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(c) Account for the acceleration above V2 by trading the climb gradient for speed increase. Integrate this climb gradient loss over the distance required to accelerate to determine an equivalent height increment to be added to all subsequent obstacles.
(5) Gradient loss in turns may be accounted for by increasing the obstacle height by the gradient loss multiplied by the flightpath distance in the turn. This will result in an equivalent obstacle height that can be analyzed as a “straight-out” obstacle in the operator’s airport analysis programs.
(6) For bank angles greater than 15 degrees, the 35-foot obstacle clearance relative to the net takeoff flightpath should be determined from the lowest part of the banked airplane.
18. ADDITIONAL CONSIDERATIONS.
a. AFM Data.
(1) Unless otherwise authorized, AFM data must be used for OEI takeoff analysis. It is recognized that many AFMs generally contain only the OEI performance for loss of an engine at V1 on takeoff. AEO performance must also be considered to determine the airplane’s flightpath in the event of an engine failure at a point on the flightpath after V1. The best available AEO data should be used consistent with best engineering practices. Operators may find appropriate acceptable data in various sources, such as community noise documents, performance engineers’ handbooks, flight characteristics manuals, and manufacturers’ computer programs.
(2) C ertain airports may present situations outside the boundaries covered by the AFM.
AFM data may not be extrapolated without an authorized deviation, as specified in §§ 121.173(f) and 135.363(h). Applications for such deviations, with supporting data, should be sent to the Executive Director, Flight Standards Service, through the Principal Operations Inspector (POI) at the Flight Standards District Office (FSDO) or CMO.
b. Acceleration and Cleanup Altitudes.
(1) For standardization of operating procedures, many operators select a standard cleanup altitude that is higher than that required for obstacle clearance at most airports. With the standard cleanup altitudes, the acceleration and cleanup may be accomplished within the takeoff thrust time limit established in the AFM. The obstacle analysis is usually based on a level-off for cleanup, but there is no operational requirement to level off except in the rare case of a distant obstacle, which must be cleared in the final segment. Obstacle clearance margins usually are improved by continuing the climb during cleanup.
(2) The terrain and obstacles at certain airports may require a higher-than-standard cleanup altitude to be used and may still allow acceleration and cleanup to be accomplished within the takeoff thrust time limit.
c. Validation Flights.
(1) Consideration should be given to conducting a flight to confirm the flightcrew’s ability to fly actual, special OEI departures and to uncover any potential problems associated
Par 18 Page 14 with those procedures, particularly if they differ significantly from the AEO procedures, or if terrain makes course guidance questionable at OEI altitudes. It should be emphasized that the purpose of this flight is not to prove the validity of the performance data or to demonstrate obstacle clearance. In addition, flight deck workload considerations and minimum control speed characteristics are best evaluated in a simulator. Under NO circumstances will validation flights be conducted during any revenue flight. If it is possible to perform the validation or part of the validation in a simulator, presuming that the simulator is appropriately modeled and qualified, it is recommended that the simulator be used for this purpose. If an actual validation flight in an airplane is required, it is recommended that a pre-validation flight be conducted in the simulator to simulate actual evaluation/validation conditions and procedures. It may also be possible that prior experience gained by another airplane type and/or operator may provide sufficient confirmation of the procedure.
(2) A confirmation flight with a simulated engine failure at V1 is not recommended.
Acceptable techniques used for these flights include:
(a) Initiating the procedure from a low pass over the runway at configurations, speeds, and altitudes that represent takeoff conditions.
(b) Using a power setting on all engines calculated to give a thrust/weight ratio representative of OEI conditions.
(c) Setting one engine to flight idle to give a thrust/weight ratio representative of OEI conditions.
19. PILOT INFORMATION.
a. Departure and Go-Around Procedures. The development and implementation of unique departure and go-around procedures should be coordinated with the operator/program manager’s flight operations and/or standardization department. Flightcrews must receive instructions through an appropriate means regarding these procedures. Based on the complexity, this instruction could be done through flight operations bulletins, revisions to selected flightcrew manuals, takeoff charts, NOTAMs, or special ground or simulator training.
b. OEI Procedure Information. The operator should advise flightcrews of the following (this may be accomplished as a general policy for all airports with exceptions stated as applicable, or specified for each airport):
(1) How to obtain V-speeds consistent with the allowable weights, with particular attention given to the effects of wind, slope, improved climb performance, and contaminants.
(2) The intended track in case of an engine failure. (Some operators have a standard policy of flying a runway heading after an engine failure; others routinely assume the AEO ground track unless specifically stated otherwise.) In any case, the intended track should be apparent to the flightcrew, and failure at any point along the track should be taken into account.
(3) Speeds (relative to V2) and bank angles to be flown—AEO and OEI.
Par 19 Page 15
(4) The points along the flightpath at which the flap retraction sequence and thrust reduction are to be initiated.
(5) Initial turns should be well defined. (“Immediate” turns should be specified with a minimum altitude for initiation of the turn or a readily identifiable location relative to the runway or navigational fix.)
NOTE: Current performance information may be provided to flightcrews by dispatch centers that is relevant to each flight operation. Such information may be passed to flightcrews as a result of a dispatch conference, using radio communications, Aircraft Communications Addressing and Reporting System (ACARS), Electronic Flight Bags (EFB), onboard printers, etc.
20. MISSED APPROACHES, REJECTED LANDINGS, AND BALKED LANDINGS.
a. General.
(1) Parts 121 and 135 do not specifically require an obstacle clearance analysis for OEI missed approaches or rejected landings. While it is not necessary to perform such an analysis for each flight, dispatch, or landing weight limitation, it is appropriate to provide information to the flightcrews on the safest way to perform such a maneuver, should it be required. The intent is to identify the best option (or options) for a safe lateral ground track and flightpath to follow in the event that a missed approach, balked landing, rejected landing, or go-around is necessary. To accomplish this, the operator may develop the methods and criteria for the analysis of OEI procedures that best reflect that operator’s operational procedures.
(2) Generally, publi shed Missed Approach Procedures (MAP) provide adequate terrain clearance. However, further analysis may be required in the following circumstances:
(a) A published missed approach has a climb gradient requirement;
(b) The DP for the runway has a published minimum climb gradient;
(c) A special OEI takeoff procedure is required; or
(d) There are runways that are used for landing but not for takeoff.
NOTE: Operators should incorporate procedures for converting required climb gradients to required climb rates in pilot and dispatcher airplane performance sections of their approved training programs.
(3) A distinction needs to be made between a missed approach and a rejected landing. An OEI missed approach from the minimum descent altitude (height) (MDA(H)), decision altitude (height) (DA(H)), or above can frequently be flown following the published MAP. A rejected landing from a lower altitude may require some other procedure (e.g., following the same OEI procedure as used for takeoff). In any case, the pilot should be advised of the appropriate course of action when the published MAP cannot be safely executed.
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b. Assessment Considerations.
(1) Operators may accomplish such assessments generically for a particular runway, procedure, aircraft type, and expected performance, and need not perform this assessment for each specific flight. Operators may use simplifying assumptions to account for the transition, reconfiguration, and acceleration distances following go-around (e.g., use expected landing weights, anticipated landing flap settings).
(2) The operator should use the best available information or methods from applicable AFMs or supplementary information from aircraft or engine manufacturers. If performance or flightpath data are not otherwise available to support the necessary analysis from the above sources, the operator may develop, compute, demonstrate, or determine such information to the extent necessary to provide for safe obstacle clearance.
(3) The operational considerations should include:
(a) Go-around configuration transitions from approach to missed approach configuration, including expected flap settings and flap retraction procedures.
(b) Expected speed changes.
(c) Appropriate engine failure and shutdown (feathering if applicable) provisions, if the approach was assumed to be initiated with AEO.
(d) Any lateral differences of the missed approach flightpath from the corresponding takeoff flightpath.
(e) Suitable balked landing obstacle clearance until reaching instrument approach, missed approach, or en route procedurally protected airspace.
(f) An y performance or gradient loss during turning flight.
(g) Methods used for takeoff analysis (such as improved climb), OEI maximum angle climb, or other such techniques may be used.
(h) Operators may make obstacle clearance assumptions similar to those applied to corresponding takeoff flightpaths in the determination of net vertical flightpath clearance or lateral track obstacle clearance.
c. Assessment Conditions for Balked Landing.
(1) A “balked landing” starts at the end of the touchdown zone (TDZ). A TDZ typically is considered to be the first one-third of the available landing distance or 3,000 feet, whichever is less. When appropriate for the purposes of this provision, operators may propose to use a different designation for a TDZ. For example, alternate consideration of a TDZ may be appropriate for runways:
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(a) That are less than 6,000 feet in length and which do not have standard TDZ markings;
(b) That are short and require special aircraft performance information or procedures for landing;
(c) That are for Short Takeoff and Landing (STOL) aircraft; or
(d) Where markings or lighting dictate that a different TDZ designation would be more appropriate.
(2) An engine failure occurs at the initiation of the balked landing from an AEO configuration.
(3) Balked landing initiation speed is greater than the reference landing speed (VREF) or go-around speed (VGA) (as applicable).
(4) Balked landing initiation height is equal to the specified elevation of the TDZ.
(5) Balked landing initiation configuration is normal landing flaps and gear down.
(6) At the initiation of the maneuver, all engines are at least in a spooled configuration.
d. “One-Way” Airports or Other Special Situations.
(1) Where obstacle clearance is determined by the operator to be critical, such as for:
(a) Airports in mountainous terrain that have runways that are used predominantly for landing in one direction and takeoff in the opposite direction (“one way in” and “opposite way out”); or
(b) R unways at which the planned landing weight is greater than the allowed takeoff weight.
(2) The operator should provide the following guidance to the flightcrew:
(a) The flightpath that provides the best ground track for obstacle clearance, and
(b) The maximum weight(s) at which a missed approach or rejected landing can safely be accomplished under various conditions of temperature, wind, and aircraft configuration.
21. ALTERNATE MEANS. The methods and guidelines presented in this AC are not the only acceptable methods. An operator who desires to use an alternate means should submit an application to the responsible Flight Standards office. The application should describe the alternate assumptions, methods, and criteria to be used along with any other supporting documentation. The responsible Flight Standards office will forward the application through the FSDO (CMO/certificate management unit (CMU)) to the Executive Director, Flight Standards
Par 21 Page 18 Service, for review and approval, if appropriate (refer to Order 8900.1, Volume 1, Chapter 1, Section 1, Table 1-5, Future of Flight Standards (FFS) Nomenclature Guide).
22. AC FEEDBACK FORM. For your convenience, the AC Feedback Form is the last page of this AC. Note any deficiencies found, clarifications needed, or suggested improvements regarding the contents of this AC on the Feedback Form.
ORIGINAL SIGNED by /s/ Robert C. Carty Deputy Executive Director, Flight Standards Service
Appendix 1 APPENDIX 1. OBSTACLE ACCOUNTABILITY AREA FIGURE 1-1. STRAIGHT-OUT DEPARTURES RUNWAY
AIRPORT BOUNDARY
RUNWAYE- 16:1 (0.0625xĐ) 2,000' 200' 300' 4,800' 32,000' NOTE: D IS THE DISTANCE ALONG THE INTENDED
FLIGHT PATH FROM THE END OF THE RUNWAY IN FEET Page 1
Appendix 1 FIGURE 1-2. TURNING DEPARTURES RUNWAY
AIRPORT BOUNDARY START OF TURN
RUNWAY - 16:1 (0.0625 x D) 200' 300° 4,800' 11 25 x D) NOTE: D IS THE DISTANCE ALONG THE INTENDED FLIGHI PATH &- -
FLIGHT PATH FROM THE END OF THE RUNWAY IN FEET * 3,000' -→ Page 2
Advisory Circular Feedback Form If you find an error in this AC, have recommendations for improving it, or have suggestions for new items/subjects to be added, you may let us know by contacting the Flight Technologies and Procedures Division at 9-AWA-AFS400-Coord@faa.gov or the Flight Standards Directives Management Officer at 9-AWA-AFB-120-Directives@faa.gov.
Subject: AC 120-91A, Airport Obstacle Analysis Date: _____________________ Please check all appropriate line items:
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