This is an oft misunderstood subject among pilots of transport category airplanes. I'll try to be as brief and clear as possible. There's two separate issues here. Part 25 and the flight rules for whichever regulation the airplane is operated under. With respect to
minimum required climb gradient, Part 25 defines the minimum OEI climb gradient for ANY operation, irrespective of obstacles. This minimum is an AFM limitation and the airplane may not be operated unless it is met or exceeded. OEI climb gradient data is also provided in the AFM for all approved WATs.
The specific regulation under which the airplane is operated determines minimum
obstacle clearance requirements for any operation conducted under that rule.The OEI net climb gradient must be sufficient to meet the obstacle clearance requirements along the takeoff flight path.
One at a time:
The climb gradient published in the AFM for each takeoff segment applies only to the OEI case.
§25.111 Takeoff path.
(a) The takeoff path extends from a standing start to a point in the takeoff at which the airplane is 1,500 feet above the takeoff surface, or at which the transition from the takeoff to the en route configuration is completed and VFTO is reached, whichever point is higher. In addition—
(1) The takeoff path must be based on the procedures prescribed in §25.101(f);
(2) The airplane must be accelerated on the ground to VEF, at which point the critical engine must be made inoperative and remain inoperative for the rest of the takeoff; and
(3) After reaching VEF, the airplane must be accelerated to V2.
Part 25 certificated airplanes may not be taken off at a WAT which will not result in meeting the minimum OEI climb gradient requirements for each segment. This data is published in the performance section of the AFM. So much for part 25.
§121.189
(2) In the case of an airplane certificated after September 30, 1958 (SR 422A, 422B), that allows a net takeoff flight path that clears all obstacles either by a height of at least 35 feet vertically, or by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries.
Note: Net takeoff flight path is defined as the OEI gross flight path (actual) with a decrement of .8% applied (for example a 3.5 % gross gradient becomes a 2.7% net gradient)
Also:
AEO climb gradient data is not generally provided in the AFM. Rather, this performance data is derived by using projected GS and AEO rate of climb data from the AFM to calculate a climb gradient. A menial task to do manually but a piece of cake for a computer. I've seen graphic tables to do this too, but haven't used them much.
Next:
Minimum obstacle clearance requirements are set forth in the regulation under which the airplane is operated. (p135, 121 etc...)
That means that any Part 121 or 135 flight in a transport category jet must be able with OEI to clear obstacles along the takeoff path by a margin defined by the applicable regulation under which the airplane is being operated. (121 and 135 are identical) DPs such as SIDs or ODPs will have a minimum required gradient and these can be met using AEO climb performance
IF OEI net takeoff path climb gradient requirements can also be met.
There's a couple of ways to meet that requirement:
1) If the OEI net takeoff path climb gradient available at the proposed WAT is at least equal to the climb gradient that will allow the airplane to clear all obstacles as per the applicable regulation under which the airplane is operated while following the DP flight path.
2) An alternate flight path is available and planned for that will allow the airplane to clear all obstacles as per the applicable regulation under which the airplane is operated. An emergency or alternate departure path may be completed by an airline performance engineering department or by a service provider such a APG to maximize the WAT at which the airplane may be taken off while meeting the minimum obstacle clearance requirements under the regulation which the airplane is operated.
Bottom line:
It is not necessary to meet TERPS or PANSOPS climb gradient requirements OEI. They are not designed under the assumption that an engine will fail. Those gradients must clear terrain and meet certain ATC requirements, but are not necessarily applicable to an engine failure. If a takeoff path obstacle analysis is not completed, then meeting the DP climb gradient requirements will suffice, but may unnecessarily limit the takeoff WAT.
On the other hand an engine failure is an emergency.(mayday) In such a case the airplane is only required to meet the minimum obstacle clearance requirements of the regulation under which the airplane is operated. If this can be done following the DP flight path, then great. If not, then either the WAT must be reduced or an alternate procedure which meets obstacle clearance requirements must be planned for in case of engine failure.
Note: In many cases, the DP route may be flown at a higher WAT using the takeoff path OEI gradient instead of the published DP required climb gradient. But that cannot be done unless an analysis is completed. That's a big selling point for performance analysis service providers
A good example of this is Aspen, Colorado in the Lear 60. At a given WAT the airplane might be limited to 19,000 lbs for the Lindz DP using AFM net gradient data to meet the minimum gradient required by the DP. That very well might limit the fuel and payload enough to scrub the trip. Or we could run an APG analysis and take off at well over 20,000 lbs. That would safely and legally allow full seats and baggage with good fuel reserves back to SOCAL. What we cannot do is take off at a weight where part 135 obstacle clearance requirements can't be met OEI. Of course an actual engine failure would require us to change our plan in the air to fly the alternate procedure. The crew briefing needs to include that contingency.
I had hoped to keep this a little shorter, but this subject gets pretty involved. I probably still didn't cover it all, but I'm tired!