Narrative:

We were in cruise at 35;000 feet and below our optimum cruise altitude which was 36;000 feet. We had some weather ahead that we decided to deviate around and climb above as well. The autopilot and autothrottles were engaged in LNAV and VNAV and FMC modes. We received a clearance and began to climb first and then entered a right 15 mile offset into the FMC. The engines were not quite up to full climb N1 as the aircraft began the turn. It banked directly to 30 degrees bank and we immediately felt an aerodynamic buffet. I immediately put my hands on the controls and checked airspeed but left the autopilot on as by then the aircraft was approaching the offset heading (45 degrees from our original track) and began to roll out. This stopped the buffet which lasted only 1-2 seconds. Our speed was about 20 KIAS above the lower buffet margin (yellow band) and 15 KIAS below the upper yellow band. I saw the red stall tape rising from the bottom (it was out of sight in level cruise) and it topped out approximately 10 KIAS below the yellow band. We both immediately felt and identified the same thing: stall buffet. As we approached the parallel course at 15 NM offset; we decided to use heading select limited to 10 degrees of bank to make the turn to prevent a reoccurrence. As we rolled onto the course we then put it back into LNAV. This was a deviation from SOP after the event to prevent a reoccurrence as we were unsure of why this had occurred and had not had time yet to process the likelihood of it happening again. We spent some time discussing the event and wondering why we got a buffet with such a large margin on the stall tape and why the autopilot would command 30 degrees of bank at 35;000 feet. The autopilot commanded too high a bank angle at high altitude. It appears that the FMC commands a 30 degree bank when using the offset mode regardless of the altitude. This is dangerous and needs to change. If it is supposed to only use 23 degrees; then it should be reprogrammed to undershoot the bank and then correct up to it rather than allowing an overshoot in bank. If the speed margins were smaller it could have caused a stall and upset. Additionally; the SOP of using only LNAV in any and all situations in etopsecond officerceanic ops needs to be reevaluated. I understand the concept of threat reduction but when doing deviations for weather it unnecessarily complicates the task which introduces a whole bunch of other threats. You have to use offset (which uses 45 degree off course turns; what if I need to turn only 30 degrees?) and potentially stair step your way around the storm or you have to figure out by trial and error a latitude/long that happens to sit between the moving ts cells.

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Original NASA ASRS Text

Title: B737-800 flight crew reports encountering aerodynamic buffet at FL350 as a 15 NM offset is entered and executed in the FMC. The aircraft had rolled into a 30 degree bank to track 45 degrees off the course line and intercept the offset. The indicated airspeed remained 10 knots above the yellow band on the airspeed tape during the maneuver and the reason for aerodynamic buffet could not be explained.

Narrative: We were in cruise at 35;000 feet and below our optimum cruise altitude which was 36;000 feet. We had some weather ahead that we decided to deviate around and climb above as well. The autopilot and autothrottles were engaged in LNAV and VNAV and FMC modes. We received a clearance and began to climb first and then entered a right 15 mile offset into the FMC. The engines were not quite up to full climb N1 as the aircraft began the turn. It banked directly to 30 degrees bank and we immediately felt an aerodynamic buffet. I immediately put my hands on the controls and checked airspeed but left the autopilot on as by then the aircraft was approaching the offset heading (45 degrees from our original track) and began to roll out. This stopped the buffet which lasted only 1-2 seconds. Our speed was about 20 KIAS above the lower buffet margin (yellow band) and 15 KIAS below the upper yellow band. I saw the red stall tape rising from the bottom (it was out of sight in level cruise) and it topped out approximately 10 KIAS below the yellow band. We both immediately felt and identified the same thing: stall buffet. As we approached the parallel course at 15 NM offset; we decided to use heading select limited to 10 degrees of bank to make the turn to prevent a reoccurrence. As we rolled onto the course we then put it back into LNAV. This was a deviation from SOP AFTER the event to prevent a reoccurrence as we were unsure of why this had occurred and had not had time yet to process the likelihood of it happening again. We spent some time discussing the event and wondering why we got a buffet with such a large margin on the stall tape and why the autopilot would command 30 degrees of bank at 35;000 feet. The autopilot commanded too high a bank angle at high altitude. It appears that the FMC commands a 30 degree bank when using the offset mode regardless of the altitude. THIS IS DANGEROUS AND NEEDS TO CHANGE. If it is supposed to only use 23 degrees; then it should be reprogrammed to undershoot the bank and then correct up to it rather than allowing an overshoot in bank. If the speed margins were smaller it could have caused a stall and upset. Additionally; the SOP of using ONLY LNAV in any and all situations in ETOPS/Oceanic ops needs to be reevaluated. I understand the concept of threat reduction but when doing deviations for weather it unnecessarily complicates the task which introduces a whole bunch of other threats. You have to use offset (which uses 45 degree off course turns; what if I need to turn only 30 degrees?) and potentially stair step your way around the storm or you have to figure out by trial and error a LAT/LONG that happens to sit between the moving TS cells.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.