289 certification of Category III all weather operations are contained in AC 20-128D, “Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout,” dated July 13, 1999.
238. Takeoff Performance Credit For Alternate Forward Center of Gravity Limits.
a. Applicable Regulations . Sections 25.23, 25.103, 25.105, 25.107, 25.109, 25.111,
25.113, 25.115, and 25.121.
b. Explanation .
(1) In the early 1970’s, approvals were granted for one alternate forward center of gravity (c.g.) limit per manufacturer’s airplane model (e.g., DC-10 Series 40, B737-300, A300- 600, etc.). These approvals included AFM performance information that took credit for the improved takeoff performance available with a further aft forward c.g. limit. The effect of this was that all operators of a manufacturer’s airplane model (e.g. B737-300) would have the same alternate forward c.g. limit that may or may not afford any takeoff performance benefit, depending on any one operator’s interior configuration, loading, and route structure.
(2) Since those early approvals, the commercial aviation market has changed considerably, with many transport category airplanes capable of fulfilling more than one mission requirement in an operator’s fleet. This is particularly true for modern extended range operation two-engine airplanes, most of which were derived from an earlier medium range version of the same basic airplane type. When these airplanes are used in long range operations, the c.g. can be considerably aft of the forward limit, upon which the takeoff performance is based, due to large fuel loadings. The advent of integral horizontal stabilizer fuel tanks has also led to a further aftward shift of the center of gravity. Some operators use these same long range airplanes on short to medium length routes, with the c.g. further forward than that of the long range operations, but still significantly aft of the forward c.g. limit.
(3) The operational flexibility of many of today’s transport category airplanes can be enhanced by taking credit for the improvements in takeoff performance afforded by a c.g. that is aft of the forward limit. The reductions in stall speeds and airplane drag render increased takeoff weights for a given field length and takeoff flight path profile. The approval of takeoff performance for two alternate forward c.g. limits would provide this type of operator with increased flexibility in operating the same airplane model on both long range, and short to medium range routes.
(4) The concept of having alternate forward c.g. limits, with associated improvements in takeoff performance, does not conflict with any of the airworthiness or operational requirements of the federal aviation regulations. Sections 25.103(b)(5) and 25.117 state that compliance with the stall speed and climb performance requirements, respectively, must be shown with the “most unfavorable center of gravity” position. Historically, this has been interpreted as a requirement to conduct the associated performance flight testing at the extreme forward c.g. limit, and present the resulting AFM performance for that same extreme forward
c.g. limit. However, no corresponding requirement exists for presentation of AFM data in §
25.1583 (Operating Limitations) or § 25.1587 (Performance Information) that would limit the performance information to being presented for only the most extreme unfavorable center of gravity obtainable for a particular airplane. In the context of the part 25 airworthiness requirements, it would be acceptable to specify alternate forward c.g. limits as operating limitations, and provide takeoff performance that represents the capability of the airplane at those c.g. limits.
(5) The primary concern in granting perfor mance credit for forward c.g. limits, aft of the extreme forward limit, relates to the diminishment of the conservative performance margin that resulted from generally operating with the c.g. some measurable increment aft of the extreme forward limit, upon which the takeoff performance is based. With the availability of alternate forward c.g. limits, this intrinsic performance margin will be decreased, and on a statistical basis, completely eliminated more often. Consequently, emphasis should be put on maintaining accurate weight and balance records, implementing accurate loading plans, and providing the necessary training and operating procedures for ground and dispatch personnel.
(6) Flight crew training is also a concern for alternate forward c.g. limit operations.
With many of today’s transport category airplanes incorporating high levels of cockpit automation (e.g., FMS), it is imperative that the flightcrew be aware of when an alternate forward c.g. limit is being used as the basis for computing takeoff performance data. This is of critical import when an alternate forward c.g. limit is used in conducting a performance-limited takeoff. Since the aforementioned automated onboard systems may or may not be reprogrammed to accommodate the necessary performance data, it should be emphasized that the forward c.g. limit provided by the ground operations and dispatch personnel takes precedence over that shown by any onboard systems.
c. Procedures . Approval may be granted for as many as two alternate forward c.g. limits (for a total of three), with associated takeoff performance credit, provided the following certification and operational criteria are complied with:
(1) Certification Criteria .
(a) No more than two alternate forward c.g. limits (three total) may be approved per operator-specific variant of a particular airplane type and model. “Airplane type” refers to those airplanes of similar design as identified on the type certificate; “airplane model” refers to different versions of an airplane type as reflected on the type certificate. An “operator specific variant” is an airplane type and model outfitted to a particular customer’s requirements (e.g., engine type, seat pitch, galley locations, etc.). For control purposes, airplanes that are classified as an “operator specific variant” should be clearly identifiable as such; this can be accomplished by definitive AFM document numbering systems and AFM’s tied to specific airplane serial numbers.
(b) The alternate forward c.g. limits should be sufficiently different that they will be treated as discrete limits and not result in confusion when determining takeoff performance adjustments.
(c) The c.g. range that results from the use of an alternate forward c.g. limit should be of sufficient magnitude to be practical and allow for expected variations in operational loading.
(d) The alternate forward c.g. limits should be identified as such and presented on the weight and center of gravity chart in the limitations section of the basic AFM. That chart should also provide a reference to a separate appendix to the AFM that contains the related airplane performance adjustments.
(e) To minimize the impact on crew training and standardization of cockpit procedures, it is preferable that onboard FMS be modified to include the performance and fuel management data associated with the alternate forward c.g. limits. If the FMS is not modified in such a manner, a limitation should be added to the AFM that prohibits the use of the FMS when using an alternate forward c.g. limit.
(f) The AFM performance associated w ith alternate forward c.g. limit operations must be substantiated by flight test data per § 25.21(a)(2). This does not mean that flight test data needs to be obtained at every alternate forward c.g. limit; the intent is for the applicant to verify the analytical predictions of c.g. effect with flight test data collected at different c.g.’s.
(g) All affected cockpit placards and displays should be revised to reflect the alternate forward c.g. limits.
(2) Operational Criteria .
(a) Training and operational procedures, emphasizing the need for accurate weight and balance control, should be implemented. These procedures should address weight and balance knowledge of any given airplane, loading plans and procedures, dispatch responsibilities and procedures, and flightcrew awareness.
(b) Before an operator implements alternate forward c.g. limit operations, each airplane that will be used in those operations should be weighed and its operating empty weight (OEW) and center of gravity location determined. This process should be repeated on an annual basis to assure accurate knowledge of the empty airplanes’ c.g. positions.
(c) Operators seeking takeoff performance credit for alternate forward c.g. limits should provide evidence that their loading plans and procedures result in the c.g. location of any affected airplane being known to within one percent mean aerodynamic chord (MAC).
(d) Operators with mixed fleets of a particular airplane type (i.e., more than one operator specific variant of a type) should provide evidence of having an accurate control system to eliminate the potential for loading errors.
(e) If the FMS databases have not been modified to reflect the effect of the alternate forward c.g. limits on performance (e.g., takeoff speeds) and fuel consumption, a
292 limitation should be placed in the appropriate operating manuals that prohibits the use of the FMS when operating with an alternate forward c.g. limit.
(f) All affected documents (weight and balance manuals, flight crew operating manual, etc.) should be revised to reflect the alternate forward c.g. limits. These documents should also note any restrictions associated with their use.
239. Performance Credit For Automatic Propeller Drag Reduction Devices.
a. Explanation . In the description of takeoff path requirements, § 25.111(c)(4) states, “The airplane configuration may not be changed, except for gear retraction and automatic propeller feathering…until the airplane is 400 feet above the takeoff surface.”
b. Procedures .
(1) Propeller pitch setting generally has a significant effect on minimum control speeds and airplane drag. The magnitude of these effects is such that continued safe flight may not be possible should a combined failure of an engine and its automatic propeller drag reduction system occur at a takeoff speed based on operation of that system. Consequently, in accordance with § 25.1309(b)(1), the probability of this combined failure must be shown to be extremely improbable (on the order of 10-9 per flight hour).
(2) In determining the combined engine and automatic propeller drag reduction system failure rate of paragraph (1) above, the engine failure rate should be substantiated.
Notwithstanding the in-service engine failure rate, the failure of the automatic propeller drag reduction system should be shown not to exceed 10-4 per flight hour.
(3) The automatic propeller drag reduction system should be designed so that it will automatically be disabled on the operating engine(s) following operation on the failed engine.
The probability of an unwanted operation of the automatic propeller drag reduction system on an operating engine, following its operation on the failed engine, should be shown to be extremely improbable (on the order of 10-9 per flight hour).
(4) If performance credit is given for operation of the automatic propeller drag reduction system in determining the accelerate-go takeoff distances, the propeller of the failed engine must also remain in the reduced drag position in determining the accelerate-stop distances.
(5) In determining the brake application speed for the maximum energy rejected takeoff demonstration of § 25.109(g), consideration should be given to the accelerate-stop configuration (all-engines-operating vs. engine failure with automatic propeller drag reduction system operation) that will produce the greatest brake energy absorption requirement.
(6) The limitations section of the AFM should contain a requirement for a flight crew functional check of the automatic propeller drag reduction system. The frequency with which
293 flight crew functional checks and ground maintenance checks of the automatic propeller drag reduction system are required will be considered in the evaluation of the system reliability.
(7) Clear annunciations should be provided to the flight crew for the following:
(a) The automatic propeller drag reduction system is “ARMED.”
(b) A malfunction exists in the automatic propeller drag reduction system.
240. Airplane Backing Using Reverse Thrust.
a. Explanation . Where compliance with applicable airworthiness requirements has been demonstrated, operational approval has been granted for the use of reverse thrust to back airplanes away from airport gates in lieu of a tug pushback. The applicable airworthiness requirements are prescribed in paragraph b, below. Note that compliance with these requirements only demonstrates the capability of the airplane, and does not constitute operational approval; operational approval should be coordinated with the applicable FAA Flight Standards office.
b. Procedures . To obtain airworthiness approval for reverse thrust backing, compliance should be demonstrated with the criteria prescribed below. Since operational approvals for reverse thrust backing specify the applicable airplanes, airports, and gates, some of these tests will be site specific and should be coordinated with the applicable FAA Flight Standards office.
(1) Both the airplane and engine manufacturer should determine the applicability of the maneuver and provide appropriate limitations for the procedure. These limitations should include:
(a) Engine power setting and operating parameter limits;
(b) Minimum and maximum allowable weights;
(c) Center of gravity (c.g.) limits;
(d) Ramp slope limits;
(e) Use of wheel brakes;
(f) Atmospheric conditions (see NOTE 1, below); and
(g) Any other factor unique to the proposed operation.
NOTE: 1. The use of reverse thrust backing with snow, ice, or slush on the ramp, or during periods of heavy rain, is not considered good operating practice.
(2) All limitations, and any normal or abnormal operating procedures, associated with reverse thrust backing should be included in the AFM. Any procedures related to ground crew functions should also be included in the AFM.
(3) Testing should be conducted to verify that the reverser efflux does not have a detrimental effect on the powerplant installations. Items to be considered include:
(a) Foreign object damage (FOD);
(b) Effects on engine cooling;
(c) Inlet flow distortion;
(d) Re-ingestion of exhaust gases; and
(e) Any effects on engine mounted accessories.
(4) Testing should be conducted to verify that the reverser efflux does not have a detrimental effect on other airplane systems such as air conditioning system inlets, APU inlets and exhausts, overboard drains, etc.
(5) It should be verified that cockpit and cabin air will not be contaminated.
NOTE: 2. It is acceptable to have a limitation prohibiting the operation of the air conditioning packs during reverse thrust b acking to avoid cockpit and cabin air contamination.
(6) Reverse thrust backing demonstrations should be conducted to evaluate the associated procedures. These demonstrations should be conducted at:
(a) Maximum ramp weight with the c.g. at the aft limit, and
(b) Any other weight that may be critical with the c.g. at the aft limit.
(7) The reverse thrust backing demonstrations of paragraph (6), above, should be evaluated to determine:
(a) The degree of aft pitching moment, including its effect on nose wheel steering;
(b) The effect of inadvertent heavy/emergency braking action;
(c) Adequate cockpit visibility and ground crew function;
(d) The adequacy of procedures for transitioning from reverse to forward thrust;
(e) The effect of thrust asymmetry due to failure of an engine to enter reverse, or recover forward thrust mode;
(f) The effects of low tire pressures;
(g) The effect of ramp slope and surface condition;
(h) The effects of ambient atmospheric temperature; and
(i) The effect of flap/slat configuration.
Appendix 1 A1-1 Appendix 1
Acronyms and Abbreviations
Acronym/ Definition Abbreviation
AC advisory circular ACARS aircraft communications addressing and reporting system ACO aircraft certification office ACN aircraft classification number ADC air data computer ADF automatic direction finder ADI attitude director indicator ADQ adequate AEO all-engines-operating AFM airplane flight manual AGL above ground level AHRS attitude and heading reference system AOA angle of attack A-PC airplane-pilot coupling APU auxiliary power unit ARSR air route surveillance radar ARTCC air route traffic control center ASR airport surveillance radar ATA Air Transport Association ATC air traffic control ATCRBS air traffic control radar beacon system ATS automatic throttle system ATT REF attitude reference ATTCS automatic takeoff thrust control system C Celsius CAR civil air regulations
Appendix 1 A1-2 CAS calibrated airspeed CAT category CAWS central aural warning system CBR California bearing ratio CDL configuration deviation list CDU cockpit display unit CFR Code of Federal Regulations
c.g. center of gravity CL lift coefficient CMM component maintenance manual CN yawing moment coefficient COM communications CON controllable CRT cathode ray tube CVR cockpit voice recorder CWS control wheel steering Δ incremental change in value δ ratio of atmospheric pressure at any altitude to the pressure at sea level dB decibel DFDR digital flight data recorder DG directional gyros DH decision height DME distance measuring equipment ECAM electronic centralized aircraft monitor EFCS electronic flight control system EGT exhaust gas temperature EICAS engine indicating and crew alerting system ELT emergency locator transmitter EMC electromagnetic compatibility EMI electromagnetic interference EPR engine pressure ratio
Appendix 1 A1-3 ESDU engineering sciences data unit F Fahrenheit F & R function and reliability FGS flight guidance system FMEA failure modes and effects analysis FMS flight management system FOD foreign object damage FS longitudinal control (stick) force FS/g stick force per g ft. foot or feet FWC flight warning computer g or G acceleration due to gravity at the Earth’s surface GNSS global navigation satellite system GPWS ground proximity warning system HF high frequency HGS head up guidance system HQ handling qualities HSI horizontal situation indicator HQRM handling qualities rating method HUD head up display Hz cycles per second IAS indicated airspeed ICAO International Civil Aviation Organization IKE initial kinetic energy ILS instrument landing system IMC instrument meteorological conditions INS inertial navigation system IRS inertial reference system ITT interstage turbine temperature IVSI instantaneous vertical speed indicator KE kinetic energy
Appendix 1 A1-4 LF low frequency LFE limit flight envelope LNAV lateral navigation M Mach number MAC mean aerodynamic chord MC master caution MCS master caution system MCT maximum continuous thrust MDA minimum descent altitude MEL minimum equipment list MFD multi-function display MF medium frequency Min. minutes min. minute(s) MLRC long range cruise Mach MLS microwave landing system MMEL master minimum equipment list MW master warning MWS master warning system max maximum friction coefficient available between a tire and the runway surface N1 rotational speed of low pressure compressor (turbine engine) N2 rotational speed of high pressure compressor (turbine engine) NAS national airspace system NAV navigate NM nautical mile(s) NFE normal flight envelope
Appendix 1 Change 1 A1-5 OEW operating empty weight OFE operational flight envelope PA public address PFC porous friction course PIO pilot induced oscillation Q dynamic pressure R/A radio altimeter RFI radio frequency interference
r.p.m. or RPM revolutions per minute RMI radio magnetic indicator RNAV area navigation RNP required navigation performance RTO rejected takeoff RVP Reid vapor pressure RVR runway visual range SAAAR special aircraft and aircrew authorization required SAS stability augmentation system SAT static air temperature, or satisfactory (when used with the handling qualities rating system) SATCOM satellite communications secs. seconds SELCAL selective calling system S/F slow/fast SR special regulation STC supplemental type certificate STOL short takeoff and landing SWC stall warning computer TAT total air temperature TCAS traffic collision avoidance system TIA type inspection authorization