Conversation so far:
When was Erich Ludendorff with Hindenburg?
Falkenhayn was replaced as Chief of the General Staff by Field Marshal Hindenburg on 29 August 1916.
Ludendorff was his chief of staff as first Quartermaster.
Are there any other interesting aspects about this article?
They toured the Western Front meeting --and evaluating-- commanders, learning about their problems and soliciting their opinions.
What rank was he during this time?
CANNOTANSWER.
What was their location?
Hindenburg was given titular command over all of the forces of the Central Powers.
Ludendorff's hand was everywhere.

Question: What was their mission?
Self-contained rewrite: What was mission of Ludendorff  and Hindenburg  ?