1972 — Dec 8, United 533 approach crash; houses hit, ~Midway Airport, Chicago, IL — 45

–45 Kimura. World Commercial Aircraft Accidents 3rd Ed., 1946-1993, V.1. 4-11-1994, p.2-13.
–45 NTSB. AAR. United…Chicago-Midway Airport Chicago, Illinois Dec 8, 1972. 1973.

Narrative Information

NTSB Synopsis:

“A United Air lines Boeing 737-222 crashed on December 8, 1972, at 1428 C.S.T. while making a nonprecision instrument approach to Runway 31L at the Chicago-Midway Airport, Chicago, Illinois. The accident occurred in a residential area approximately 1.5 miles southeast of the approach end of Runway 31L. The aircraft was destroyed by impact and subsequent fire. A number of houses and other structures in the impact area were also destroyed.

“There were 55 passengers and 6 crewmembers aboard the aircraft. Forty passengers and three crewmembers were killed. Two persons on the ground also received fatal injuries.

“The aircraft was observed descending below the overcast in a nose-high attitude and with the sound of high engine power just before it crashed into structures on the ground.

“The National Transportation Safety Board determines that the probable cause of this accident was the captain’s failure to exercise positive flight management during the execution of a nonprecision approach, which culminated in a critical deterioration of airspeed into the stall regime where level flight could no longer be maintained.

“As a result of this accident the Safety Board again emphasized the unique demands for crew coordination and constant vigilance during non-precision approaches. The Board also made several safety recommendations to the Federal Aviation Administration dealing with the use of flight spoilers and the occupant survival and evacuation aspects of this accident.” (NTSB 1973, p. 1)

“United Air Lines Boeing 737-222, N9031U, operating as Flight 553 (UA-553) on December 8, 1972, was a scheduled passenger flight from Washington National Airport, Washington, D. C., to Omaha, Nebraska, with an intermediate stop at the Chicago-Midway Airport, Chicago, Illinois. There were 55 passengers, including 5 children and 2 infants, and a crew of 6 aboard the aircraft.

“UA-553 departed Washington at 1250 on an Instrument Flight Rules (IFR) clearance and was assigned an en route altitude of 28,000 feet by Air Traffic Control (ATC). The flight proceeded in accordance with its IFR flight plan. After its arrival in the Chicago Air Route Traffic Control Center area, UA-553 was cleared to descend to 4,000 feet and was given radar vectors to intercept the Midway Airport Runway 31L localizer course. At 1419, Chicago Center effected a radar handoff and transfer of the flight to Chicago Approach Contro1. After contacting approach control, UA-553 was advised that radar contact had been established. The flight was also advised to maintain a heading of 290° and to intercept the Runway 31L localizer course.

“At the same time, approach control was handling other traffic, includ¬ing Aero Commander N309VS which had executed a missed approach at Midway and was being vectored back to the Kedzie outer marker (OM) to intercept the localizer for a second approach to Runway 31L.

“Approach control requested UA-553 to decrease airspeed to 180 knots at 1421:56, and to slow to 160 knots 80 seconds later. A clearance to descend to 2,000 feet was issued at 1423:42. Shortly thereafter, the separation between UA-553 and the preceding Aero Commander prompted the controller to request UA-553 to begin slowing to its approach speed. All these advisories were acknowledged by the flight.

“At 1424:10, the controller advised the Aero Commander to turn inbound to intercept the localizer and cleared it for the approach to Runway 31L. At 1424:45, the Aero Commander was switched to the Midway Tower frequency with a request to, “… keep up as much speed as long as you can.” Accord¬ing to the approach controller, the spacing between the Aero Commander and UA-553 was approximately 31/2 miles at that time. At 1424:51, when the Aero Commander reported passing the OM, it was cleared to land on Runway 31L and At 1425:35, when UA-553 was approximately 2 miles outside the OM and on the localizer course for Runway 31L (as observed on the approach control radar), the flight contacted the Midway Tower and reported that it was out of 3,000 feet for 2,000 feet. After requesting the flight to report passing the OM inbound, the tower controller advised UA-553 that it was number two on the approach. At 1426:30, UA-553 reported passing the OM inbound and was advised by the tower, “United five five three continue inbound. You’re number two on the approach. I’ll keep you advised.”

“At 1426:41, the Aero Commander reported the runway in sight and received clearance to land on Runway 31L. About 9 seconds later, the tower controller considered having the Aero Commander land on Runway 31R instead; but when he saw its proximity to Runway 31L, he reissued the clearance to land on that runway. At 1427:04, UA-553 was issued a missed approach clearance as follows: “United five fifty-three execute a missed approach, make a left turn to a heading of one eight zero climb to two thousand.” UA-553 replied, “Okay left turn to one eight zero left turn Okay.” At 1427:36, the controller advised, “United five-five-three contact departure control now one-one-eight point four.” UA-553 did not acknowledge that transmission; there were no further communications with the flight.

“The approach controller stated that after the tower controller had coordinated with him regarding the missed approach clearance issued to UA-553, he noticed that the radar target associated with the aircraft had drifted approximately 1/8 to 1/4 mile to the right of the localizer center¬line. He observed the target for two sweeps of the radar antenna after which he saw it disappear from the radarscope.

“According to cockpit voice recorder (CVR) information, the captain called for the final descent check at 1426:24, about 4 seconds after the sound of the Kedzie OM identifier ended. The checklist was completed at 1427:03; about 1 second later the first officer called, “Ah, thousand feet.” Less than 2 seconds after this call, the sound of stickshaker activation (a device designed to alert the pilot to approaching stall) could be heard on the CVR tape and remained audible until the recording ended at 1427:25. The beginning of the stickshaker sound coincided with the word “execute” in the tower controller’s missed approach clearance.

“According to surviving passengers, the last public address announcement from the cockpit, made about 5 minutes before impact, indicated that the flight was over Gary, Indiana, at 4,000 feet, and would be landing in about 5 minutes. Some survivors stated that the engine noise decreased at the time the announcement was made, and that this lower noise level remained constant until shortly before impact. Most survivors agreed that there was a rapid application of power just before impact, accompanied by the rotation of the aircraft to a nose-high attitude. The sound level of the engines at this time was described in terms such as “full throttle” and “sounded like on takeoff.” One passenger stated that the aircraft “seemed to jerk as the engines came on.” Two of the three surviving cabin attendants and one ground witness were of the opinion that there was more than one power “surge.”

“Several survivors said that the aircraft shuddered following the nose- up pitch change; four of them estimated that the aircraft’s nose rose at least 300. One passenger stated that the nose pitchup occurred in two phases: the first, gradual and to a moderate angle; the second, abrupt and to a high angle.

“Several eyewitnesses heard loud engine sounds and observed the aircraft in a nose-high attitude. A licensed pilot stated that when he saw the air¬craft break out of the overcast at 400 to 450 feet above the ground, it was descending in a level attitude. He said: “There was a surge of power and there was an abrupt attitude change in the aircraft. The nose went to a very high angle of attack”.…

“The aircraft was destroyed by impact and postcrash fire.” (NTSB 1973, 2-4)

“The impact and subsequent fire destroyed five wood and brick frame houses and one garage, and damaged three other houses and two garages.” (NTSB 1973, 5) ….

“None of the eyewitnesses near the scene of the accident reported seeing fire while the aircraft was in flight, or hearing sounds other than those associated with engine operation. The investigation revealed no evidence of in-flight fire, or of structural damage not related to impact and ground fire.

“The first witnesses at the crash site stated that structures on both sides of the aircraft fuselage were burning, and that white smoke was emanating from the fire. They also stated that the fire was very intense around the center section of the aircraft, and that thick black smoke obscured part of the fuselage. The overall conflagration involved the aircraft as well as the destroyed dwellings and their contents.

“The Chicago Fire Department was first notified of the crash at 1429. Five engines, three truck companies, one helicopter, one dry chemical unit, and three ambulances responded immediately. The first radio calls report¬ing that units were “on the scene” were made at 1431 and 1432. Additional alarms were struck at 1437 and 1449.

“The fire was put out almost entirely with water; 20 gallons of foam were used in the rear service door area. The main fire was controlled within 20 to 30 minutes after the fire fighting equipment arrived at the scene. Smoke, heat, and small “flareups” continued for more than 3 hours after the crash.” (NTSB 1973,11)

1.14 Survival Aspects

“The first sounds of impact were recorded about 1 second before the end of the CVR recording. The aircraft either damaged or destroyed several houses before coming to rest across the foundation of one of them. Except for the aft portion of the coach section, the empennage, and the left side of the cockpit, the fuselage was destroyed by impact and fire. Therefore, the analysis of the conditions in the cabin and the related survival aspects, immediately after impact, is based on survivor observa¬tions.

“The only survivor in the fuselage section forward of the wing was the first-class flight attendant who occupied the aft-facing jumpseat adjacent to the left forward entry door. She was seriously injured when her seat collapsed and she was trapped by debris from the aircraft and the house. She was freed from the wreckage after an intensive 30-minute rescue operation by Chicago Fire Department personnel. No first-class section seats were recovered intact.

“There were 17 survivors in the coach section, including the 2 uninjured flight attendants who occupied the jumpseat in the rear of the cabin. According to the survivors, ceiling panels and hat racks with their contents fell on the passengers and in the aisle of the coach section during the impact sequence; seats dislodged from approximately row 12 to 15, as well as other debris, obstructed the aisle. A survivor who was seated in the center cabin section reported that there was no floor structure under his seat. He released his seatbelt and exited through the cargo compartment and a break in the fuselage. Another survivor stated that he “had the feel¬ing that there were people moving underneath” him as he tried to find an exit. A female survivor reported that “people were scrambling over the seat tops and I was kicked and my hand was stepped on.”

“Survivors reported that all cabin lights went out after the impact, and that no lights were visible during the evacuation. Six survivors escaped through breaks in the fuselage. Nine passengers who exited through the rear service door were assisted by the two flight attendants; these attendants were the last to leave the aircraft.

“The left side of the cockpit and the left forward entry door area were relatively intact. The captain’s seat was intact and sustained only minor fire damage. The floor attachments for this seat were in place; the 4-point seatbelt and shoulder harness release mechanism was found unlocked and operable. Shoulder harness straps were found retracted in the inertial reel without signs of scorching or discoloration.

“The first and second officers’ seats were destroyed by impact and fire. The first officer’s shoulder harness straps were found retracted inside the inertial reel and showed no thermal damage.

“Injuries sustained by the survivors included fractures of the vertebrae, pelvis, and extremities, as well as first-degree burns. (NTSB 1973,12) ….

“The captain sustained a fractured arm, fractured ribs, and lacera¬tions. The cause of his death was attributed to “smoke inhalation with carbon monoxide asphyxia and blood cyanide accumulation.” The first officer’s death was attributed to “injuries multiple extreme with severe burns” and other trauma; the cause of the second officer’s death was listed as “extensive burns”.” (NTSB 1973,14) ….

“The rather abrupt level-off reflected in the…data suggests that, because the captain’s attention was occupied by other instruments and checklist activities, the realization that he was rapidly approaching MDA may have come suddenly, and late. Considering the pilot reports of a 500-foot ceiling between the OM and the airport, it is also possible that visual ground contact, coupled with a high descent rate, prompted the immediate level-off.

“The rush of cockpit activities at this point, the first officer’s routine callout that the spoilers were “armed,” and the fact that the spoilers are seldom used during the final segment of an instrument approach, may well have caused the captain to overlook the position of the spoilers at level-off. This probability is supported by the events that followed….The Board further believes that the captain, caught in a rapid tempo of unusual events, was unable to analyze the situation in time to apply effective corrective action.”

The engine acceleration after level-off produced an asymmetrical thrust of 8,000 pounds on one engine and 5,900 pounds on the other, a total of 13,900 pounds. A thrust in excess of 12,500 pounds should have been sufficient to accelerate the aircraft out of the stickshaker regime if the flight spoilers had been stowed. With the spoilers in the flight detent position, however, a total thrust of 14,500 pounds would have been required merely to maintain unaccelerated level flight within the stickshaker regime. With less thrust, any attempt to maintain level flight would require an increase in pitch attitude resulting in a continuing deceleration and the eventual reaching of the stall angle of attack.

“The specified recovery procedure for an approach to a stall is to lower the nose, apply takeoff thrust, retract the flaps to 15°, and retract the gear when a positive rate of climb is achieved. The performance and simulator studies indicate that the B-737 has sufficient thrust capability to accelerate out of the approach-to-stall regime, even with the spoilers extended. If takeoff thrust is produced within 2 or 3 seconds of stick- shaker activation, little or no altitude has to be sacrificed.

“The stickshaker sound started while the engines were still accelerating in response to the captain’s application of level-off power. CVR evidence suggests that instead of applying more power, the captain’s immediate reaction was to reconfigure the aircraft; within 2 seconds of stickshaker onset, there was a sound indicative of flap lever movement. If the flaps were retracted to 15° at this time, the associated loss of lift would cause the aircraft to settle. It is quite likely that the captain would counter this situation by increasing the nose attitude even further. Eyewitnesses and surviving passengers both attested to such an increase in pitch attitude The subsequent CVR comments, “want more flaps,” “flaps fifteen,” “I’m sorry,” and the sound of another click similar to flap lever movement can be interpreted as the crew’s realization of the adverse effect of flap retraction and their corrective action by selecting 40° flaps. Such a final selection was verified by wreckage examination. Although the CVR sound spectrogram does not conclusively show a subsequent power increase, it seems probable, based upon witness observations and engine examination, that takeoff thrust was eventually applied. At this point, however, the angle of attack may have been so high as to make recovery impossible even with full thrust developing….

“In summary, the preponderance of evidence indicates that the rush of cockpit activities during the final descent caused a breakdown of the safe-deploy-guards inherent in the tasksharing of a crew. The error-provoking environment set the stage for the crew’s failure to notice that the spoilers were still extended at level-off and to arrest the rapid deterioration of air-light speed that followed.

“Although the greater portion of this analysis deals with the events surrounding the level-off, the Board wishes to emphasize that the accident sequence was triggered by the captain’s failure to exercise positive flight management earlier during the approach.” (NTSB 1973, 28-31)
…..

“Recommendations

“….In view of the fact that adherence to established operational proce¬dures and practices would probably have prevented this accident, the Board reiterates its often-expressed concern about the apparent lack of crew coordination and cockpit discipline during nonprecision approaches.

“Two of the accident reports released by the Board in 1972 (NTSB-AAR¬72-11 and NTSB-AAR-72-31) contained specific recommendations in this regard. In the first report, the Board included the complete FAA’s Air Carrier Operations Bulletin No. 71-9 in the recommendations section. The subject of the bulletin is: Training Emphasis on Nonprecision Approach Procedures and Interpretation of Low Visibility Weather Reports. This bulletin, in essence, summarizes the common faults noted in nonprecision approaches and makes several pertinent recommendations. The following quotation from this bulletin illustrates its main theme:

“Perhaps we should stop using the philosophy of non- precision and face up to the need for standards that all phases of flight should be based upon precision and pro-fessionalism. Still another area in the conduct of non- precision approach has to do with the attitude, cockpit discipline and crew coordination of the flight crew. Recent events strongly indicate a widespread lack of appreciation for the importance of these factors. Substandard attitude, discipline and coordination are apparent to the degree that many approaches are being flown in a hit-or-miss fashion rather than in a disciplined by-the-book procedure.”

“In the second report, the Board recommended that the FAA:

Reemphasize to all flightcrew members the necessity for total crew coordination and adherence to approved procedures.

Insure that all flightcrew members are currently apprised of the contents of Air Carrier Operations Bulletin 71-9, emphasizing that a “nonprecision” approach requires as much, if not more, crew coordination than a “precision” approach because of the lack of precise guidance from electronic navigational aids outside the aircraft.

As an additional step in drawing attention to this bulletin, the Board will forward copies to the organizations listed below [omitted here] with the recommendation that its contents be used, together with this accident report to stress the unique demands for crew coordination and vigilance during nonprecision approaches.” (NTSB 1973, 33)

Sources

Kimura, Chris Y. World Commercial Aircraft Accidents 3rd Edition, 1946-1993, Volume 1: Jet and Turboprop Aircrafts. Livermore, CA: Lawrence Livermore National Laboratory, Risk Assessment and Nuclear Engineering Group. 4-11-1994.

National Transportation Safety Board. Aircraft Accident Report. United Air Lines, Inc., Boeing 737, N9031U, Chicago-Midway Airport Chicago, Illinois December 8, 1972 (NTSB-AAR-73-16). Washington, DC: NTSB, adopted August 29, 1973, 65 pages. Accessed at: http://libraryonline.erau.edu/online-full-text/ntsb/aircraft-accident-reports/AAR73-16.pdf