Flight Crew Air Transport Pilot (ATP)
Distraction; Workload; Situational Awareness; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Executed Go Around / Missed Approach; Flight Crew FLC complied w / Automation / Advisory; General Maintenance Action
Aircraft; Chart Or Publication; Procedure
We were on a Visual Approach to Runway XX.  Intercepting the glideslope; I called for gear/flaps 15.  Calling for flaps 30; I looked at the flap gauge and saw the yellow Leading Edge Flap Transit light on.  I looked up and saw a yellow Slat #6 light. We executed a go-around to run the checklist. ATC gave us instructions and then asked 'State reason for the go-around.'  We cleaned up the gear and got the flaps up to 10.  I was hand flying and checking for a controllability issues and the FO got the QRH out. We ran the checklist and setup and briefed the change to flaps 15 and ran the brief for landing again for flaps 15. The FO also did a visual check of Slat 6 and saw that it matched the others. ATC gave us instructions numerous times while running the checklist.  We were vectored back in for the visual to Runway XX and were instructed to maintain 180 knots until final; we started our second approach.  Glideslope intercepted; I called for the gear and flaps 15. I was flying at 148 to 155 knots as we were heavy and on glideslope.  At approximately 1500 ft we got a 'Airspeed Low' call out. I was at 145 knots.  It only lasted for a moment.  It looked like a logic error to the both of us as the speed was well above the stall warning or approach to stall; but we bumped the speed 10 knots flying at 150 knots.  That is when we realized that we had fat fingered the Vref15 plus 15 and had done the usual plus 5. We had 134/139 in the box not 148/153 we should have.  No further problems were encountered and we made an uneventful landing. Maintenance found a bad sensor on the #6 Slat and a MEL was issued.The QRC and brief are still very new.  ATC chatter did not help.  We talked it over and beat ourselves up over our screw up.  We are just not sure how it happened; but we both messed it up somehow. We handled the situation fine and we both learned a valuable lesson from the experience.
B737 Captain reported the leading edge flap transit light illuminated as flaps 30 was selected. Maintenance found a faulty slat sensor after landing.