novice
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Instrument Approach Using Vnav

Tue Apr 02, 2013 5:56 am

From a Boeing 737-800 checklist I've been looking over under the title 'Instrument approach using VNAV' it states;

"The recommended roll modes for the final approach are:

For a VOR or NDB approach use LNAV

For a LOC, approach use VOR/LOC"

What is the difference between a VOR and a Localiser approach? as it states for a VOR approach use LNAV and for a LOC use VOR/LOC
 
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Starlionblue
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 7:05 am

Quoting novice (Thread starter):
What is the difference between a VOR and a Localiser approach? as it states for a VOR approach use LNAV and for a LOC use VOR/LOC

A VOR approach uses a VOR (or more than one) for reference. A localiser approach uses the localiser component of an ILS, in other words lateral guidance but not vertical (glidepath) guidance. In a VOR approach the VOR is frequently off airport, while a localiser approach implies the localiser will lead you to the runway.

For example at Kissimmee:
- The VOR/DME-A approach uses the ORL VOR, which is at Orlando Executive 16.4 NM away. As you can see in the airport diagram, this approach will plonk you down quite offset from the runways, so there are only circling minima available. This is also evidenced by the lack of a runway number in the approach name. http://155.178.201.160/d-tpp/1303/05793VDA.PDF
- The "ILS or LOC RWY 15" approach is simply the ILS approach without vertical guidance, and leads you directly to runway 15. In the minima section you can see how the minimums is lower with the full ILS, and is a Decision Altitude, while with LOC only it is higher and is a Minimum Descent Altitude. http://155.178.201.160/d-tpp/1303/05793IL15.PDF

[Edited 2013-04-02 00:07:21]
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Pihero
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 7:45 am

Starlionblue:
It's a bit more complicated than that : An approach based on navaids has a defined path, for obstacle protection purposes and one has to follow that defined trajectory.
Therefore, offset and all, one is stuck to the published let-down chart.

BUT: LNAV is much more accurate than a VOR - or even more so an NDB - and , everything normal, you're still within the protected envelope of the approach.

BUT # 2 : You have to display the VOR needle and continuously crosscheck that your trajectory is within the VOR accuracy ( generally, we accept 5° ).

This system has been on the 'Bus since its EIS, worked for a long time by Airbus and their clients and have been now standard OPS for , IIRC, the past twenty years.

A last remark : the procedure demands a thorough preparation on the FMS, validation of all the remarkable aspects of the approach - in particular the fix, the descent plan, the stabilisation altitude... - and it's done in TRACK /FPA. On the other hand, execution is a lot easier than with *classic* methods.

This procedure is killing the *dive and drive* method so loved by old timers .   

Cheers
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Starlionblue
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 7:57 am

Thanks for expanding Pihero. I had never thought of the "defined path" in this way for VOR/NDB approaches versus an ILS/LOC. But wouldn't you need to follow such a defined path on a GPS/RNAV approach as well?

[Edited 2013-04-02 01:02:02]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Pihero
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 12:11 pm

Quoting Starlionblue (Reply 3):
. But wouldn't you need to follow such a defined path on a GPS/RNAV approach as well?

Yes, but this time the descent plane - i.e. the glide path - is part of the data base.
As you can't crosscheck the navigation with ground-based navaids - after all, it's the purpose of the GNSS - you have to validate *high accuracy* and two GPS in Nav mode..
The preparation is even more thorough : check the threshold cordinates, insert bearing / Distance to said threshold...
The PFD display looks like an ILS when you're established.
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Starlionblue
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 12:23 pm

Thanks for info. Very interesting. Ever learning here.

Quoting Pihero (Reply 4):
The PFD display looks like an ILS when you're established.

IIRC on the G1000 there is a subtle difference. The gllideslope bug is magenta for GPS approach and green for ILS. Same same for the pilot of course.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
IAHFLYR
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 1:10 pm

A most interesting read, thanks for your expertise guys.
Any views shared are strictly my own and do not a represent those of any former employer.
 
KELPkid
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 3:36 pm

Quoting Pihero (Reply 2):
This procedure is killing the *dive and drive* method so loved by old timers .

Even if you're using the navaids, don't modern FMS'es compute a smooth vertical descent profile to avoid the "chop and drop" methodology that us private pilots are taught?  

One of my favorite approaches for this: http://155.178.201.160/d-tpp/1303/00330LD21.PDF  
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
Pihero
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 4:47 pm

Quoting KELPkid (Reply 7):
Even if you're using the navaids, don't modern FMS'es compute a smooth vertical descent profile to avoid the "chop and drop" methodology that us private pilots are taught?

If the approach is in the database, yes.
Otherwise, you'd have to fly the approach using FPA , the profile gradient manually inserted. (TRK / FPA is the setting used instead of HDG / VS). Your chart for instance gives the passing altitudes at different waypoints : NURRE at 5300', YIPUE 4400', COVDU at 3500'...etc...that gives you an average 300 ft / Nm of descent, therefore a 3° glide path / a 5% ILS-like gradient. There's no * dive and drive* step.
So, in order to fly that path, you just select FPA -3° at .3 Nm from the descent point - taking the inertia of the aircraft into consideration.
WARNING : it doesn't prevent you or you PM to announce every each one of these waypoints and the passing altitude to check that you're on the right accurate trajectory.
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Pihero
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RE: Instrument Approach Using Vnav

Tue Apr 02, 2013 4:54 pm

Quoting Starlionblue (Reply 5):
IIRC on the G1000 there is a subtle difference. The gllideslope bug is magenta for GPS approach and green for ILS.

Yes I believe it is now the norm : the VDEV pointer - as for *vertical deviation* is magenta.
The same bug is used when one follows a VNAV profile - like a *managed* descent.
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barney captain
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RE: Instrument Approach Using Vnav

Tue Apr 09, 2013 2:30 am

Maybe I missed it in the replies, but I believe the OP was asking why the various ROLL modes were specified for the different approaches. The quick answer is accuracy. LNAV is the more accurate roll mode when on an NDB or VOR approach and VOR/LOC is the preferred roll mode when on a LOC approach.
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novice
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RE: Instrument Approach Using Vnav

Tue Apr 09, 2013 10:56 pm

Thanks for the reply's guys  
 
SPREE34
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RE: Instrument Approach Using Vnav

Wed Apr 10, 2013 4:10 am

The scale and accuracy of the VOR and LOC signals also play into this.

The Localizer beam is fixed, and varies from 3 to 6 degres in width. It provides more accurate path guidance than a VOR course, which broadcast in 360 degrees of azimuth, with a second signal transmitted to compute the specific azimuth (radial) from the station. The equipment in the aircraft must be configured to indicate correctly depending on the NAVAID being used.
I don't understand everything I don't know about this.
 
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Starlionblue
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RE: Instrument Approach Using Vnav

Wed Apr 10, 2013 9:19 am

Is LOC really more accurate than VOR? Given a VOR receiver must be accurate to within a few degrees I'd think you can hit the thing pretty accurately. Granted, once you are very close to your target you enter the cone of confusion with a VOR, while a localizer will get you right to the runway.

Quoting SPREE34 (Reply 12):
The Localizer beam is fixed, and varies from 3 to 6 degres in width.

3-6 degrees at the end of the runway. It spreads with distance.
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Mir
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RE: Instrument Approach Using Vnav

Wed Apr 10, 2013 3:33 pm

Quoting Starlionblue (Reply 13):
Is LOC really more accurate than VOR?

The sensitivity of the course needle in LOC mode is far greater than it is in VOR mode, which means you can fly the approach more accurately.

Quoting Starlionblue (Reply 13):
3-6 degrees at the end of the runway. It spreads with distance.

No it doesn't. The beam is always a certain number of degrees wide, no matter how far you are from the runway. The width that corresponds to in feet, however, will vary, and will increase the farther you get from the runway. So while one dot off may indicate an error of 100 feet at the threshold (for instance), the same deflection 5 miles out might put you 1500 feet off course. It's the same principle as with a VOR - the farther away you are, the more off course you will be for a certain deflection.

-Mir
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Starlionblue
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RE: Instrument Approach Using Vnav

Wed Apr 10, 2013 11:43 pm

Quoting Mir (Reply 14):
Quoting Starlionblue (Reply 13):
Is LOC really more accurate than VOR?

The sensitivity of the course needle in LOC mode is far greater than it is in VOR mode, which means you can fly the approach more accurately.

Sorry I was unclear. Yes the course needle is. However isn't that an artifact of the instrument? That is, would it not be possible to navigate just as accurate to a VOR as to a localizer if we had an instrument with that sensitivity? Just wondering idly.

Quoting Mir (Reply 14):
Quoting Starlionblue (Reply 13):
3-6 degrees at the end of the runway. It spreads with distance.

No it doesn't. The beam is always a certain number of degrees wide, no matter how far you are from the runway. The width that corresponds to in feet, however, will vary, and will increase the farther you get from the runway.

You are correct of course. I had a brainfart. I'm forgetting how angles work. Is this what old age feels like AAAAH!
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

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