Investigation Summary
The flight was a proficiency check for three Delta pilots, accompanied by an FAA inspector. After performing several maneuvers and one successful landing at Greater Southwest International Airport (GSW), the crew requested a circling approach to Runway 13. They were sequenced behind an American Airlines DC-10 that was performing touch-and-go landings. Although the tower controller issued a 'caution turbulence' advisory, the term 'heavy' was not yet a standard part of the air traffic control lexicon for wake separation; consequently, the crew did not fully appreciate the magnitude of the hazard. The DC-9 followed the DC-10 at a distance of approximately 2.1 to 2.25 nautical miles with a time separation of about 55 to 60 seconds. As the DC-9 descended below the flight path of the DC-10 near the runway threshold at an altitude of approximately 100 feet AGL, it encountered the left wingtip vortex. The aircraft began to oscillate and then rolled violently 90 degrees to the right. Despite a late attempt to initiate a go-around, the right wingtip struck the runway, and the aircraft flipped onto its back, followed by a severe post-crash fire.
Final Conclusions
The NTSB determined the probable cause was an encounter with a trailing vortex generated by a preceding 'heavy' jet, resulting in an involuntary loss of control. The investigation highlighted that existing FAA procedures for VFR flight did not provide adequate protection from vortex encounters. The board also noted that the crew likely underestimated the hazard, possibly due to a 'cry wolf' syndrome regarding frequent but often benign turbulence advisories, and a general industry belief that wake turbulence was primarily a threat to small general aviation aircraft rather than transport-category jets.
Photographic Evidence (1)
Video Analysis
Catastrophic Training Flight at Texas' Greater Southwest Airport - Delta Flight 9570 — A detailed look at the 1972 crash of N3305L, a DC-9 that fell victim to the wake turbulence of a DC-10, leading to major changes in aircraft separation rules.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Systems & Failure Modes
Wake Turbulence Separation
The regulatory framework for spacing aircraft to avoid wingtip vortices. This accident led to the creation of 'Heavy', 'Medium', and 'Light' categories with specific mileage buffers.
Flight Data Recorder (FDR)
Used by investigators to identify the exact moment of vortex encounter via vertical acceleration traces and roll axis oscillations.
Interesting Facts
- 01The aircraft was a McDonnell Douglas DC-9-14 delivered in 1965.
- 02The preceding aircraft was a McDonnell Douglas DC-10, which is significantly larger and classified as a 'heavy' jet.
- 03The DC-9 was approximately 2.1 miles behind the DC-10 at the time of the encounter.
- 04Prior to this accident, there were no mandatory wake turbulence separation standards based on aircraft weight.
- 05The NTSB conducted wake vortex testing using colored smoke and a loaned DC-10 to confirm the hazard magnitude.
Safety Actions & Advisories
Mandatory Separation Standards
The FAA established new minimum aircraft separation standards based on maximum takeoff weight, requiring 5 miles of separation for aircraft following a 'heavy' jet.
Heavy Designation
Introduced the 'Heavy' radio callsign suffix for aircraft weighing over 300,000 pounds (later revised to 255,000 pounds) to alert other pilots and controllers to significant wake vortex risks.