Aircraft Accident Report: United Airlines Flight 389/Part 2

by the National Transportation Safety Board

 

2. ANALYSIS AND CONCLUSIONS

 

2.1 Analysis

 

A review of the available evidence has eliminated all causal areas other than those involving the operation of the aircraft. No evidence was found that would indicate other than normal operation of the powerplants and systems at the time of impact. There is no evidence that will support a finding of sabotage, flight crew incapacitation, or any malfunction of the aircraft.

There is no indication that the weather played any part in this accident. The weather observed and reported by the U. S. Weather Bureau and the crew of the B-707 aircraft that was operating behind UAL 389 revealed that the weather was suitable for this type of operation. The only possible problem the weather may have presented was limited visibility, but there is no evidence to indicate that after descending through the clouds, the flight was required to operate in less than VFR weather conditions. Light airframe icing could have occurred in the descent down to about 13,000 feet however, this should have had no effect on the safe operation of the flight. The thunderstorm activity in the area was all to the north of the aircraft's flightpath and there is no indication of other than light turbulence during the latter part of the flight.

The damage to the structure indicated the aircraft struck the water slightly nose up, with a slight right-wing-down attitude. The breakup of the fuselage and wings was extensive with the area below the cabin floor fragmented, and little of this area was recovered. The wreckage pattern indicates the aircraft struck the water on a heading of approximately 256 degrees magnetic, and the slight right-wing—down attitude caused the wing to dig in and rotate the aircraft to the right as it broke up. Damage to the right wing was more extensive than to the left and the leading edge devices received considerable damage, much more so than the leading edge devices on the left wing. The fractures on the horizontal stabilizers had a downward component indicating forces which overloaded the spars, resulting in a complete failure. All the fractures examined were caused by gross overloads and there was no evidence of pre—impact damage or fatigue.

The aircraft‘s mass, the rate of descent of the aircraft, and the hydraulic action of the water caused the destruction of the lower portion of the fuselage and started the failure of the No. 2 engine mounts. Immediately after the initial impact the forward lower section of the airframe and the lower wing surfaces struck the water. The amount of destruction to the air-frame indicated a high rate of speed. The distortion of the engine tubing attached to the No. 3 engine strut attests to the fact that the engine separated from the strut in a forward and downward direction. The location of the recovered components of this engine indicates separation of the engine at the time of initial impact or immediately thereafter. The separation of the engine this early in the accident sequence explains its severe disintegration

The separation of the No. 1 engine was in an outward and rearward direction based on fracture examination and the engine's location in the wreckage area. It did not separate at initial impact but rather after the aircraft started to rotate to the right.

Portions of all the control surfaces and trim tabs were in the recovered wreckage. There was no evidence of unusual wear, distress, or pre-impact malfunction in any of the control system components recovered.

There was no evidence of an inflight fire or explosion. Only a few small pieces of wreckage were recovered with soot or smoke discoloration. This condition was caused by exposure to burning fuel on the water after impact.

The aircraft was in a clean flight configuration with the landing gear, trailing edge flaps, leading edge devices, and the speed brakes retracted, and all structural components were capable of normal operation prior to impact with the water.

The No. 1 engine was the least severely damaged of the three installed. While the bending of compressor and turbine blades opposite the direction of rotation indicates that the engine was rotating at impact with the water, the condition of the compressor turbine drive shaft, with no noticeable twisting, indicates that deceleration rate was lower than that of the other two engines. The bending of the shaft indicates that there was a considerable side load imposed on the engine when it struck the water. There was no evidence of any lubrication deficiency or bearing operational distress that would have impeded normal engine rotation. There was no evidence of any pre-impact failure, overtemperature operation, or structural failure of this engine before impact.

The No. 1 engine was rotating at a higher speed than No. 1 when No. 2 struck the water. This is evidenced by the more severe bending of the compressor and turbine blades in a direction opposite to rotation as well as the twisting fractures of the two compressor drive shafts. There is no evidence of structural failure of the basic engine components prior to impact with the water, nor is there any evidence of lubrication or bearing distress. There is no evidence of an overtemperature condition existing in the engine prior to the accident.

Examination of the fragments of the No. 3 engine that were recovered indicates that the engine was rotating at the time of impact. While there is a possibility of an inflight catastrophic problem with this engine, there is no evidence such as fire damage or shrapnel damage to the recovered portions of the engine cowling to support this possibility. Catastrophic engine failures ordinarily make themselves known through fire damage and puncture damage to the surrounding structure and the engine cowling. Also, the crew was conducting a radio conversation with approach control until just before the accident and gave no indication that any problem existed with the aircraft or any of its systems.

Examination of the fuel boost pumps revealed no evidence of fire, overheating or other operational distress. The fuel valves were found in the positions prescribed by the carrier for normal flight operation. One piece of the recovered fuel system evidenced some soot or smoke damage but the rest of the system was free of any evidence of fire. This sooting was probably the result of the explosion that occurred on impact.

The thrust reversers were found to have been in the forward thrust, normal flight position, at impact. While the No. 1 thrust reverser doors were slightly out of the forward thrust position when recovered, this was Judged to have been the result of impact damage.

There is no evidence on which we can base a finding of engine failure before impact. All three engines were operating when the aircraft struck the water. Based on the relative damage to the compressor blades of the Nos. 2 and 3 engines they were operating at a higher rotational velocity than No. 1. This further supports the theory that the aircraft was right-wing-down at initial impact and the REM of the No. 1 engine had time to be reduced more than the other two engines before No. 1 hit the water. The fuel system valve positions indicate that no intentional shutdown of any engine was initiated by the crew. Finally, there is no evidence of intentional or inadvertent thrust reversal in flight.

The recovered air conditioning ducting revealed no evidence of fire or smoke. The pressurization switch and valve positions were those established for normal operation and both bleed air valves were open. In the event of fire, the checklist requires these valves be closed, as is also required in the event of a pressurization emergency.

While there had been a history of autopilot writeups, the discrepancies were not of such a nature as to affect the control of the aircraft, but were rather a nuisance to flight crews. The only effect this problem would exhibit to the crew would have been a stabilizer out-of-trim light being "On".

There was no evidence found to indicate any pre-existing malfunction or problem with the electrical system. The only burn damage found on wire insulation is attributed to the flash fire which occurred after impact. There was no evidence of overheating, arcing, or any unusual appearance resulting from a malfunction of the electrical system. The generator control switches were recovered set in the positions prescribed by the carrier for normal flight operation.

The only discrepancy found in the flight control system that could not be attributed solely to impact damage was the condition of the stabilizer brake pawl. The position in which it was found made it useless as a brake device. This pawl is designed to prevent unwanted aircraft nose up movement of the stabilizer. Because the pawl is located in such a position as to be protected from damage during impact a possibility exists that the crew actuated the pawl to stop an unwanted nose up maneuver of the aircraft. It is also possible that the crew initiated an aircraft nose up maneuver just prior to impact and that impact forces caused a subsequent movement of the control column that activated the stabilizer brake and displaced the pawl. However, the position of the stabilizer jack screw in a 0.5 unit aircraft nose up at impact would indicate the pawl position was probably the result of impact. The low torque of the stabilizer trim actuator and the failure of the four flight spoilers to pass a cold soak test are not considered to be in causal relationship to this accident. The worst possible result of this situation would be one or more spoilers failing to extend when selected to the extended position. This would cause an unexpected and unwanted roll condition. However, the evidence indicates that the spoilers were in the down or retracted position at impact and thus an unwanted roll probably was not a problem to the crew. Furthermore, the review of pilot reports of spoiler difficulties indicated they were a high altitude phenomena. The crew of this flight indicated no concern in their last radio contact with the approach controller and were within l5 seconds of impact at the time of the last transmission. The crew had already descended below their assigned altitude of 6,000 feet and the accident was about to occur. Lastly, the evidence indicates the aircraft initially struck the water in a normal descent attitude.

Examination of the recovered components of the hydraulic system reveal no evidence of pre-impact malfunction. The four anti-icing valves were recovered in the closed position which indicates the anti-icing system was "off" at impact. One pilot's directional indicator showed that the course selected, 251 degrees and the heading of 260 degrees, were in consonance with the inbound course of 254 and the wreckage distribution direction of 270 degrees.

All the recovered VHF navigation and communications equipment was tuned for an approach to O'Hare and for communications with Approach Control. The air data computer provided only one piece of meaningful data, an indicated air speed of 200-210 knots. This reading was at the time of electrical power failure sometime after the initial impact with the water. The recovered altimeter had approximately the proper barometric setting for O'Hare Airport. It is noted however that this setting is also approximately the setting which should have been used in cruising flight, 29.92. Furthermore, all altimeter settings given the crew were within one or two hundreths of the 29.92 setting and it is possible the crew did not reset the altimeter because the maximum difference in indicated altitude would be approximately 30 feet which would not be significant during a VFR approach and landing, particularly with an ILS glide slope available for the approach.

The investigation of medical records, pathological findings, and toxicological results revealed nothing indicative of pre—existing disease or inflight incapacitation of any flight crewmember. Further, a study of associated psychophysiological factors such as recent schedules, recent psychological environment, and miscellaneous background data led to the conclusion that no physical or emotional impairment existed in the cases of any of the flight crewmembers when the aircraft departed New York.

The evidence indicates that the flight from New York to the point where the descent was initiated was normal and routine without any reported discrepancies or difficulties. A review of the air traffic control transcriptions reveals no evidence of any irregularities or signs of unusual operation on the part of the crew. The record also indicates that there was no known or observed traffic that conflicted with UAL 389 during the period of its observation on radar inbound from the Pullman VORTAC. Additionally there were no components of another aircraft in the recovered wreckage nor were any aircraft reported missing in the accident area.

The flight's first descent clearance was issued at 2103 and called for an immediate descent through FL 310. Based on the shipboard witness testimony the impact measured to the last whole minute was at 2120. The pilot reported leaving 35,000 feet at 2103 and leaving 28,000 feet at 2108. At 2109 he reported leaving 26,000. At 2114.36 the SAGE computer reported track number A 039 to be at an altitude of approximately 16,500 feet, and finally at 2119.54 SAGE reported track K 047 to be at approximately 2,000 feet. At the time this latter altitude was recorded the flight was holding a clearance which limited its descent to 6,000 feet. Furthermore, the crew was in radio contact with the approach controller at this time in routine radio transmissions relating to their approach to the Chicago area. Included in this last transmission was a reference to the latest altimeter setting. Track K 047 is assessed as being associated with UAL 389 because there was no other known or observed radar traffic in the area where the track appeared plus its correlation with track A 039 which in turn can be associated with UAL 389 by comparison of the ARTCC controller's recollection of the flight path and positions of UAL 389 over known fixes. Again there was no other known or observed high altitude traffic that could have appeared in the computer as this track except UAL 389.

A study of the calculated flightpath of the aircraft using these points reveals that the flightpath was well within the normal operating parameters of a B-727 making a clean descent when operated in conformance with UAL's operating procedures and techniques. The resultant flight profile (See Attachment No. 1), shows an average rate of descent from 35,000 feet to the lake level of approximately 2,000 feet per minute. UAL 389 was cleared from cruising flight at 350 to several lower altitudes; however, these clearances were given in a manner which precluded the necessity of leveling the aircraft and holding any one altitude for a period of time. The descent was continuous and the pointers on the altimeters would have been in continuous motion, making them more susceptible to misreading. Upon breaking out of the clouds between 8,000-10,000 feet, the crew's attention would have been divided between their routine duties preparing for an approach and landing, and maintaining a lookout for other traffic.

Additional calculations were performed based on the flight's reports and the controller's recollection of its location at various times. Based on the extremes of their recollection the ground speed of the flight in the Sturgeon area would have been between 240-300 knots. Using 240 knots a straight line descent from 2,000 feet m.s.l. would have resulted in an impact with the water at 2120:38. If the speed were calculated to be 270 knots the impact would have been at 2120:27 and 2120:14 for 300 knots. All of these performances are well within the operating envelope of the aircraft.

If we assume the flight leveled off at 6,000 feet as their clearance required, and were at 6,000 at the time of their last radio communication with the approach controller, in order to proceed four miles at a speed of 240 knots and strike the water at 2120:38, the aircraft would have to average a descent rate of 9,430 feet per minute, and arrest it so as to strike the water in a nearly level attitude. This calculation does not include any time allowance for initiation or recovery from this steep descent attitude. This was determined to be well outside the operating capabilities of the aircraft. Using the higher speeds and earlier impact times the rate of descent would go up to 15,000 and 30,000 feet per minute while covering ground commensurate with the aircraft's velocity. Therefore, it is determined that the aircraft was below 6,000 feet while conducting the last radio transmission with approach control.

The statements of the traffic controllers and a review of the SAGE readout indicate that the aircraft was decelerating as it approached the Sturgeon Intersection. This action is in accord with the speed restrictions on operating turbojet aircraft in a terminal area.

The statement of the flight crew operating three minutes behind UAL 389 indicated that the base of the broken cloud deck was approximately 8,000-10,000 feet m.s.l., and they could see the lights on shore from about 15 miles off shore. They also stated that there was haze in the area and visibility was fuzzy and unclear. UAL 389 was descending into an area of high traffic density and the crew may well have directed their attention to looking for other aircraft after breaking out of the clouds, rather than descending by use of their flight instruments. Although these considerations may have taken a majority of the crew's attention outside the cockpit no reasonable explanation for their failure to level the aircraft at 6,000 feet, their assigned altitude, can be offered. This is particularly true when one considers the fact that the last communication from the flight which ended at 2220.03 made reference to the altimeter setting.

There has been no evidence recovered to date which will substantiate any pre-impact difficulties with the aircraft. The crew was in radio contact with approach control at a point in time that was about 3–4 miles, or about one minute away from the accident site, and reported no difficulties. There has been no evidence recovered that will substantiate a finding regarding a malfunction of the altimeters. In fact, the SAGE altitude data correlates favorably with the aircraft's reported altitude when such correlation is made in cruising flight. There is no history of altimeter problems in the aircraft's maintenance records and there were no altimeter writeups reported by the crew that flew the aircraft into New York.

A review of Attachment No. 2 shows the position of the hands of the altimeter could be misinterpreted under certain operating conditions and the crew could have misread 6,000 feet to be l6,000 feet. It is believed the first officer was flying the aircraft. The captains voice was identified on the air traffic control tapes and it is a normal custom in air carrier operations to have the pilot who is not flying the aircraft make the radio transmissions. If the captain were looking outside the aircraft for traffic or occupied with cockpit duties such as the completion of a checklist, and the first officer misread the altimeter, this error could escape undetected. To have such an error occur, however, it would be necessary for the pilot or pilots to fail to see the gradually increasing display of the cross–hatched warning section of the altimeter and the gradually decreasing display of the white arc associated with the 10,000 feet pointer. This possibility is supplemented by the literature available regarding the various investigations of the readability of the three pointer altimeter which indicates it is the most susceptible to misreading of any of four types presently in use in commercial aviation.

Therefore, the Board believes that the crew for reasons unknown allowed the aircraft to descend below the assigned altitude of 6,000 feet and ultimately crashed into the water.

 

2.2 CONCLUSIONS

 

A. Findings

 

1. The known medical information concerning the flight crewmembers is not indicative of any pre-existing disease or inflight incapacitation.

2. Post mortem examinations indicate that there is no evidence of incapacitation or pre-impact injury to the crew.

3. There is no evidence of any system failure prior to impact.

4. The aircraft was approximately on the correct inbound course.

5. The cockpit VHF radio controls were set in a manner appropriate for a descent and approach to O'Hare.

6. The recovered altimeter had a setting that could have introduced an error of approximately 20 feet in the instrument reading.

7. Based on the identification of the captains voice on the ATC recording tape and common custom and practice in commercial aviation, it is believed the first officer was flying the aircraft.

8. The SAGE altitude reports referred to UAL 389 and recorded the altitude of the flight at 2119:54 as 2,000 feet with an accuracy of plus 500 – minus 1,000 feet.

9. The air data computer provides an indication of 200-210 knots at the time of electrical power interruption.

10. There was no major mechanical failure of the Nos. 1 and 2 engines before impact.

11. There is no evidence of a major mechanical failure 1n the forward fan section, forward bearing, or lubrication system of the No. 3 engine.

12. The rotational speeds of Nos. 2 and 3 were higher than No. 1 at engine impact.

13. The fuel system revealed no evidence of an intentional engine shutdown by the crew.

14. There is no evidence of thrust reversal inflight.

15. The aircraft was in a clean flight configuration at impact. The landing gear, trailing edge devices, leading edge devices, and speed brakes were fully retracted.

16. All structural components were capable of normal operation prior to impact.

17. There was no evidence in the recovered engine cowling and surrounding structure to indicate that anything had been ejected through the engine cowlings prior to impact.

18. There was no evidence of inflight explosion. The only fire that occurred during this accident was the flash fire which accompanied the impact with the water.

19. The aircraft's fire extinguisher system was not used.

20. Weather was not a causal factor in this accident.

21. The air traffic control handling of this flight was routine.

22. There was no evidence of any distress or inflight difficulties on the recorded transmission from the crew.

23. The aircraft made a continuous descent at an average rate of approximately 2,000 feet per minute from 35,000 feet to impact with the water.

24. The flight was cleared to descend to and maintain 6,000 feet.

25. At the time of the last radio communication w1th approach control, the aircraft had descended through 6,000 feet and was at an altitude of approximately 1,000–2,500 feet m.s.l. This is approximately 500 to 2,000 feet above the water.

26. The impact with the water occurred between 2120 and 2121.

 

B. Probable Cause

 

The Board is unable to determine the reason for the aircraft not being leveled off at its assigned altitude of 6,000 feet.

BY THE NATIONAL TRANSPORTATION SAFETY BOARD

(Signature Omitted)
JOSEPH J O'CONNELL, JR
Chairman

(Signature Omitted)
OSCAR M LAUREL
Member

(Signature Omitted)
JOHN H REED
Member

(Signature Omitted)
LOUIS M THAYER
Member

(Signature Omitted)
FRANCIS H McADAMS
Member

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