B737-700
Flight Crew Last 90 Days 392
Human-Machine Interface; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented
Aircraft; Human Factors
While being vectored for final approach; I called for landing gear down; flaps 15; then flaps 30 and proceeded to slow to target speed of 134 knots. As I called for the Before Landing Checklist; the First Officer (F/O) noted that the flaps had stopped moving. I glanced and noted the flaps were indicating midway between one and two with the green LED FLAPS EXT light illuminated. I elected to go-around at about 800' to 1000'. We requested vectors back around and informed Approach Control that we'd need a little extra room to run a procedure. The First Officer and I referenced the QRH; running the Flaps; Trailing Edge: Symmetrical Non-Normal Checklist. While stepping through the procedure; I armed the alternate flap master switch as directed. As the F/O continued to read the next checklist item; I noticed that the amber LE FLAPS TRANSIT light had illuminated and that the flaps were retracting. I glanced at the flap handle and verified it was in position 15 and glanced up at the flight control panel and did not see anything that could cause the flaps to be retracting. I simultaneously increased thrust to pick the speed up to 200 knots to maintain adequate airspeed for an unknown aircraft configuration. I quickly reassessed the situation and realized I must have missed something and looked back at the overhead flight control panel; concentrating on the ALTERNATE FLAPS Position Switch.  I then noticed that the switch had been previously left in the UP position and quickly moved it to the OFF position. The flaps stopped moving between 0 - 1 with the amber LE FLAPS TRANSIT light illuminated. This evolution covered only five to seven seconds. I then honored the required 15-second off period for alternate flap direction reversal before continuing with the checklist and turning back to the airport. As we extended the flaps to 15 using the alternate system; I noted the flaps and leading edge devices were performing normally.  The approach and landing were unremarkable.  If this had happened with flaps extended past 15 at a low airspeed and the pilots were slow to respond; a loss of aircraft control could easily occur. If it occurred at low altitude; recovery could be in doubt.NOTE: the Alternate Flaps Position switch is a small toggle switch and usually dark; metallic colored from oxidation over the years. It's very hard to see what position it's in against the dark-gray panel when sitting directly under it; hence; I missed the fact it was left in the UP position when I first glanced up at the panel. It's a momentary DOWN; normally OFF; selectable UP switch. To extend flaps you must hold the switch in the DOWN position and it will return to OFF when released; but you can select it to the UP position and it will remain in that position until manually moved to OFF.Since a loss of aircraft control could result from this happening again; I make the following recommendations: 1. Flt Ops immediately issue a [bulletin] instructing Captains to verify the Alternate Flaps Position switch is in the OFF position on their originating flow. (I've already added it to mine.) 2. ALL QRH procedures involving the use of the Alternate Flaps Master Switch be modified to include the following step PRIOR to selecting the Alternate Flap Master Switch to the ARM position:  Alt Flap Switch ..........................VERIFY OFF [w][W] WARNING - activating the Alternate Flaps Master switch with the Alternate Flap switch in the UP position will cause the flaps to retract... blah blah blah...3. Immediately contact manufacturer and recommend they modify the AFM to include the above recommendations to prevent an inadvertent loss of aircraft control.
B737-700 Captain reported inadvertent flap retraction when arming the alternate flap position switch in response to a trailing edge flap abnormality. The reporter went on to recommend changes to the procedure to minimize the possibility of reoccurence.