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Author Topic: Instrument approach - final altitude...  (Read 823 times)

jcomm

  • Join date: Dec 2011
  • Location: LPMT
  • Posts: 1573
Instrument approach - final altitude...
« on: Thu, 31 Dec 2020 11:16 »
Good morning,

wishes of a Great 2021 to All of you and your beloved !  Full of Health, Joy and whatever more of good we can  get :-)

A question that results from a video posted by a simmer at another forum, while operating a 744 n X-Plane 11.

He sets, at some point during VNAV approach, the final altitude as the closest to the rw threshold altitude.

IRL is there any circumstance under which such a reference altitude is set on the MCP ALT window ?  Thought the lowest you should set, under any type of approach, would be the MDA(H) and then, after passing the FAF, the missed approach ALT ?


localiser

  • Join date: Aug 2009
  • Posts: 61
Re: Instrument approach - final altitude...
« Reply #1 on: Thu, 31 Dec 2020 11:35 »
Hi jcomm,

On a VNAV approach, we set the MDA rounded down to the nearest 100 feet.

For example if the published MDA is 670’, we will set 600 on the MCP.

At the point of setting the MCP altitude, you will be in VNAV PTH, and it’s generally helpful to have levelled off at the platform altitude for the approach (for ex 3000’).

The VDI will be present on the ND, and you will be able to see the VNAV glide slope descend to meet the aircraft position as you approach the FAF.

When the aircraft is descending, and a minimum of 300 feet below the missed approach altitude, we then set the missed app alt in the MCP, and the aircraft will continue to descend on the VNAV glide.

jcomm

  • Join date: Dec 2011
  • Location: LPMT
  • Posts: 1573
Re: Instrument approach - final altitude...
« Reply #2 on: Thu, 31 Dec 2020 13:00 »
Thx loc !

skelsey

  • Join date: Jan 2017
  • Posts: 104
Re: Instrument approach - final altitude...
« Reply #3 on: Thu, 31 Dec 2020 17:35 »
However, I believe this is something which is airline-specific.

I'm familiar with setting MDA (rounded down) in this circumstance, but recently I was chatting with a friend who flies for a major US airline and it turns out their procedure for a VNAV approach is to set threshold elevation.

So horses for courses, as they say...

jcomm

  • Join date: Dec 2011
  • Location: LPMT
  • Posts: 1573
Re: Instrument approach - final altitude...
« Reply #4 on: Thu, 31 Dec 2020 20:09 »
Very interesting to learn that Skelsey. Thank you !

cavaricooper

  • Join date: Dec 2009
  • Location: KTPA
  • Posts: 1218
Re: Instrument approach - final altitude...
« Reply #5 on: Sun, 3 Jan 2021 13:27 »
Simon is correct- ASA routinely "Dials the Dirt" and part of the GA procedure is for the PNF to set MA ALT.
Carl Avari-Cooper, KTPA

dhob

  • Join date: Sep 2019
  • Posts: 67
Re: Instrument approach - final altitude...
« Reply #6 on: Mon, 4 Jan 2021 13:26 »
For the 747-400 specifically, whether the MCP altitude is set to the MDA or TDZE is irrelevant when using VNAV. The MCP just needs to be set far enough below the FAF altitude to be out of the way. For the Legacy FMC, inside the FAF the FMC is assured to be in the "on approach" mode, meaning the MCP altitude can be set above the airplane altitude without causing VNAV ALT to display. When inside the FAF and more than 300' below the MAA, set the MCP to the MAA.
The NGFMC transitions to the approach mode when flaps are out of up. As such, the MCP can be set to MAA before the FAF for VNAV Non-ILS approaches. The Boeing procedure changed recently for the NGFMC to the following:
With flaps out of up and 300 below MMA, set the MCP to the MAA.
As all our 747-400s are NGFMC, we transitioned to this procedure last year.

Mariano

  • Join date: May 2009
  • Posts: 730
Re: Instrument approach - final altitude...
« Reply #7 on: Mon, 4 Jan 2021 15:04 »
Jcomm,

Happy New Year to you (and everyone) as well.

At our airline, we set the MCP to MDA by adding 50 feet and then rounding to the next highest 100 feet (670 feet becomes 800 feet).

Under FAA rules (which govern our operation), no part of the aircraft must descend below MDA unless certain visual and positional requirements are met. By setting MDA+50 feet, should we have to execute a missed approach, we ensure that as the engines spool up and the aircraft momentum transitions from downwards to upwards, the landing gear and aft fuselage/empennage will remain above MDA, not violating regulations. The “Minimum” aspect of MDA is what dictates this procedure. It’s a MINIMUM altitude.

In case of a non-precision approach with a DA, we set the MCP to DA rounded to the next 100 feet (we do not add 50 feet), since DA is a DECISION altitude AT which we are allowed to decide whether to initiate a missed approach, therefore penetrating the DA “layer” while transitioning is allowed.

The easy way to remember this is MDA=“Must Do Arithmetic”, DA=“Don’t Add”.

As you can clearly see, it’s all about each carrier and the way the respective regulating authority approved their procedures.

Best regards,

Mariano
« Last edit: Mon, 4 Jan 2021 15:34 by Mariano »

jcomm

  • Join date: Dec 2011
  • Location: LPMT
  • Posts: 1573
Re: Instrument approach - final altitude...
« Reply #8 on: Mon, 4 Jan 2021 15:59 »
Mariano, dhob, Carl, skelsey, localizer...

Thank you all again for the precious info !!!

A Great 2021 !

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 12310
    • Aerowinx
Re: Instrument approach - final altitude...
« Reply #9 on: Mon, 4 Jan 2021 16:11 »
Mariano, so you will never set the MCP to the missed approach altitude?

What if you push TOGA and the MCP ALT is below you?

And what if you continue the approach, won't your 767 VNAV level off at the MCP ALT and change to VNAV ALT?

Or are you talking about the MDA bug, not the MCP ALT bug?


Cheers,

|-|ardy

Mariano

  • Join date: May 2009
  • Posts: 730
Re: Instrument approach - final altitude...
« Reply #10 on: Mon, 4 Jan 2021 17:47 »
Hardy,

Thanks, good catch. I got all caught up with the whole MDA additives issue that I forgot to mention that this procedure (MDA plus 50 feet) is for the barometric altimeter bug, ensuring that a missed approach won’t violate MDA by calling for it with enough of an altitude margin. The MCP altitude is initially set to MDA plus 50 feet rounded to the next higher 100 feet, but we reset it to the missed approach altitude once inside the FAF and below 300 feet below the missed approach altitude. I omitted this last (crucial) point in my initial post.

Best regards,

Mariano



Hardy Heinlin

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Re: Instrument approach - final altitude...
« Reply #11 on: Mon, 4 Jan 2021 21:05 »
I'm relieved to hear that :-)

Mariano

  • Join date: May 2009
  • Posts: 730
Re: Instrument approach - final altitude...
« Reply #12 on: Mon, 4 Jan 2021 21:11 »
😂

If someone is going to immediately pick up on mistakes/omissions/nuances, etc., it’s definitely going to be you. ;-)

Happy 2021,

Mariano