Another low altitude alert

Reply Subscribe

Page 2 of 9

Last »

Thread Tools

Search this Thread

10-02-2024 | 06:41 AM

#11

rickair7777

Prime Minister/Moderator

Joined: Jan 2006

Posts: 45,164

Likes: 803

From: Engines Turn or People Swim

Quote:

Originally Posted by dera

In approach logic the FMS will descend through MCP altitude when inside the FAF.
Or to be more exact, you can have MCP set above you and it will keep descending.

I guess our procedure accounts for the situation where the missed alt might be below your current alt. We set field elevation pretty early on an RNAV/RNP. On an RNP it can be on the downwind, once established on the app course and vertical path and cleared.

So in LNAV/VNAV it will descend through an MCP after the FAF? Or does the MCP always have to be above current alt?

10-02-2024 | 06:43 AM

#12

e6bpilot

Gets Weekends Off

Joined: Apr 2013

Posts: 4,590

Likes: 434

Quote:

Originally Posted by rickair7777

On RNAV apps? Or just ILS?

With an RNAV I understood that it can capture, if you set an altitude below your current altitude.

As long as you are at least 700' below the MAA and descending in path, you can set the MAA and it will continue the descent on the gradient path that it has set in the FMC. This is the procedure we have been using for a good while.

10-02-2024 | 06:48 AM

#13

e6bpilot

Gets Weekends Off

Joined: Apr 2013

Posts: 4,590

Likes: 434

Also looking at other sources, it appears that this may have been bad data and certainly not worthy of the headline. Another ADSB tracking website only had them <300' below the published altitude and they quickly corrected. Clearly it triggered a tower low altitude alert, but I don't think this issue was on par with the previous incidents and I am guessing that SWA being the latest for their turn in the barrel has a lot to do with why this even came out at all.
Certainly not great being below any published altitude, don't get me wrong, I just don't see this being worthy of panic and mayhem.

10-02-2024 | 06:53 AM

#14

rickair7777

Prime Minister/Moderator

Joined: Jan 2006

Posts: 45,164

Likes: 803

From: Engines Turn or People Swim

Quote:

Originally Posted by e6bpilot

So you can dial it up THROUGH your current alt and it won't capture as you spin it? Assuming the MCP Alt was originally set below you to allow descent in Path?

We don't use that feature and I'd guess it's because of all the varied RNP's in Alaska. There probably wouldn't be a consistent point in the profile where you could set the missed so it could get skipped. For LNAV/VNAV we just always set it when executing the missed... it's always at the same place in that profile: Gear Up, Set Missed App Alt".

10-02-2024 | 06:57 AM

#15

rickair7777

Prime Minister/Moderator

Joined: Jan 2006

Posts: 45,164

Likes: 803

From: Engines Turn or People Swim

Quote:

Originally Posted by e6bpilot

Yeah I think we are seeing more news about such things due to random people being able to monitor ATC via the interwebs, and then quickly produce a youtube video anytime they hear "low altitude alert" or "Go Around". You can get a tower low altitude alert on some non-precision approaches just by descending below the published MDA on a 3* path after visually acquiring the runway.

10-02-2024 | 07:01 AM

#16

biigD

Line Holder

Joined: Oct 2010

Posts: 1,991

Likes: 187

Quote:

Originally Posted by rickair7777

On LNAV/VNAV apps? Or just ILS?

With VNAV I understood that it can capture, if you set an altitude below your current altitude?

LNAV/VNAV. It'll capture, but instead of setting zeroes, we set the missed approach altitude (inside the FAF and 300' below the missed approach altitude). As you noted, there might not be a lot of time to accomplish this if the missed approach altitude isn't much higher than the minimums - something we'll brief as a threat if that's the case. The other big thing is to make absolutely sure we're in PATH, for obvious reasons.

10-02-2024 | 07:06 AM

#17

biigD

Line Holder

Joined: Oct 2010

Posts: 1,991

Likes: 187

Quote:

Originally Posted by rickair7777

So you can dial it up THROUGH your current alt and it won't capture as you spin it? Assuming the MCP Alt was originally set below you to allow descent in Path?

Yep!

::filler::

10-02-2024 | 09:32 AM

#18

Superviking

On Reserve

Joined: Feb 2020

Posts: 48

Likes: 10

As long as your 300 feet below the missed alt you can set it and Vnav won't flip out and try to level off. Personally I prefer doing ILS as once the GS capture you can just set the missed vs having to wait and possibly forgetting it like with an Rnav

10-02-2024 | 10:34 AM

#19

EasternATC

Line Holder

Joined: May 2010

Posts: 364

Likes: 0

Quote:

Originally Posted by e6bpilot

..., I just don't see this being worthy of panic and mayhem.

Quite right! But how else would them Youtubers get their clicks?

10-02-2024 | 11:09 AM

#20

luv757

Line Holder

Joined: Aug 2005

Posts: 534

Likes: 15

From: 18%er but I’ll enforce UPA23 to the last period.

We don't set 0s at my shop either. We set TDZE (rounded up to nearest hundred so 48' we'd set 100) at GS capture for an ILS or on an LNAV/VNAV we can set TDZE when cleared the approach and on a published segment (and in VNAV). Outside of that the lowest we can set is FAF altitude (again if in VNAV). We set MA altitude at our 1000' foot call.