Is the Tower (Local) Controller responsible for "separating" VFR traffic in the pattern in the U.S. in Class D airspace?

Question

For example, in U.S. Class D airspace, if an inbound aircraft operating VFR is instructed by the Control Tower to "enter left downwind for runway 18, report entering downwind," is the controller responsible for providing specific separation between that aircraft and other VFR traffic in the pattern (not on the runway or taxiway)?

If the controller instructs an aircraft on downwind to "turn base leg now, (VFR) traffic is on a 4 mile final on a straight in." Is the controller now responsible for the separation?

I realize these are two questions, but they are related.

Answer 1

Separation requirements depend on the class of airspace, per ICAO Annex 11.

Class D: VFR aircraft are not separated from any other aircraft. ATC only provides traffic information about VFR aircraft.

Note the separation can be done visually. Considering the size of the typical class D airspace, this is the usual method for multiple VFR aircraft in the area.

Once the aircraft get to the runway, the ATC regulation JO 7110.65 3−10−3 states that ATC is responsible for making sure an arriving aircraft doesn't cross the threshold before an aircraft ahead of it on the same runway is either landed and clear of the runway (or a certain distance past the threshold), or has departed and crossed the opposite end of the runway.

Also per 7110.65 2-1-1:

The primary purpose of the ATC system is to prevent a collision involving aircraft operating in the system.

So while there may be no hard requirements for ATC to separate VFR aircraft, they will do their best to make sure that the VFR aircraft have the best chance of seeing and avoiding each other.

Answer 2

In my experience the tower will ensure separation for aircraft on approach and in the pattern. I fly out of KPNE frequently (a class D airport) and often am told to modify my pattern for incoming jets or faster aircraft. However they will not provide separation services for aircraft simply flying around in their airspace:

Class D airspace is under the jurisdiction of a local Air Traffic Control Tower (ATCT). The purpose of an ATCT is to sequence arriving and departing aircraft and direct aircraft on the ground; the purpose of Class D airspace is to provide airspace within which the ATCT can manage aircraft in and around the immediate vicinity of an airport. Aircraft operating within this area are required to maintain radio communication with the ATCT. No separation services are provided to VFR aircraft. The configuration of each Class D airspace area is unique. Class D airspace is normally a circular area with a radius of five miles around the primary airport. This controlled airspace extends upward from the surface to about 2,500 feet AGL. When instrument approaches are used at an airport, the airspace is normally designed to encompass these procedures.

In your scenario:

If the controller instructs an aircraft on downwind to "turn base leg now, (VFR) traffic is on a 4 mile final on a straight in." Is the controller now responsible for the separation?

If both aircraft are flying VFR both are responsible to adhere to "see and avoid" rules so it is ultimately the PIC's job to ensure separation but broadly speaking the tower will provide it.