VNAV altitude

[Alex-lloyd]

Hi, Apologies if this has been answered elsewhere, but I haven't been able to find the answer. When you set up VNAV for a Skyview GPS VFR approach, you enter the distance and "altitude above the threshold" that you wish the Autopilot to fly you to. My question is assuming that the barometric pressure (as opposed to GPS altitude) is used to determine this then should the Baro be set to QFE or QNH? It's confusing to me as (in the UK) its not normal to talk about an altitude above the threshold - its either a height above the threshold or an altitude (above sea level).

Also secondary question, what happens when this height (altitude) is reached? Does the autopilot level off or continue the descent? What is the relationship if any to the Altitude bug or the setting of the Minimum Altitude?

Many thanks

 

[Stevec]

It is relative to whichever you have set. I understand it will just use what is displayed on the display. Remember the UK is pretty much the only country that I know of that uses QFE so system is probably not designed to differentiate between QFE and QNH. When you carry out a VNAV approach it should be done relative to QNH.

 

[Alex-lloyd]

I'm not sure I fully understand your explanation. My assumption given what you say about QFE not being used outside of the UK, is that QNH should be set. I guess the system uses the database to tell it the threshold altitude and from that works out what height above the threshold to use for its aiming point. So, for example:

1) I set up the system to be 100ft above the threshold at 0.5 mile out
2) The Airfield is at 500 ft above sea level
3) I set the altimeter to read QNH
4) The system adds the threshold elevation to the requested aim point
5) The autopilot will direct me down a glideslope to the point I have selected at which point my altimeter will read 600ft

Given that I have made assumptions here it would be really helpful if someone who knows could confirm my understanding - thanks is advance.

Also still interested to know what happens after this point as per my original question and any interactions with the Alt bug and the Minimum descent setting.

Many thanks

 

[Stevec]

Your assumptions are correct that is exactly how it works. QNH should be used and my understanding, however I am not 100% sure how it is used these days, is that QFE should only be used in or around the circuit. I am sure there is someone here on the forum from UK who can confirm this for you.
The altitude and minimum bugs are relative to the altitude displayed on the screen.

 

[Stevec]

To answer the second part of your question. I have tried out the VNAV descent this morning to confirm it works as I thought. If you are at say 2500ft and your airfield is at 20ft amsl. With the AP engaged in VNAV and NAV the aircraft will remain at 2500ft tracking to the airfield. I have set my system up so that I arrive overhead the airfield at 1500ft. When it reaches the point it wants to go down on the slope and rate of descent you have programmed It changes the altitude bug to 1520 and starts the descent showing the rate of descent required to maintain slope with a magenta V/S pointer. On reaching 1520ft it flys level and captures that altitude. The minimums bug is merely a warning.
Hope this answers your questions.

 

[Alex-lloyd]

That's great - many thanks!