McDonnell Douglas DC-10-30 accident at Boston, Massachusetts, 17 Dec 1973

Accident narrative

On December 17, 1973, at about 1543 e.s.t., Iberia Lineas Aereas de Espana Flight 933, a McDonnell Douglas DC-10-30, crashed while making an instrument landing system (ILS) approach to runway 33L at Logan International Airport in Boston, Massachusetts. The aircraft struck approach light piers and an embankment short of the runway, shearing its right main landing gear before skidding to a stop. Of the 153 passengers and 14 crewmembers aboard, 13 passengers were slightly injured, and two passengers and one flight attendant were seriously injured during the evacuation. The aircraft was substantially damaged.

### The flight

Flight 933 was a scheduled international passenger flight from Madrid, Spain, to Boston. The flight into the Boston area was routine, with no reported problems with the aircraft or its systems.

At 1534, Boston Approach Control cleared the flight to descend to 3,000 feet and provided radar vectors to intercept the ILS localizer course for runway 33L. At 1540:30, the Boston tower local controller advised the crew that the runway visual range was out of service, visibility was three-quarters of a mile in moderate rain and fog, and the surface wind was from 310 degrees at 10 knots. The controller asked the flight to report the lights in sight.

The captain flew the ILS approach with the No. 1 autopilot coupled and both autothrottle systems engaged in the speed mode, set at 145 knots. The tower cleared Flight 933 to land and reported that braking action was fair to poor.

When the flight engineer called "300 feet," the first officer saw the approach lights to his right and reported them. The captain responded, "Ok, lights in sight," disconnected the autopilot, and banked right to align with the runway. He did not disengage the autothrottle system.

According to the captain, the aircraft was aligned with the runway when the flight engineer called "minimum decision height." The captain knew the aircraft was low but thought there was no problem. He overrode the autothrottle system to advance the throttles and slightly increased back pressure on the control column. After the first officer and flight engineer told him the aircraft was still low, he advanced the throttles farther but felt the aircraft continuing to descend. The flight engineer rapidly called out, "50, 40, 30, 20, 10," and the aircraft struck the approach light pier about 500 feet short of the runway threshold.

The aircraft then struck an embankment between Boston Harbor and the airport, shearing the right main landing gear. The captain's seat slid to its aft limit of travel, preventing him from seeing the runway. He pushed forward on the control column, and the aircraft struck the runway hard, skidded off the right side, and stopped about 3,000 feet beyond the threshold. A fire erupted on the left side as it skidded. The captain declared an emergency and ordered an evacuation.

### What the investigation found

Investigators found no evidence of pre-impact failures or malfunctions in the aircraft's structure, engines, flight controls, or instruments. The captain's seat was found to have moved freely because impact forces likely distorted the gear rack support, disengaging the drive pinion.

During the evacuation, the floor in the aft cabin was found displaced upward, blocking the aisles and trapping five people. Four escaped through a break in the top of the fuselage, and one was rescued by the crew. The right forward exit door could not be opened pneumatically because of a bent backstop bracket, which the Board noted was likely deformed before delivery to Iberia. Two aft doors also failed to open; the Board stated it was possible the flight attendant did not apply sufficient force to the handle.

Flight data recorder (DFDR) data showed that as the aircraft descended below 500 feet, it began deviating left of the localizer, rose slightly above the glide slope, and increased airspeed. Pitch and thrust decreased. The autopilot was disengaged 3 seconds after passing the middle marker. Nine seconds after disengagement, the pitch attitude was nose-up and thrust was increasing, but the rate of descent averaged 1,060 feet per minute.

By comparing the DFDR data with a no-wind plot, investigators derived the winds aloft along the approach path. The data showed a low-altitude wind shear. Between 500 and 200 feet, the longitudinal wind changed from an 18-knot tailwind to a headwind, and the left crosswind decreased from 23 to 4 knots.

Tests in a DC-10 simulator programmed with these winds demonstrated that the shear caused reductions in thrust and pitch attitude as the aircraft descended through 200 feet. When the autopilot was disengaged at 200 feet, the rate of descent increased to 1,179 feet per minute within 7 seconds. Pilots had to significantly increase pitch attitude within 6 seconds to prevent landing short of the runway. The simulator pilots agreed that the visual cues from 200 feet were not alarming enough to cause them to initiate a missed approach.

The Board also examined the ILS glide slope for runway 33L. The threshold crossing height (TCH) of the glide slope beam was 34.3 feet, a figure not published on the approach charts. The glide slope was unusable below 200 feet. The Board noted that for a DC-10-30 on the glide slope, this low TCH provided only 24.6 feet of clearance over the approach light pier and 7.8 feet over the runway threshold. The Board believed these clearances were too low for the existing weather minimums.

### Probable cause

The National Transportation Safety Board determined that the probable cause of this accident was "that the captain did not recognize, and may have been unable to recognize, an increased rate of descent in time to arrest it before the aircraft struck the approach light piers. The increased rate of descent was induced by an encounter with a low-altitude wind shear at a critical point in the landing approach where he was transitioning from automatic flight control under instrument flight conditions to manual flight control with visual references. The captain's ability to detect and arrest the increased rate of descent was adversely affected by a lack of information as to the existence of the wind shear and the marginal visual cues available. The minimal DC-10 wheel clearance above the approach lights and the runway threshold afforded by the ILS glide slope made the response time critical and, under the circumstances, produced a situation wherein a pilot's ability to make a safe landing was greatly diminished."