Douglas DC-9-15 accident at Sioux City, Iowa, 27 Dec 1968

Accident narrative

At approximately 0711 c.s.t. on December 27, 1968, Ozark Air Lines Flight 982, a Douglas DC-9-15, crashed while taking off from the Sioux City Airport in Sioux City, Iowa. The aircraft rolled violently shortly after lift-off and struck the ground, sliding into a grove of trees. There were no fatalities among the 64 passengers and four crewmembers. Ten passengers and three crewmembers were injured, and the aircraft was destroyed.

### The flight

Flight 982 was a regularly scheduled passenger flight from Sioux Falls, South Dakota, to O'Hare International Airport in Chicago, Illinois, with an en route stop at Sioux City.

The flightcrew consisted of Captain Patrick G. Sweeney, 48, and First Officer John T. Schmeltz, 33. Captain Sweeney had accumulated 19,145 total flight-hours, of which 63 were in the DC-9. First Officer Schmeltz had 6,048 total flight-hours, with 20 in the DC-9. The crew had reported for duty the previous morning at 0705, terminating their first sequence of flights at 1211. Following an 8-hour and 24-minute rest, they reported again at 2035 and terminated at Sioux Falls at 0026 on the morning of the accident. After approximately five hours of rest, they reported for duty at Sioux Falls at 0525.

The flight departed Sioux Falls at 0625 and climbed to an assigned altitude of 11,000 feet. The aviation area forecast called for moderate to locally heavy mixed icing in clouds and precipitation. The surface weather observation at Sioux City at 0657 reported an 800-foot overcast ceiling, three miles visibility in fog and light freezing drizzle, a temperature of 22°F, and wind from 360 degrees at 10 knots.

### The sequence of events

The flight arrived at the Sioux City ramp at 0658 following a routine instrument approach. The captain stated he used engine anti-ice during the descent through the cloud layer but did not use airfoil anti-ice, as he did not think it was needed.

During the aircraft's ten minutes on the ramp, ground personnel observed ice on the wings. One refueling employee noted ice with sharp points extending back six to eight inches under the wings, about one-sixteenth of an inch thick. The other refueler, a certified flight instructor, noticed rime ice on the leading edge about one-half inch thick.

A station agent extending the handrails observed that the entire leading edge of the left wing was covered with a layer of rough ice. He started to get the deicing equipment, but a second agent advised him that the captain did not want the aircraft deiced. A third agent entered the aircraft, informed the captain of the ice on the leading edges, and asked if he would care to look at it. The captain replied that he did not, and that he would turn on the heat after becoming airborne.

The flight departed the ramp for Runway 35. The crew turned on the engine anti-ice and fuel heater systems while taxiing. Control tower personnel and ground witnesses observed the takeoff roll, with several noting that the aircraft's acceleration seemed slow.

According to the crew, the aircraft lifted off and began its climb. The captain called for the landing gear to be raised and began turning off the landing and taxi lights. As the gear began to retract, the captain stated that the right wing suddenly and violently dropped to what seemed to be a 90-degree angle of bank. He applied rudder and power, and then applied left aileron. The right wing came up, but the aircraft continued to roll to the left to an angle of about 45 degrees until the left wing contacted the runway.

The captain discontinued the takeoff, leveling the wings and reducing power just before final ground contact. The aircraft touched down approximately 110 feet beyond the departure threshold of Runway 35. It slid straight ahead, swerved several times while striking objects, and came to rest in a grove of trees 1,181 feet beyond the runway end.

There was no fire. The fuel tanks ruptured and spilled an estimated 2,200 gallons of fuel, but absorption by the 22 inches of snow and reduced vaporization at the 22°F temperature prevented ignition.

During the final swerve, the captain and first officer were thrown violently to the right and sustained back injuries. Neither was wearing a shoulder harness. The captain also struck his head on the center pedestal radio panel. The forward stewardess received head injuries when her seat separated at the left attach point, causing her to strike a protruding handhold.

The first officer closed both fuel valves, released the captain's seat belt, and proceeded to the cabin. The forward airstair door was blocked by a tree, but the first officer and a passenger opened it enough to permit evacuation. The galley exit could not be opened, and the aft stairway exit was blocked by damage. Passengers in the coach section evacuated through the over-wing window exits, noting that the wings were icy and slippery.

The left engine continued to run for two hours because fuselage damage caused the engine control linkage to bind. A mechanic eventually arrived by automobile from Sioux Falls and stopped the engine by pulling the firewall shutoff handle.

### What the investigation found

The Board found no evidence of pre-impact failure or malfunction in the aircraft's structures, systems, or powerplants.

Examination of the wreckage revealed a sheet of semi-clear, rough, solid ice firmly adhered to the left wing immediately aft of the leading edge. The ice was estimated to be one-eighth of an inch thick at the forward edges, tapering to one-sixteenth of an inch at the aft edges. The leading edges of the entire empennage were completely covered by solid ice estimated to be three-eighths of an inch thick. The contour pattern indicated the ice formed during flight, not during the ramp time at Sioux City.

The aircraft's computed departure gross weight was 88,149 pounds. This weight required the use of -1 power, which provides 14,000 pounds of thrust per engine. This requirement was noted on the completed weight and balance form given to the captain. However, investigators found the engine pressure ratio (EPR) gauges set to 1.85, which corresponded to -5 power, or 12,250 pounds of thrust. The crew confirmed they selected this lower power setting. The Board concluded the crew failed to check the form, read it erroneously, or were not thoroughly knowledgeable of the aircraft's power requirements.

The flight data recorder showed that the aircraft's acceleration was near the computed expected acceleration for the -5 power setting actually selected. Lift-off occurred near the computed performance lift-off speed.

The cockpit voice recorder was recovered undamaged but the tape was jammed. A playback revealed it had last recorded two days prior to the accident. The Ozark prestart checklist required testing the recorder, which would have revealed the malfunction. The captain stated he complied with the checklist but could not positively state the recorder was tested.

The Board analyzed the aerodynamic effects of the ice. Studies confirmed that airfoil icing results in higher stall speeds and lower stall angles of attack. The Board noted that the approach and landing at Sioux City were likely completed without incident because they were flown at nearly the same angle of attack at which the ice accumulated, aided by ground effect during the landing flare. During lift-off, however, the aircraft was rotated to an angle of attack 7 to 9 degrees greater.

The Board concluded that the aircraft became airborne due to the increased lift of ground effect. As it reached the area of reduced lift and increased induced drag near the upper limits of ground effect, the aerodynamic penalties of the ice caused a stall. The popping and torching from the engines reported by passengers were determined to be compressor stalls caused by the crew's throttle manipulation and disturbed airflow.

The investigation reviewed two previous incidents involving DC-9 training flights where unexpected violent rolls and delayed recoveries occurred during approach-to-stall maneuvers with known or suspected airfoil ice. The Board noted that rotation of an iced airfoil to angles of attack other than that at which icing occurred can cause large changes in pitching-moment coefficients.

The Board found that company policy and regulations prohibited takeoff with ice adhering to the wings. The company manual also required airfoil anti-icing to be turned on when the temperature was +6°C or lower and moisture was present, or whenever icing was anticipated. The Board concluded that the captain failed to recognize the aerodynamic penalties of airfoil icing and failed to have the aircraft deiced.

### Probable cause

The Safety Board determines that the probable cause of this accident was a stall near the upper limits of ground effect, with subsequent loss of control as a result of the aerodynamic and weight penalties of airfoil icing. The flightcrew failed to have the airfoil ice removed prior to the attempted takeoff from Sioux City. The Board also finds that the crew selected an improper takeoff thrust for the existing gross weight condition of the aircraft.