Narrative:

A (left) air data computer (air data computer) failure triggered multiple EICAS warnings and cautions; and caused the captain's (pilot flying) instruments to be erratic/unreliable. While in cruise flight at FL320; night/IMC; in light turbulence with the left autopilot engaged; the captain (pilot flying) and I (pilot monitoring) were alerted by an EICAS 'ovspd' warning. The captain assumed manual control of the aircraft while trying to interpret his erroneous instruments; initially indicating a rapid climb. Together we concluded that his instruments were unreliable; and that both the standby instruments and my (first officer) instruments were in agreement and reliable. We suspected an air data computer problem at that point. Additionally; we noted that the engine EPR indications on both pw engines were 0.00; while all other engine indications appeared normal. 'Aileron lockout;' and 'rudder ratio;' cautions accompanied the above; and appeared intermittently during remainder of flight. During the transition from automated flight to manual flight while referencing primary instruments in IMC (not yet determined to be unreliable); altitude awareness was temporarily compromised. We immediately broadcast our awareness of flight instrument errors and unreliable altitude information to ATC. We requested; and were granted block-altitudes. Transfer of aircraft control to me was made as we determined that my instruments were reliable; and I became the pilot flying. I remained pilot flying through the remainder of the flight; including the landing. The captain selected the alternate air data computer switch; which restored reliable indications on his instruments. The relief first officer returned to the cockpit from his rest break to assist. ATC coordination for diversion; QRH procedures; cabin preparation; and subsequent descent/approach/landing were uneventful (overweight landing; but vsi less than 100 FPM). Specifically; we requested block-altitudes as we executed a diversion to the closest airport with a suitable runway and VMC conditions. Throughout this event we made several queries about our altitude to ATC to verify/confirm altimeter reliability. The altitudes reported back to us by ATC did not agree with what both my altimeter and the standby altimeter were indicating. We suspected that altitude disagreement between captain and first officer altimeters rendered mode-C our transponder unreliable; and might have explained the disparity between ATC and our indications. At one point; ATC reported losing mode-C readouts. Preventative measures human performance: this scenario; particularly given the night/IMC conditions; could easily lead to spatial disorientation. Although system knowledge will provide the pilot with an accurate diagnosis of the problem; the diagnosis is not likely to be immediate since the air data computer failure is not annunciated;' i.e.; 'air data computer fail' does not appear on the EICAS. Accordingly a similar air data computer failure while in close proximity to terrain; e.g.; the approach environment; could significantly raise the potential for CFIT while the crew attempts to silence warnings(s) and determine a) that some instruments are actually erroneous; and B) which ones are reliable. In terms of training; I would recommend more frequent exposure to simulated scenarios of this nature; particularly at low altitudes (AGL). I would also suggest that the selection of alternate air data computer should be considered a time-critical action; especially in a low altitude; IMC environment.

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Original NASA ASRS Text

Title: B767-300 flight crew experienced ADC failure in night IMC and diverted to the nearest suitable airport.

Narrative: A (left) Air Data Computer (ADC) failure triggered multiple EICAS warnings and cautions; and caused the Captain's (pilot flying) instruments to be erratic/unreliable. While in cruise flight at FL320; night/IMC; in light turbulence with the left autopilot engaged; the Captain (pilot flying) and I (pilot monitoring) were alerted by an EICAS 'OVSPD' warning. The Captain assumed manual control of the aircraft while trying to interpret his erroneous instruments; initially indicating a rapid climb. Together we concluded that his instruments were unreliable; and that both the standby instruments and my (First Officer) instruments were in agreement and reliable. We suspected an ADC problem at that point. Additionally; we noted that the engine EPR indications on both PW engines were 0.00; while all other engine indications appeared normal. 'AILERON LOCKOUT;' AND 'RUDDER RATIO;' cautions accompanied the above; and appeared intermittently during remainder of flight. During the transition from automated flight to manual flight while referencing primary instruments in IMC (not yet determined to be unreliable); altitude awareness was temporarily compromised. We immediately broadcast our awareness of flight instrument errors and unreliable altitude information to ATC. We requested; and were granted block-altitudes. Transfer of aircraft control to me was made as we determined that my instruments were reliable; and I became the pilot flying. I remained pilot flying through the remainder of the flight; including the landing. The Captain selected the alternate ADC switch; which restored reliable indications on his instruments. The Relief First Officer returned to the cockpit from his rest break to assist. ATC coordination for diversion; QRH procedures; cabin preparation; and subsequent descent/approach/landing were uneventful (overweight landing; but VSI less than 100 FPM). Specifically; we requested block-altitudes as we executed a diversion to the closest airport with a suitable runway and VMC conditions. Throughout this event we made several queries about our altitude to ATC to verify/confirm altimeter reliability. The altitudes reported back to us by ATC DID NOT agree with what both my altimeter and the standby altimeter were indicating. We suspected that altitude disagreement between Captain and First Officer altimeters rendered Mode-C our transponder unreliable; and might have explained the disparity between ATC and our indications. At one point; ATC reported losing Mode-C readouts. Preventative Measures Human Performance: This scenario; particularly given the night/IMC conditions; could easily lead to spatial disorientation. Although system knowledge will provide the pilot with an accurate diagnosis of the problem; the diagnosis is not likely to be immediate since the ADC failure is not annunciated;' i.e.; 'ADC Fail' does not appear on the EICAS. Accordingly a similar ADC failure while in close proximity to terrain; e.g.; the approach environment; could significantly raise the potential for CFIT while the crew attempts to silence warnings(s) and determine A) that some instruments are actually erroneous; and B) which ones are reliable. In terms of training; I would recommend more frequent exposure to simulated scenarios of this nature; particularly at low altitudes (AGL). I would also suggest that the selection of alternate ADC should be considered a time-critical action; especially in a low altitude; IMC environment.

Data retrieved from NASA's ASRS site as of April 2012 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.