Medium Transport; Low Wing; 2 Turbojet Eng
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 46; Flight Crew Total 5400; Flight Crew Type 470
Workload; Confusion; Communication Breakdown; Situational Awareness
Airspace Violation All Types; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter VFR In IMC
Flight Crew Took Evasive Action
Human Factors; Weather
[We were] operating in a military restricted area where range rules [were] to remain in VMC. Due to increasing cloud coverage; we had to knock off our planned flight early and began preparing for return to base (ZZZ). The PIC was managing the descent while I managed the checklists. We leveled at 10;000 ft. MSL to stay below cloud deck. We were flying southbound with mountainous terrain to the west. Cloud coverage was along the entire route ahead of us; and a hot restricted area was to the east (with cloud coverage). As a crew; we discussed the forward visibility which we estimated to be about 30 NM from our present location. It looked like some rain showers ahead down the valley; and cloudy conditions over the mountains (with maybe a little better clearance in the immediate direction). On board; radar was not indicating anything appreciable returns; and weather showed just a few pixels of green.As we approached the weather; the PIC said we should turn right as he thought the weather looked better towards the mountains. I stated that the weather ahead looked better in my opinion; as it looked like we would go under some of the rain showers in the valley. PIC said he thought it was IMC ahead; and started turning right towards the mountains. I reiterated that going down the valley was our best option; especially due to the mountainous terrain to our right. He mentioned he has flown in this area hundreds of times and he knew this would be better. I again reiterated that not only were we turning towards mountains; but there was a National Park over there which required us to stay 3;000 ft. above the terrain; and would limit how low we could go to avoid weather. I estimated that going down the valley could have got us as low as 5;000 ft. MSL safely (about 2;500 ft. AGL). PIC stated that going down the valley would be impossible for us to stay VMC and maintained the heading towards the mountains. I told him that I did not see what he was seeing; and he stated it would work fine.As we approached the first ridge the PIC climbed to 11;000 ft. MSL to stay safely above the terrain (although I estimate we needed to be about 13;000 ft. MSL to avoid National Park overflight). As we continued south; we could see that it was closing in and that the bases were getting lower; so the PIC started stepping down in altitude to approximately 8;500 ft.' MSL. At one point; the PIC was getting nervous; and asked me if we should turn around. As I was super focused on trying to stay ahead of the PIC; I told him whatever you do; do not turn right; as the Terrain display was showing a 10;200 ft. peak while we were descending through about9;500 ft. MSL. The visibility was becoming much reduced and we entered some light rain showers. As we were navigating mountainous terrain in visibility that were maybe 1 NM at times; the GPWS alerted us to 'TERRAIN; TERRAIN' followed immediately by 'PULL UP; PULL UP; PULL UP.' I immediately called climb-climb-climb. The PIC's only response was to pull back on the yoke (as opposed to our training which calls for disconnect AP (Auto Pilot)/AT (Auto Throttles); pitch to PLI (Pitch Limit Indicator) and max power). I put my hand on the throttles and started to push up about the same time the AT's responded to increasing power. As the climb commenced the Terrain Warning silenced; however we did encounter solid IMC. Shortly after we broke through the line of showers; [we] proceeded back to the airfield [in] VFRThere were multiple opportunities to prevent this from happening. The first and foremost that comes to mind is that as soon as we disagreed on the course of action; we should have entered a hold to discuss it. This would have given us time to regroup and may have resulted in a better decision. Additionally (in hindsight); we could have requested an IFR flight plan back to the landing airfield per the range rules. However; we were so focused on remaining VMC that we overlooked that option; probably due to task saturation. Personalities and organizational culture are also paramount to conducting safe flight operations; and these subjects will be presented to leadership as a result of this incident.
Corporate aircraft pilot reported flying VFR and entering IMC over mountainous terrain resulting in GPWS alert and evasive action taken in order to return to VFR flight.