Date: 201309
Location: ZZZ.ARTCC, US
Flight conditions: IMC
Light: Night
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pitot-Static System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Human-Machine Interface; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Landed As Precaution; Flight Crew Became Reoriented; Flight Crew Diverted; General Declared Emergency
Contributing factors: Weather; Human Factors; Aircraft
Primary problem: Aircraft

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.


Second narrative: Cruise Flight at FL380. On top of the weather. Tops reported at FL360. FMC message; VERT NAV Disconnect. As the pilot not flying; I advised the Captain the VNAV has disconnected. I began to scan my instruments to determine the cause; and my instruments looked normal; but we began a shallow descent and the Captain disconnected the autopilot. I called out; we are descending. The Captain acknowledged and said; I think we are stalling. Due to the stall recovery the Captain was performing; I called ATC and advised we are descending. Coordination was difficult with ATC due to congestion. ATC asked if we were in a mountain wave and could we maintain FL350. I began to scan my instruments and saw .82 Mach and increasing; along with a VSI pegged below 5;000 FPM. I immediately looked and the Captain's instruments and realized his airspeed indicator malfunctioned. I crosschecked my airspeed and altimeter with the STBY and determined his airspeed was INOP. I voiced my concern by pulling the thrust levers back to idle; and said your airspeed is INOP. The PLANE is FLYING; STOP your descent. I verified the probe heat was on; and the Captain said your airplane. I took the controls; and pulled us out of a 5;000 FPM descent with speeds exceeding .85 Mach. I was able to gain control of the aircraft; climb back up to FL350; and maintain .74 Mach. We then declared an emergency and were handed to Center. The Captain and I began to run the checklists and determine the state of the aircraft. We determined through coordinating with ATC; STBY instruments; HUD; and the FMC that the Captain's airspeed was INOP and the First Officer's altimeter was INOP. We ran the appropriate checklist and decided to divert. Weather there was VFR. We began a shallow descent with the Captain flying the HUD and the First Officer controlling the airspeed. As we descended through FL250; our instruments began to return to normal; and all indications through cross reference became normal. I elected to fly the visual since my airspeed did not malfunction. We landed in an overweight condition. The aircraft taxied to the gate uneventful with CFR to follow.Remain calm and analyze the situation. Don't make quick reactions without voicing your intentions. Use CRM. Maintain a better situation awareness by following the memory items and the checklists.


Synopsis: 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.