This wasn't quite the problem I've seen - the use of Alt Intv I'm fairly comfortable with. A clearer idea of the scenario:
Approx FL280 descending direct to a 10nm final at our destination, with say .76/280 in the DES page, with a restriction of 250/100, when without warning the speed restriction goes to 207/FLAPS, the bug goes to 207 knots (no UP speed at this FL!) and wants to start decelerating the aircraft. Thereafter the path keeps talking rubbish as it starts to tell us we are high, where if it were calculating for 207 kts, we are clearly low.
We just ignored VNAV for the rest of the descent, but my colleague was trying to see what he could do with the FMC to make it work. Every time he made a new entry, the target speed changed to something sensible, then straight back to the FLAPS restriction. The only altitude constraint initially in was 180B/3000A at the centrefix.
I'm mainly interested in how to get out of it, as doing an ILS it's no issue, but if one were to have the same problem when planning to fly a VNAV NPA, we would have zero confidence that the VNAV Path info was any good.
Last edited by Jwscud; 24th Sep 2013 at 09:02.