Date: 202107
Local time of day: 1201-1800
Location: E10.TRACON, CA
Flight conditions: Mixed
Weather elements / visibility: Turbulence; Rain; 2
Light: Daylight
Aircraft: Medium Transport; Low Wing; 2 Turbojet Eng
Operator: Corporate
Operating under: Part 91
Flight plan: None
Mission: Test Flight / Demonstration
Flight phase: Descent
Airspace: Class E E10
ATC / advisory: TRACON E10
Reporter organization: Corporate
Reporter function: Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Confusion; Communication Breakdown; Situational Awareness
Anomaly: 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
Detector: Automation Aircraft Terrain Warning; Person Flight Crew
When detected: In-flight
Result: Flight Crew Took Evasive Action
Contributing factors: Human Factors; Weather
Primary problem: Human Factors

Narrative: [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.




Synopsis: 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.