2nd segment revisited...
This whole topic is perhaps one of the most misunderstood aspects encountered when you transition into flying jets. Let me try to explain it the best I can.
Second segment climb is part of a certification requirement under FAR 25 (Sec. 25.121 Climb: One-engine-inoperative). In each "phase" of the takeoff, takeoff-gear down, takeoff-gear up, and takeoff-final, a Part 25 transport category aircraft must exhibit a minimum one-engine inoperative climb gradient (rise/run) except for those phases that involve level acceleration.
These phases or segments must be specified by clearly defined changes of configuration, thrust, & speed. They begin from lift off and continue until the aircraft is at least 1,500 feet above the runway elevation or to point where transition to the enroute configuration is complete (clean, MCT thrust, 1.25 Vs). This is known as the "Takeoff Path" of the aircraft.
The gradient of climb is depended on the number of engines installed, giving greater safety margins to those aircraft with more engines (the idea is more engines means a heavy, less maneuverable aircraft, and thus should have greater minimum climb requirements) For a two-engine transport category aircraft, the 1st segment (lift-off to gear retraction) requires a positive gradient out of ground effect. A 2.4% gradient is required from gear retraction to 400 ft. in the takeoff configuration, gear up, takeoff thrust, at V2. At the end of the final segment, clean, MCT, 1.25 Vs (otherwise known as "enroute configuration"), the aircraft must capable of a 1.2% climb.
Both FAA approved Aircraft Flight Manual and the FAR's are going to require the pilot/operator to limit takeoff weight so as to meet this certification requirement. For Part 91 operators of aircraft certified after 9/30/58, it's 91.605(b)(1) and for Part 135 operators it's 135.379(a). There will also be a limitation in the AFM limiting the takeoff weight such that these one engine-out climb requirements are met. Usually it's in the form of a statement such as "the maximum allowable takeoff weight shall not exceed that shown in the takeoff weight limit chart", but that can vary between manufacturers. So you must always meet your 2nd segment climb requirements along with the minimum climb requirements for other segments. Comply with the AFM restrictions and the approprate charts in this matter and you'll be ok.
So far, most everyone gets it correct up to this point. It's when you start Taking obstacle clearance and TERP's requirements that folks tend to get things screwed up. FAR 25.115 defines the takeoff flight path. For each weight, temperature, and pressure altitude the manufacturer must determine the one engine-out takeoff flight path. They must use the same segments as defined in the certification climb requirement above. The takeoff flight path begins at end of the one engine-out takeoff distance (35 feet above the runway or a point known as "reference zero") and continues to the end of the takeoff path described above (1,500 feet or transition to the enroute configuration complete, whichever is higher). This is the actual performance of the aircraft as demonstrated from flight testing based on procedures developed for "in-service" operation, flown by crew's of "average skill", with allowances for any time delays that may be expected in service (25.101).
The net takeoff flight path represents the takeoff flight path described above diminished by a percentage of climb gradient depended on the number of engines installed, -0.8% or two engine aircraft, -0.9% for three engine aircraft, and -1.0% for four engine aircraft. Since both the actual (gross) takeoff flight path and the net takeoff flight path begin at 35 feet, you can see that net flight path will be 0.8% lower (2 engine aircraft with an engine inoperative) than the actual flight path giving a difference of 48' per nautical mile between the two flight paths. The concept here is that the difference between the actual flight path and net flight path yields an ever increasing spread between the two path. At 10 miles, they'll be 480 feet separation between the two flight paths. As you'll see below, obstacle clearance is based on the net flight path to as to ensure an increasing margin for obstacle clearance as the aircraft climbs further from the runway.
The net takeoff flight path is used in meeting the one-engine out takeoff flight path requirements of FAR 121.189(d) and FAR 135.379(d) for turbine aircraft. This regulation states that the net takeoff flight path must clear obstacles by 35 feet for those obstacles 200 feet either side of the flight path inside the airport boundaries and 300 feet either side after passing the airport boundaries. Each operator must perform a detailed analysis of all obstacles, and using either the AFM flight path charts or computer-based performance programs demonstrate that they can meet the above obstacle clearance requirements. Obstacle data is obtained from a variety of sources including airport obstruction charts, USGS terrain charts, etc. It's not something you are going to do on pre-flight. You'll have to obtain this obstacle data from a engineering source like Jeppesen Ops' Data. They'll give you obstacle data for a particular airport/runway and any special engine-out turning procedures that may need to be applied. If Jeppesen Op's Data has the AFM data, they can do all this ahead of time and print out an "Airport Analysis". From this analysis, you have the maximum weight allowable for that airport/runway in terms of a runway limit weight and a climb limit weight for a given temperature. Correction factors are provided for non-standard conditions, wind, A/I on or off, etc. The airport analysis is the method used by the FAR 121 air carriers.
Now, let talk TERP's, DP (a.k.a. SID) and climb performance. First, the one engine-out obstacle clearance requirements stated above and the TERP's IFR obstacle clearance requirements are two entirely different subjects! They are not related in any way. A transport category aircraft suffering an engine failure on takeoff will likely NOT meet TERP's climbs requirements and IS NOT REQUIRED TO DO SO!!!!!!!!!!! (enough emphasis?) This is where folk get things wrong. The AIM alludes to this in 5-2-6(e)(4) by stating that the pilot should "consider the effects of degraded performance and the actions to take in the event of an engine loss during the departure." The Canadian supplement and ICAO PAN-OPS state it more clearly that IFR SID's and departure procedures are based on normal, read all-engine performance, and not one-engine out performance.
TERP's climb requirements for DP's are based on a 40:1 obstacle identification surface (OIS) beginning no higher than 35 above the departure end of runway and continuing to the minimum altitude for enroute operations. If no obstructions penetrate the 40:1 OIS then the standard IFR climb gradient of 200 ft/NM applies. This provides at least 48 feet per nautical mile of obstacle clearance. If obstacles penetrate the OIS, then the DP climb gradient will be raised a sufficient amount to regain the 48 ft/NM clearance. Careful note must be given here. There could be obstacle which would require higher than standard climbs but only to heights less than 200 feet above the runway. In this case, higher than standard climb gradients are NOT provided
The problem with a Part 25 aircraft is that we are not provided with all engine climb performance data in the AFM. When the performance rules were first written for the first jet transport aircraft (SR-422), one engine-out performance was considered more restrictive than all engine performance data. This engine out performance data was used to develop the takeoff flight path concept we discuss above. The climb gradient data from the AFM is used ONLY to construct the net takeoff flight path as described above and cannot be used in meeting TERP's criteria. Any attempt to use the climb gradient charts from a Part 25 AFM in determining TERP's obstacle clearance would be using this data in a manner for which it was never intended and never approved for.
There are several reasons you cannot use AFM 2nd segment climb data in meeting TERP's criteria. First, all the one engine-out climb flight path data (1st, 2nd, and final segment climb charts) are only good to 1,500 above the runway (some manufacturers have gone beyond this point up to 3,000 to 5,000 feet) and CANNOT be used to the heights demanded of certain DP's such as is the case of KASE, 7,800 ft to 14,000 ft. 2nd segment climb data is generally only valid to 400 ft above runway, the point where 2nd segment ends. Second, the differences in climb terminology. Most 2nd segment climb gradient charts in AFM's give the available NET climb gradient so that when you apply this to the flight path charts or computer program you get a resulting NET flight path. TERP's climb requirements are based on actual performance, NOT an already an already reduced NET climb gradient. There's the consideration of 5 minute limitation on takeoff thrust. Many DP's require climb gradients to significantly high altitudes that would exceed the limitation on takeoff thrust, not to mention a shallowing climb gradient due to density altitude changes as you climb. Remember if you try to use 2nd segment climb, you'll only meet that climb gradient if you keep V2, takeoff flaps, and takeoff thrust and that climb gradient is generally valid only at 400 ft above the runway. Finally, as we said above, TERP's performance is stricly considering normal aircraft performance, not an emergency situation such as engine out climb.
'Sled
