# Doubt about VOR/DME approach with the FMC. Who control the descent rate?

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Hi, I'll be trying a VOR/DME approach to Mallorca, Runway 06L... and I have a doubt.

I understand how a VOR/DME approach must be taken.

You arrive the last segment of your approach... called FAF - Final Approach Fix. In that point, you start a descent rate, which would vary according the recommended landing speed for your aircraft.

Basically you have a table in your IAC - Chart, in which you see the descend rate in feet per minute according the a range of speeds.

For example, let's imagine my aircraft has a recommended landing speed of 100 knots.

When I arriva the last segment from my approach, FAF, I must start to descend the aircraft in a certain range of feet per minute, indicated in a table provided in the IAC chart.

See the table here:

In this case... if my recommended landing speed is 100 knots, when I arrive the FAF I would have to start to descend my aircraft with a Rate of Descend ROD of 573 feet per minute. You can see this indicated in the table.

When you arrive the FAF (you know that because the FAF is indicated in the IAC chart and it is indicated at a distance from the VOR in nautical miles), then, when you arrive the FAF you start to descend the aircraft programming a descend rate in the autopilot with the VS button.

This is the way you fly a VOR/DME approach in light aircrafts, like Bonanzas, Cessnas and so on...

However, what happen when we try a VOR/DME approach using a FMC?

Okay, the first thing I want to post is the table that Kabronicus (amazing guy from this forum) provided me. Extremely useful, telling all the waypoints names and its meaning.

I have highlighted some relevant information in that table.

Okay, so we are doing a VOR/DME approach to Mallorca, Runway 06L... let's see what the FMC says.

Okay, two important things for a VOR/DME approach:

1. I have VNAV selected in my autopilot, okay? so my aircraft is following the altitudes indicated in my flight plan.
2. Sadly, I don't have autothrottle in this aircraft... so I cannot program speeds... I have to control it using what God gave me. "The Hand", basic, but effective

The first segment of my approach, IAF, it's called MJV okay?

FMC LEGS

I have numbered the information in the LEGS page from the FMC.

I will explain what numbers from 4 to 6 means exactly, according the Jeppesen table for names and conventions.

N. FMS /FMC      SYMBOL    RUNWAY     MEANING
---------------------------------------------------------------------------
4. CD06L         CD        06L        CD = Final Approach Course Fix (FACF)
5. FD06L         FD        06L        FD = Final Approach Fix
6. MD06L         MD        06L        MD = MAPT - Miss Approach Point

So we know what means CD, FD, and MD...

Okay...

So observe we arrive to the Final Approach Point - FAF - (number 4) FD06L and we have an altitude of 1400 feet.

At that point we should start the descend rate, according the table provided in our IAC chart.

However we are flying with the FMC, and we have the altitudes expressed automatically... so when I arrive the FAF I am to the right altitude 1400 feet.

What do we have next?

Next we have...

MD06L... and what is that? That is the MAPT Miss Approach Point.

Then, we have MD06L, the Miss Approach Point to an altitude of 420 feet.

What that means?

That means the aircraft arrive to the FAF to the right altitude, 1400 feet...

The aircraft continue navigating... it goes on... and arrive the MAPT to 420 feet.

:unknw:

So I think I am not descending the aircraft. It is clear... the aircraft is descending itself...

I have my VNAV activated...

So my doubt is who control the Rate of Descend for my VOR/DME approach? I am in control ? or the aircraft is in control ?

If the aircraft is controlling this? would this work fine? I ask because the descend rate is linked to the speed... so what happen if I do not use the recommended landing speed? would the descend works?

I guess yes... because.... this is not like VS... in which you have to apply a descend rate according your speed.

I guess the FMC have this simplified... in the sense that when you arrive the FAF you must stay to a certain altitude. FULL STOP.

Then... you continue navigating (whatever your speed be) and when you arrive the MAPT you must stay to a certain altitude. FULL STOP.

So the aircraft will take control of the vertical speed (VS) in at every moment, according the speed.

Summarizing, with the FMC... even if you don't apply the right speed to land, and even if you don't have autothrottle... you won't have any problem landing, because VNAV take total control of your altitudes every time.

Is this right?

Cheers

By the way, I have the MAP or MAPT (Miss approach point) to a different altitude of what is indicated in the real chart????

I have in the FMC 420 feet and in the chart I have 490 feet???

See it yourself...

Wow that's a long post. I will read it and try to answer you mate!

Jaime Beneyto

System: i9 9900k OC 5.0 GHz | RTX 2080 Super | 32GB DDR4 3200MHz | Asus Z390-F

Wow that's a long post. I will read it and try to answer you mate!

I am lucky having you here buddy. Really

The first thing to understand is that the FMC is only there to assist you, not to make your life more complicated

The approach should be flown in essentially the same way.

However, what happen when we try a VOR/DME approach using a FMC?

In the FMC you should have the important waypoints of the approach plate with their corresponding altitude and speed. Past the IAF this is reference information only.

I have VNAV selected in my autopilot, okay? so my aircraft is following the altitudes indicated in my flight plan.

I have my VNAV activated...
So my doubt is who control the Rate of Descend for my VOR/DME approach? I am in control ? or the aircraft is in control ?

I wouldn't use VNAV past the IAF. LNAV is okay as long as you constantly monitor your raw data and verify it's flying the correct path. If LNAV doesn't do it correctly, revert to HDG SEL or manual.

For the vertical path I recommend you use Vertical Speed instead of VNAV. It's best for step descents past the IAF.

The trick is the following:

- Descend to the IAF altitude

- Once at IAF altitude and in ALT HOLD mode, set the next lower altitude in the MCP

- Open the V/S window and set it to ZERO so the aircraft doesn't climb/descend

- Some 0.3 nm before reaching the IAF, put -1000 or -1500 fpm in the V/S window.

- Check for the green arc and make sure it "lands" before your next waypoint. Adjust accordingly.

- Once you reach the new altitude, repeat the previous steps.

For the final approach segment, that is, from FAF to the threshold, do the same, set the MCP altitude to runway elevation (round it up) and start rolling the V/S wheel aggressively or else you'll end up too high. The green arc should land on the runway threshold. Make sure you don't descend below MDA without visual with the runway environment.

Also if you're interested in flying VOR approaches, check these videos: LFMN VOR04 and LEJR preparation of VOR02 and LEJR VOR02

Jaime Beneyto

System: i9 9900k OC 5.0 GHz | RTX 2080 Super | 32GB DDR4 3200MHz | Asus Z390-F

So basically you recommend me to disconnect the VNAV button as soon I reach the first segment, the IAF.

Then, I continue descending according what is told in the charts... and I apply my descend rate using V/S as always...

you recommend me this even for Airbuses and Boeings? right? always?

And of course, if I do this with the VS, I must follow the right descend rate indicated in the chart, and the right speed... as always right?

However... I suspect that if I don't do this... due the aircraft have different altitudes... for the IAF, IF, FAF and MAPT... I guess it would descend automatically the aircraft to the different altitudes... even if I don't follow your advice and allow the aircraft to do that iself, right?

Personally I prefer the way you recommend, but I wanted to be sure of this, to understand properly the FMC working.

Also... there is a point that I don't see in the Jeppesen convention...

Have you seen the point I have numbered as... 3...?

(NTSC) what would be that?

It repeat also 1900 feet...

As you can see in the altitude profile... MJV is the IAF - 3000 feet. Then we have the IF 1900 feet... in the fmc is represented as MJV too... point 2...

Then we have point 3... (NTSC) also 1900 feet, as the IF ...

What would be (NTSC) ?

Any idea?

Then, I continue descending according what is told in the charts... and I apply my descend rate using V/S as always...

you recommend me this even for Airbuses and Boeings? right? always?

When what you want to achieve is a certain vertical speed, then V/S is the best mode to use.

"Always" and "never" are very strong words, I don't like to use them because there's ALWAYS an exception and you can NEVER be sure about anything, hahaha

I know for instance there are LNAV/VNAV approaches, but that's a different thing.

And of course, if I do this with the VS, I must follow the right descend rate indicated in the chart, and the right speed... as always right?

Yeah but you won't be able to command a 573 fpm descent. In this case you'll put in 600 and that will do it. Adjust accordingly.

The rule of thumb is take your final approach speed, multiple by 10 and divide by 2. Say your speed is 135, times 10 is 1350, divided by 2 is 675 fpm, so you'd round that up and 700 fpm would be your target.

Have you seen the point I have numbered as... 3...?

Those are floating points. In this case it's the intersection with the localizer I believe, the point where you will INTerCpt the LOC.

Floating points are put in by the FMC only, you can't put them in yourself (or I believe so). These points can be something like (500) during a missed approach, meaning the waypoint depends on your altitude and does not have a geographical position (it depends on other things, it has no coordinates)

Jaime Beneyto

System: i9 9900k OC 5.0 GHz | RTX 2080 Super | 32GB DDR4 3200MHz | Asus Z390-F

What aircraft is this and what nav data? I ask because, well it's not quite correct. Ignoring the Jepessen naming conventions there are exactly six defined waypoints for the approach. The first one is at MJV and it does not include the hold. I believe you would only fly a hold there if specifically directed by ATC. The next waypoint is defined as being 8NM away from MJV on R261. The next waypoint is defined as being 11NM away from JOA on R237. It is what is termed a course intercept and as such is usually preceeded by an (INTC) waypoint which defines the predicted intercept position. The next waypoint is defined as 6NM from JOA on R237 and it is the FAF. The next waypoint after that is the MAP and it is defined without an altitude which means it will be the touchdown zone elevation plus 50 feet.

In short you should see only six waypoints from start to MAP. This is based off the raw data, not someone else's parsed data (which is what you would be using).

As for the VNAV... what you should and shouldn't do is all based on the aircraft's avionics/autopilot and since we don't know what one this is, I couldn't possibly offer you any information on whether VNAV should be allowed to control descent that far or not.

Ed Wilson

Mindstar Aviation
My Playland - I69

Hey sorry buddies, I was a bit busy these days so I was away from the forum.

I'm back again

Thansk a lot for the replies.

SPECIALLY TO YOU ALPHA FLOOR, AS ALWAYS AMAZING REPLY WITH A LOT OF GREAT INFORMATION.

Buddy you are the best.

Thank you

You're welcome compañero

Jaime Beneyto

System: i9 9900k OC 5.0 GHz | RTX 2080 Super | 32GB DDR4 3200MHz | Asus Z390-F

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