Date: 201310
Local time of day: 0601-1200
Location: ZZZ.TRACON, US
Flight conditions: VMC
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
ATC / advisory: TRACON ZZZ
Aircraft component: Autothrottle/Speed Control
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Confusion; Situational Awareness
Second reporter function: Pilot Flying; First Officer
Anomaly: Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: While being vectored on a left base in VMC to a 12-15 mile final we were at Flaps 10; autopilot on; and slowing towards 175 knots for traffic ahead.  Approach Control gave us a heading of '120 degrees to join the localizer; cross XXX at 3;000; 170 knots until 5 mile final; cleared for the LOC XXX approach'.  The Pilot Flying accidentally reached for the IAS knob instead of the Heading knob; and selected 120 knots.  Pilot Monitoring was verifying the crossing restriction on the FMC and looking for the indicated traffic; and did not visually backup the Pilot Flying in the heading selection.  The Pilot Flying looked down at the FMC to validate for himself the LNAV/VNAV setup.  We both noticed that the aircraft was not responding to the heading selection; looked up to the Mode Control Panel to check the control inputs; realized the error; then quickly noticed that the airspeed had dropped well below the Flaps 10 Minimum Maneuver speed to approximately 135 knots.  No secondary indications occurred.  Pilot Flying immediately applied power and recovered the airspeed to 170 knots. The remainder of the approach and landing were completed uneventfully.  Debrief thoughts: Though we had an early wake up; we both felt well rested before departure.  Pilot Flying did not follow his hand with his eyes when he reached for the desired control; then diverted his attention to the FMC for ATC compliance.  Pilot Monitoring allowed himself to become preoccupied with outside traffic; locating the assigned runway; and verifying FMC compliance instead of backing up the Pilot Flying in his heading control selection.  Although two experienced and conscientious pilots were involved; this sharply reinforces the need for both pilots to be actively attentive to control inputs.


Second narrative: No additional information contained in this report.


Synopsis: While being vectored for an ILS in VMC conditions the crew of a Boeing 737 was given a heading of 120 degrees and the pilot flying; utilizing the autopilot and autothrottle functions; mistook the IAS selector for the heading select dial and set 120 knots.  When the airspeed had decayed to about 135 knots; well below the 10 degree selected flap minimum maneuver speed; the PF noticed the deteriorated energy state and immediately restored power and airspeed back to 170 knots.