Looking at Jamieone's link to Debretts on
Use of an Officer's Rank Without His Surname
Debretts says
In social correspondence certain officers may be addressed at the beginning of a letter by their rank alone, e.g. 'Dear Admiral' in place of 'Dear Admiral Smith', but some officers dislike this practice, especially when used by those outside the Services.
In any event this practice is limited to the following ranks:.
.
.
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Royal Air Force: Flight-Lieutenant (shortened to 'Air Commodore'); Air Vice-Marshal and Air Chief Marshal (shortened to 'Air Marshal'), and Marshal of the Royal Air Force (shortened to 'Marshal').
Surely Debretts have not made a mistake?
Flt Lt Bloggs
RAF Little Airfield Still Open
Dear Air Commodore.....