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Heathrow to Dublin

Problem with RNAV approach - Vnav not descending.

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I'm a newbie and I don't think I'm doing it right.  I'm trying to do RNAV approaches into LOWI, specifically the RNAV 08 Z from the West.  The charts say that you should intercept the profile at a waypoint at 13000.  I descended to that level in time, got the LNAV Approach and VNAV Path and nothing happened, there was no descent, no glide path.  The vnav deviation bar stayed level.  Any help?  I'm sure many have attempted this popular approach.


Oh, and a quick follow up question: what fix should you be aiming at when selecting a regular VNAV descent in the FMC?  Should you put the FAF or the last waypoint?  The reason is that the VNAV has brought me in too steep and late and had me overspeeding on the way down.


Thanks 

Edited by Heathrow to Dublin

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Iv'e found a similar issue trying to do the Rwy 26 RNAV App as well. The approach goes OK from RTT to W1002 where the descent is supposed to begin, VNAV switches to approach mode however I notice there is an altitude of 9,500' showing for the next waypoint WI103 which is not evident in the flight plan page & the aircraft remains at 9,500'. 

I had been doing a series of flights between LOWI & LOWW to get the hang of ILS & RNAV approaches but am yet to have a successful RNAV approach to LOWI & have had to disconnect & hand fly it without vertical guidance. Odd thing was I dropped the HUD on one occasion & it appeared to be doing the descent OK however not certain on that.

Not having any issues with cruise VNAV descents & find them great, along with other little tricks like calling up airport weather briefings if you have a weather engine (AS). Landings also are no issue with a little power on till touch down as recommended.

Apart from this quirky RNAV system to someone used to the Boeing format I'm very impressed with this aircraft.


Cheers, Ross

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The fslabs has a hard time with it too.  Does not descent at the FAF.  Also at Sion, does not descend at the FAF.  I'm sure there is something amiss here..

 

Edited by Heathrow to Dublin

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I've noticed that the Q400 sometimes has difficulty flying arrivals which are designated RNP, showing similar altitude problems to what you describe. The few RNAV RNP arrivals I have attempted suffered from the aircraft being too high crossing some way points and experiencing difficulty when transitioning from the arrival to the approach. Setting the RNP in the FMC (shown on the chart) helps a bit. Mostly if I can fly the arrival, using a combination of VNAV/LNAV or VS, the aircraft will usually fly the RNP approach in VNAV ok. As I said, my Q400 struggles with the RNP arrivals.

Did the first poster have the Alt Sel set to a lower altitude? Alt Sel needs to be set lower than the cruise or present alt in order for the aircraft to descend.

I have since decided not to use any charts designated RNP and only use RNAV/GNSS or ILS charts, which work mostly perfectly in my q400 FMC. Using these arrivals & approaches I can't remember having a problem. Just keep an eye on the MDA, if using RNAV, and let the AP fly, or use the ILS

DP

 

Edited by DP10

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