NTSB Factual Report

HISTORY OF FLIGHT

On December 28, 2001, at 1431 central standard time, a Boeing 737-522, N941UA, operated by United Airlines as flight 1260, declared an emergency due to a reported control system problem while descending into the Chicago O'Hare International Airport (ORD), Chicago, Illinois. Visual meteorological conditions prevailed at the time of the incident. The revenue passenger flight was operating under the provisions of 14 CFR Part 121 while on an instrument flight plan. The 2 pilots, 3 flight attendants, and 99 passengers reported no injuries. The flight departed the Eastern Iowa Airport, Cedar Rapids, Iowa, at 1357, and was en route to ORD at the time of the incident.

According to an interview with the captain, the aircraft was descending from 10,000 feet mean sea level (msl) when he noticed a "subtle vibration" in the cockpit that lasted a period of approximately 2 seconds. Following the vibration, the aircraft began an uncommanded roll to the right. The captain reported that he disconnected the autopilot system at approximately 20 degrees right roll, and he regained control of the aircraft using manual control inputs of left rudder and left aileron. The captain stated that during the uncommanded roll the rudder controls remained stationary and in a neutral position. The captain also reported that he had "complete usage of the rudder" during the recovery.

Subsequent to the recovery, the captain applied two short inputs (1 second each) of left wing down trim. Following the trim input, the yoke returned to a neutral position and he was able to relax his control pressure inputs. The captain reported that at approximately 8,000 feet msl "everything was sorted out" and the airplane "flew like an new airplane."

The captain stated the first officer (FO) declared an emergency to Chicago approach control and requested to land on runway 27R (7,967 feet by 150 feet, grooved concrete). The airplane touched down on runway 27R at 1439 without further incident.

Written statements, crew interview summaries, and a Chicago approach control transcription are included with the docket material associated with this factual report.

PERSONNEL INFORMATION

Captain:

The captain was the pilot flying (PF) and was seated in the left cockpit position. The captain held an airline transport pilot (ATP) certificate with an airplane multiengine land rating. He was type-rated for a Boeing 737 airplane. The Federal Aviation Administration (FAA) issued his current ATP certificate on May 5, 2001.

FAA records indicate his last medical examination was completed on December 26, 2001, and that he was issued a first-class medical certificate with no restrictions or limitations.

The captain reported having accumulated approximately 4,000 hours in a Boeing 737 airplane, of which approximately 1,000 hours were as pilot-in-command (PIC). He flew 128.9 hours during the last 3 months, all in a Boeing 737 airplane. He had flown 61.8 hours during the previous 30 days and 4.0 hours during the prior 24 hours.

The captain began his flight career with the United States Navy, where he flew the P-3 Orion airplane for 8.5 years. He began his civilian flying career with Trans World Airlines (TWA), where he was a copilot in a Boeing 727 airplane. The captain reported United Airlines hired him on May 20, 1991. He flew as a copilot in the Boeing 737, 767, and 777 airplane models prior to upgrading to captain for Boeing 737 airplanes.

First Officer:

The first officer (FO) was the pilot-not-flying (PNF) and was seated in the right cockpit position. The FO held an ATP certificate with an airplane multiengine land rating. The FO additionally had a rating for commercial privileges in single-engine land airplanes. The FO did not have a type-rating for a Boeing 737 airplane. The FAA issued his current ATP certificate on November 9, 1999.

FAA records indicate his last medical examination was completed on March 15, 2001, and that he was issued a second-class medical certificate with no restrictions or limitations.

The FO reported having a total flight time of approximately 2,600 hours, of which approximately 1,300 hours were as PIC. He stated he had flown 800 hours in the Boeing 737 airplane, and all were as second-in-command. He flew 148.3 hours during the last 3 months, all in a Boeing 737 airplane. He had flown 59.9 hours during the previous 30 days and 4.0 hours during the prior 24 hours.

The FO reported United Airlines hired him on May 8, 2000, and all of his Boeing 737 airplane flight experience was acquired with the airline.

AIRCRAFT INFORMATION

The Boeing 737-522, N941UA, serial number 26676, is a pressurized, low-wing, narrow-body transport category airplane. The Boeing 737-522 model has all-metal full-cantilevered wing and tail surfaces, a semi-monocoque fuselage, and a fully retractable landing gear. The two wing mounted CFM International CFM56-3B1 turbofan engines each produce 20,000 lbs of thrust. The 737-522 model uses the basic structure of a 737-300 series airplane; however, incorporates a 94-inch shorter body and a revised forward and aft wing-body fairing.

The 737-522 model accommodates 108 passengers in a mixed class configuration, has a maximum range of 2,996 nm at a maximum operating altitude of 37,000 feet, and has an maximum operating speed of 0.82 mach (340 knots calibrated airspeed). A 737-522 airplane has a zero-fuel weight of 103,000 lbs and a maximum gross takeoff weight of 133,500 lbs.

According to company records, the airplane was delivered on October 6, 1992, and had accumulated 27,409 hours total time in service. The incident airplane was being maintained under the provisions of a FAA approved continuous airworthiness program. The last major maintenance check was completed on January 11, 2001, and the airplane had accumulated 2,602 hours since the inspection.

METEOROLOGICAL INFORMATION

A weather observation station, located at ORD, recorded the weather as:

At 1356: Wind 300 degrees true at 13 knots; 10 statute mile visibility; broken clouds at 4,000 feet above ground level (agl); temperature -05 degrees Celsius; dew point of -13 degrees Celsius; altimeter 29.63 inches of mercury.

At 1456: Wind 290 degrees true at 14 knots; 10 statute mile visibility; scattered clouds at 4,200 feet agl and broken clouds at 5,000 feet agl; temperature -05 degrees Celsius; dew point of -13 degrees Celsius; altimeter 29.64 inches of mercury.

According to measurements transmitted by a radiosonde balloon launched near Davenport, Iowa, at 1800, the winds between 9,000 - 12,000 feet msl were:

Altitude (feet msl); Wind direction/speed (degrees true/knots)

9,000; 290/35

9,117; 290/35

9,753; 288/36

11,758; 281/40

12,000; 280/40

TESTS AND RESEARCH

Aircraft radar track data was obtained from Federal Aviation Administration (FAA) Air Traffic Control. The provided data indicated the incident airplane (UAL Flt 1260) was following a Boeing 777-200, operated by American Airlines as flight 154 (AAL Flt 154). Both aircraft were en route to ORD and were flying the Janesville Four Arrival. Both aircraft approached KRENA intersection on a 096-degree magnetic flight path. Both airplanes crossed the intersection at 10,000 feet msl, with AAL Flt 154 passing at 1426:00 followed by UAL Flt 1260 97-seconds later at 1427:37. After crossing KRENA intersection both aircraft flew a 133-degree magnetic flight path. Both airplanes had ground speeds of approximately 307 knots [250 knots indicated airspeed (KIAS)] and were maintaining a lateral separation of approximately 8.3 nm or about 97 seconds.

At 1428:23, 12.56 nm from KRENA intersection, AAL Flt 154 began descending from 10,000 feet msl. At 1428:37, Chicago approach control instructed UAL Flt 1260 to descend from 10,000 feet msl to maintain 7,000 feet msl and UAL Flt 1260 acknowledged the clearance at 1428:40. UAL Flt 1260 began the descent at 1428:46, 6.23 nm from the intersection.

According to data recovered from the digital flight data recorder (DFDR), UAL Flt 1260 began a shallow left roll at 1428:54 that reached a maximum left bank angle of 1.4 degrees at 1429:00. The left bank was immediately followed by a right roll, which reached a maximum right bank angle of 21.4 degrees at 1429:14. During the left and right rolls the autopilot maintained the desired course by commanding appropriate opposing aileron and spoiler inputs. The uncommanded right roll began at 1429:04 when the airplane was at approximately 9,700 feet msl. A calculated average roll rate of 1.5 degrees/second was encountered during the uncommanded right roll.

At 1429:14, the autopilot was disconnected and manual inputs were used to recover from the right roll. Large manual flight control inputs were recorded for approximately 100 seconds subsequent to the autopilot disconnection. At approximately 1430:54, at approximately 8,000 feet msl, all flight control movements had reduced in amplitude and were consistent with those before the uncommanded roll event.

During the uncommanded roll and subsequent recovery, all rudder control surface movements correlated with the position of the rudder pedals.

At 1431:15, Chicago approach control instructs UAL Flt 1260 to "reduce speed two one zero, caution wake turbulence you'll follow a heavy on the final."

At 1431:29, UAL Flt 1260 replies with, "two one zero and ah slow to two ten here united ah two twelve sixty, yeah we're going to have to declare an emergency, we had a little bit of a rudder hard over here we'd like to get into runway two seven as fast as we can."

Chicago approach control issued UAL Flt 1260 vectors for runway 27R, and an uneventful landing was made at 1439.

United engineering and maintenance personnel performed visual inspections and testing of the rudder and lateral control system while following, but not limited to, guidance provided in Boeing Service Letter 737-SL-27-110-A 'Unexpected Roll and Yaw Event Troubleshooting'. Visual inspections of the flight control system, functional inspections, and testing were performed before any component removal. No flight control anomalies were found during the post-incident examination.

A copy of Boeing Service Letter 737-SL-27-110-A is included with the docket material associated with this factual report.

On December 30, 2001, the incident airplane successfully completed a verification test flight, during which no anomalies were detected with any flight control systems.

ADDITIONAL DATA/INFORMATION

According to FAA Advisory Circular (AC) 90-23F 'Aircraft Wake Turbulence', the wing-tip vortices produced by large aircraft "can impose rolling moments exceeding the control authority of the encountering aircraft." The AC also states, "Flight tests have shown that the vortices from larger (transport category) aircraft sink at a rate of several hundred feet per minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft." The AC advises, "Pilot's should fly at or above the preceding aircraft's flight path, altering course as necessary to avoid the area behind and below the generating aircraft." The AC further states, "The probability of induced roll increases when the encountering aircraft's heading is generally aligned or parallel with the flight path of the generating aircraft."

A copy of AC 90-23F is included with the docket material associated with this factual report.

Parties to the investigation included the FAA, United Airlines, The Boeing Company, and the Airline Pilots Association.

NTSB Probable Cause

The wake turbulence encounter.