Rather than being the "new" aircraft of the original W.9, the W.9a was an upgrade to the military Hyderabad with an extended wingspan for more lift and moving the propeller discs apart to remove the vibration apparent in the W.8f.
After initial flight testing it was found that the rudder suffered from excessive snatch from the airflow from the central engine, a problem that had been already noticed on the W.8f and caused difficulty for accurate bomb aiming on the Hyderabad.
Consideration was given to a V/1500 style triple rudder, but a simple flat rudder was wind tunnel tested and fitted and test flown. The test flying revealled that the new fin was too large, evidently the wind tunnel model had been made with too short a fuselage, so the fin was cropped back. The new fin was then adopted as the standard, and retro-fitted to all still flying civil W.8's and military Hyderabads.
The W.9a gave excellent service for Imperial Airways, and on retirement was sold to New Guinea to operate into the mountains to Wau, finally being written-off after crashing in 1930.