Eastern Flight 401

A missing green light

Eastern Air Lines Flight 401 left New York's John F. Kennedy International Airport at 21:20 on 29 December 1972 for Miami. The Lockheed L-1011 carried 163 passengers and 13 crewmembers. Its flight to Florida was uneventful until the landing preparations. When the landing gear handle was moved down, the green light confirming that the nose landing gear was extended and locked failed to illuminate. The captain recycled the gear, but the indication remained dark.

At 23:34:05 the crew told Miami tower that it would have to circle because it still lacked a nose gear light. The tower instructed the aircraft to climb straight ahead to 2,000 feet and return to approach control. The crew reported reaching that altitude over the airport. Approach control then directed it onto a northerly heading while maintaining 2,000 feet above mean sea level. The interrupted approach became a period of troubleshooting away from the airport.

Troubleshooting at 2,000 feet

At 23:36:04 the captain instructed the first officer, who had been flying, to engage the autopilot. Approach control subsequently directed a left turn. The first officer removed the nose gear light lens assembly, then jammed it while trying to reinstall it. The captain sent the second officer into the forward electronics bay below the flight deck to inspect the nose gear position through an optical sight. The sight allowed the alignment of two rods on the gear linkage to be checked when the wheelwell was illuminated.

At 23:37:24 a brief downward acceleration began a loss of about 100 feet. A pitch correction arrested that initial loss. The aircraft then turned west on the controller's instructions. Troubleshooting continued, and the captain again directed the second officer to check the gear indices. At 23:38:46 the crew requested permission to continue farther west while trying to restore the light. Approach control agreed. The captain and first officer remained occupied with the lens assembly and the way it had been inserted.

An alert during the descent

At 23:40:38 the cockpit recorder captured a half-second C-chord altitude alert. The warning indicated a departure of 250 feet from the selected altitude. No crewmember commented on the sound, and the flight data showed no correcting pitch change. The Board later found that the crew had not heard the alert as the aircraft passed through 1,750 feet. On Eastern's configuration, the accompanying flashing amber altitude warning light was inhibited below 2,500 feet of radar altitude.

Shortly after 23:41 the second officer reported that he could see nothing through the gear inspection system because it was dark. An Eastern maintenance specialist, who had occupied the forward observer seat, discussed the wheelwell light with the crew and then went into the electronics bay to help. At 23:41:40 the approach controller asked how things were progressing. He had just noticed an altitude indication of 900 feet in the aircraft's radar data block. The crew requested a return to the airport, and the controller cleared a left turn toward a southerly heading.

Recognition seconds before impact

The first officer noticed the altitude problem at 23:42:05. Two seconds later he questioned whether they were still at 2,000 feet. Radio altimeter warning tones began at 23:42:10. At 23:42:12, during the left turn, the aircraft struck the Everglades in a 28 degree left bank, 18.7 statute miles west-northwest of Miami International Airport. The weather was clear, visibility was unrestricted, and the night was moonless.

The left outer wing struck first, followed by the left engine and left main landing gear. The airliner broke apart across flat marshland. A fuel fire followed impact. Coast Guard helicopters located the wreckage about 15 to 20 minutes after launch, and rescue operations took approximately four hours. Sixty-eight survivors were flown to local hospitals. The report's injury table classified 99 injuries as fatal and 77 as nonfatal. Its footnote records two deaths more than seven days later, bringing the total who died to 101 and leaving 75 survivors. The seven day definition then used kept those later deaths in the table's nonfatal category.

Wreckage, seats and survival

The crash site lay approximately eight feet above sea level, with soft mud beneath six to twelve inches of water. Wreckage extended over an area about 1,600 feet long and 300 feet wide. The passenger cabin broke into four main sections and many smaller pieces. Most survivors were found near the cockpit, midcabin service area, overwing area and tail section, toward the far end of the wreckage path. Most fatalities were found nearer the center of that path. Crushing chest injuries were the predominant cause of death.

Investigators attributed much of the survival to the way occupants moved with the larger sections as the wreckage slowed. Some seats remained attached to large pieces of floor; some occupants were thrown clear after their speed had substantially decreased. The passenger seats incorporated energy absorbers and were bolted to platforms attached to tracks in the aircraft structure. Many units stayed attached to those platforms. The Board believed the seat design materially contributed to survival. It also found that the destruction had exceeded the normal conditions for a survivable accident, through loss of occupant protection and restraint.

The aircraft and the recorded evidence

Eastern had received N310EA in August 1972. By the accident it had accumulated 936 flight hours and 502 landings. Its recorded weight and balance were within the applicable limits at departure and at the accident. The digital flight recorder supplied investigators with 62 performance parameters, including engine thrust, control surface positions, bank angle and pitch attitude. These measurements enabled analysis of the automatic controls alongside the aircraft's movement. The cockpit voice recorder tape was recovered intact, providing the conversations and warning sounds from the initial call to Miami tower through the impact.

The indicator and the automatic controls

Examination found both bulbs in the nose gear indicator burned out. The lens assembly was jammed sideways and protruded from its normal position. The recovered optical sight and wheelwell light were operative. The right main landing gear remained down and locked; the nose gear and left main gear had separated with parts of their supporting structure. The Board found no preimpact failure of the aircraft's structure, engines or systems apart from the burned-out indicator bulbs.

Investigators checked the automatic flight controls through component tests, simulation and flight tests using equipment recovered from the aircraft. They concluded that the autopilot had operated in control wheel steering during the final descent. In that mode, light pressure on the control wheel changed aircraft attitude. Altitude hold could be overridden by force on the control column while the autopilot continued to stabilize pitch attitude. Its engagement lever could remain in its selected position, with the altitude mode light and annunciation providing the indication that altitude hold had ended.

The Board considered an inadvertent force on the captain's control wheel as he turned to speak to the second officer as a possible explanation for the initial altitude hold disengagement. It also examined mismatched pitch computers with different override force thresholds. The Board concluded that the mismatch was not a critical factor. In pitch attitude stabilization, changes in thrust could alter vertical speed unless the pilots supplied compensating pitch control.

Power changes and divided attention

Reductions in engine power began 160 seconds before impact. The Board attributed the ensuing descent to those reductions combined with small nose-down control movements. Tests of the recovered speed control equipment produced a different pattern from the accident recordings. The recorded reductions affected different engines at different times, whereas the autothrottle drove the throttles together. Investigators concluded that the speed control system had not caused the reductions. They believed one or both pilots had intentionally reduced thrust, probably to control airspeed.

The altimeters, vertical speed indicators, airspeed indicators, pitch attitude indications and vertical speed selector would have shown that the aircraft was descending. The Board found that the pilots had not understood the small control forces needed to change attitude in control wheel steering. Eastern's training met FAA requirements, but company operating policy required command mode operation and limited practical experience with control wheel steering. The Board called for stronger checks during ground school and an early simulator proficiency follow-up after pilots began scheduled L-1011 flying.

The NTSB's probable cause was the flight crew's failure to monitor the instruments during the final four minutes and detect the unexpected descent in time to prevent impact. Preoccupation with the nose gear indication diverted attention from the instruments. The captain helped with the jammed assembly while directing the other checks, and positive delegation of aircraft control was not accomplished. The final findings said that the captain had failed to ensure that a pilot monitored the aircraft's progress throughout the troubleshooting.

Controller assistance and safety recommendations

The controller believed the aircraft was safe after its prompt response to his query. He knew the radar system could display incorrect altitude information for several scans and continued handling five other flights. The Board acknowledged that the system had not been designed to provide terrain clearance and that FAA procedures did not require that service. It nevertheless urged procedures enabling controllers to help crews when they noticed marked altitude deviations, and it pressed the FAA to accelerate rulemaking for ground proximity warnings.

Recommendations also addressed the gear inspection task and the altitude alert. The Board sought a wheelwell light switch next to the optical sight, instructions at that location, and a flashing altitude alert that worked below 2,500 feet. In its 14 May 1973 response, the FAA said it was studying the recommendations. Additional survival recommendations called for shoulder harness provisions at cabin attendant seats and accessible high intensity portable lights. Testimony described how darkness at the Everglades wreckage hampered survivors and cabin attendants trying to help others.