Boeing 707-331B accident at Los Angeles, California, 28 Aug 1973 (N8705T)

Accident narrative

On August 28, 1973, at 2150 Pacific Daylight Time, a Trans World Airlines Boeing 707-331B experienced severe pitching oscillations, or porpoising, while descending approximately 35 miles west of Los Angeles, California. The aircraft underwent more than 50 oscillations over about two minutes, subjecting the occupants to peak acceleration forces of +2.4g to -0.3g. Of the one hundred forty-one passengers and 11 crewmembers aboard, one passenger was fatally injured, and one flight attendant and two passengers were seriously injured.

### The flight

Trans World Airlines Flight 742 was a scheduled international passenger flight from Bangkok to San Francisco, with intermediate stops including Honolulu and Los Angeles. The flight departed Honolulu at 1709.

The flight crew consisted of Captain John Wilber Harpster, who had accumulated about 26,171 total flight-hours with 8,170 hours in the Boeing 707; First Officer Robert Cooper Evans, with 6,128 flight-hours including 4,378 in the Boeing 707; and Flight Engineer Don Wilbur Jackson, who had about 19,000 flight-hours.

### The sequence of events

The flight operated routinely at a cruise altitude of 33,000 feet (Flight Level 330). The crew reported good weather with clear skies and no turbulence. At about 2110, the flight contacted the Los Angeles Air Route Traffic Control Center. At 2129, the controller cleared Flight 742 to descend to 11,000 feet. The crew acknowledged the clearance, disengaged the autopilot, reduced power, and began the descent.

In the cabin, entertainment had just ended, and flight attendants were cleaning the galley and preparing for landing. The "fasten seatbelt" sign was off, and five or six passengers were standing near the aft galley and lavatories.

As the aircraft descended through 22,000 feet at an indicated airspeed of 350 knots, it pitched up abruptly, then pitched down, and began an oscillatory motion. The aircraft's nose pitched from about 5 to 7 degrees noseup to 5 to 7 degrees nosedown. The crew turned on the "fasten seatbelt" sign, verified the autopilot was disengaged, reduced engine power to idle, and turned off the rudder power, mach trim, and yaw damper. The captain, assisted by the first officer, attempted to counteract the porpoising by applying inputs through the control column.

In the aft coach section, unrestrained flight attendants and passengers were thrown repeatedly from the floor to the ceiling. According to the passengers, the vertical acceleration forces were progressively less farther forward in the cabin.

The aircraft continued to descend and decelerate. As the indicated airspeed reduced to about 300 knots at an altitude of 19,500 feet, the pitching oscillations abated and stable flight was regained. The crew noted that controllability was normal except for a slightly high resistance to forward control column movement. They declared an emergency, requested medical assistance, and were cleared for an instrument landing system approach. The flight landed at Los Angeles International Airport at 2243 without further incident.

### What the investigation found

An inspection of the aircraft revealed no structural damage attributable to the accident. The flight data recorder confirmed approximately 55 oscillation cycles over a two-minute period. Investigators noted that the same aircraft, registration N8705T, had experienced a similar pitching oscillation on July 18, 1972, which resulted in no damage and was concluded at the time to be a turbulence encounter.

Extensive flight tests and inspections were conducted on the aircraft's longitudinal control system. Initial inspections found a burned contact on the stabilizer trim control relay and higher-than-allowable elevator hinge line friction, but the Board believed neither discrepancy was sufficient to cause the accident.

During test flights, pilots observed that the control column push forces necessary to counter aircraft noseup, out-of-trim conditions were weaker than desirable. The force required to displace the control column deviated from acceptable criteria. Instrumentation revealed that the left elevator deflected further than the right elevator for a given control column displacement, and the left control tab hinge moment was significantly lower than predicted.

Ground tests showed excessive spanwise waviness on the upper skin of the left stabilizer under load, and residual waviness under no-load conditions that exceeded fabrication limits. Additionally, the left elevator upper nose surface contour at the hinge line was predominantly below the faired contour extension of the upper stabilizer surface, creating a "step down."

Boeing Company engineers theorized that the skin waviness disrupted the airflow and thickened the boundary layer on the stabilizer upper surface. This thickened boundary layer was further disturbed by the step down at the hinge line. The disturbed airflow altered the pressure distribution on the elevator, resulting in a lower-than-normal hinge moment. This, in turn, produced the lower-than-normal pilot force requirements. Acceptable control force characteristics were restored during testing by removing shims to eliminate the step down.

The Board believed that during the cruise flight, fuel burn-off required an aircraft nosedown trim change. The autopilot likely effected this change through elevator displacement. Because of the lower-than-normal elevator control loads, the threshold required for the autopilot to run the stabilizer trim was not reached, leaving the aircraft out-of-trim in the noseup direction.

When the captain disengaged the autopilot to descend, he immediately applied force to the control column to correct the pitch change. Because of the low force gradient, the push force felt normal, masking the out-of-trim condition. The Board believed that a slight relaxation of pressure on the column caused an abrupt pitchup. The crew reacted naturally by applying more push force, but the low force required led to overcorrection. Subsequent control column motions aggravated the oscillation until the crew reduced power and the aircraft slowed, making it less sensitive to control inputs.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was "a combination of design tolerances in the aircraft's longitudinal control system which, under certain conditions, produced a critical relationship between control forces and aircraft response. The atypical control force characteristics which were present in this particular aircraft's control system were conducive to overcontrol of the aircraft by the pilot. The pilot's normal reaction to an unexpected longitudinal disturbance led to a pitching oscillation which was temporarily sustained by his subsequent application of control column forces to regain stable flight."

The Board determined that the cause of the death and injuries was "the impact of unrestrained persons with unyielding objects in the cabin environment."