The aircraft ATR72-500 Reg.HS-PGA was operating a scheduled flight PG281 from U-Tapao airport to Phuket airport on 22nd July 2014. After
level-off and cruising at FL180 for awhile, the flight was experiencing light to moderate turbulence. I, as a PIC, felt that it was not safe nor
comfortable for cabin crew to service passengers, so I decided to give the seat belt sign "ON". The weather along the route at that time was
overcast at approximately FL200, but there was a few cloud at cruising level and flight visibility from the cockpit was more than 10 kilometers.
There was no sign for any precipitation from radar echo intensity. There was no green, yellow nor red radar echo intensity from the weather radar.
The weather was seem to be good, except light to moderate turbulence that gave us felt bump intermittent, so I decided to leave the seat belt sign
"ON" during the flight. After the seat belt sign had been "ON" for about 10 minutes, suddenly the flight experienced with unexpected clear air
turbulence that gave vertical acceleration approximately 1.82G. Cockpit crew and all passengers are safe, except both cabin crew were injured,
FA.Jareeya's head was hit with the top roof, FA.Kunnatam was hit with the cabin door's stair. During that time, they were clearing galley stuff and
toilet at the aft of the aircraft. After the incident, FA.Jareeya told me that she felt pain in her head and FA.Kunnatam felt pain in her neck and
back, so I requested a medical assistance upon landing. After landing, a nurse came to see them at the aircraft. The nurse gave them some pills and
told them to wait to see the symptom if it will get worst or not. I asked them whether they want to see a doctor at hospital right away, they decided
to wait for the improvement of the symptom. However, if their symptom will not improve or getting worst, they will let me know and I will take
them to a hospital immediately.