Common mode errors
Forgive me if this has already been covered - I have tried to keep up on the thread, but the posting has been very prolific.
I work with sensor driven control systems and have noted that when exposed to slightly abnormal conditions, sensors using the same design initially tend to degrade toward failure in a very similar manner. In redundant sensor systems this can result in a common mode error which may be difficult or impossible to detect. It is only when abnormal conditions become excessive and the sensors become erratic that they provide enough differential error to be detected.
Since the ADR's depend on differential errors to detect potential sensor faults, it seems to me that if all pitot tubes initially began to degrade at about the same rate (perhaps due to water buildup at the drains), such a common mode degradation would not be detected initially and would fool the system into believing that the A/C was traveling faster than it really was. The AP would respond accordingly and slow the A/C down. Then, as water/ice build up continued, the differences in readings would have exceeded the error detection threshold for the system, causing the AP, etc to disconnect. But when that finally happened, the A/C would be going slower and closer to the stall speed than the computed flight envelope would expect.
Could a scenario like this have robbed the crew of precious time needed to make corrections? Are there parameters recorded on the FDR that could be used to determine retroactively that something like this occurred?