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WilloW_737

V NAV Approach Straight In - clarification please ?

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Greetings:

 

I need clarification on straight in V NAV approaches.  I will try and be succinct; I obviously lack knowledge in this approach type.

 

(1)  I notice that the L NAV approach course is slightly off center to the runway alignment.  Is this normal for a V NAV Approach ?

 

(2)  The only way to realign with the runway is to push the LOC button on the MCP.  Is it correct procedure to have the  LOC button activates during a L NAV Approach?

 

(3)  RNP as per the LEGS page of the FMC during approach reads 0.30 / 0.05.  Is the 0.30 the offset the L NAV is reading for the runway ?  (approach course is slightly off center to the runway alignment)

 

(4)  During FMC set-up it's recommended to enter the ISA deviation and QNH.   Should the QNH be set to radio or baro ?  Is ISA only necessary to change if outside temperature is below ) Degrees Celcius?

 

A lot of questions.  I hope someone has the answers which I am sure will help other as well as myself.

 

Thank You,    WilloW

 

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(1)  I notice that the L NAV approach course is slightly off center to the runway alignment.  Is this normal for a V NAV Approach ?

 

Yes.  There are many approaches that are slightly offset from the runway centerline for airspace or obstacle reasons.

 

Also, don't confuse terms:

The LNAV + VNAV approach you're referring to is an RNAV approach.

 

 

 


(2)  The only way to realign with the runway is to push the LOC button on the MCP.  Is it correct procedure to have the  LOC button activates during a L NAV Approach?

 

Make sure you are not confusing approaches here.  Certain approaches are meant to have an offset.  I'm not sure what your pressing the LOC button did because I'm not sure what your settings were when you pressed it.  Using an RNAV approach, you should not be using LOC at any time.

 

Here's an example with an offset:

http://155.178.201.160/d-tpp/1305/00765RY21.PDF

 

It is intentional.

 

 

 


(3)  RNP as per the LEGS page of the FMC during approach reads 0.30 / 0.05.  Is the 0.30 the offset the L NAV is reading for the runway ?  (approach course is slightly off center to the runway alignment)

 

Nope.

 

It's RNP/ANP.  The required navigational performance (RNP) is 0.30 accuracy.  Your aircraft's is actual navigation performance (ANP) 0.05.  Since the ANP is lower (more accurate) than the RNP, it means you're okay to fly the approach.  The values are in reference to how well the aircraft can pinpoint its exact location, and therefore fly the RNAV approach accurately (RNAV approaches provide no ground-referenced transmissions to follow, like LOC/NDB/VOR - this means the aircraft is on its own to track between two points, so it must be accurate).

 

It has nothing to do with runway offsets.

 

The belief that all approaches will always dump you right onto a runway is a simism.  Autolands are not as normal as many in the sim realm are lead to believe.  As an example (while not an RNAV example), this approach would definitely have no chance of allowing an autoland:

http://155.178.201.160/d-tpp/1305/00443LDAD19.PDF

 

Yeah, that's a LOC course of 145 to land on a runway with a heading of 184.

 

Offsets are thrown in whenever an approach cannot created to align with the runway (in the case immediately above, prohibited airspace is north of the field in line with the runway).

 

 

 


(4)  During FMC set-up it's recommended to enter the ISA deviation and QNH.   Should the QNH be set to radio or baro ?  Is ISA only necessary to change if outside temperature is below ) Degrees Celcius?

 

QNH is a pressure value, not your minimums selector.  QNH is the altimeter setting for the destination field.

 

ISA DEV should always be entered.  It is the difference between standard temperature (15 Celsius at sea level) and the current temperature.

 

This is covered in Tutorial #2. 

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scandinavian13, on 23 May 2013 - 08:54 AM, said:

The belief that all approaches will always dump you right onto a runway is a simism. Autolands are not as normal as many in the sim realm are lead to believe. As an example (while not an RNAV example), this approach would definitely have no chance of allowing an autoland:

I'll echo the above statements. Most approaches, whether RNAV or ILS are not flown all the way to touchdown. The autopilot is usually disengaged once the field is in sight and at around 1500 ft AGL. Full autolands are rare and in the case of a CATIIIb or c approach, almost never flown. Many approaches utilize offset approaches either for obstacle and terrain clearance, or operational restrictions. If the prevailing weather minima requires a full CATIIIa,b,or c approach, the aircraft, flight crew and the airport has to be CATIIIa,b,or c certified. Most airlines impose a DH and RVR limit considerably higher than even a CATIIIa approach which is, I believe a DH of 50' and an RVR of 250m.

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Another issue with FSX is it is 7 years old now? So the Navdata in the fmc and FSX database are off with respect to magnetic bearings. So even if your approach is right down center it will appear to be off in the ND . There is a sticky in the PMDG general forum with a link to some nice people who have a couple utilities to make this situation better.

 

Eric W

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Hello;

 

Thank you for your very helpful comments.  I have some further reading to do as I was off-subject on a few things.  My QNH question was quite novice as I already knew the answer, but failed to see it. 

 

@scandnavian13; I was using the LOC button on the MCP to reallign with the runway (automatically) as the RNAV approach, as indicated by the FMC was offset to the right.

 

Attached is the RNAV chart for YMHB (Hobart, Tasmania Australia).  This is the airport I have been using.

 

 

 

When I fly the RVAV approach in the FMC the course is offet to the right, however, on the plate there is no offset.

 

The nav data in FSX is up to date.

 

Am I reading the plate incorrectly.

 

Thanks,  WilloW

 

 

 

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Are you saying that with LNAV you are flying offset from the magenta line or the extended runway center line?

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Are you saying that with LNAV you are flying offset from the magenta line or the extended runway center line?

Extended runway center line...   bad use of the word "offset" which means something else  :)   (unfortunately I could not edit after I posted...)

 

The RNAV approach places the aircraft to the right of the track indicated on the approach plate (about 400 meters).  To land you must either turn to the left to reach the threshold, or push LOC on the MCP which causes the aircraft to align.

 

Note that the NAV1 is set to 109.90 (which is the ILS frequency) and NAV2 is set to 112.70 (which is VOR frequency).

 

Thanks,   WilloW

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To land you must either turn to the left to reach the threshold, or push LOC on the MCP which causes the aircraft to align.
 
Note that the NAV1 is set to 109.90 (which is the ILS frequency) and NAV2 is set to 112.70 (which is VOR frequency).

 

I see what you were doing now.

 

Based on the chart, the approach is not offset from the runway at all (unlike those that are designed that way).  If the initial position was not set correctly, ANP degrades, or the airport is not in the correct position in FSX, then any of these could cause your issue.  The issue does not show with the LOC approach, however, because the LOC is a radio beam from a fixed point at the end of the runway.

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I see what you were doing now.

Based on the chart, the approach is not offset from the runway at all (unlike those that are designed that way). If the initial position was not set correctly, ANP degrades, or the airport is not in the correct position in FSX, then any of these could cause your issue. The issue does not show with the LOC approach, however, because the LOC is a radio beam from a fixed point at the end of the runway.


That makes sense. I'll try another RNAV approach at a different airport and see how that goes.

I still have quite a way to go to understand RNAV in its entirety, and having all the ducks "lined up" to start with would certainly help. Thank you, WilloW

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I still have quite a way to go to understand RNAV in its entirety, and having all the ducks "lined up" to start with would certainly help.

 

RNAV as we use it today is just navigating using GPS coordinates over ground-based navigation aids (like VORs, NDBs, and LOCs).

 

The only reason it's special is that, in dispensing with ground-based aids, it becomes very important for the aircraft to be able to calculate where it is at all times.  A notable example would be KAL007.  One of the ideas as to why the aircraft strayed into hostile airspace was an improper position entry.  Because the position was not supplemented by GPS like today's systems, this error went unnoticed and the aircraft strayed into hostile airspace and was intercepted and shot down.

 

Now, take yourself out of a cruise perspective and shoot an approach in mountainous terrain.  That position error won't get you shot down - it'll get you put into a mountain.

 

In order for the aircraft to ensure it knows where it is (in the case of the 737NG), it uses GPS to supplement the inertial reference system (IRS).  The IRS provides extremely accurate position data, but over time, this degrades.  An pictorial analogy would be something similar to hurricane track predictions:

085338W5_NL_sm.gif

 

As the errors inherent in the IRS continue over time, the track becomes less precise.

 

GPS is able to extend the accuracy of this data by updating the IRS positions which allows it somewhat of a fresh start, accuracy-wise.

 

 

 

TL;DR:

It's the same as the "old way," where you follow the magenta line (and for approaches, the same two magenta dots on the PFD), but instead of relying on a ground signal, you're relying on the aircraft's understanding of where it is.

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A good analogy (storm prediction).

 

Thank you for your time and help.  I seem to have things sorted now with RNAV Approaches.   Cheers,    WilloW

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