference path. ILS characteristics should be considered as defined in ICAO Annex 10, unless otherwise specified by FAA.
U.S. ILS systems are classified by Type, as defined in FAA Order 6750.24 as amended (e.g., II/D/2).
4.3.10.2. Microwave Landing System (MLS). The MLS provides a reference signal aligned with the runway centerline and deviation signals when the airplane is left or right of the extended centerline.
The linear coverage area is approximately 40 degrees either side of the extended runway centerline emanating from a point at the far end of the runway. The MLS provides a vertical flight path to the runway similar to ILS. MLS characteristics should be considered as defined in ICAO Annex 10, unless otherwise specified by FAA. U.S. MLS systems are classified by Type, similar to ILS.
4.3.10.3. GPS/GNSS Landing System (GLS). GLS is a landing systems based upon the Global Navigation Satellite System (GNSS). For Category III operations the landing system typically includes a local area differential augmentation system in the vicinity of the runway for which a Category III procedure is specified. The local area system may serve one or more runways, or nearby airports, depending on its classification for each particular runway. The classification of a GLS service may be different for different runway ends (e.g., III/E/3 for Runways 14L and 14R, but I/D/1 for RW 22L).
Desired path, centerline, and deviation signals as applicable, are computed by airborne avionics. The coverage area for GLS is typically at least within a 25 mile radius of a primary airport, but extended service volumes are permitted. GLS provides for both vertical and lateral flight path specification to the touchdown zone of the runway(s) served, and a lateral path for rollout or takeoff guidance. GLS characteristics should be considered as defined in ICAO Annex 10, unless otherwise specified by FAA
(e.g., FAA accepted references to RTCA SC-159 MASPS).
U.S. GLS systems should be operationally classified by "Type" for each runway end served, similar to ILS (e.g., GLS II/D/2). The operational classification is intended to address at least the service provided in terms of “Type of service or performance level”, “area of service” relative to each specific runway served, and level of availability or continuity. As with ILS, the classification schema should not be specified in terms of specific minima Categories (e.g., Category III), since onboard aircraft equipment capability or other factors may determine actual minima to be authorized.
GLS systems are intended to provide equivalent or better service to that provided by a U.S.
Type I ILS, including: 1) Adequate guidance over the runway for takeoff or landing rollout
(e.g., autoland), and 2) Vertical guidance on approach down to at least 100’HAT. GBAS local area augmentation systems (e.g., LAAS) are usually considered to support this objective. Wide area augmentation systems (e.g., WAAS, EGNOS) are typically considered to be RNAV systems, and are not alone considered to be GLS.
Authorization for use of GLS is for each specific air carrier, aircraft, and GLS system type until pertinent GLS international standards accepted by FAA are promulgated.
4.4. RNAV/Flight Management Systems (FMS). RNAV/Flight Management Systems (FMS) are typically used in conjunction with Category III Instrument Approach Procedures only for initial or intermediate approach segments, or for missed approach.
For departure, RNAV/Flight Management Systems (FMS) may be used for non-visual takeoff guidance after passing the height at which LNAV or VNAV may respectively be engaged or made active, or above 35' AGL, whichever is higher. Other applicable FAA criteria (e.g., section 121.189) must be addressed for takeoff. For development or authorization of departure procedures which follow completion of a low visibility takeoff, FAA Orders 8260.40A, 7100.1, or other applicable RNAV/RNP criteria should be consulted.
Procedures based on 3D or 2D RNAV may or may not include use of RNP. For RNP operations, see section 4.5 below.
4.5. Required Navigation Performance (RNP). A definition for Required Navigation Performance (RNP) is specified in Appendix 1. Standard Levels of RNP typically used for various initial, intermediate approach and missed approach segments for Category III procedures may be based on specific landing systems (e.g., ILS, MLS, or GLS), on multisensor RNAV (e.g., FMS with IRS, VOR, DME inputs), or on other aircraft navigation systems having FMS like capabilities (e.g., GPS Navigation Systems).
RNP applications used for a final approach segment supporting a Category III procedure are typically be based on use of a specific landing system sensor (e.g., ILS, MLS, or GLS), or on multisensor RNAV systems having suitable flight critical performance.
Aircraft/systems RNP capability is typically determined by assessing RNP total system error components, Path Definition Error (PDE), Position Estimation Error (PEE) and Flight Technical Error (FTE). In additional to total system error (TSE) components, associated assurance limits are considered to determine what levels of navigation performance may actually be achieved with integrity. Assessment may specify that certain RNP levels may only be achieved with flight director or autopilot coupled operation, or that for particular applications availability must be based on use of dual independent navigation systems
(e.g., as with the/E equipment classification).
4.5.1. Standard RNP Types. Standard values of RNP supporting initial, intermediate, or final approach segments, or missed approach segments applicable to Category III procedures are as specified in Table 4.5.1-1 below.
Table 4.5.1-1 STANDARD RNP TYPES APPLICABLE TO Category III RNP Type Applicability/Operation (Approach segment) Normal Performance (95%) Containment Value RNP 1 Initial/Intermediate/ Missed approach +/- 1 nm +/- 2 nm RNP 0.5 Initial/Intermediate/ Missed approach +/- 0.5 nm +/- 1 nm RNP 0.3 Initial/Intermediate/ Missed approach +/- 0.3 nm +/- 0.6 nm RNP levels as specified for lowest Category I Final approach/ initial missed approach (but not below 200' HAT) (See AC 120-29, as amended (See AC 120-29, as amended) RNP levels as specified for Category II Final approach/ initial missed approach (but not below 100' HAT) (See AC 120-29, as amended) (See AC 120-29, as amended) RNP 0.003/15 Final approach/ initial missed approach (any altitude) +/- 0.003 nm +/- 15 ft. (*) +/- 0.006 nm +/- 30 ft. (*) (*) Note: vertical accuracy does not apply below 100 ft. HAT - below 100 ft. HAT vertical performance is determined by applicable standards for touchdown performance.
RNP is a required navigation performance level described by the specification of a numeric value indicating the required navigation accuracy for a specific operation, typically specified laterally in nautical miles - e.g., RNP 1 is a Required Navigation Performance of ± 1 nautical mile (95% Probability).
RNP containment is specified as RNP(X) times two (i.e., RNP(X) x 2).
Standard RNP Levels are defined for lateral performance, or lateral and vertical performance, if applicable.
Regarding containment, the probability of exceeding the containment value without annunciation should be assessed by some acceptable method. If assessed during an airworthiness demonstration program during type design or STC, this probability should be considered to be at least remote (e.g., a major failure
condition). If based on “fleet qualification” (e.g., use of criteria such as contained in RTCA DO-236 Appendix D for RNP) alternative methods, procedures or buffers may be used to assure protection of containment.
Operationally, containment is typically used in procedures or separation standards in conjunction with other operational verification procedures, other systems (e.g., TCAS, EGPWS), or operational buffers to satisfactorily address assurance of safe operations and contingencies. This is to provide for safety in the unlikely event operation within the containment region cannot be maintained.
In the event that operational procedures or other technical methods are not available to assure safety in the event of loss of containment, then potential for loss of containment situations may need to be addressed in a hazard assessment specifically keyed to an example of the intended application or operation. More demanding uses of containment may require a higher level of assurance or a more complex or comprehensive backup or contingency mechanism (e.g., use of an RNP Level for specification of a lateral path for Category III flare and rollout).
4.5.2. Non-Standard RNP Types. Non-Standard RNP Types are those RNP values other than as specified in paragraph 4.5.1. Non-Standard RNP Types are authorized by FAA on a case by case basis where an applicant has a demonstrated need for such use.
4.6. Flight Path Definition. The following parameters, path points, or definitions may be used as a technical basis for a variety of other references or terms eventually necessary to prepare or implement procedures or development criteria. This section is intended to provide a technical basis for preparation of operational concepts for use with RNAV and RNP based systems (e.g., to serve as a technical basis for preparation of procedures criteria, operations concepts, operational manuals, or operational authorizations). Actual procedural information, manuals, training materials and other documents used by operators, pilots, procedure developers or others who may need to apply this information may use simplified or summarized forms of this information, as necessary, to satisfy their particular operational requirements.
Note: The path points and segments addressed below are those considered necessary to support definition and operational use of systems intended for Category III. For commonality, their definition and use are intended to be consistent with Category I and Category II. It should be noted, however, that various GNSS or other RNAV applications or criteria currently use or propose use of other similar points or waypoints, use different terminology, or use different definitions for points (e.g., Landing Threshold Waypoint). Accordingly, since U.S. and international terminology and definitions for such points are evolving, it is important to confirm and assure suitability of each point used, for consistency of terminology, suitable definition, and suitable application.
Landing and Rollout Flight Path - The following criteria specifies certain reference points and other criteria necessary to effectively implement landing and rollout operations using a landing system where the required flight path is not inherent in the signal structure of the navigation aid (e.g., satellite systems).
The location of points used to describe the landing and rollout flight path are shown in Figure 4.6-1.
Runway Datum Point (RDP) - The RDP is used in conjunction with the FPAP and a vector normal to the WGS-84 ellipsoid at the RDP to define the geodesic plane of a final approach flight path to the runway for touchdown and rollout. It is a point at the designated lateral center of the landing runway defined by latitude, longitude, and ellipsoidal height. The RDP is typically a surveyed reference point used to connect the approach flight path with the runway. The RDP may or may not necessarily be coincident with the designated runway threshold.
Flight Path Alignment Point (FPAP) - The FPAP is used in conjunction with the RDP and a vector normal to the WGS-84 ellipsoid at the RDP to define the geodesic plane of a final approach and landing flight path. The FPAP may be the RDP for the reciprocal runway.
Flight Path Control Point (FPCP) - The FPCP is a calculated point located above the RDP in a direction normal to the WGS-84 ellipsoid. The FPCP is used to relate the vertical descent of the final approach flight path to the landing runway.
Datum Crossing Height [DCH] - The height (feet) of the FPCP above the RDP.
Glide Path Angle [GPA] - The glide path angle is an angle, defined at the FPCP, that establishes the descent gradient for the final approach flight path of an approach procedure. It is measured in the geodesic plane of the approach (defined by the RDP, FPAP, and WGS-84 ellipsoid’s center). The vertical and horizontal references for the GPA are a vector normal to the WGS-84 ellipsoid at the RDP and a plane perpendicular to that vector at the FPCP, respectively.
Glidepath Intercept Reference Point [GIRP] - The GIRP is the point at which the extension of the final approach path intercepts the runway.
Figure 4.6-1
Datum Crossing Height (DCH)
Runway Datum Point (RDP) Glidepath Intercept Reference Point (GIRP) Flight Path Control Point (FPCP) Flight Path Alignment Point (FPAP) η η = Glide Path Angle The locations established for, and the values assigned to, the RDP, FPCP, DCH and GPA should be selected based upon the operational need to establish the GIRP. Operational considerations include:
1. Path of wheels over threshold,
2. Need for coincidence with other aids and systems - visual and non-visual, or markings
3. Runway characteristics (e.g., local slope in the touchdown zone, general runway upslope or downslope, runway lateral crown),
4. Actual displaced thresholds or multiple runway thresholds,
5. Actual clearways or stopways
6. Construction temporary requirements Takeoff Flight Path. The flight path for low visibility takeoff (while on the runway) should be defined by the RDP and FPAP.
4.7. Engine Inoperative Category III. The basic concept addressing expected flightcrew response to non-normal situations, such as typically represented by an engine failure, is described in Section
6.1.8.
Engine Inoperative Category III is specifically addressed because for some modern aircraft and systems it is possible to retain and address Category III operating capability, as an alternate configuration. This is to aid operators and pilots in planning for and selecting more desirable and fully considered landing airports that may be closer in distance or time, have less adverse weather in terms of icy or short runways with adverse winds, are better equipped to handle a diversion or emergency, or provide a timely return to land at a takeoff airport or a closer takeoff alternate airport, in the unlikely event of engine failure or other serious problem related to the engine failure.
With pre-planned engine inoperative Category III capability, airports and minima that otherwise may not be considered acceptable for use could be selected by the pilot or operator without having to subsequently justify its use based on emergency authority. This capability also has the advantage of allowing for full pre-assessment of the aircraft capability, and engine inoperative aircraft configurations
(e.g., flap settings, electrical system capability, hydraulic system capability), approach procedure characteristics, missed approach performance and other factors that may be difficult to assess in real time if not previously assessed.
This capability also can permit an operator some additional flexibility in selecting alternate airports.
Accordingly Section 5.17 describes aircraft and airborne system criteria for credit for engine inoperative Category III capability. Sections 7.1.3 item 10, and 7.2.6 address crew qualification. Section 10.8 addresses operational authorization and selection and use of alternates.
5. AIRBORNE SYSTEMS.
5.1. General.
5.1.1. Airborne Systems. Airworthiness criteria for airborne systems intended to meet criteria in this AC are specified in paragraph 5.1.3 through 5.19 below for operational authorizations, or Appendix 2 for takeoff, or Appendix 3 for landing and rollout for airworthiness demonstration of new or modified aircraft types or systems.
Aircraft shown to meet provisions of Appendix 2 or 3 respectively, are considered to meet provisions of this section.
For aircraft approved using earlier versions of this AC, airworthiness criteria for airborne systems intended for Category III operations are as specified in criteria referenced by the approved AFM.
Airborne equipment listed in this section needs to be operative for Category III, in accordance with provisions of applicable standard Operations Specifications (OpSpecs) and Operations Specifications for that operator. Airframe manufacturers and individual operators may also include other optional equipment as part of the Category III configuration, however, that equipment does not need to be operative to conduct a Category III approach unless required by that operator's OpSpecs.
Airborne systems making use of RNP capability for approach segments or missed approach segments should be approved in accordance with or consistent with any one or more of the following criteria or processes:
1) FANS I or FANS A systems approved through a certification program acceptable to FAA, or
2) Through reference to an acceptable standard such as RTCA DO-236 for RNP, or
3) Through reference to RTCA DO-236 Appendix D for “fleet qualification” of existing aircraft, or
4) Through other FAA or FAA accepted equivalent criteria for intended for application or approval of RNP (e.g., TERPS or PANS-OPS RNP provisions as adopted).
5.1.2. Non-Airborne Systems. Unless otherwise specified in the Appendices to this AC, NAVAID/landing system characteristics, including facility classification, should be considered as specified in Section 4.3.10 above and AC 120-29 for ILS, MLS or GLS (e.g., U.S. use of ICAO Annex 10 Criteria, FAA Order 6750.24 as amended, and the applicable NAVAID facility classification for Category III). NAVAID facility use is predicated on applicable ILS, MLS, or GLS Type classifications (e.g., ILS III/E/2, GLSII/D/2) or equivalent classification at non-U.S. facilities. Specific Navigation Services are addressed in Section 5.12.
5.1.3. Takeoff Guidance Systems. When takeoff minima are predicated on use of a takeoff guidance system, the takeoff guidance system should be demonstrated to meet provisions of this paragraph or provisions specified in Appendix 2 by an airworthiness demonstration. Takeoff guidance systems which have been shown to meet Appendix 2 by airworthiness demonstration and have a corresponding AFM reference are typically considered to meet requirements of this paragraph.
A takeoff guidance systems shall be demonstrated to show that the airplane will not deviate significantly from the runway centerline during takeoff while the system is being used within the limitations established for it. Compliance may be demonstrated by flight test, or by a combination of flight test and simulation.
Flight testing must demonstrate repeatable performance, and cover those factors affecting the behavior of this airplane (e.g., wind conditions, ILS characteristics, weight, center of gravity). Compliance with the performance envelope should be demonstrated with pilots appropriately qualified to use the airborne system, and should not require extraordinary skill, training or proficiency.
Demonstrated winds should be 150% of the winds for which credit is sought, but not less than 15 knots of headwind or crosswind.
In the event that the airplane is displaced from the runway centerline at any point during the takeoff or rejected takeoff, the system must provide sufficient guidance to enable the "pilot flying", or the pilot in command who may assume control and become the "pilot flying", to control the airplane smoothly back to the runway centerline without significant overshoot or any sustained nuisance oscillation.
Figure 5.1.3-1 provides the performance envelope for evaluating takeoff command guidance systems for the following scenarios:
a. Takeoff with all engines operating
b. Engine Failure at Vef - continued takeoff
c. Engine Failure just prior to V1 - rejected takeoff
d. Engine Failure at a critical speed prior to Vmcg rejected takeoff: (Vef < Vmcg) Note: For that portion of the flight path following liftoff, the demonstrated lateral path may be adjusted for any effect of wind drift when showing compliance with the performance envelope below.
Airplane Speed Failure/Continued Take-off Runway Centerline
VFAILURE + 7m 0 + 14m Airplane Speed All Engines Take-off Runway Centerline V1 VLOF + 7m 0 Airplane Speed Failure/Rejected Take-off Runway Centerline
VFAILURE + 7m 0 + 14m h=35' + 14m h=35' STOP FIGURE 5.1.3-1.
5.2. Airborne Systems for Category III Minima Not Less than RVR600. The following equipment in addition to the instrument and navigation equipment required by 14 CFR part 91 for IFR flight is the minimum aircraft equipment considered necessary for Category III: