Narrative:

Departed and climbed to our cruise altitude of FL350. In IMC with increasing bumps; we elected to climb to FL370; almost on top. Convective activity was well below and off our route. No indication of icing or precipitation. After several minutes at FL370; my airspeed began a sudden; rapid drop all the way to zero indicated. Simultaneously; my altimeter showed severe; rapid altitude loss down from FL370 to FL250. We immediately judged that my instruments were incorrect as the standby and first officer's instruments showed level at FL370 and steady at mach 0.78 as before. As pilot flying; with my instruments fluctuating wildly; I elected to transfer controls to the first officer; and then ran immediate action items for unreliable airspeed. After the immediate action items and working through the unreliable airspeed QRH; we confirmed the captain's side data was erroneous and selected alternate air data. My indications returned to normal; but with autothrottles off and my flight director off; per the QRH; I designated the first officer as pilot flying for the remainder of the flight. We soon received rudder ratio and aileron lockout EICAS messages and complied with the QRH. These are associated messages that confirmed unreliable airspeed; as per the QRH. We also received a ground proximity system EICAS; even after selecting ground proximity flap override; as previously directed. At this point; the flight briefly returned to normal and I began entering write-ups in the aircraft logbook. Soon after; our jump seater noticed the cabin altitude gauge and brought it to our attention. Although we had no warnings indicated; the cabin altitude was slowly climbing above 8;500 feet with the outflow valve fully closed. We descended to FL350 and then FL300 and advised ATC of the possibility of an emergency descent. As a precaution; we briefed the rapid decompression checklist and agreed to divert to [a nearby alternate]; if an emergency descent was needed; as it was closest. Once level at FL300; the cabin altitude reached 9;000 feet and stabilized. We remained at FL300 until given a normal descent and landed flaps 20 as directed by the QRH.at night; in IMC; at high altitude is about the worst possible time to experience unreliable airspeed (at least we weren't ETOPS). Luckily; I had fantastic support from my first officer and the deadheading captain. The first officer immediately took the controls when directed and kept us stable; despite having to briefly hand-fly at FL370 in IMC and turbulence; a much safer option than me flying with confusing; rapidly-changing indications while we ran checklists. The deadheading captain's sharp eye gave us a heads-up on a pressurization problem that otherwise would've caught us completely by surprise. Had we stayed at FL370; I'm confident we would have eventually received a cabin altitude warning; requiring an emergency descent and diversion. Fantastic coordination and support from the entire crew. When time permitted; I sent an ACARS message to dispatch advising of our situation. I received no response whatsoever; despite still being at least an hour from landing. At least I can rely on my crew.

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Original NASA ASRS Text

Title: Boeing 767-300ER Captain reported diverting to an alternate airport after experiencing air data computer and pressurization system anomalies.

Narrative: Departed and climbed to our cruise altitude of FL350. In IMC with increasing bumps; we elected to climb to FL370; almost on top. Convective activity was well below and off our route. No indication of icing or precipitation. After several minutes at FL370; my airspeed began a sudden; rapid drop all the way to zero indicated. Simultaneously; my altimeter showed severe; rapid altitude loss down from FL370 to FL250. We immediately judged that my instruments were incorrect as the standby and First Officer's instruments showed level at FL370 and steady at Mach 0.78 as before. As pilot flying; with my instruments fluctuating wildly; I elected to transfer controls to the First Officer; and then ran immediate action items for Unreliable Airspeed. After the immediate action items and working through the Unreliable Airspeed QRH; we confirmed the Captain's side data was erroneous and selected Alternate Air Data. My indications returned to normal; but with autothrottles off and my Flight Director off; per the QRH; I designated the First Officer as pilot flying for the remainder of the flight. We soon received Rudder Ratio and Aileron Lockout EICAS messages and complied with the QRH. These are associated messages that confirmed Unreliable Airspeed; as per the QRH. We also received a Ground Proximity System EICAS; even after selecting Ground Proximity Flap override; as previously directed. At this point; the flight briefly returned to normal and I began entering write-ups in the aircraft logbook. Soon after; our jump seater noticed the cabin altitude gauge and brought it to our attention. Although we had no warnings indicated; the cabin altitude was slowly climbing above 8;500 feet with the outflow valve fully closed. We descended to FL350 and then FL300 and advised ATC of the possibility of an emergency descent. As a precaution; we briefed the Rapid Decompression checklist and agreed to divert to [a nearby alternate]; if an emergency descent was needed; as it was closest. Once level at FL300; the cabin altitude reached 9;000 feet and stabilized. We remained at FL300 until given a normal descent and landed flaps 20 as directed by the QRH.At night; in IMC; at high altitude is about the worst possible time to experience Unreliable Airspeed (at least we weren't ETOPS). Luckily; I had fantastic support from my First Officer and the deadheading Captain. The First Officer immediately took the controls when directed and kept us stable; despite having to briefly hand-fly at FL370 in IMC and turbulence; a much safer option than me flying with confusing; rapidly-changing indications while we ran checklists. The deadheading Captain's sharp eye gave us a heads-up on a pressurization problem that otherwise would've caught us completely by surprise. Had we stayed at FL370; I'm confident we would have eventually received a Cabin Altitude warning; requiring an emergency descent and diversion. Fantastic coordination and support from the entire crew. When time permitted; I sent an ACARS message to Dispatch advising of our situation. I received no response whatsoever; despite still being at least an hour from landing. At least I can rely on my crew.

Data retrieved from NASA's ASRS site 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.