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Setting MAA on a VNAV app VS / NPA (v/s, NDB VOR etc)

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I have read in my FCOM that on a VNAV app set the MAA when at least 300 feet below the MAA.

 

Then i read for a V/S approach you set the MAA approximately 300 feet above the MDA(H) 

 

Why is this ? 

 

Regards.

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Because you don't want VNAV to revert modes in that situation (either into VNAV ALT - level off - or into V/S, not sure atm which one would happen).

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Also like to add 

 

For a VOR, NDB Approach

 

 

Note: If not visual when ALT HLD annunciates, a missed approach is
mandatory.

 

At minimum descent altitude:
 
Verify ALT HLD mode annunciates.
PNF ....................................................... Set missed approach altitude
PF .....................Push TOGA switch and complete go-around actions
 
Note: If ALT HOLD is deselected from the MCP and airplane
altitude is within 100 feet of MCP altitude, the autopilot
flies to and captures the MCP altitude.
 
 
This is for a VOR and NDB approach, so why does the pilot set MAA when arriving at the MDA unlike the other NPA

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Because you don't want VNAV to revert modes in that situation (either into VNAV ALT - level off - or into V/S, not sure atm which one would happen).

 

Found this

 

On the V/S approach, the missed approach altitude is set when 300 feet above the MDA(H) to use the guidance of the altitude range arc during the approach and to prevent altitude capture and destabilizing the approach.

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Found this

 

On the V/S approach, the missed approach altitude is set when 300 feet above the MDA(H) to use the guidance of the altitude range arc during the approach and to prevent altitude capture and destabilizing the approach.

 

This.

 

Use the green banana to guide your descent. I typically monitor the DESC page FPA and V/B, too.

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Can you explain this -

 

VNAV is always preferential to V/S if the profile is in the database as you fly a geometric path instead of a rate.

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Different companies different SOPs. But I will tell you this. IF you are doing, say and NDB approach, and your MDA is sat 800 feet ASL. Wait until you reach 800 MSL and you see ALT HLD on the FMA. Make sure its ALT HLD and not ALT ACQ. If you set the MAA BEFORE you have ALT HLD on the FMA, then the autopilot will keep descending right into the at the V/S you were descending at. So after the FAF descend to the MDA so you arrive at a Visual Descent Point. When you reach MDA AND you have ALT HLD on the FMA THEN set the MAA altitude. When you reach the VDP then decide to land or fly the missed approach

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Worth mentioning as well that EU law requires all NPAs to be flown as CDFA: 'dive and drive' is no more, so that is probably where the instruction to set the missed approach altitude 300ft or so above the MDA comes from: if you're flying a CDFA you should never be in a situation where ALT HLD annunciates as you'll either be continuing to land or going missed (I think the US differs on this so US operators may have different procedures).

 

Don't forget to add 50ft to the MDA when flying a CDFA unless the chart gives you a DA!

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All NDB and VOR approaches are going to be coded for a CDFA in the FMS these days, well i think? Like you said no passenger planes can fly dive and drive to the MDA now if i stand correct?

 

But in the FCOM it mentions MDA(H) only and not DA(H) ?

When you reach MDA AND you have ALT HLD on the FMA THEN set the MAA altitude. When you reach the VDP then decide to land or fly the missed approach

 

Thanks alot for that makes sense now! How do you calculate your CDA from the VDP?

Worth mentioning as well that EU law requires all NPAs to be flown as CDFA: 'dive and drive' is no more, so that is probably where the instruction to set the missed approach altitude 300ft or so above the MDA comes from: if you're flying a CDFA you should never be in a situation where ALT HLD annunciates as you'll either be continuing to land or going missed (I think the US differs on this so US operators may have different procedures).

 

Don't forget to add 50ft to the MDA when flying a CDFA unless the chart gives you a DA!

 

Seems that quotes outdated then?

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Just use the 3:1 rule for calculating the VDP. For every 1000 AGL the VDP is 3 miles back from the threshold. Since the MDA is usually lower than 1000 feet, just take the MDA (in AGL) X 3. So 500 AGL is a VDP of 500X3 would be 1.5 miles back. 800 AGL would be 2.4 miles back and so on. That would give you a 3 degree glide path. BUT remember that it takes time to get that plane from a level attitude to one that provides a 3 degree FPA so you might want to add an additional .3 miles on the VDP. As stated, dive and drives are a thing of the past. If we want to be a part of airline code sharing (IATA) then we can not do dive and drives any longer.

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how come in my FCTM or FCOM it only mentions MDA(H) for V/S approaches? Since we know MDA can't be used because they now use the CDFA technique so why don't they specify DA(H) or both?

Someone explain this please -

 

VNAV is always preferential to V/S if the profile is in the database as you fly a geometric path instead of a rate.

 

As i understand geometric descend is idle all the way from TOD but what is actually a rate include?

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how come in my FCTM or FCOM it only mentions MDA(H) for V/S approaches? Since we know MDA can't be used because they now use the CDFA technique so why don't they specify DA(H) or both?

 

Because NPAs are designed with MDAs, not DAs.

 

If you initiate a go-around at DA, inertia means you will descend slightly below the DA before you begin climbing away again. The obstacle clearance for the missed approach on a precision approach with takes this in to account.

 

The same protection does not exist with an MDA. This means that you must not dip below the MDA when carrying out a missed approach, even momentarily. In the days of dive and drive, this was fine because you'd be level at the MDA when you got to the MAP. However, now we carry out continuous descent NPAs, you have to add 50ft to the MDA to ensure that you do not bust it when carrying out a go-around.

 

Some companies have customised charts or procedures that display DAs on NPAs which effectively takes this process in to account, but the original design will still have been with an MDA.

 

 

 


As i understand geometric descend is idle all the way from TOD but what is actually a rate include?

 

A geometric path is not necessarily an idle path.

 

Geometric (adj):

1. Relating to geometry, or according to its methods

2. Characterised by or decorated with regular lines and shapes

 

A geometric path means that the FMC draws a straight line from one altitude constraint to the next and follows it. This may or may not involve idle thrust, but it will certainly involve variable rates of descent as your groundspeed changes and as the gradient between constraints changes.

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Because NPAs are designed with MDAs, not DAs.
 
If you initiate a go-around at DA, inertia means you will descend slightly below the DA before you begin climbing away again. The obstacle clearance for the missed approach on a precision approach with takes this in to account.
 
The same protection does not exist with an MDA. This means that you must not dip below the MDA when carrying out a missed approach, even momentarily. In the days of dive and drive, this was fine because you'd be level at the MDA when you got to the MAP. However, now we carry out continuous descent NPAs, you have to add 50ft to the MDA to ensure that you do not bust it when carrying out a go-around.
 
Some companies have customised charts or procedures that display DAs on NPAs which effectively takes this process in to account, but the original design will still have been with an MDA.

 

So the charts are basically referring to DA then/

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So the charts are basically referring to DA then/

 

Only if it says DA on the chart.

 

If it says MDA you need to add 50ft.

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geometric path is not necessarily an idle path.
 
Geometric (adj):
1. Relating to geometry, or according to its methods
2. Characterised by or decorated with regular lines and shapes
 
A geometric path means that the FMC draws a straight line from one altitude constraint to the next and follows it. This may or may not involve idle thrust, but it will certainly involve variable rates of descent as your groundspeed changes and as the gradient between constraints changes.

 

So it calculates the paths to the next altitude restraint, and then navigates them with thrust as required.


It doesn't state on the charts if its DA or MDA 

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Only if it says DA on the chart.

 

If it says MDA you need to add 50ft.

 

This will vary with different operators.  We are allowed to use the MDA as a DA without adding 50 feet under any of the following situations:

 

- if the runway has a PAPI or VASI installed.

- If the runway has a published ILS approach we can also use the published MDA as a DA when conducting the ILS with the glideslope inop or a localizer approach.

- the approach has a ball note that reads "Only authorized operators may use VNAV DA in lieu of MDA.

 

In all other cases we would add 50 feet to the MDA, this is rarely the case.

 

Edit:  The approach must also have a coded glide path in the FMC database.

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We are allowed to use the MDA as a DA without adding 50 feet under any of the following situations:

 

Edit:  The approach must also have a coded glide path in the FMC database.

 

Is this why operators use DA 

 

 

 

AERODROME OPERATING MINIMUMS ACCORDING TO EU-OPS 1

In May 2008 we asked commercial operators about their plans for this EU-OPS implementation. The following items are directly related to the results of that survey:

 

a. All non-precision approaches will be reviewed to show CDFA (Continuous Descent Final Approach) profile and minimums.

 

b. In case of CDFA only, a DA(H) is shown instead of the previously published MDA(H). The missed approach point is still shown according to state source but the missed approach initiation arrow is moved to the point where the DA(H) is reached.

 

c. Jeppesen charted AOM do not include an add-on when current MDA(H) is replaced by DA(H). Pilots are reminded to check their operator’s Flight Operations Manual or similar documents whether they have to apply an add-on or not.

 

d. For CDFA profiles, Jeppesen will show DME vs altitude bands, distance vs altitude bands or timing vs altitude tables. If not provided by the State source those altitudes will be calculated by Jeppesen.

 

e. Non-CDFA profiles and minimums will be shown in exceptional cases only and may be combined with CDFA profiles and minimums.

 

f. For CAT I operations with full approach light system (FALS) Jeppesen will include RVR values below 750m together with the higher values. Pilots are reminded to check their operator’s Flight Operations Manual or similar documents to fulfill the requirements for using the lower RVR values.

 

g. Lower than standard CAT I minimums are charted on request on customer tailored charts.

 

h. Other than standard CAT II minimums will be charted if the procedure is approved for such operations by the state of the airport.

 

i. Circling minimums must not be lower than the minimums of preceding instrument approach procedure. If circling MDA(H) and/or visibility must be raised due to higher straight-in values, only one set of circling minimums is shown which relates to the highest straight-in minimums.

It mentions below  1. Approach methods: 2D: without vertical guidance, flown to an MDA(H) so why does an LNAV NPA only allow you to fly to the MDA(H)

 

 

The new scheme introduces two types of approach methods (2D and 3D), two types of approach minima (Type 1 and Type 2), in addition to redefining three types of approach procedures (NPA, APV, and PA).

 

1. Approach methods: 2D: without vertical guidance, flown to an MDA(H) 3D: with vertical guidance, flown to a DA(H)

 

2. Approach minima: Type A: minimums 250 ft (75m) or greater Type B: minimums below 250ft (75m), and further divided into CAT I, II and III

 

3. Approach procedures: Non-Precision (NPA): an instrument procedure (IAP) designed for 2D Type A

 

Approach with Vertical Guidance (APV): an IAP designed for  3D Type A

 

Precision Approach (PA): an IAP designed for 3D Type B

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- if the runway has a PAPI or VASI installed.

- If the runway has a published ILS approach we can also use the published MDA as a DA when conducting the ILS with the glideslope inop or a localizer approach.

 

Interesting. How does a PAPI keep you from dipping below MDA? I'd like to know how they sold that to your POI.

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Interesting. How does a PAPI keep you from dipping below MDA? I'd like to know how they sold that to your POI.

 

The way it was explained to me was that if the runway has visual or electronic glideslope guidance it complies with the TERPS obstruction clearance plane, or something to that effect.  You can initiate a go-around at MDA and you will still be clear of obstacles.

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Seems like there are different ways operators do things that's for sure.

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