B737-800
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented
Aircraft; Company Policy
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.
Reporter states that company procedures have been changed to allow manual entry and exit from offset routes (not due to his experience). He is still curious why buffet was encountered with IAS 10 knots above the yellow band. The B737NG FMC is designed to enter and exit offset routes using up to a 30 degree bank angle regardless of the altitude.
During cruise flight over the ocean in ETOPS airspace we detected weather ahead. We asked for and received a clearance for a deviation right of course. We were below optimum altitude due to ATC so we also asked for and received a clearance to climb 1;000 feet to our new optimum altitude. We had a very comfortable and wide airspeed envelope between the high and low limits displayed on the airspeed tape. We programmed the FMC for the new altitude and we started the climb first; then we programmed R15 (right 15 miles) and executed that. Per our company procedures we stayed in LNAV and let the autopilot fly the deviation. As soon as we executed the deviation the aircraft rolled quickly to 30 degrees of bank and we immediately felt an aerodynamic buffet. This got our attention real quick and I immediately glanced at the airspeed indicator which showed the yellow band coming up from the bottom but we were still at least 20 KIAS above the yellow band. I contemplated pushing the nose down or reducing the bank angle with force on the yoke and/or disconnecting the autopilot but I didn't want to risk aggravating the situation with a large control input. Also by then the aircraft was starting to come out of the turn and of course the buffet immediately went away. At no time did the airspeed ever get even close to the high or low yellow buffet band; and yet there is no question that I felt an aerodynamic buffet. There are two issues here. One; according to the 737NG systems handbook; LNAV bank angles are limited to 'not more than 23 degrees' and yet the autopilot took the plane quickly to a full 30 degrees and held it there. Later on this same flight I noted that when making LNAV course changes (even large course changes); the aircraft banked at 23 degrees. For some reason when doing a course offset; it went to a full 30 degrees of bank. I have noticed other aircraft overbanking to approximately 27 degrees and then shallowing out the bank to 23 degrees and holding it there. I would think that above 20;000 feet it would be better to under bank rather than over bank initially and then fine tune from there. The second issue is what the airspeed indicator was showing. The 737NG systems handbook says the yellow band on the airspeed indicator shows 'low and high speed buffet margins' and the note says '1.3 G maneuver capability occurs at 40 degrees of bank in level flight.' I know we weren't in level flight but we were established in a climb and we were nowhere close to 40 degrees of bank; and we were also nowhere close to the yellow band on the airspeed tape; so why did we get the aerodynamic buffet? I think the requirement to do all ETOPS deviations in LNAV needs to be re-evaluated. An LNAV track offset only gives you one option and that's a 45 degree turn off course. What if I need 25 degrees to go between cells? Secondly if you need to fine tune your offset you get large heading changes (45 degrees again) and large bank angles that are unnecessary and sometimes downright dangerous; especially if you're in turbulence which is obviously common when deviating around thunderstorms. Not only that but creating offsets is a whole lot more work and button pushing that in itself introduces threats and errors which I think are more hazardous than the potential to forget you're not in LNAV. It also causes you to be heads down typing when you could be easily navigating between cells; heads up; and using the heading select option to smoothly avoid the cells.
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.