maintenance manual gives conditional inspections for numerous things that may be encountered,
Suspected as much but do not have immediate access to chapter & verse on multiple A/C types.
Do all the extra checks in AMM Chap 5 (or wherever they crop up on different types) depend on physical evidence of ash found on / in parts of the aircraft? Or do they say things like: 'If high sulphur levels are found in engine oil AND the aircraft may have had a volcanic ash encounter, you MUST borescope the hot section of the affected turbine'?
It still seems to me from other information that there's unwise dependence on definite visual evidence before assuming the worst about less-accessible parts such as inside the turbine. Checks on filters on engine bleeds seems a sure-fire test for actual ash exposure, leading to a QUANTITATIVE result after weighing and/or microscopic examination of washings from the filter(s). Finding ash on a surface somewhere or high sulphur in the oil will remain only QUALITATIVE tests, indicating more checks need to be done...