Date: 201810
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Weather elements / visibility: Turbulence
Aircraft: MD-11
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Initial Approach
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Flap/Slat Control System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Troubleshooting
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While configuring for the approach on a dog-leg to intercept the localizer XXL at ZZZ; received level 2 SLAT DISAG after extending flaps to 28. The alert initially stayed on for probably a minute and then went away. No uncommanded rolling moment. Discontinued the approach and referenced the QRH. [Notified] Approach. Brought the jumpseater into the cockpit to assist and provide oversight. Called the dispatcher on SATCOM and requested a conference call with [Maintenance Control]. [Maintenance Control] said datalinked telemetry from the aircraft reported a single sensor had faulted to a value of zero. I discussed and investigated the potential of an actual asymmetric slat situation vs only a sensor problem. I was satisfied with the level and depth of the information provided by [Maintenance Control] that [Maintenance Control] believed it was only a sensor problem. I ran through a decision tree of following the QRH level 2 SLAT DISAG procedure. A #1 thrust reverser MEL precondition; wet runway and poor data in the QRH procedure for landing distance with an reverser inop led me to understand that a 0RET/28 landing would probably result in stopping near the last brick of the runway. ZZZ1 was our alternate but their long runway was under construction and the runways available there were shorter than ZZZ XXR. We didn't have enough fuel to go anywhere else. After a conference with the First Officer; and bringing the senior jumpseat pilot into the decision process; I decided that due to a lack of uncommanded rolling moment; the intermittent nature of the problem and the telemetry data from the aircraft; the best course of action was to fly a fully configured CAT II Autoland to a full stop on the long runway at ZZZ. During approach intermittent SLAT DISAG alerts triggered intermittent stick shaker and red speed boxes on the airspeed tape. Configuration changes were made mid-airspeed range for each new configuration in consideration of the possibility of an actual asymmetric deployment of the flight controls. Added 5 knots to approach speed as a safety margin; but flying qualities of the aircraft remained nominal throughout every configuration change and while stabilized on final. No uncommanded rolling moment was noted in any phase. Uneventful landing and taxi in. Suspected aircraft slat position sensor fault.


Second narrative: Turning to intercept localizer for rwy XXL ZZZ. After selecting flaps 28 began receiving intermittent SLAT DISAG alert - approx. 3 seconds of alert on; 5 or so seconds off. Requested approach break us out of the approach. Approach did a nice job vectoring us at 6000 during our troubleshooting. We cleaned up to 15/ EXT then 0/ EXT. FO (First Officer) was PF (Pilot Flying) for initial approach and troubleshooting. Captain referenced the QRH; then took controls so FO could read thru the procedure. FO then took controls and radios. Captain brought the jumpseater to the cockpit to assist; then SATCOMMED Dispatch and Maintenance. Maintenance had telemetry that confirmed the Slats were all extended and the LH no. 4 indicator was faulty. This fact; the intermittent nature of the alert and that we experienced no rolling moment during slat deployment led us to decide on a CAT II 50/EXT auto land. Captain took controls and we returned to ZZZ runway XXR. On approach had intermittent SLAT DISAG alert. No rolling moment. Uneventful CAT II Auto land. Fire trucks followed us to the ramp. Left flaps / slats at 50/EXT. Nice job by ZZZ Approach; tower; ground and crash fire rescue. Faulty LH #4 slat position sensor.


Synopsis: MD11 flight crew reported evaluating the landing options with a faulty LH #4 slat position sensor.