Why does the Vertical Profile activate far from destina...

[Guest cliff]

The Manual states that "regardless of whether you actually engage VNAV or not the FMC still calculates the vertical profile and when you cross into the descent portion of the profile it activates a path indicator on the side of the navigation display"I'm flying at 20,000 ft over Valencia (In Spain) bound for London, Heathrow and nowhere near the descent point.However the Vertical Profile had suddenly appeared (VNAV is inactive)with the indicator at the top and showing a figure of about 4400. What purpose does it serve at this Stage of the flight? And another question, if I may. I'm flying with "Radar Contact" and following ATC instructions. They periodically give me a "QNH" figure. What does that mean please?Cliff

[Guest Qantas Guy]

the figure (4400) is how low you are in the descent profile, it shows you are this "low" because you havent started your descent yet, as you get closer to the ToD, the figure will get smaller and should hit zero or center in the G/s indictor on the ND just as you reach the ToD. if you descend correctly it will stay centred the entire descent.when ATC give you a 'Q' number you are required to set your altimeter barometre to this QNH this is the Height pressure for the area. I.e it would be different over England than from Spain due to elvation and temprature differences etc. shows height above Mean Sea level. everytime it changes or you travel to an area with a new QNH you will be notified if under control and IFR.Daniel

[Guest GeoffC]

My guess would be that the aircraft thinks you have started to descend for some reason. Due you have the pressure set to standard or local? If it's set to local it may change as the weather updates and make the altimeter think you have dropped.Just a thought.In response to Daniel's post, wouldn't the pressure be left at standard at FL200, rather than adjusting to reflect the updates.

[Guest cliff]

Daniel may I pose a couple of supplementary questions?1 That 4400 is at the top of the vertical line. Doesn't that mean that I'm too high rather than too low?2. Please confirm my understanding that this is the Q number that I set on the ground at 220 with the Baro knob and that I should now alter it with the Baro knob to the QNH figure instructed by ATC. In this case the current figure at 20,000 ft is given as 1212Cliff

[ConstVoid 110]

Cliff,1) If the indicator is at the top of the vertical line it means you are too low - think of it as being the direction you need to fly to bring the indicator to the centre of the line.2) 20,000 ft in Spain is way above the transition altitude - the altitude at which you should reset the alitimeter to STD, and at which you refer to alitudes by 'flight level' rather than feet - ie 20,000 ft would be FL200. If Radar Contact is still giving you barometer settings at that altitude then it suggests you've got the wrong values in the 'Trans Lev' fields on the 'Controller Info' page. You need to specify the flight level at the departure and destination airports (typically FL40 to FL60 in Europe).Ian

[Guest cliff]

Ian you've opened a subject that I'm hoping you can clarify for me.1. I shall now refer to levels above the transition level as FLXXX. That's clear.2. When passing the transition level I shall now reset the altimeter to STD. That's clear.3. The part that confuses me are the instructions from ATC with QNH numbers. Why would it be neccesary to set these numbers if we have reset the barometer to STD at the appropriate time?Cliff

[ConstVoid 110]

When you above the transition altitude, Radar Contact should no longer give you barometer settings. As you are above the TA you should have the barometer set to STD (29.92 in / 1013 mb).Since you mentioned in an earlier post that you were at 22,000 ft and still getting QNH settings from RC there must be something wrong. The most likely problem is that on the Controller info page the TL is set too high. For example if the departure transition altitude is 4,000 ft should should enter it in the box as '40' (ie the FL) not '4000', which would mean a transition altitude of 400,000 ft. If you are flying at 22,000 ft with the TA set that high RC would continue to give you QNH sttings, as you are below what it believes is the TA.Hope that makes it more clear.Ian

[Guest cliff]

Hah! That the answer Ian................Thanks for that!Cliff

[ConstVoid 110]

You're welcomeIan