me thinks this is being thought .... for what it's worth, in the US climb gradient mins were needed as a basis for designing a safe IFR system for TERPS ....these mins were based on performance of average aircraft at the time long ago, which wasn't great compared to today, especially single engine performance.
they basically came up with a low minimum gradiant that it didn't matter as most aircraft could meet it. even with failed engine(s). As most airports were on flat ground with no big structures, it worked well.....there are some standard assumptions such as no turns before DER and 400' but it works well and one could reach a safe IFR altitude and route most direct regardless of runway..... because this min gradiant is not easy for pilots to figure using math skills based on their planes performance, it's dummied down for us in ground speed vs VSI required rates tables to stay above the gradient, so knowing the TERPS basis is not needed....just use the table. ...BTW, common missed question on ifr ride is this table based on IAS or ground speed.
so, TERPS require airports that have higher climb gradients to clear obstacles and terrain have procedures such as ODPs and/or higher climb VSI rates. The climb rate table will indicate this higher gradiant.....the greater the gradiant, the greater the VSI rate..
However. a pilot can come up with his own procedure in what's called "in lue of" ODP if he can meet the min gradient......common example is say a twin can meet ODP VSI rate on both engines but not single engine, pilot can come up with a different climb procedure that allows him meet mins off the ODP or SID to join enroute structure.
I saw this a lot flying B727s in mountainous South America where we could not meet some min climb performance on SIDs with engine failure, and would have to come up with our own procedure or maybe use different runway even with tailwind that pointed towards lower ground. However, later flying B757s, we rarely had such issues due to much greater SE performance. In more complicated airports, we would brief routes to take if engine failed in a SID that we could not meet if engine failed....so we could depart the SID and head towards lower ground...... this is procedure in lue of the standard. BTW, out side US TERPS counter part is PANOPS which is an ICAO document but mostly mimics TERPS.
sorry, this went way longer than thought ...but end with this ...don't read too much into ODPs...takeaway is if no ODP published, most aircraft can meet the gradient.....if you cannot, figure a routing in lue of that you can....maybe another runway or immediately turn. You are not required to fly a ODP (unless in atc clearance which you can amend) but the ODO is usually the path of least resistance and all the TERPS calculations are done for you and dummied down to pilot level with the minimum climb VSI indication on the GROUND SPEED table for that specific gradient. simply check your performance will meet the min climb (remember to adjust GS for tail wind if needed). hope this helps out.
