Prohibited VHF and HF Frequencies

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Plowing through a tech refresher presentation (for the B737-800), the section on Radio and Navigation Systems mentions frequencies (120.000MHZ on VHF 2 or 3) and a list of several HF Frequencies (including 2 relating to a few specific airframes) that should not be used. No reason was given and can’t see anything in the technical manual. Can someone explain the reason? (Out of interest, not querying the need to observe the limitations.)
I remember reading one of those old From The Flight-deck books years ago, that mentioned transmission on a certain HF frequency on a 747 Classic would drive the outflow valve open!

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In the context of the 747 (100, 200 and 400) don’t recall forbidden frequencies (HF or VHF) but memory being what it is….be interesting to get further opinion.

Did see one instance of outflow valves on a -400 briefly motoring towards open for no apparent reason in the cruise - the suspicion was it was mobile phone related.

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To be clear, the book mentioned it as applying to that one airframe (a BA 747-200) as an entry in the Tech Log when the crew took it over rather than applying to all 74’s.

[wiggy]

Ah, OK :ok: