Secondary air supply system with feed pipe(s) having sonic orifice(s)

A secondary air system (SAS) of an aircraft engine having a main gas path extending through an engine core, a central axis of the aircraft engine extending through the engine core, the aircraft engine producing secondary airflow from a source of secondary air, the SAS comprising:

• two or more hollow struts extending radially through the main gas path and forming part of an inter-compressor case of the aircraft engine, each of the two or more hollow struts defining therein a strut conduit extending between a strut inlet and a strut outlet at opposite ends of the hollow strut, the strut outlet in fluid flow communication with a buffer cavity for feeding the secondary airflow to the engine core, the buffer cavity formed by a plenum axially aligned with the two or more hollow struts and defined within the inter-compressor case, the buffer cavity extending partially circumferentially within the inter-compressor case to interconnect the strut outlets of the two or more hollow struts, the buffer cavity extending around the central axis from a first end positioned at a strut outlet of a first hollow strut of the two or more hollow struts, to a second end positioned at a strut outlet of a second hollow strut of the two or more hollow struts circumferentially offset from the first end, a portion of a circumference of the inter-compressor case devoid of the buffer cavity;
• and a SAS feed pipe having an inlet receiving the secondary airflow from the source of secondary air, and an outlet in fluid flow communication with the strut inlet to feed the secondary airflow into the strut conduit, the SAS feed pipe having a sonic orifice therein between the inlet and the outlet thereof.