I've flown around that area a lot in the past. It's nice and scenic in VMC. But between Mt. Salak and Mt. Pangrango, one has to have their wits around and not have a lapse of situational awareness. Some very strange winds are up there, and conditions are unpredictable even in VMC, and in IMC, it's damn awful. I must thank my fortunes to not have to go through the pass between those two mountains anymore these days!
Flying to the south of Jakarta, is dangerous if only armed with a Jepessen Map. No terrain depiction, only the MSA is there.
If they had reached FL150 as reported, they would have had to descend prior to reaching the 30NM mark. Sounds easy enough. BUT, late afternoon in that area is normally not good for flying. I've seen clouds "collapsing down the side of Mt. Salak and spread across and cover the whole pass towards Pangrango before (I turned back... being in an unpressurized prop twin w/ no WXR). Quite often in the mid afternoon, clouds would cover both mountains, and only reveal Pangrango, and Salak is totally covered... looking like a big cumulus over small area undercast...
If these guys were only armed with Jepp approach, STAR, and enroute charts, a simple "fly out along R206 to 30NM and turn back", would be fine in VMC, but deadly in IMC and bad localized weather. A simple urge to duck under over there is hard to overcome unless you know the terrain there. The crew was unfamiliar with the area. Last time someone crashed into Mt. Salak, it was an air force C-212 on the way back to Halim from the south coast after some remote sensing equipment testing. Been there, also did the same, test remote sensing equipment down the south coast, heading back, huge temptation to simply press, "Direct to" Halim airport on GPS or FMC.
My crew used to hate me for forcing them to plot a course through the pass on the GPS and Mission console display, added with the instructions "In IMC Do not proceed direct HLM unless west of R-200 HLM and within 20DME HLM. Strict adherence to planned course through the pass in IMC, maintain 6000 above until 20DME. If GPS u/s or unable to maintain 6000, revert to alternative IMC return plan" (which involved a long detour west to avoid terrain over 1hr30 extra on that prop). It wasn't until they went through the pass in IMC did they appreciate the instructions. On a bad day, the pass collects northerly winds like a compressor on the north side, then spews it up the middle and down the sides in an erratic manner. I went through it in IMC once on one of those bad days, I'd rather not go through that again ever.