Hey
@Dugie8, I'm really interested to see what you have.
Sorry, sorry. Got home last night and proceeded to puke my guts out. Stupid Gas Station sushi.
Ok, talking transport category jets here, nothing more. 135.379 ((2) For an airplane certificated after September 30, 1958 (SR422A, 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.) The sister reg under 121 reads almost verbatim, if not verbatim.
To put it simply, you have to miss the obstacles and KNOW before you take off that you will miss the obstacles. Seems simple, yet folks still cling to "see and avoid" and "VFR departures". Neither exist under 135, at least in the context that you can ignore take off performance limitations, ie weight.
So, most, if not all, 142 schools, CAE, FSI, SimComm, etc, teach this as all you have to do is make sure your second segment climb performance is equal or greater to the SID climb gradient requirements. Sounds good at first glance BUT you have to consider most airplances Second segment end at 1500 feet then you nose over and accelerate (3rd segment), clean up and then climb at Venr or some other term (4th segment). Now when someone points this out somebody likes to be smarter than the system and says, "Well I'll just look at the second segment performance at a higher altitude, ie take off from a 1500' MSL field and the SID tops out at 3500' MSL, so they look at second segment performance at 3500' MSL". No Bueno. You can interprolate you cannot extrapolate a chart. Meaning, that isn't how the chart is meant to be used and that isn't how the airplane is going to perform. You need to take into account things like ground effect, engine power reduction/loss, etc.
Dugie, where the hell you going with all this. Glad you asked!
TERPS is for all aircraft, not just us jet drivers. TERPS starts at the end of the runway (35 feet above it) and goes out for 200'/NM. It's a sheet of plywood that you should not penetrate. That rate goes up depending on obstacles. Let's go back to the 3rd segment climb thing. More than likely that nose over and acceleration will put you on the wrong side of the plywood. So your 2nd Segment climb from gear retraction to 1500' or so may exceed the SID requirement, right up to you nose over, then BLAMMO. Granite. Also you have to take into account your climb penalty for turning.
So what does APG do, again Good Question. APG says, HEY! Even with an engine failure at V1 very few jets are going to cross the end of the runway at 35 feet. We will start your climb gradient from where the AFM says you will be with an engine failure, ie distance down the runway. Imagine a TOFL of 4000 (accelerate go) on a 9000 foot runway. Lots of real estate between you and where TERPS thinks you are. So you may have the same paultry climb performance on one engine but you just bought yourself another mile or so before you even come close to an obstacle (not a lot of towers right on the runway).
Most of these APG procedures are either fly a heading after acceleration altitude or they simply overlay the existing SID/ODP. The lateral track is easy, APG does all the magic for the vertical track.
Could you figure all this out on your own. Short answer, no. While we have all the charts to calculate all those segments, we don't have all the obstacle data. TERPS takes into account the big stuff but not the low close in stuff. Let that sink in for a moment. There could be obstacles out there that penetrate your departure path and you don't know about them but APG does. Thus APG is not just for "mountainous" airports, it is for EVERY, DAMN, TAKEOFF.
I keep using APG but there are other providers, APG is just what I am familiar with.
Hope this helps
