Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Situational Awareness; Workload
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented
Human Factors; Weather
Flight was uneventful until cleared the ILSZ RWY 36 via direct QIT VOR at 18;000 feet assigned. A large build up was directly situated over QIT therefore ATC was asked for a left deviation due to the buildup. During the decent briefing the approach and contingencies were briefed; however; a storm disrupting the approach was not anticipated. A left deviation was started and I decided to proceed direct to point D111G as the best course of action. The FO had his terrain display in terrain mode allowing me to see any conflicts ahead. The FO correctly pointed out that going direct point D254I would be a better course of action and allow more time for the approach. I agreed and we asked for and was given direct to that point by ATC. At this point the situation was less than ideal. The weather was mostly cloudy with only brief glimpses of the terrain. I had the altitude of 10;500 feet in my mind thinking I was heading for my original point (D111G) when in reality we were required to be at 14;200 feet point D254I. I had started the descent when the FO pointed out the error and I immediately started to climb. At this point we were still not on the approach due to the weather. We were still proceeding direct to D254I. I was also slowing and configuring for the landing. I ended up disconnecting the autopilot in order to reestablish myself on to a stable approach. The terrain display never showed any conflicts and no verbal warnings of the system was encountered. I knew approximately where the aircraft was at all times. My error was in the decent altitude while forced off of the approach due to weather. We eventually did get on the approach; were stable when we broke out of the weather and had an uneventful landing. The large cell situated over the UIO VOR forced an unprepared diversion around it resulting in an off route decent in order to land. This in turn resulted in attaining a lower than desired altitude in heavily mountainous terrain. This was my first time into UIO. Although I reviewed the required airport familiar special qualification briefing; in this situation it was not adequate. I consider myself very experienced in mountainous terrain and I believe this particular field should require a Check Airman checkout. 'Picking the brain for gotchas' would have been invaluable.
Air Carrier Pilot Flying reported descending below the assigned altitude in mountainous terrain.