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

[Driver170 42]

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.

[Don t moderate me this way 882]

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).

[Driver170 42]

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

[Driver170 42]

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.

[Spin737 102]

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.

[Driver170 42]

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.

[Don t moderate me this way 882]

For sure VNAV respects altitude constraints, those would be ignored in V/S.

[Jack C 8]

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

[skelsey 765]

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!

[Driver170 42]

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?

[Jack C 8]

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.

[Driver170 42]

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?

[Don t moderate me this way 882]

Rate is ROD = Rate Of Descent = constant V/S, I reckon.

[skelsey 765]

 

 

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.

[Driver170 42]

 

 

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/