Can someone who flies the queen explain?

[anthonyrees 0]

On a flight from Windhoek (Namibia) to Cape Town (South Africa) on the jump seat of a 747-400, I noticed that on decent from 12,000ft to 16,500ft they did a strange 'step decent' manouvre. All was automated by the fmc. On decent, the airbrakes would deploy for about 20 seconds and then the plane would decend and loose speed rapidly. (The drag was well felt) They would then release for a further 10 seconds and then deploy all over again for 20 secs again. This happened again and again in the same order continually for about 8 mins until the target altitude and airspeed was reached.Anyone care to shed some light on strnage this procedure for me, as it was not something I have read about before. Can this be achieved in the sim?Anthony ReesCape TownSouth Africa

[cowpatz 243]

You might want to re-check those altitudes :)It would be hard to know what was really going on there without knowing what ATC instructions they were trying to comply with or what pitch mode they were using. A 'normal' descent (if there is such a thing) usually kicks off in VNAV (after all that is what it is there for). If a clearance restirction from ATC or a modification of any description is made to the FMC, then VNAV will recompute the descent path. As it is doing this the pitch mode will change to VNAV speed. When the path is recomputed you may find yourself in a position of being too high or fast or both. If your airspeed is restricted by ATC (say for traffic sequencing) then one of the options avail to asssist you in getting down is speed brake. The crew you observed were probably getting speed control and possible even track shortning and each time they executed the modification in the FMC found that they ended up being higher than the calculated profile and had to use speed brake to slow down and descend. Either that or they were just hopeless pilots!!!Going to watch the tri nations test tomorrow????CheersSteveNZ