Background
The aircraft was conducting an IFR arrival and was vectored for an NDB runway 23 approach to the Westmoreland County Airport. The crew had previously terminated the same NDB approach earlier that day, approximately 1 mile east of the airport.
Approach
At the NDB final approach fix, located 8.1 miles from the runway, the crew turned inbound and continued toward the airport. They contacted the tower and reported over the ILS outer marker, an intermediate point on the NDB final approach situated 3.9 miles from the runway. The tower issued local landing advisories and cleared the aircraft to land.
Just before beginning the approach, the wind at Latrobe changed from 230° at 4 knots (a headwind) to 340° at 18 knots (a strong quartering tailwind). The crew had calculated their timing to the missed approach point using a ground speed of 120 knots.
Impact
Moments after the copilot advised the captain that they were 15 seconds from the missed approach point, the aircraft collided with trees. The impact occurred 3.8 miles southeast of the airport at the minimum descent altitude (MDA) of 1,900 feet above mean sea level. All seven occupants were injured, five seriously.
Aircraft and Equipment
An examination of the aircraft records indicated a recurring and current squawk concerning the horizontal situation indicator (HSI) slaving system.
Probable Cause
Occurrence #1: in flight collision with object. Phase of operation: approach - FAF/outer marker to threshold (IFR). Findings: (1) Flight/nav instruments, horizontal situation indicator (HSI) - failure, partial; (2) Operation with known deficiencies in equipment - performed - pilot in command; (3) Light condition - dusk; (4) Weather condition - low ceiling; (5) Weather condition - fog; (6) Weather condition - rain; (7) IFR procedure - improper - pilot in command; (8) Compensation for wind conditions - improper; (9) Proper alignment - not maintained; (10) Terrain condition - high terrain; (11) Object - tree(s).