I'll try to explain as detailed as possible what happened (yesterday's Groupflight), so maybe someone might shed some light on this mistery:I was flying normally with VNAV and LNAV, and then at one point, I decided to slow down, so that I'm not so much faster as everyone else. I switched from the VNAV to Speed, and dropped the speed. Ok, that was easy part. At that point, I was contacted by the US ATC, who gave me instructions to descend to FL110 by blahblah. I don't know how distant any more I was to that VOR, but I think it was around 100nm. I was at FL350 at that moment, and he told me to maintain maximum forward speed for spacing. I confirmed, set at the LEGS page for that waypoint desired speed/alt, which I believe now was around 290IAS/FL110, changed the ALT also to 11,000, and then "directed" aircraft to that waypoint and pressed VNAV. First of all, neither did aircraft start to descend, and then speed went much too high, to about 320 IAS, which would overspeed also (was already below the red tape).So my question is 1) why didn't the aircraft start to descend? 2) why was the speed set so high, and not to the FMC desired speed?As soon as I then turned off VNAV, I was able to set my speed as I wanted, and I then went into manual descend, which one the end worked out ok.And well, not everything went ok, on the end of the flight, I was unable to change the radio frequency, so I was not able to set the ILS. Can someone explain me that?ATC told me to prepare for the visual approach, but why wasn't I able to change the radio freq?!? Did ATC know that my radio is "broken"? :-D

VNAV computes a path in order to reach your target altitude at the approprate waypoint. VNAV thus does not begin a descent until it reaches the computed descent point for an idle thrust descent at the programmed (usually around 290 KIAS) speed, unless you press the DES NOW key on the DEScent page.So, the airplane may not have started a descent for a couple of reasons:You hadn't reached the T/D point yet.The T/D point was reached a while back, in which case you should have gotten a DES PATH UNACHIEVABLE message.Did you EXEC your change after you made it?Regardless of what happened with VNAV, here's how you could have complied with your clearance:Put the aircraft in LVL CHG and set the speed using the MCP speed knob to keep the speed right below the red part of the speed tape.When you got down to around 15000, set the speed to 290 and used V/S to modify your rate of descent for the last 4000 feet.You could have also set the speed manually (SPD), and used the DEScent page to give you a hint of what VS to use to cross XXX at 11000, provided that the restriction was showing on the DEScent page.Don't have a clue what happened with your radios.

Hey Tim, thanks for the answer!First, I don't know if I reached T/D. But didn't get DES PATH UNACH.EXEC...hm, I think I did, but not sure now any more. Will remember next time :)I proceeded with the manual descend anyway, similar to what you described.That with the radios was really weird. I was simply unable to change the freq.I hope the VNAV is not as buggy as it is with PSS (especially old 747 and 777).And what about the speed? I'll try now a similar situation, try to replicate that from yesterday.

As a suggestion:With all things aviation a logical process is the way to go.The first instruction is to make best speed so we go first to CRZ page and set Mach.80 and 320 kts (or as fast as you want to go) or so and in DES page we set same numbers.Then you were given an ALT restriction so go to the LEGS page and enter that. If you have not already passed TD (Top of Descent) then you will see the TD listed on the DES page and shown on the ND in MAP mode. If you have already passed TD then you should see from the DES page how far too high you are and the VS and FPA required to achieve the target you set in LEGS. Best way if this is the case is to go to LVL CHG and wind teh speed right up while watching the DES page to see if you are catching up .... if not then pull Speed Brake and if that won't get you in then the controller is making an unreasonable demand and you would tell him so politely :)Brad