2011-07 · NASA ASRS report 959280
A CRJ-700 flight crew experienced ADC and AHRS failures in cruise at FL270. A loss of 300 feet of altitude resulted before they regained the necessary instrumentation.
In cruise at FL270 I was manually flying the aircraft when my PFD went blank for a few seconds. The autopilot had been deferred so I tried to hold course and altitude while cross-reading the Captain's PFD; which had just come back on after blanking out momentarily as well. When my screen returned it was showing erroneous attitude and navigation information. It was clear that AHRS 2 had failed. At that point I realized I was losing altitude and suggested that the Captain take the controls because I didn't know if I was receiving reliable altitude data. I also noticed our vertical speeds were not in agreement. The Captain had pulled out the checklist so he could not take the controls. When the altitude alerter came on I began to pitch up more aggressively. Upon reaching our assigned altitude I transferred the controls to the Captain and we notified ATC of the deviation. We completed the checklist and continued on. The only undesired state that occurred was the 300-foot loss of altitude due to the distraction described above.In hindsight I should have cross-checked the standby instruments for additional info. Also I should have offered to transfer the controls before the Captain pulled out the checklist since I was distracted and struggling to hold altitude. Quicker communication probably could have prevented the deviation.
We were lucky to come out of the clouds able to use visual reference and prevent another deviation while we got our system slaved to the Captain's side which was working. IVSI info was also not correct.Training for this situation in the past helped. The altitude deviation was due to erroneous information from the aircraft systems.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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