Seeds of doubt were planted today upon hearing a conversion in which 2 experienced pilots were telling a less experienced pilot that 'Wilco' is unacceptable once inside controlled airspace and can't be used to reply to an instruction. (Which I thought was weird since Wilco = Will Comply and I don't know how you can comply with something you aren't told to do, but thought I perhaps misunderstood)
Certain instructions and clearances must be read back. Reporting instructions are not instructions that must be read back, and in fact, doing so can be hazardous because the readback can be confused with the report itself.
"Birdseed 123 report passing FL190"
"[unintelligible] Passing FL190, Birdseed 123"
"Birdseed 123 thank you break Duckling 456 climb FL180"
oops... the missing word was "report" and Birdseed 123 is still passing 150 in the climb, now in conflict with Duckling 456.
In CAP413, the UK's RT manual it's inconsistent. Examples:
Ch 3 1.15.5 (which deals explictly with readbacks)
G-ABCD, cleared to cross A1 at Wicken, maintain FL70 whilst in controlled airspace. Report entering the airway
Cleared to cross A1 at Wicken, maintain FL70 in controlled airspace, Wilco, G-ABCD
but Ch 3 1.18.6
BIGJET 347 , when ready descend FL170, Report leaving FL210
When ready descend FL170. Report leaving FL210, BIGJET 347
The majority are "wilco" but there are clearly examples of "report ..." as a readback.
In fact, looking at Chapter 4 1.5:
Edition 15 had:
G-CD taxi to the flying club, via A1 cross runway 24 report vacated
Taxi to the flying club cross runway 24 via A1 will report vacated G-CD
Edition 20 now has:
G-CD, taxi to the flying club, via A1 cross runway 24 report vacated
Taxi to the flying club, via A1 cross runway 24, Wilco, G-CD