The ADF is the least accurate of the nav aids due to many errors it is subject to, when discussing ADF errors one must also factor in such errors as precipitation static, electrical static such as st. elmos fire, mountain/valley errors, sea/land errors and on and on go the reasons for the ADF being inaccurate.
Several years ago I was required to write the U.S. Commercial rotorcraft exams for a U.S. licence and to my surprise there were seven ADF time and distance / bearing change questions on the exam. Can anyone tell me what advantage knowing the answers to such inaccurate methods would give me?
Of course it is nice to be able to chat about the errors that are inherent in the ADF but at the end of the conversation we still are left with the same result...the ADF is an aid for cloud breaking procedures when flying IFR and is without doubt a true non precision aid. Knowing this very simple fact we also must be aware that it will soon go the way of the radio range method of navigation.
As a closing comment I defy anyone that is flying the ADF to convince me that any given swing of an ADF needle is due to any one cause.
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The hardest thing about flying is knowing when to say no.