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When a pilot loses radio contact with ATC, there are standard procedures to follow, e.g. squawk 7600, observe the traffic pattern, observe the red / green light gun.
On the other side, if a controller attempts to contact an aircraft and gets no response, what will they do? Assume that radar contact is not lost, or the controller is not equipped with radar.
EDIT: Let say a controller spotted a 7600. What's next?
You didn't mention any specific jurisdiction but for the FAA, the expected actions are in the ATC orders. Section 5-2-8 says:
When you observe a Code 7600 display, apply the procedures in para 10−4− 4, Communications Failure
Section 10-4-4 gives general instructions for comms failure, it's too long to quote in full but the first paragraph summarizes it:
In the event of lost communications with an aircraft under your control jurisdiction use all appropriate means available to reestablish communications with the aircraft. These may include, but not be limited to, emergency frequencies, NAVAIDs that are equipped with voice capability, FSS, Aeronautical Radio Incorporated (ARINC), etc.
The instructions suggest some specific actions:
If comms haven't been reestablished within 5 minutes, ATC is instructed to consider it as suspicious:
If radio communications have not been (re) established with the aircraft after 5 minutes, consider the aircraft’s or pilot’s activity to be suspicious and report it to the FLM/CIC per FAA Order JO 7610.4, Chapter 7, Hijacked/Suspicious Aircraft Reporting and Procedures
Section 10-2-5 also says that lost comms are an emergency from the ATC perspective and should be escalated to the ARTCC. If comms aren't restored within 30 minutes then the aircraft is considered overdue, which triggers another set of procedures listed in section 10-3.
As for clearing airspace or whatever, there are no specific instructions in the orders, it just says that:
When an IFR aircraft experiences two-way radio communications failure, air traffic control is based on anticipated pilot actions.
In other words, ATC expects IFR pilots to follow lost comms procedures as per 14 CFR 91.185 and will direct other traffic accordingly.
Finally, one 'routine' lost comms scenario is when ATC can't contact an aircraft because of poor radio coverage. This happens a lot with smaller aircraft (which fly relatively low) operating in remote areas and ATC might ask another aircraft, usually a higher-flying airliner if possible, to contact the first aircraft and relay their call: N12345, this is BigAir 123 with relay from Denver Center.
As you note, there are procedures to follow in lost comm scenarios, and ATC will expect you to follow these procedures. In turn, they will continue to hand you off to subsequent controllers toward your destination and keep other aircraft out of your way. They will attempt to establish one-way comms or comms via alternate methods (ACARS, SELCAL, satellite phone). If comms are impossible, they will work other airplanes to maintain separation with you and the ability to do this easily comes from established lost comm procedures for pilots to follow.
For IFR lost comms, the procedures they expect you to follow are given in 14 CFR 91.195.
If you are in VMC or encounter VMC you will continue flight under VFR land as soon as practical. If ATC sees you break off of your route and descend toward an airport, they will coordinate to make sure you successfully land and otherwise dispatch S&R to find you.
If you are in IMC or cannot comply with continuing under VFR (e.g. your operation is limited to IFR ops) then ATC will expect you to:
and to:
The regs basically boil down to "fly your clearance and anything we told you to 'expect'". If you lost comms shortly after takeoff while on radar vectors, you'd fly direct to the fix you were being vectored to (e.g. the first fix on your flight plan) while climbing to your expected flight level (e.g. your cruising altitude). You'll then fly your cleared route to your destination, descending in accordance with your arrival procedure and then flying direct to an approach fix and flying the approach.
ATC will expect this from you and they will vector / sequence other airplanes out of your way to maintain separation. They can reliably do this because your behavior is well-defined and as long as you do what is expected of you, you are relatively easy to work around.
