In the first movement of Pini di Roma, "I pini di Villa Borghese", Ottorino Respighi writes an F#A3 tremolo in the first flute part.
This tremolo is awkward to play with regular fingerings, and hard to sound with harmonic fingerings (F#1 & A1) without any modifications.
Over the years, many flute players have requested this fingering in the FLUTELIST forum, which I posted on 2005-11-09.
Here are some examples:
This first fingering is similar to the harmonics of F#1 and A1 with the exception of the right hand little finger not pressing the Eb key.
Since the tremolo is marked as starting from A3, start the tremolo with the trilling finger down.
There are several other fingerings for this tremolo. Even some that use the C#-Trill key. Experiment to find the fingering that best suits you.
Note: Pyotr Ilyich Tchaikovsky uses this same tremolo in his Swan Lake ballet in Act IV, No.29, 2 measures before rehearsal #22.
Historical Note: Ottorino Respighi (1879-1936) composed Pini di Roma in 1924.