Radio failures. PANS Doc 4444/RAC (1996)
If IFR-IMC under radar maintain the last assigned speed and level, or minimum flight altitude if higher, for a period of seven minutes following: setting 7600/failure to report position/reaching last assigned level, whichever is the later then proceed iaw filed FPL
If IFR-IMC when being radar vectored or having been directed by ATC to proceed offset using RNAV without a specified limit, proceed in the most direct manner possible to join current FPL route no later than the next significant point, taking into account the applicable minimum flight altitude.
So, it seems to me that the difference between a radar heading and a 'non' radar heading may be 7 minutes.