United Airlines Flight 553

Flight

United Airlines Flight 553 was a scheduled service from Washington National Airport to Omaha, Nebraska, via Chicago Midway International Airport. On December 8, 1972, the aircraft used for the flight was Boeing 737-222 City of Lincoln, registration N9031U.

The flight-deck crew consisted of Captain Wendell Lewis Whitehouse (44), First Officer Walter Coble (43) and Second Officer Barry Elder (31). The captain, a highly experienced pilot with approximately 18,000 flight hours to his credit, had been with the airline since 1956 and had logged 2,435 hours in the Boeing 737 cockpit. First Officer Coble had more than 10,600 flight hours under his belt and Second Officer Elder had close to 2,700 hours.

Crash

The accident occurred as the airplane was making its final approach to Midway Airport.

At 14:24 Central Standard Time, Flight 553 was cleared by air traffic control to Midway Airport on an approach to runway 31L. The localizer approach for runway 31L used an Outer Marker Beacon (OMB) named "Kedzie," located 3.3 nmi (6.1 km; 3.8 mi) prior to the threshold of runway 31L. Under the published landing procedures, the aircraft was to maintain a minimum altitude of 1,500 feet (460 m) until it passed the OMB, at which point the flight was allowed to descend to a minimum altitude of 1,040 feet (320 m). Published procedures, and pilots operating under instrument flight rules, use mean sea level (MSL) to measure altitude; at Midway Airport, an altitude of 1,040 feet (320 m) MSL corresponded to an actual height above ground level (AGL) of only 429 feet (131 m). The area was overcast at the time Flight 553 landed, and pilots landing at Midway Airport immediately before Flight 553 reported that the airport was only visible below 500–600 feet (150–180 m) AGL.

When Flight 553 reached the Kedzie OMB, the aircraft was still at an altitude of 2,200 feet (670 m) MSL, a full 700 feet (210 m) above the minimum crossing altitude of 1,500 feet (460 m) MSL. Realizing the aircraft was too high, the captain extended the spoilers (speed brakes) and steepened the aircraft's descent to 1,550 feet (470 m) per minute. The aircraft continued to descend at a rate of 1,500 feet (460 m) per minute until it reached its level-off altitude of 1,040 feet (320 m) MSL.

At the time it reached its level-off altitude and emerged from the clouds less than 500 feet (150 m) above the ground, Flight 553 was descending at a rate of more than 1,500 feet (460 m) per minute. This exceeded the descent rate of 600–700 feet (180–210 m) per minute required by the precision approach path to runway 31L, and United Airlines' recommended descent rate for the Boeing 737 of 1,000 feet (300 m) per minute. At 14:27, the first officer called out "thousand feet" in reference to the plane's altitude reaching 1,000 feet (300 m) MSL, a height of only 380 ft (120 m) above the ground. The captain leveled the plane off and increased engine power. The throttles were not advanced fully and, with the spoilers still extended, did not provide enough thrust to climb or even to maintain level flight without losing speed. The stick shaker, a stall warning device attached to the pilots' control yoke, activated 6–7 seconds after the aircraft leveled off and continued to sound as the aircraft entered an aerodynamic stall.

At 14:28, the aircraft struck trees and then roofs along W. 71st Street before crashing into a house at 3722 W. 70th Place, 1.89 miles (3.04 km) short of the runway, in a residential area of the city's West Lawn community, one and a half blocks west of Marquette Park. The three-man flight crew died, along with 40 of the 55 passengers. The crash destroyed five houses and damaged three others, killing two people on the ground.

Investigation

The National Transportation Safety Board (NTSB) was notified of the accident at 14:40 CST and immediately dispatched an investigation team to the scene. Agents of the Federal Bureau of Investigation (FBI) were on the scene about 45 minutes after the crash, before any investigators from the NTSB.

The Flight Data Recorder on board the aircraft was not functioning at the time of the crash due to a mechanical failure. Fortunately, the ARTS-III (Automated Terminal Radar Services) system at nearby O'Hare International Airport was in operation at the time of the accident, and saved recorded transponder data on magnetic tape. Those tapes were analyzed extensively and compared to Boeing flight profile data to develop the course, speed, rate of descent, and altitudes of the plane as it made its approach to Chicago Midway. The Cockpit Voice Recorder was working normally and the tape in that "black box" was relatively undamaged, which enabled the NTSB to sequence it in time with the readings of ARTS-III. The NTSB then was able to determine the power output of the engines, at any given point in time, with CVR tape sound analysis. That correlation (CVR with ARTS-III) allowed the NTSB to reconstruct the flight's performance, and to determine that the stick shaker first sounded 6 to 7 seconds after the plane leveled off at 1,000 ft (300 m) MSL (380 ft (120 m) AGL) and continued until ground impact.

That ARTS-III system tracked the plane from a position of 55 miles (89 km) east of its antennae site to the point when the plane stalled at 1,000 ft (300 m) MSL.

The official finding of the NTSB was that the probable cause of the accident was the stalling of the airplane (airspeed too low) because the captain failed to ensure that the flight remained within the required airspeed and altitude parameters for that non-precision approach profile. No evidence was ever found of sabotage or foul play.

From their performance study and simulator tests, the NTSB concluded that the spoilers must have been extended to at least the flight detent position during the rapid descent that preceded the stall. It was thus likely that when the Captain attempted to level off and then to go-around, the crew failed to immediately retract the spoilers which made it all the more difficult to recover from the stall before ground impact. Although the spoilers were found to be fully retracted in the wreckage, it was possible that the spoilers could have retracted on their own as a result of the impact forces and loss of hydraulic pressure.

The final mistake was inappropriate manipulation of the flaps, from 30 degrees to 15 degrees, while the plane's airspeed was still too low, with spoilers extended. This was incorrect while attempting to go-around with the stick shaker activating because the decreased lift resulted in increased stall speed.

Conspiracy theories

Dorothy Hunt's death led to the accident becoming caught up in rumors and conspiracy theories related to the unfolding Watergate scandal. Mrs. Hunt was carrying $10,000 in $100 bills when the plane crashed, and some alleged that this money was meant for people connected to Watergate. James McCord alleged that Mrs. Hunt supplied the Watergate defendants with money for legal expenses. The FBI's rapid appearance at the crash scene was also regarded by some as unusually fast. Skeptics of the official narrative speculated that the plane was targeted due to Mrs. Hunt's presence on board, and that sabotage of the flight was covered up by government agencies. As a result, the accident became known as "the Watergate crash."

Proponents of Watergate-related theories included Sherman Skolnick, a Chicago-based private investigator, who alleged that the aircraft had been sabotaged by the CIA. This claim was echoed by Nixon's special counsel Chuck Colson in an interview with Time magazine in 1974. However, the same article speculated that Colson was accusing the CIA of the broad Watergate conspiracy in a desperate attempt to stave off President Richard Nixon's impeachment in the scandal, and that Colson may have "lost touch with reality" as he faced a prison sentence.