During a lot of climbs I have seen the following: Using VNAV the airplane climbs and accelerates perfectly until FL300. Reaching M0.80 V/S increases as usual to maintain the mach number. Then the V/S, still with VNAV engaged, begins to oscillate between about 1000 and 2000 ft/min leading to a constantly changing pitch until level out at T/C. ZFW is about 111 tons, ca. 45 tons fuel, airplan is correctly trimmed, elevator almost in neutral position, Climb completely performed in VNAV after having reached Vref+80 at about 3000 ft AGL. One funny addicional observation: When performing the climb from FL300 to T/C with time compression mode, the V/S will first remain quite steady and then decrease smoothly as expected.Any suggestions?

Had the same effect one or two times. Seems VNAV sometimes becomes unstable at higher altitudes. Try to switch to V/S, wait until climb rate and speed are stable, then reengage VNAV.Regards,Matthias (EDDN)

OK, will try that!Thanks for your help.