The Air Tindi Beechcraft 200 King Air aircraft departed controlled flight during its initial descent and crashed near What, Northwest Territories, Canada. Both pilots were killed. The aircraft operated flight 503, an instrument flight rules flight itinerary from Yellowknife to What Airport, WekweŠt Airport, and Ekati Aerodrome. During the flight preparations at Yellowknife the first officer (FO) noted that the right-side vacuum-driven attitude indicator was not erect. The captain assured the FO that the instrument would start to operate. The flight took off from Yellowknife at 08:51. During the climb, while working the after-takeoff checklist, it appeared that the right-side ADI still wasn’t working. The captain suggested to the FO that he should tap the attitude indicator to see if it was stuck or frozen. The FO replied that the attitude indicator was still not erect. While en route at FL120 the pilots attempted to troubleshoot the right-side attitude indicator, but to avail. At 09:11, after having commenced the descent, the captain’s attitude indicator on the left-side instrument panel displayed a red “GYRO” flag and the autopilot disconnected. At this time the captain began to manually fly the aircraft with partial flight instruments. The aircraft continued to descend, and the captain attempted to initiate a climb. The aircraft climbed briefly before beginning to descend again. The aircraft subsequently entered a gradual left turn that progressed into a steep descending left turn (spiral) from which the aircraft never recovered. The investigation established that both the captain and the FO had an iPad mini equipped with the ForeFlight Mobile application which had the capability to provide a backup attitude indication and synthetic vision view to both pilots. Since it was a backup feature, Air Tindi did not have a formal training program in place for using the attitude and heading reference system (AHRS) feature of ForeFlight. Findings as to causes and contributing factors: 1. For undetermined reasons, the left-side attitude indicator failed in flight. 2. Although just before take off the crew acknowledged that the right-side attitude indicator was not operative, they expected it to become operative at some point in the flight. As a result, they did not refer to the minimum equipment list, and departed into instrument meteorological conditions with an inoperative attitude indicator. 3. The crew’s threat and error management was not effective in mitigating the risk associated with the unserviceable right-side attitude indicator. 4. The crew’s crew resource management was not effective, resulting in a breakdown in verbal communication, a loss of situation awareness, and the aircraft entering an unsafe condition. 5. The captain did not have recent experience in flying partial panel. As a result, the remaining instruments were not used effectively and the aircraft departed controlled flight and entered a spiral dive. 6. The captain and first officer likely experienced spatial disorientation. 7. Once the aircraft emerged below the cloud layer at approximately 2000 feet above ground, the crew were unable to recover control of the aircraft in enough time and with enough altitude to avoid an impact with terrain.