What might lead to an “acoustical change”?
A light propeller driven aircraft’s certificated noise level is dependant on two factors, the aircraft’s noise at source and the aircraft’s take-off performance as defined the AFM. An adverse change to either could lead to a finding of an “acoustical change”. Factors that might adversely affect an aircraft’s noise at source The two principle sources of noise are the propeller and the engine. Propeller noise is highly dependent on the propeller helical tip Mach number. Other factors that influence propeller noise include the number of blades, blade tip shape, blade thickness and the inflow angle of air flowing into the propeller. Any change that could affect any of these factors is potentially an acoustical change. Such changes would include: • An increase in take-off rpm (“red line”), or highest rpm in the normal operating range (“top of green arc”); • A change leading to an increase in the best rate of climb speed (Vy); • An increase in propeller diameter; • A change to the propeller
