Date: 201208
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Landing
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: At about 1;500 feet AGL on a visual approach; the flaps were set to 30; the final descent checklist was accomplished; and the target speed of 123 knots was set.  The performance computer was on an MEL so all performance data had been received from Dispatch.  When the Vref and target were entered into the FMC; there was only one knot difference from what Dispatch gave us and what the FMC was showing for a Vref speed.  At about 100 feet AGL; we got an automated 'airspeed low' call out.  The First Officer; who was the flying pilot; added power but our airspeed was at 123 knots +/- two knots.  He was right on speed.  At this time I noticed that the 'LE flaps transit' light was illuminated.  I also noted that the MASI speeds looked like they were set for flaps 15 and that the 'speedbrake do not arm' light was illuminated.  As the First Officer added power; the 'airspeed low' automated call out stopped and the MASI speeds looked like they went back to a flaps 30 setting.  Out of reflex; I disarmed the speedbrake knowing that I could manually deploy it at touchdown.  I almost always make sure that the speedbrake has fully deployed by hand anyway.  At about 30 feet AGL the MASI [Mach airspeed indicator] speed went back to what looked like a Flaps 15 setting and we got the automated 'airspeed low' call out again.  Within a second or so we landed and I deployed the speedbrake by hand.  After we landed the LE FLAPS went from amber 'LE flaps transit' to green 'LE flaps ext' and the MASI went back to what looked like a setting for Flaps 30. At the gate I wrote up the LE flaps and notified Maintenance.  They came out and inspected the flaps and did a bite check on the PSEU [proximity sensor electric unit].  The only fault noted was an over voltage on some of the LE flap sensors.  This whole event lasted about four seconds plus or minus one or two seconds.  On reflection; it may have been better for me to command a go-around and try to figure out what the aircraft was doing; but I really believe that by the time we would have reacted and commanded a go-around; we would have touched down anyway.  We were too low; and it all happened too fast with multiple related and unrelated system indications. After talking to Maintenance; I believe that we received some sort of power surge that made the aircraft computers lose track of how it was configured.  One second it thought it was at Flaps 15 and the next second it thought it was back at Flaps 30.  Then back to Flaps 15 and then at touchdown; back to Flaps 30.  The flap indicator indicated Flaps 30 the whole time and I did not have time to look on the overhead panel and see if it was showing the LE Flaps fully extended.  I don't know if this has ever happened to anyone else; but maybe we should get our Safety Department to take a look and see if something like this has ever occurred before.




Synopsis: B737-700 Captain reports 'airspeed low' call out at 100 feet during approach along with a LE flaps transit light.  Power is added by the flying First Officer; although the aircraft is on speed; and the warning silences.  The approach is continued and the cycle repeats once more prior to touch down.