Air metering device for gas turbine engine

A gas turbine engine having an axis of rotation, the engine comprising:

• a radially-extending annular wall comprised of a metal having a first thermal expansion at an engine running temperature;
• a cylindrical surface extending axially past the wall, the surface and wall cooperating to define adjacent secondary airflow cavities, one secondary airflow cavity on either side of the wall, the airflow cavities in serial communication with a source of pressurized secondary air, the cylindrical surface comprised of a metal having a second thermal expansion at an engine running temperature, the second thermal expansion unequal to the first coefficient;
• and
• an annular ring radially-slidably mounted to the radial wall, the ring cooperating with the cylindrical surface to define a non-zero gap between them, the gap providing a minimum opening between the cavities and hence in use metering serial secondary airflow communication between the cavities, the ring having a face which in use sealingly cooperates with the wall to impede secondary airflow between the ring and wall, the ring comprised of a metal having a thermal expansion at an engine running temperature substantially equal to the second thermal expansion.