If you get cleared to a waypoint not on your Flight Plan route, then tell ATC. It might indicate that we hold a different Flight Plan from that in the cockpit (more likely with a charter airline who are filing different plans to get round slot restrictions). ATC can then give you a reroute instruction which will either take you the correct way (sometimes Flight Plans filed don't obey the Standard Routes or file along routes which are not available), or back on to your planned route at a later waypoint. In either case, the onus is on the pilot to tell us that he has a disconnect in his route, followed by an onus on ATC to pass an appropriate clearance for the pilot to follow. I definitely wouldn't just assume that ATC want me to go to 'DDD' next, I'd get a positive clearance from them. In the absence of a positive clearance from them, you could ask them to 'confirm routeing after 'XXX' ?'. No egg on your face and ATC then have the opportunity to pass the clearance you should maybe have got in the first place
The weather one can again take two forms. One restricted, and one not. For example, you might ask for 20 degrees left as per your example. The controller can either instruct you to turn left 20 degrees, in which case you are now on a vector and you can't come off it until cleared to do so. Advising you are now clear of the weather might be a good cue for the controller to derestrict you of course
. Or ATC might say something like 'own navigation to 'DDD' avoiding weather'. In this case, the weather deviation and how you then get back on track to 'DDD' is at your discretion. Direct routeing to 'DDD' after the weather deviation would be the most logical.
The 'cleared to destination' is for RT Failure purposes and means that you are expected to follow your flight planned route to destination in that event. If you are on a heading and RT Fail, or have been given a clearance limit point somewhere along the line, then you follow the published procedures for those events. Similarly, if you have been given level restrictions or intermediate cleared levels, you follow the RT Failure procedures associated with those. Note that although most basic procedures follow the ICAO ones, it is worth checking the exact procedures for the State you are flying in, and any local variances published for the airfield you are flying in to or out of.
The UK basic procedures for IFR flight are:
IMC Procedures for Pilots
1 A pilot of a flight experiencing communication failure in IMC shall:
a) select SSR Mode A, Code 7600 with Mode C.
b) i) maintain for a period of 7 minutes, the current speed and last assigned level or minimum safe altitude, if this is higher. The period of 7 minutes begins when the transponder is set to 7600 and this should be done as soon as the pilot has detected communications failure.
ii) if the failure occurs when the aircraft is following a notified departure procedure such as a Standard Instrument Departure (SID) and clearance to climb, or rerouting instructions have not been given, the procedure should be flown in accordance with the published lateral track and vertical profile, including any stepped climbs, until the last position, fix, or waypoint, published for the procedure has been reached. Then for that part of the 7 minutes that may remain, maintain current speed and last assigned level or minimum safe altitude, if this is higher.
iii) thereafter, adjust the speed and level in accordance with the current flight plan and continue the flight to the appropriate designated landing aid serving the destination aerodrome. Attempt to transmit position reports and altitude/flight level on the appropriate frequency when over routine reporting points.
c) i) if being radar vectored, or proceeding offset according to RNAV, without a specified limit, continue in accordance with ATC instructions last acknowledged for 3 minutes only and then proceed in the most direct manner possible to rejoin the current flight planned route. Pilots should ensure that they remain at, or above, the minimum safe altitude.
ii) if being radar vectored by an Approach Control Radar Unit (callsign DIRECTOR/RADAR/APPROACH), comply with the loss of communications procedures notified on the appropriate Radar Vectoring Chart as detailed in the AD 2 section of the UK AIP.
d) i) arrange the flight to arrive over the appropriate designated landing aid serving the destination aerodrome as closely as possible to the ETA last acknowledged by ATC. If no such ETA has been acknowledged, the pilot should use an ETA derived from the last acknowledged position report and the flight-planned times for the subsequent section of the flight.
ii) arrange the flight to arrive over the appropriate designated landing aid serving the destination aerodrome at the highest notified Minimum Sector Altitude taking account of en-route terrain clearance requirements.
iii) if following a Standard Instrument Arrival (STAR), after the 7 minute period detailed in paragraph 6.1 b) i) has been completed, pilots should arrange descent as close as possible to the descent planning profile. If no descent profile is published, pilots should arrange descent to be at the minimum published level at the designated Initial Approach fix.
e) on reaching the appropriate designated landing aid serving the destination aerodrome, begin further descent at the last acknowledged EAT. If no EAT has been acknowledged, the descent should be started at the ETA calculated in d) i), above, or as close as possible to this time. If necessary, remain within the holding pattern until the minimum holding level, published for the facility, has been reached. The rate of descent in holding patterns should not be less than 500 ft per minute. If ‘Delay not determined’ has been given, pilots are not to attempt to land at the destination aerodrome, but instead divert to the alternate destination specified in the current flight plan or another suitable airfield.
f) carry out the notified instrument approach procedure as specified for the designated navigational aid and, if possible, land within 30 minutes of the EAT or the calculated ETA. When practical, pilots should take account of visual landing aids and keep watch for instructions that may be issued by means of visual signals from the ground.
g) if communications failure occurs during an approach directed by radar, continue visually, or by using an alternative aid. If this is not practical, carry out the missed approach procedure and continue to a holding facility appropriate to the airfield of intended landing for which an instrument approach is notified and then carry out that procedure.