McDonnell Douglas DC-9-14 accident at Fort Worth, Texas, 30 May 1972

Accident narrative

On May 30, 1972, at 0724 central daylight time, a Delta Air Lines McDonnell Douglas DC-9-14 crashed while attempting a go-around following a landing approach to Runway 13 at Greater Southwest International Airport in Fort Worth, Texas. As it passed the runway threshold, the airplane rolled rapidly to the right and struck the ground in an extreme right-wing-low attitude. The aircraft was destroyed by impact and fire. The four occupants—three Delta pilots and a Federal Aviation Administration air carrier operations inspector—sustained fatal injuries.

### The flight

Delta Air Lines Flight 9570 departed from Love Field in Dallas, Texas, at 0648. It was a training flight scheduled to qualify two captain-trainees for type ratings in the DC-9. The airplane’s takeoff gross weight was 77,300 pounds, which included 2,000 pounds of sandbags loaded in the forward cargo compartment for ballast. The gross weight and center of gravity were within prescribed limits.

Upon arriving in the Greater Southwest International Airport (GSW) area, the flight requested and received clearance for an instrument landing system (ILS) approach to Runway 13. The tower advised the crew that an American Airlines McDonnell Douglas DC-10, also on a training flight, was in the traffic pattern conducting "touch and go" landings. The DC-9 completed its first ILS approach with a full-stop landing, then took off again with instructions to maintain visual flight rules (VFR).

A second ILS approach was terminated by a voluntarily executed missed approach. The flight then requested and received clearance for a VOR approach to Runway 35, to be terminated by a circling approach to land on Runway 17.

### The sequence of events

During the circling approach, the DC-9 pilot requested approval to change the landing to Runway 13, following behind the DC-10 which was inbound on the ILS. The local controller approved the request, stating, "Okay that'll be fine use one three for full stop. Caution turbulence."

Intracockpit conversation indicated the DC-9 crew visually assessed their spacing behind the DC-10. A crewmember noted, "All right, there's twenty seconds," indicating the downwind leg was extended 20 seconds beyond a position abeam the end of Runway 13. At that time, the pilot initiated a left turn to the final approach. The DC-9 rolled out onto the final approach at 0722:56, slightly to the right and below the path traversed by the DC-10. The time separation between the two airplanes was 53 to 54 seconds, with the DC-9 positioned 2.25 nautical miles behind the DC-10, which was lifting off after its touch-and-go landing.

The flight proceeded normally until the DC-9 passed the runway threshold. At approximately 0723:23, the flight data recorder registered a vertical acceleration excursion to +1.7 g at an altitude of approximately 670 feet mean sea level, or 100 feet above the airport elevation. The check pilot in the right seat commented, "A little turbulence here."

The airplane began to oscillate about its longitudinal axis. At 0723:28.2, the check airman ordered a "go-around," followed immediately by a call for "takeoff power." At 0723:30, the stall warning stick shaker activated. After two reversals, the airplane rolled rapidly to the right. Having achieved approximately 90 degrees of roll, the right wingtip struck the runway surface 1,242 feet beyond the threshold. The airplane continued to roll to a nearly inverted attitude before the main body struck the runway. The fuselage and empennage separated upon impact and slid.

### What the investigation found

The Board found no evidence of preimpact structural failure, fire, or malfunction of any airplane system or powerplant. The weather at 0723 included a visibility of 30 miles, a temperature of 67° F, and surface winds from 340 degrees at 7 knots. A radiosonde ascent disclosed a temperature inversion and stable air from the surface to approximately 1,600 feet.

The investigation determined that the DC-9 had descended into the circulatory airflow of the left wingtip vortex generated by the preceding DC-10. Flight track data and computer simulations indicated the encounter occurred at an altitude of approximately 60 feet above ground level. The Board noted that the meteorological conditions—specifically the stable air and surface winds—were conducive to the persistence of a vortex. The crosswind component prevented the vortex from moving laterally, while a slight tailwind component moved the turbulent air mass back into the runway threshold area, where it remained stationary and invisible to the DC-9 crew.

Simulations showed that the vortex's clockwise circulation initially produced upward airloads on the DC-9's right wing, causing a moderate left roll. The Board believed the pilot's reflex reaction would have been to apply right aileron control to maintain a wings-level attitude. This action carried the airplane deeper into the vortex core, where the right wing encountered downward loads. The resultant load reversal, combined with the right aileron command, produced a sharp right roll. The induced rolling moment exceeded the maximum lateral control capability of the DC-9, making recovery impossible within the available altitude.

The Board found that the DC-9 was operating under VFR procedures at the time of the accident. Under existing FAA procedures, the controller was only required to issue a wake turbulence cautionary advisory to VFR arriving aircraft not under radar control, leaving the responsibility for maintaining safe separation with the pilot. The Board noted that pilot compliance with recommended vortex avoidance procedures was extremely difficult, as a pilot's ability to accurately judge air-to-air range and the vertical descent path of a preceding airplane is limited.

The Board believed that because of the moderate nature of the initial roll, and possibly because of uneventful past encounters with less severe vortices, neither the flightcrew nor the FAA inspector recognized the severity of the turbulence in time to execute a missed approach and avoid the accident.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was "an encounter with a trailing vortex generated by a preceding 'heavy' jet which resulted in an involuntary loss of control of the airplane during the final approach. Although cautioned to expect turbulence the crew did not have sufficient information to evaluate accurately the hazard or the possible location of the vortex. Existing FAA procedures for controlling VFR flight did not provide the same protection from a vortex encounter as was provided to flights being given radar vectors in either IFR or VFR conditions."