than the one on which the blades were first used it is understood that the blades may be replaced on the same aircraft from which they were earlier removed but the aircraft may have been flown in the meantime using an alternative set of rotor blades in that event the accumulated stress history retained in the data storage device of computer 42 representative of the stress history of the rotor blades during its initial period of service will have been erased by appropriate means and new data pertaining to the replacement set of blades entered in its place prior to erasure the accumulated stress history of the removed set of blades is entered into an alternate data storage device such as a ground based information system which may include digitial computers or manual entry log books upon returning the refurbished set of blades to service the recorded stress history is retrieved from the alternate information system and entered by means of the data entry device 46 into the data storage of computer 42 of the helicopter in which the blades are to be installed so as to provide base data of the stress history of the rotor blades in an alternate embodiment of the invention the computer indicator and alarm devices may be omitted from the system and the transducer output merely recorded on a recording device 44 such as a magnetic tape recorder for subsequent analysis on the ground while such a system would not provide real time monitoring of the soundness of the rotor blades it will nevertheless represent.