Incident Overview
On an unspecified date, two aircraft were conducting visual approaches to parallel runways at an airport. Reeve flight 726, a Boeing 727, and Wilburs flight 304, a Cessna 402B (registration N969JW), both received radar advisories for visual approaches to runways 06L and 06R, respectively. Both flights were subsequently cleared to land.
Sequence of Events
At 1555:04, the local tower controller transmitted wind information: at the approach end of runway 06R, wind was from 340° at 9 knots; midfield wind was 330° at 7 knots. Approximately at 1557, flight 726 landed on runway 06L. About 69 seconds later, flight 304 was crossing the threshold of runway 06R at approximately 100 feet above ground level when it encountered wake turbulence and rolled sharply to the right, byd 90°. The pilot corrected with aileron and power, but the aircraft lost altitude and struck the ground before recovery.
Environmental and Operational Factors
The weather observation at 1601 indicated wind from 020° at 5 knots. The centerlines of runways 06L and 06R were 700 feet apart. The threshold of runway 06L was 4,600 feet beyond the threshold of runway 06R. Calculations showed that vortices could have drifted from runway 06L to runway 06R in as little as 28 seconds. The approach control did not advise either flight of the other's position, nor did the tower controller issue a wake turbulence advisory to flight 304.
Regulatory Context
The FAA handbook addressed wake turbulence advisory requirements for parallel runway options with less than 2,500 feet separation, but did not address offset thresholds. All five occupants of flight 304 were injured.
Official Findings
The probable cause as determined by the investigation includes: Occurrence #1: vortex turbulence encountered; phase of operation: approach - VFR pattern - final approach; findings: (f) light condition - dusk, (f) ATC clearance - improper - ATC personnel (dep/apch), (f) unsafe/hazardous condition warning - not issued - ATC personnel (lcl/gnd/clnc), condition(s)/step(s) insufficiently defined - FAA (other/organization), (c) in-flight planning/decision - improper - pilot in command. Occurrence #2: in flight collision with terrain/water; phase of operation: approach - VFR pattern - final approach.