In the UK this is used and also has specific requirements for the pilots should an RTF failure occur, but I cannot find any ICAO reference to this. Can anybody help as to where the UK's requirements come from?
Do all pilots know what to do when they hear "Delay not determined"?
Quote Mats Part 1, Chapter 4, Section 4.8
If ‘Delay not determined’ has been given, pilots should not attempt to land at the destination aerodrome, but instead divert to the alternate destination specified in the current flight plan or another suitable airfield.
However........
ICAO Doc4444 Chapter 15.3.5
commence descent from the navigation aid or fix
specified in 4) at, or as close as possible to, the
expected approach time last received and acknowledged;
or, if no expected approach time has been
received and acknowledged, at, or as close as
possible to, the estimated time of arrival resulting
from the current flight plan;