In most accidents the very basic information, attitude and altitude, were still correct. Secondary data like speed, vertical speed, trends, FD signals, FMS signals etc. were partly or grossly wrong.
In
most accidents involving commercial airliners,
there's nothing wrong at all with the airplane, or with the sensors, or with any indications, or with any automation.
Boeing estimates that up to
80% of all crashes are caused solely by human error. The vast majority of these are pilot errors, but this category also include errors made by ATC, and errors made by mechanics.
So the largest safety initiatives are focused to reduce human error, e.g., by: 1) improving the pilot decision making process; 2) using tools & automation to help prevent mistakes.
Improving manual handling in response to possible mechanical issues, while important, would only factor in a diminishing minority of accidents.
This wasn't always the case. Per Boeing data, in early days of aviation only 20% of crashes were caused by human error, while 80% were caused by machine causes.
Today, the situation has reversed. 80% of accidents are now caused by human errors, while only 20% of crashes involved any mechanical issue.