a safety margin for expected variations under operational  conditions. Subpart I of Part 121 and Part 135 require that  the OEI net takeoff flight path be at least 35 feet above  obstacles that are located within the prescribed lateral  distance either side of the flight path The actual obstacle  clearance capability, under optimum conditions after  experiencing an engine failure on takeoff, is equal to the  difference between gross and net flight path, plus the  additional 35 feet. [Figure 1-19]  Advisory Circular (AC) 120-91, Airport Obstacle Analysis,  provides guidance and acceptable criteria for use in  determining the safe lateral clearance from obstacles,  when developing takeoff and initial climb out airport  obstacle analyses and engine out obstacle avoidance  procedures to comply with the intent of these regulatory  requirements. Pilots departing an airport under IFR and  operating under Part 121 or 135 are required by 14 CFR  91.175(f )(4) to use an engine-inoperative takeoff obstacle  clearance or avoidance procedure that assures compliance  with the obstacle clearance requirements (subpart I)  of those rules. The assessment of OEI takeoff obstacle  clearance is separate and independent of the IFR departure  procedure and associated all-engines-operating climb  gradient requirements. While the Part 91 operating rules  governing large, commuter, and turbine-powered aircraft  do not require the use of an OEI takeoff obstacle clearance  or avoidance procedure, such use is encouraged for Part  91 operators of these aircraft.  Unlike TERPS, which assesses obstacle clearance beginning  at the DER, the OEI net takeoff flight path obstacle  assessment begins at the point where the aircraft reaches  35 feet above the runway at the end of the OEI takeoff  distance. Therefore, the OEI net takeoff flight path  assessment may begin before the DER allowing for the  use of a portion of the runway for the OEI climb. The OEI  net takeoff flight path obstacle clearance assessment must  also account for clearance of the low, close-in obstacles  that are noted on the IFR departure procedure, but are not  necessarily cleared when complying with the TERPS-based  IFR climb gradient.  The OEI net takeoff flight path is unique for each aircraft  type and is assessed on each takeoff for the required  obstacle clearance directly against those obstacles located  beneath the OEI flight track and within the prescribed  lateral distance from the flight path centerline. TERPS, on  the other hand, provides a required climb gradient that  represents a surface that the aircraft’s all-engines-operating  climb profile must remain above throughout the IFR  climb until reaching the en route environment. These two  methods of assessing obstacle clearance are necessarily  quite different. TERPS is used by the procedure designer  The actual, or gross, OEI flight path represents the vertical  OEI climb profile that the aircraft has been demonstrated  capable of achieving using takeoff procedures developed  for line operations based on the aircraft’s weight,  configuration, and environmental conditions at the time  of takeoff. The OEI net takeoff flight path represents the  actual OEI takeoff flight path that has been degraded by  an amount specified by the certification rules to provide  Part 25 transport category and Part 23 commuter category  airplane certification rules define the one-engine­ inoperative (OEI) takeoff flight path, which is normally  constructed from a series of segments beginning from 35  feet above the runway surface at the end of the OEI takeoff  distance and ending at a minimum height of 1,500 feet  above the runway elevation. However, the OEI net takeoff  flight path assessment may continue above 1,500 feet if  necessary to ensure obstacle clearance.  Large and turbine-powered, multiengine transport  category airplanes and commuter category airplanes  operated under Part 121 or Part 135 have additional takeoff  obstacle clearance requirements beyond the scope of the  IFR departure procedure requirements addressed by TERPS.  These obstacles are especially critical to aircraft that do not  lift off until close to the DER or which climb at the minimum  rate. [Figure 1-18]  are less than 200 feet above the DER elevation, within 1 NM  of the runway end, and do not require increased takeoff  minimums. The standard ROC to clear these obstacles  would require a climb gradient greater than 200 ft/NM  for a very short distance, only until the aircraft was 200  feet above the DER. To eliminate publishing an excessive  climb gradient, the obstacle above ground level (AGL)/  MSL height and location relative to the DER is noted in the  Takeoff Minimums and (Obstacle) Departure Procedures  section of a given TPP booklet. The purpose of this note  is to identify the obstacle and alert the pilot to the height  and location of the obstacle so they can be avoided. This  can be accomplished in a variety of ways:  • The pilot may be able to see the obstruction and  maneuver around the obstacle(s) if necessary;  • Early liftoff/climb performance may allow the aircraft  to cross well above the obstacle(s);  • If the obstacle(s) cannot be visually acquired during  departure, preflight planning should take into account  what turns or other maneuver(s) may be necessary  immediately after takeoff to avoid the obstruction(s).  One-Engine-Inoperative (OEI) Takeoff Obstacle  Clearance Requirements  1-20 