N9348

History of FlightOn July 12, 1997, at 1735 eastern daylight time, a McDonnell Douglas DC-9-15, N9348, operated by Northwest Airlines, experienced an uncommanded right roll while on final approach into Memphis International Airport, Memphis, Tennessee. The airline transport pilot landed the airplane without incident. Neither the pilot, co-pilot, two flight attendants, or the 71 passengers were injured. The passengers were deplaned normally. The 14 CFR Part 121, Flight 944, had departed Indianapolis International Airport, Indiana, and was scheduled to land at Memphis International Airport. Northwest Airlines removed the airplane from service to investigate the incident. Visual meteorological conditions prevailed at the time and an instrument flight plan had been filed.

The pilot reported that at 150 feet AGL, the aircraft performed an uncommanded roll to the right requiring maximum left aileron control input to correct. The pilot reported that the uncommanded roll repeated itself three times between 150 to 50 feet AGL.

Personnel Information

The airline operator reported that the Captain held an Airline Transport Rating and had 8,531 total hours of flight time in the DC-9-15.

The airline operator reported that the Copilot held an Airline Transport Rating and had 1,178 total hours of flight time in the DC-9-15.

Aircraft Information

The airplane was a McDonald Douglas DC-9-15, serial number 45787, fuselage number 127, equipped with two Pratt & Whitney (P&W) model JT8D-7B engines. It was manufactured on June 26, 1967, and had 64,560 total airframe hours.

On May 28, 1997, a structural crack was detected at St Louis, Missouri (STL), and the airplane was ferried to Minneapolis, Minnesota (MSP), for repair. While at MSP, further inspection revealed additional right wing cracks and corrosion. The airplane was then ferried to Triad International Maintenance Corporation (TIMCO), a Federal Aviation Administration (FAA) certified Class IV Repair Station located at Piedmont Triad International Airport (GSO), Greensboro, North Carolina, for repair and other scheduled maintenance. The airplane arrived at TIMCO on June 4, 1997.

Part of the scheduled maintenance accomplished by TIMCO included removing and replacing the upper and lower spar caps and fuel tank refurbishment. Spar cap replacement required that the entire lateral control system be removed from the aft spar, including all cables, pulley brackets and associated actuators, and control surfaces. In order to facilitate other maintenance (FOM), Job Card 28009 required TIMCO to disconnect spoiler control cables 17B and 18B, and later re-connect and rig per NWA 90-2000-1-9000 control cable procedures. Since spoiler cables were not considered primary flight control cables, they were not designated by NWA as Required Inspection Items (RII). After scheduled maintenance was completed, N9348 was released for service by TIMCO on July 1, 1997.

The airplane was positioned by a ferry flight to DTW, and then began passenger service on the same day. N9348 flew 52 flights before the first incident. During that period there were no discrepancies noted concerning flight control problems.

On July 9, 1997, at about 0015, the first reported uncommanded roll incident occurred while landing on runway 18R at MEM. The flight had departed Detroit Metro Wayne County Airport (DTW), Detroit, Michigan. The uncommanded roll, "right wing dips excessively with no known wind and no aircraft ahead, seems to occur during ground effect," prompted the flightcrew to initiate a go-around maneuver. On the next approach, a less severe uncommanded roll was experienced, and the airplane was landed successfully.

The flightcrew previously had flown 6 other flight segments with the airplane, and did not report any flight control discrepancies. The airplane remained on the ground for 6 hours and 32 minutes, before its next flight.

Corrective actions taken by NWA maintenance at MEM were: "Checked flaps with protractor and checked control surfaces per MM Chapter 27 (flight controls); Troubleshot flight spoilers, ground speed spoiler brakes, flaps, and flap vanes; No loose or floating problems were noted; Airplane needs further troubleshooting."

After these maintenance actions, N9348 was released for service.

The airplane departed MEM at 0700 on July 9, 1997 for DTW, with a different flightcrew. No flight control discrepancies were noted. At DTW, the flightcrew changed again, and N9348 flew 2 more segments, ending up in Minneapolis-St Paul International Airport (MSP), Minneapolis, Minnesota. No flight control discrepancies were noted. The flightcrew changed again, and N9348 departed for Cedar Rapids Municipal Airport (CID), Cedar Rapids, Iowa.

At 1715, on July 9, 1997, after flying four flight segments since the first occurrence, the second reported uncommanded roll incident occurred. The "airplane abruptly rolled right, full left control wheel input was required to stop the right roll, aircraft then rolled sharply left, this sequence occurred several times," while the airplane was on an approach to runway 9 at CID. The airplane landed successfully, and remained on the ground for 15 hours and 50 minutes before its next flight.

Following this incident, NWA maintenance at CID found the right inboard spoiler cables extremely loose, and re-rigged the cables per MM 27-60-1. The airplane was released for service.

After being released for service, the airplane flew 16 more flight segments before departing Indianapolis International Airport (IND), Indianapolis, Indiana, and arriving at MEM on July 12, 1997. There were no discrepancies noted concerning flight control problems during those 16 flights.

At 1730, on July 12, 1997, at MEM, the third reported uncommanded roll incident occurred. The pilot indicated the, "airplane abruptly rolled right, full left aileron was required to maintain control." The airplane remained on the ground for over 4 days before its next flight. Corrective actions taken by NWA maintenance at MEM that involved flight controls were: 1. Looked at right elevator and found the Right Elevator Damper bad. Removed and replaced the Right Elevator Damper per MM 27-30-5 (Elevator Dampers).

2. Checked cable runs on right and left wings for aileron and spoilers. Cables checked normal, no crossed cables or turn-barrel interference.

3. Left and Right Mixers looked normal. 4. At NWA Maintenance Control request, replaced right hand (RH) inboard (IB) and RH outboard (OB) Spoiler Actuators per MM 27-61-2.

5. Found RH OB spoiler cables under-tensioned. Re-tensioned per MM 27-61-0. Spoiler rig and operational checks within limits per MM 27-61-0.

6. Left IB Flap Actuator leaking. Replaced IB Flap Actuator per MM 27-52-2. Functional Check is good.

7. Performed Hydraulic Decay ITCAN (inspect, test, and correct as necessary) per MM 29-10-0, and found left hand (LH) OB Spoiler Actuator bypassing severely. Replaced the Spoiler Actuator per MM 27-61-2.

8. At NWA Maintenance Control request, replaced Rudder Power Pack per CITEX 09-2722-1-9801.

9. During rudder check, found Hydraulic Shut-Off Valve bypassing. Replaced Rudder Power Shut-Off Valve per MM 27-20-9. Leak Check and Function Check are good.

10. RH Flap Moveable Vane Track Cable rubbing at top. The bolt and swivel appear to be worn. Removed and replaced IB Bus Cable End Fitting. No help. After flap valve adjustments were made, ample clearance was achieved.

11. At NWA Maintenance Control request, removed the Stability Augmentation Computer and installed a serviceable unit per MM 22-11-1.

12. LH and RH Flap Moveable Vanes are out of limits. Adjusted and tested vanes per MM 27-51-2.

13. RH Flap Moveable Vane Track has flat spot. Flat spot is within limits of MM 27-50-1. Wear is normal and no action is required.

14. Spoiler Deployed Light does not come on until 45 degrees of control wheel travel to right. Checked spoiler extend function per MM 27-61-1. Indicator illuminated before 10 degrees of spoiler deflection.

15. Found cable run 17B caught between cable guards at pulley for 18B and riding on lower side of pulley for 18B. Freed cable. Checked routing and rig checked/throw checked good per MM 27-61-0.

Tests and Research

The flight spoiler system consists of four spoiler panels, four actuators, two pressure reducer valves, two pressure switches, the speedbrake control lever, portions of the left and right lateral control mixers, and connecting cable systems, and mechanical linkages.

The spoiler panels supplement the flight control surfaces during flight and during the landing roll. There are two spoiler panels on the upper surface of the trailing edge of each wing, forward of the ailerons. The spoiler mechanical system is a hydraulically actuated system that controls the movement of the spoiler panels to supplement the ailerons in lateral control of the airplane. The system also acts as a speedbrake.

There are two lateral control mixers, one located in each main gear wheel well. The mixers allow the spoiler system to be operated by the speedbrake control lever, by the aileron control wheels, or by both simultaneously. The mixer brackets also serve as common attachment points for cable pulleys and drums of the spoiler system, aileron control system, aileron trim control system, and the flap followup system.

Spoiler panel float, due to loss of hydraulic pressure, is prevented by a cam attached to a torsion bar and positioned against rollers on the actuator aft hinge pin. The cam exerts a force against the hinge point so that the drive linkage from the actuator to the spoiler panel is locked in an over-center position, when the panel is retracted.

The cable rigging is described as follows:

Cable run 17B movement is commanded to open the control valve to raise spoiler panel from inputs by the Speed Brake Handle and Aileron Control Wheel (movement past 8 (+/-3) ...