Posted on behalf of one of our readers:
Dear Roger Dodge,
Not covered in the responses so far to your post of 22 Oct was a possibility that I have experienced with a similar FMC on the B744 as follows. Once the new "direct to" clearance is acknowledged the PNF (Pilot not Flying) line selects the desired new waypoint which writes it into the scratch pad. He then places it to the top of the legs page by selecting the top line select key and then (normally after confirmation by the PF [Pilot Flying] ) selects execute.
It takes as long to do as it takes to read but can be done quicker if no-one checks the new leg on the map on the Nav Display. So if you cleared the subject aircraft just as he was approaching BPK, in the time he acknowledged you and then selected DIDGO or at least the line where he thought it was, the FMC could have moved one waypoint closer to destination. The result is that the aircraft is now going direct to the waypoint AFTER DIDGO! Does this concur with the 090 heading flown?
Long haul and familiarity with the systems coupled with tiredness drops the "check before action step" out of the loop and consequently I have seen the above scenario on more than one occasion. Even if the FMC did not transition to the next waypoint at the time of selection, the "fat finger of fate" is still capable of missing the correct one and usually just before the caffeine unfurrows the now puzzled brow, the "on the ball" ATCO has adjusted the track.
I am, for the moment, retired and regret that I cannot show you how easy this and other scenarios can be effected in electronic airspace but feel sure your refreshingly enquiring mind will find the answers you require. Suffice to say that any external instruction whilst approaching a waypoint, approaching a level, or during a configuration change may test the multi tasking abilities of the average male pilot.
Remembering fondly the peerless UK air traffic control, I remain,
Yours faifhfully
CX B744 Captain retired.