EOSID Methods of Analysis

The net takeoff flightpath must clears all obstacles by either 35 feet vertically or 200 feet laterally inside the airport boundary, or 300 feet laterally outside the airport boundary. To operate at the required lateral clearance, the operator must account for factors that could cause a difference between the intended and actual flightpaths and between their corresponding ground tracks. For example, it cannot be assumed that the ground track coincides with the extended runway centerline without considering such factors as wind and available course guidance (reference paragraph 14). This AC will focus on two methods that may be used to identify and ensure clearance of critical obstacles: the Area Analysis Method and Flight Track Analysis Method.

The two methods may be used in conjunction with each other on successive portions of the analysis. For example, an operator may choose to use an area analysis for the initial portion of the takeoff analysis, followed by a flight track analysis, and then another area analysis.

a. The Area Analysis Method defines an obstacle accountability area (OAA) within which all obstacles must be cleared vertically. The OAA is centered on the intended flight track and is Page 6 Par 8 5/5/06 AC 120-91 acceptable for use without accounting for factors that may affect the actual flight track relative to the intended track, such as wind and available course guidance.

b. The Flight Track Analysis Method is an alternative means of defining an OAA based on the navigational capabilities of the aircraft. This methodology requires the operator to evaluate the effect of wind and available course guidance on the actual ground track. While this method is more complicated, it can result in an area smaller than the OAA produced by the Area Analysis Method.

AREA ANALYSIS METHOD.

a. During straight-out departures or when the intended track or airplane heading is within 15 degrees of the extended runway centerline heading, the following criteria apply:

(1) The width of the OAA is 0.0625D feet on each side of the intended track (where D is the distance along the intended flightpath from the end of the runway in feet), except when

limited by the following minimum and maximum widths.

(2) The minimum width of the OAA is 200 feet on each side of the intended track within the airport boundaries, and 300 feet on each side of the intended track outside the airport boundaries.

(3) The maximum width of the OAA is 2,000 feet on each side of the intended track.

b. During departures involving turns of the intended track or when the airplane heading is more than 15 degrees from the extended runway centerline heading, the following criteria apply:

(1) The initial straight segment, if any, has the same width as a straight-out departure.

(2) The width of the OAA at the beginning of the turning segment is the greater of:

(a) 300 feet on each side of the intended track.

(b) The width of the OAA at the end of the initial straight segment, if there is one.

(c) The width of the end of the immediately preceding segment, if there is one, analyzed by the Flight Track Analysis Method.

(3) Thereafter in straight or turning segments, the width of the OAA increases by 0.125D feet on each side of the intended track (where D is the distance along the intended flightpath from the beginning of the first turning segment in feet), except when limited by the following maximum width:

(4) The maximum width of the OAA is 3,000 feet on each side of the intended track.

c. The following apply to all departures analyzed with the Area Analysis Method:

(1) A single intended track may be used for analysis if it is representative of operational procedures. For turning departures, this implies the bank angle is varied to keep a constant turning radius with varying speeds.

(2) Multiple intended tracks may be accommodated in one area analysis by increasing the OAA width accordingly. In a turn, the specified OAA half-widths (i.e., one-half of the OAA maximum width) should be applied to the inside of the minimum turn radius and the outside of the maximum turn radius. An average turn radius may be used to calculate distances along the track.

(3) The distance to an obstacle within the OAA should be measured along the intended track to a point abeam the obstacle.

(4) When an operator uses the Area Analysis Method, the operator does not need to separately account for crosswind, instrument error, or flight technical error within the OAA.

(5) Obstacles prior to the end of the runway need not be accounted for, unless a turn is made prior to the end of the runway.

(6) One or more turns of less than 15 degrees each, with an algebraic sum of not more than a 15 degree change in heading or track, may be analyzed as a straight-out departure.

(7) No accountability is needed for the radius of the turn or gradient loss in the turn for a turn with a 15 degree or less change in heading or track.

FLIGHT TRACK ANALYSIS METHOD.

The Flight Track Analysis Method involves analyzing the ground track of the flightpath. This paragraph discusses factors that the operator must consider in performing a Flight Track Analysis.

a. Pilotage in Turns.

The operator should consider the ability of a pilot to initiate and maintain a desired speed and bank angle in a turn. Assumptions used here should be consistent with pilot training and qualification programs.

b. Winds.

(1) When using the Flight Track Analysis Method while course guidance is not available, operators should take into account winds that may cause the airplane to drift off the intended track.

(2) The operator should take into account the effect of wind on the takeoff flightpath, in addition to making the headwind and tailwind component corrections to the takeoff gross weight used in a straight-out departure.

(3) When assessing the effect of wind on a turn, the wind may be held constant in velocity and direction throughout the analysis unless known local weather phenomena indicate otherwise.

(4) If wind gradient information is available near the airport and flightpath (e.g., wind reports in mountainous areas adjacent to the flightpath), the operator should take that information into account in the development of a procedure.  

-o0o-

Source: AC No:  120-91  Airport Obstacle Analysis