B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Aircraft; Weather
During descent we planned on using wing and engine anti-ice during approach tops where around 6;000 feet. We turned on the anti-ice around 12;000 feet on descent. Indianapolis approach was vectoring us for approach to Runway 23L. During descent we discussed a recent article posted on social media about icing and increased stalls speeds. The article discussed buffet during a flaps 25 or 30 approach on the 767. Indianapolis approach slowed us to 180 knots during vectors to approach causing us to have to use flaps 15 for a prolonged time. We were picking up moderate rime icing; as confirmed by at 'cone' forming on the windshield wiper attachment point. We were vectored onto the approach outside LARRY and told to intercept the LOC. We were then cleared for the approach and told to hold 180 knots to PACER. VREF was calculated at 138 knots with an approach speed of 143. We added a gust factor to VAPP and planned on an approach speed of 148. During flap extension from 20-25 we started to notice a buffet and the 'Hook' popped up to 143 knots. The First Officer called for flaps 30 and the buffet increased. We increased the approach speed to 155 knots and the buffet disappeared. We landed uneventfully. It was interesting that the hook was 5 knots above the VREF speed of 138. It was exactly as explained by the article. After landing I took several pictures of the left wing root; and left inboard flap and aileron. Even 10 minutes after landing there was significant ice on the inboard aileron and front of the flap fairing.If I had not read the article posted on social media I would not have been aware of the possible condition caused by moderate icing with prolonged flap extension in the 767. At no time did I feel that the aircraft was in a degraded condition; however if we had flown the calculated VREF+5 we would have been near the top of the 'Hook' and possibly with buffet. With the knowledge I learned to increase the approach speed I feel there was no safety of flight issue. I would suggest that [company] put out an official file in regards to this known condition.
B767-300 Captain reported experiencing airframe buffet probably related to ice accumulation on approach to IND.