1. A method for performing diagnostics for an engine using operational data for the engine, the operational data pertaining to a plurality of performance measures of the engine, the method comprising the steps of: determining a symptom of the engine from the operational data, the symptom pertaining to a measure of health of the engine that is unexpected, undesirable, or both; identifying an engine component as potentially being related to the symptom using the operational data; calculating a component deviation from a first model, wherein the component deviation relates the engine component to an adjustment to the first model with respect to a variable of the first model, the adjustment believed to be potentially caused by the engine component, based at least in part on the operational data; determining a plurality of root cause deviation measures offline via a second model using simulated operational data representing various different possible conditions for the engine; comparing the component deviation with the plurality of root cause deviation measures, each root cause deviation measure relating one of a plurality of potential root causes for the symptom to the first model with respect to the variable of the first model, thereby generating a plurality of comparisons; and selecting one of the plurality of potential root causes as a likely root cause of the symptom based on the plurality of comparisons.