A flow measuring system for one or more oil nozzles of a lubrication system of an aircraft engine comprises an external supply of a testing liquid and a pump fluidly connecting the external supply of the testing liquid to an inlet of the lubrication system via a first conduit. The inlet to the lubrication system is disposed upstream of the one or more oil nozzles. A second conduit fluidly connects an outlet of the lubrication system to the external supply of the testing liquid. The outlet of the lubrication system is disposed downstream of the one or more oil nozzles. A flow measuring device is operable to measure a flow resistance through the one or more oil nozzles.