Hi,

 

When doing a RNAV Approach using LNAV/VNAV i believe you have to set MDA (or DH baro i guess ) in the MCP for VNAV to continue the approach/descend , but i had actually set 1700´ in the MCP for LESOR when the aircraft still continued descending to Masor to the MDA in A/P VNAV/LNAV mode. The FMC was correct setup for the approach with INTC 093 active so is it a ticket or my bad?

 

http://www.bostonvirtualatc.com/charts/TNCM/IAP/RNAV.pdf

 

 

This was from a saved flight

 

The originally flight i did not noticed this because i had the MDA altitude set in the MCP

 

 

PPS!

 

Whats the different between MDA(H)  and DH(H) anyway?

Michael

 

 

The FMC was correct setup for the approach with INTC 093 active so is it a ticket or my bad?

 

Your bad, based on how I'm reading it.

 

 

 

When doing a RNAV Approach using LNAV/VNAV i believe you have to set MDA (or DH baro i guess ) in the MCP for VNAV to continue the approach/descend , but i had actually set 1700´ in the MCP for LESOR when the aircraft still continued descending to Masor to the MDA in A/P VNAV/LNAV mode.

 

When doing any approach, you should have the MDA/DH set in the BARO/RADIO MINS as appropriate.

 

This is an advisory value only.  In other words, the value is only set there to trigger an "approaching minimums" and "minimums" warning callout.  It also displays the value on the PFD.

 

If you're restricting the autopilot to an altitude, this altitude must be set in the altitude window of the MCP (where you set all other altitudes).

 

 

 

Whats the different between MDA(H) and DH(H) anyway?

 

Google would've quickly answered that question, but to save you the effort:

 

MDA/H - minimum descent altitude/height

Minimum descent altitude is an altitude calculated off your altimeter.  Minimum descent height is a height above the ground that is calculated with a radio altimeter.

 

You cannot descend below this altitude/height, whatsoever.  You descent to this altitude/height and remain at it while making your go/no-go decision (or you make it prior to reaching this altitude/height).

 

DA/H - decision height

Decision altitude is an altitude calculated off your altimeter.  Decision height is a height above the ground calculated with a radio altimeter.

 

You make your decision at this altitude/height, and as such, may descend below the altitude/height while making that go/no-go decision as part of your normal descent on profile (on the G/S or VNAV PTH).  Only minimal altitude loss is allowable, though.  The idea is that you make the decision upon hearing "minimums" at the very latest.  Since you're in a descent at that point (on the G/S or VNAV PTH), you will continue past that altitude while making your decision for a second or two.  Given the vertical guidance, this is acceptable provided the loss is kept to a minimum (which would be the result of making at decision upon hitting DA/H).

So you had 1.700 ft set in the ALT window of your MCP?

 

F/D, A/T, A/P, LNAV and VNAV all engaged?

but i had actually set 1700´ in the MCP for LESOR when the aircraft still continued descending to Masor to the MDA in A/P VNAV/LNAV mode. The FMC was correct setup for the approach with INTC 093 active so is it a ticket or my bad?"

 

I have noticed this once doing an RNAV approach as well. I'm not sure it's a bug, this might very be a case of "this is not the NG" type of thing. So I will let someone else more qualified address this part.

 

The difference between MDA and DH is of course the altitude source and the type of approach. MDA is typically used on non precision and CAT I approaches and is referenced to barometric altitude.

 

DH is usually for CAT II and III APPROACHES and is referenced to the radio altimeter.

 

Radio altimeter is more accurate than barometric altimeter as long as the terrain is flat which is why it is preferred for CAT II/III approaches. And of course barometric altimeter isn't affected by terrain but the minimums referenced to this requires visual acquisition at the MDA.

 

I'm sure someone else can provide more informative answers but I hope this will help you in the meantime

but i had actually set 1700´ in the MCP for LESOR when the aircraft still continued descending to Masor to the MDA in A/P VNAV/LNAV mode. The FMC was correct setup for the approach with INTC 093 active so is it a ticket or my bad?"

 

I have noticed this once doing an RNAV approach as well. I'm not sure it's a bug, this might very be a case of "this is not the NG" type of thing. So I will let someone else more qualified address this part.

 

The difference between MDA and DH is of course the altitude source and the type of approach. MDA is typically used on non precision and CAT I approaches and is referenced to barometric altitude.

 

DH is usually for CAT II and III APPROACHES and is referenced to the radio altimeter.

 

Radio altimeter is more accurate than barometric altimeter as long as the terrain is flat which is why it is preferred for CAT II/III approaches. And of course barometric altimeter isn't affected by terrain but the minimums referenced to this requires visual acquisition at the MDA.

 

I'm sure someone else can provide more informative answers but I hope this will help you in the meantime

 

 

Ahh thanks forgot the "Altitude" and "H" height differencies. Sometimes its confusing me because they do put MDA(H) this (H) behind which then must be MDH . Thanks Very easy and effective reading/understanding  (read none engineering)

 

Yes i had 1700 in the MCP and everything engage  but Kyle says that the plane should continue descend to MDA even though the MDA is at 700´. Maybe he did not read my post very well ?

 

Thanks

Michael

Yes i had 1700 in the MCP but Kyle says that the plane should continue descend to MDA even though the MDA is at 700´. Maybe he did not read my post very well ?

 

I know you said "MCP," but I wanted to ensure you weren't setting your minimums on the MINS knob and expecting the plane to abide by them without setting the altitude in the altitude window of the MCP.  I was simply trying to rule that out.

 

You can see that Olli's post also checked to ensure the same thing, as you hadn't specifically stated you had put it in the altitude window.

 

 

Sometimes its confusing me because they do put MDA(H) this (H) behind which then must be MDH

 

It's written as MDA(H) because the figure next to it is written like this:

700'    (686')

^MDA  ^(MDH)

 

...the latter value is mostly advisory in nature.

Thanks for your time 

 

Michael

There's a caveat to the MCP altitude limiting the descent concept when doing a VNAV approach in the 777. In VNAV approach mode (there's a series of conditions that define this in the FCOM2 somewhere) if you are 300 feet or more below the missed approach altitude the MCP effectively no longer limits the descent - it assumes that you're flying the approach down to MDA and that you're going to set the MCP to the missed approach altitude, which is higher than your current altitude and would normally cause the airplane to level off in VNAV ALT. In the case of this approach that'd occur at 3700 and you'd set the MCP altitude to the missed approach altitude of 4000 there.

There's a caveat to the MCP altitude limiting the descent concept when doing a VNAV approach in the 777. In VNAV approach mode (there's a series of conditions that define this in the FCOM2 somewhere) if you are 300 feet or more below the missed approach altitude the MCP effectively no longer limits the descent - it assumes that you're flying the approach down to MDA and that you're going to set the MCP to the missed approach altitude, which is higher than your current altitude and would normally cause the airplane to level off in VNAV ALT. In the case of this approach that'd occur at 3700 and you'd set the MCP altitude to the missed approach altitude of 4000 there.

 

 

Thanks Ryan

 

I see and understand but the case was that the T7 hit 1200-700 feet BEFORE even passing LESOR (which had 1700 feet in the FMC and 1700 feet in the MCP Altitude because i would change this step by step to Mapon and then to the Missed Approach Altitude of 4000 feet after passing Mapon at 700) and i was in VNAV/LNAV Mode.

 

I do understand that VNAV would think i would continue down to my DA or MDA in this case the 700 feet but why did it descent below 1700 feet at LESOR? I thought it would follow the FMC VNAV altitudes.

 

Well it might be an issue because it was from a saved flight . The problem was not there during the originally approach.

 

Thanks

 

Anyway

 

And btw is RNAV 10 MDA and DA equal 700feet at TNCM at the same time? (770feet no speed restrictions of 205knots to ONBED)

 

Michael 

Thats weird, I replied to this post but it is gone!

Quite a long reply too :-(

 

Not sure if it got deleted or something was messed up by myself?

 

Anyway to keep this short:

I believe that the airplane should have leveled off in 1700ft if that was set in the MCP.

(99% sure of it, but I have never tried to purposefully screw up a Vnav approach, by setting an intermediate level off altitude and leave it like that, to see if the aircraft levels off. You normally first set MDA and then the missed approach altitude once 300ft below your previous altutude AND below the missed approach altitude)

(I also believe that the airplane would level off at the MDA if you forget to change that to the missed approach altitude. And that is no different from leaving 1700ft dialed in)

 

 

Not sure if it got deleted or something was messed up by myself?

 

They were doing server maint yesterday, so a few things might have been dropped as part of that, unfortunately.

ok, thx Kyle.....Sh.. happens.