What are these two tail-side elements of British Aerospace Avro RJ100 and what is their function:
Photo source: Eric Verplanken, jetphotos.net.
The one behind is an aerobrake. The tail opens up to increase drag. It is much better seen in this Wiki image:
The one in front looks instead like the APU exhaust.
Airbrake and APU exhaust, just so.
Some aircraft use airbrakes on the tail cone, instead of airbrakes on the fuselage sides, the wings, or on the fuselage belly. The tail cone airbrake can easily be combined with a brake chute, should one be needed during testing.