If you compare in the picture below the headword in BDAG with its rendition in the context menu, the former has a circumflex accent while the latter (and the index of BDAG) has a tilde.
Now I copy here a selection from wikipedia on greek accents:
"The circumflex (Ancient Greek: περισπωμένη perispōménē "twisted around") – ᾶ — marked high and falling pitch within one syllable. In distinction to the angled Latin circumflex, the Greek circumflex is printed in the form of either a tilde or an inverted breve. It was also known as ὀξύβαρυς oxýbarys "high-low" or "acute-grave", and its original form (like a caret: ^ ) was from a combining of the acute and grave diacritics. Because of its compound nature, it only appeared on long vowels or diphthongs".
So while it may be acceptable in some circumstances to use the tilde instead of the circumflex, the latter is the standard in all the Greek textbooks I have used (quite a few). In fact I was not even aware of this use of the tilde before today. And, no matter what, one would expect consistent usage, that is, the headword, the index entry and the context menu should match, not reflect different conventions.
