Narrative:

We were at cruise at FL380; night; IMC; light chop; intermittent light turbulence; with the autopilot and autothrottles engaged. I had put my head down to put away my departure charts. As I looked up; I noticed my airspeed decreasing through 200 KTS; in the yellow band. I put my hand on the controls in case I needed to disengage the autopilot. I saw my airspeed decreasing through 170 KTS and we received a FMC warning 'VNAV disengaged' at night; in the weather. I disengaged the autopilot and applied emergency thrust and started a gradual descent to increase airspeed. I called out to the first officer; 'we're stalling!' I told him to tell ATC we needed an immediate descent. As we were in the descent; we got the clacker. The first officer and I crosschecked the instruments and deducted that the first officer's airspeed was reliable while captain's was not. We performed the memory items for unreliable airspeed. We transferred control of the aircraft. The first officer leveled off at about FL358 and reduced airspeed. I lowered the HUD and decided to maintain altitude with the HUD. So I put the airplane reference symbol on the horizon while my airspeed was indicating 150 KTS on the HUD and captain airspeed tape. We declared an emergency and told ATC that we were going to attempt to maintain FL350. Using the HUD; I leveled off at FL350. We then noticed that the first officer's altimeter was showing FL358. We decided not to use the autopilot since the captain's airspeed and first officer's altimeter was unreliable. We declared an emergency and diverted. Landing was uneventful. The situation was amplified because of night IMC. I thought the autothrottles were malfunctioning because of turbulence. I thought we were stalling at FL380. The first officer and I were both calling out airspeed. Of course mine was too low and his was too high. After we realized that my airspeed was unreliable; we transferred control to the first officer who had good information. A quicker crosscheck of instruments would have prevented the airframe overspeed; but at that time; things were a little hectic.

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Original NASA ASRS Text

Title: B737-700 flight crew experiences failure of the Captain's airspeed indication at FL380 and the Captain reacts to the slow indication by descending. The aircraft reaches Mach .85 descending at 5;000 FPM before the First Officer can determine which airspeed indication is accurate. The crew elects to divert to a suitable alternate and the Captain's airspeed begins to function normally passing through FL250.

Narrative: We were at cruise at FL380; night; IMC; light chop; intermittent light turbulence; with the autopilot and autothrottles engaged. I had put my head down to put away my departure charts. As I looked up; I noticed my airspeed decreasing through 200 KTS; in the yellow band. I put my hand on the controls in case I needed to disengage the autopilot. I saw my airspeed decreasing through 170 KTS and we received a FMC warning 'VNAV disengaged' at night; in the weather. I disengaged the autopilot and applied emergency thrust and started a gradual descent to increase airspeed. I called out to the First Officer; 'We're stalling!' I told him to tell ATC we needed an immediate descent. As we were in the descent; we got the clacker. The First Officer and I crosschecked the instruments and deducted that the First Officer's airspeed was reliable while Captain's was not. We performed the memory items for Unreliable Airspeed. We transferred control of the aircraft. The First Officer leveled off at about FL358 and reduced airspeed. I lowered the HUD and decided to maintain altitude with the HUD. So I put the airplane reference symbol on the horizon while my airspeed was indicating 150 KTS on the HUD and Captain airspeed tape. We declared an emergency and told ATC that we were going to attempt to maintain FL350. Using the HUD; I leveled off at FL350. We then noticed that the First Officer's altimeter was showing FL358. We decided not to use the autopilot since the Captain's airspeed and First Officer's altimeter was unreliable. We declared an emergency and diverted. Landing was uneventful. The situation was amplified because of night IMC. I thought the autothrottles were malfunctioning because of turbulence. I thought we were stalling at FL380. The First Officer and I were both calling out airspeed. Of course mine was too low and his was too high. After we realized that my airspeed was unreliable; we transferred control to the First Officer who had good information. A quicker crosscheck of instruments would have prevented the airframe overspeed; but at that time; things were a little hectic.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.