B737-800
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 210; Flight Crew Total 12000; Flight Crew Type 7081
Distraction; Situational Awareness
Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented
Human Factors
Captain was pilot flying and First Officer was pilot monitoring. Descending into PHX on the Eagle arrival; runway 26 transition. We were in an older 737-800 without speed intervention. We were flying the arrival in VNAV and LNAV. Altitude was set to 4;000 feet. We were given a speed reduction from 250 to 210 knots. Flaps 5 were selected. As speed approached 210 knots the auto throttles were taken off to maintain the speed of 210 knots. A second reduction of speed was requested around 7;000 feet of 170 knots. Flaps 10 were selected. Around 5;700 feet we were instructed to contact the final approach frequency. Upon checking in we were asked if we wanted Runway 25L and we confirmed yes. We were then given a heading and to maintain 5;000 feet. I told the Captain I would set it up in the box and frequencies. Unfortunately; the Captain who was still the flying pilot wanted to do it himself. He went heads down to program the box as the plane leveled at 5;000 feet. Flaps were 10 and flap 10 speed was 164 knots. Current airspeed was close to 168. I was at this point as the monitoring pilot watching the CA enter the box; get my correct approach plate out myself and monitor the plane. I saw the airspeed at 158 knots (immediate recognition of our error; no auto throttles; flight idle and level) and stated 'Check Speed' as I pushed the thrust levers to the maximum realizing we had entered an undesired aircraft state. Speed decayed to around 149 - 151 knots before the engines accelerated enough from flight idle to recapture the flaps 10 speed of 164 knots. Approach continued visually as now I adamantly stated to him to fly the plane and don't look at the box again and I would get everything set up. Second runway change was given to runway 25R which we accepted and from that moment the flight was uneventful.At no time did we receive a buffet or a stick shaker. The yellow low speed arc was around 138 knots. Our mistake and primarily mine was I let the Captain go heads down into the box as the flying pilot. I have been uncomfortable in this exact situation before when no one is flying the plane. I will never let this happen again. If we are in the approach phase; I will now use the appropriate CUSS words to make sure someone is flying the plane and to let the monitoring pilot fly do all of the box work. This is exactly how I do it as the flying pilot. We get a change and I instruct the Non-Flying pilot to do the change in the box and let them set me up as I fly the plane. I will no longer let this happen when I am the monitoring pilot or we will just go around if we can't get a new runway programmed in the box in time.
On long final we were told to slow to 210 knots. Auto Pilot on. I disconnected Auto Throttle (AT) and adjusted. Then they gave us the option for runway change twice. After that we slowed to flap 10 speed; 165 knots; on the 2nd change; I looked down and started changing runways in the box. A few seconds later; very few!; the First Officer called slow speed; and we were at 149 knots; not in the yellow; and not in the stick shaker. First Officer added power and I disconnected AT. We recovered and landed.
B737-800 flight crew reported letting the airspeed get low on approach when their attention was diverted to FMC programming.