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ILS approach works out differently each time

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I've been flying the same (ILS) approach a few times now because it seems to happen differently each time. In the beginning of the approach everything appears normal. The FMC has the landing speed at 170, and when the G/S is captured the speed indicator on the MCP should read 170 right? Well, sometimes it does, and sometimes it sets it to 179 for some reason, even though it still says 170 in the FMC. Any idea what would cause this?

 

In addition, sometimes Land 3 engages, sometimes not. I'm assuming the plane needs to be within certain limits in order for Land 3 to activate - any idea what those are?

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Hi,

 

VREF 170kt?  :blink:  What is you landing weight and flaps setting? It seems rather high to me.  :wink:

 

Anyway, are talking of the VAPP or the VREF? VREF depends of the weight, VAPP depends also on the wind (VAPP=VREF + wind margin, usually 5kt when A/T is ON).

 

The speed on the MCP is set by you and the last speed you will set is VAPP.

 

For the condition to get Land 3, I encourage you to have a look at the FCOM v2 and on the 777 forum (if you search Land 3, you will find a couple of threads regarding this).

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Any idea what would cause this?

If you have not done so, you need to do Tutorial 1 and read the Introduction manual. You need to control the speed on final not the FMC. It's all in the two documents.

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Hi,

 

VREF 170kt?  :blink:  What is you landing weight and flaps setting? It seems rather high to me.  :wink:

 

Anyway, are talking of the VAPP or the VREF? VREF depends of the weight, VAPP depends also on the wind (VAPP=VREF + wind margin, usually 5kt when A/T is ON).

 

The speed on the MCP is set by you and the last speed you will set is VAPP.

 

As I'm just getting back into simming after a while there is a high probability I've put lot of weird stuff into the FMC :pardon:.  On the Init Ref-page I see that my Vref i actually 138 at flaps 30. But what I wonder then is that on the legs-page it has 170 kts at the runway. What dictates this number?

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Don't take the speed in the leg page into account for the final approach. You true speed reference is the VREF on the approach ref page of the CDU + the margin you add to get the VAPP.

 

The speed at runway is just a predicted speed (I cannot find exactly how it is calculated though my guess is is generic).

 

In any case, the FMC of the T7 doesn't calculate the speed reduction like the one of the NG does. It will only slow you down if there is a speed constraint on the approach procedure. But in any case, you have to manage yourself to slow down to be at the good speed for final approach.

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I've been flying the same (ILS) approach a few times now because it seems to happen differently each time. In the beginning of the approach everything appears normal. The FMC has the landing speed at 170, and when the G/S is captured the speed indicator on the MCP should read 170 right? Well, sometimes it does, and sometimes it sets it to 179 for some reason, even though it still says 170 in the FMC. Any idea what would cause this?

 

Hi Thomas,

 

Just want to add that once you are locked onto the GS, the MCP will ignore speed constraints set on the Legs page of the FMC. It only follows those in VNAV (unless you manually intervene in the MCP Speed). Once the GS is locked on, VNAV disengages, the Speed window on the MCP is no longer blank, but shows the current speed of the aircraft. The speed must be controlled manually from then on.

 

Correction: the MCP shows the last target speed set by the FMC.

 

On a different kind of approach, an RNAV approach (no use of Loc or GS, and they won't even be autotuned), the MCP will follow the speed constraints set on the FMC Legs page, and you could copy the final approach speed from the FMC Approach page to the runway waypoint of the Legs page. But an RNAV approach is eventually going to be a visual, MCP off approach, so you will still need to make sure the correct final approach speed is showing on the MCP. An autoland is not allowed on an RNAV approach (although I've tried it and it does land pretty well in reasonably calm weather).

 

In the past I've confused the two types of approaches, but they really are separate (and involve selecting different approaches on the FMC), although it's perfectly normal to transition from LNAV/VNAV to LOC/GS during the approach. But on final it's one or the other.

 

I would guess the 170 is on the runway entry of the Legs page in case of a goaround that starts before that waypoint is crossed. In a goaround LNAV and then VNAV reactivate.

 

Mike

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"Set Vref plus 5."

 

One of the last calls before landing :-)

 

Normally the 777 sets 170 on final if you are in managed mode (don't know if it's called like this in a Boeing, when VNAV is on).

Then you set it manually to Vref + x. Or let your Copilot do this (FS2Crew)

 

But it was explained perfectly by the others :-)

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