The mandatory descent requirements stated that 16,000ft be maintained until overhead TACAN29, and then commence .the approved let down.
This descent directive looks and feels like an SOP, but Mahon found that this was
not a requirement they were lawfully obliged to observe. And on the evidence, I fully agree with him.
Sometimes, what appear to be SOPs are not really SOPs. I wrote an essay (post #100) called "SOPs ain't SOPs".
Mahon's reason for why this one was not an SOP comes in at number 4 on my list. I listed 5 circumstances where SOPs could be later found by a court to be not SOPs afterall. There are probably a few more.