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Mephic

VOR/DME - lateral guidance?

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Hi

 

I am highly confused by behavior of my NGX. (read: must be pilot error :) )

 

I tried the LGKV approach today to runway 23L using VOR/DME approach. 

 

I passed 108.8 VOR using LNAV/VNAV and changed to hdg mode and entered 068 degrees course. After 10 nm from the airport I did left turn turning to 232 degrees. In the same time I set course to 232 to intercept 108.8 VOR from this course and pressed VOR/LOC button. Green light was on but aircraft did not capture right course (two lights were green: both under heading and vor/loc). I was still flying 232 as set on heading but with offset from VOR and not tracking the 232 radial to it. 

 

What am I doing wrong here? 

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Hi,

 

What was displayed on the FMA? I suspect that you stayed on HDG SEL with the LOC armed. On the chart I have, the radial is 233. Maybe flying 232 you never got to intercept the loc.

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Should I first switch off hdg select and then choose loc?

Fma said hdg select I think but need to repeat that tomorrow I guess to make sure

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You should be on a convergent heading to the radial you want to intercept on HDG SEL and press VOR/LOC to arm it when you are on a intercept heading. In your case, if you fly a heading 232 and try to intercept a radial 233 assuming you try to intercept from the south, then you will never make it.

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Finally got updated charts. The old ones were showing different information. I'm going to repeat the flight today but for sake of completeness I should:

 

1. Get to the Kavala vor and fly 68 radial from it

 

2. When I'm slightly above 10 nm away I should make left turn using hdg select

 

3. When turning I should set course to 233 and press vor loc and when it intercepts 10 nm from VOR I should set v/s to -790 considering my approach speed is 150 kts. Is that all correct or I'm misunderstanding something?

 

Thanks!

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That's correct. However you may need to adjust your course to allow the FMS to intercept the radial.

 

Once reaching the heading 233, you will have to check if the LOC mode is active on the FMA (or try o anticipate).

If not, then you have to check on which side on the radial you are and change your course to intercept the radial.
Keep in mind that to intercept the radial when the VOR/LOC is armed, you still need to be on intercept heading to the radial, otherwise you will remain on HDG SEL and fly parallel to the radial without ever catching it.

 

It is important to monitor the FMA, it will help you understand what your aircraft does.

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This is exactly the error I did yesterday. It's funny never had such issues with Cessna or Commanche but 737 is really overwhelming and you forget the basics. I'm off now to get to cockpit and fly this leg again. Thanks for your help!

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