Could it not therefore be ... the attitude indications they were responding to were actually erroneous?
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Especially when dealing w/ systems 'issues' that might impact attitude displays, a pilot MUST ALWAYS verify the validity of their attitude indications the best they can. If they had an operable standby attitude indicator (they should have), it most definitely needed to be utilized in their crosscheck/verification process, and potentially used as primary attitude reference for straight - level flight.
KC135777