What happened
During a VOR/DME approach to Washington National Airport, an aircraft was observed by witnesses descending below its normal descent path and horizontally straying off course. The appearance and noise of the aircraft caused concern among occupants of nearby buildings and persons on the ground. Radar plots and flight data recorder (FDR) data confirmed the aircraft's position during this event.
One building located 2.5 miles from the airport is approximately 396 feet MSL. The VOR/DME approach procedure includes an altitude restriction of 900 feet MSL at 3 nautical miles DME and a minimum descent altitude of 720 feet at 2.5 nautical miles DME.
The pilot was interviewed and stated that he believed everything was normal and that he was at the altitude restriction at the 3-mile DME point. However, radar data showed his altitude as approximately 517 feet at the 3-mile DME and about 400 feet at the 2.5-mile DME.
Radar and witness accounts agree that the aircraft made a turn and climb at this point toward the river and resumed the normal approach path.
The investigation
The investigation examined radar plots and flight data recorder (FDR) data to confirm the aircraft's position during the approach. The pilot was interviewed regarding his perception of the approach and his altitude at key DME points.
Findings
The aircraft deviated from its normal descent path and strayed off course during a VOR/DME approach to Washington National Airport. Radar data confirmed that the aircraft was significantly below the required minimum descent altitudes at critical DME points, despite the pilot's belief that he was at the correct altitude.
Safety message
Pilots must strictly adhere to published approach procedures and minimum descent altitudes, especially during non-precision approaches like VOR/DME. Cross-checking navigation instruments with radar data or other available information can help prevent controlled flight into terrain or proximity to obstacles.