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Author Topic: MCP ALT alert logic  (Read 260 times)

Hardy Heinlin

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MCP ALT alert logic
« on: Sat, 6 Feb 2021 09:01 »
Good morning,

for reference: Aerowinx manual, page 158.

As we know, when there's less than 900 to go to the MCP ALT, the PFD altitude box gets a thick white frame and (if installed) the C-chord sounds.

When the MCP ALT has been more than 900 away and we now reset the MCP ALT to a value within 900 from the aircraft, and then wait 2 seconds, the alert will also occur. This has been tested on the ground. But I'm wondering if the alert, in this case, just occurs because the aircraft is not departing the MCP ALT. On the ground, we have a "level flight" condition.

For example, in PSX, when I just started a final approach descent and the aircraft is more than 300 below the missed approach altitude, I set the MCP ALT to the missed approach altitude. After 2 seconds the white box appears and the C-chord sounds. This is because the aircraft was outside 900 and now it's within 900 due to the MCP ALT reset.

Whenever the "white alert" occurs after an MCP ALT reset -- while the aircraft is leaving the MCP ALT -- I always have the feeling it shouldn't occur. I think the "white alert" should occur when within 900 and when not departing the MCP ALT.

Well, "not departing": How could that be defined? E.g. when the sinkrate is greater than, say, 100 fpm, and the MCP ALT is above the aircraft, the "white alert" should be inhibited. Vice versa for a 100+ climb rate while the MCP ALT is below the aircraft. Just a theory.

Any reports from the real deck? When you are on a final approach descent and you reset the MCP ALT from a relative value outside 900 to within 900 (for the missed approach altitude), do you get that "white alert"?


Regards,

|-|ardy

John H Watson

  • Join date: May 2010
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Re: MCP ALT alert logic
« Reply #1 on: Sun, 7 Feb 2021 04:32 »
My notes say that the altitude alert function is inhibited with the [(gear down and locked) AND (the flaps are in landing config.)] OR [glideslope captured]. Setting the park brake will override any inhibits.

If you look at the schematics logic diagram, both the advisory and alerts are inhibited. Since this logic goes on to drive both the displays and the aurals, I assume indications and aurals can be inhibited (for alert and advisory).


Hardy Heinlin

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Re: MCP ALT alert logic
« Reply #2 on: Sun, 7 Feb 2021 05:42 »
Thanks for the refresher. This logic is also implemented in PSX, but I forgot to think of it during my last test flights when I was focussing the amber band stuff discussed in that other thread :-) I flew continuous descents and sometimes reset the MCP ALT before the flaps were in landing configuration. Brain tunnel ...


Cheers,

|-|ardy

dhob

  • Join date: Sep 2019
  • Posts: 67
Re: MCP ALT alert logic
« Reply #3 on: Sun, 7 Feb 2021 20:14 »
Boeing changed their procedure for the NG FMC to set the MCP to MAA when at 300' or more below MAA and flaps out of up. We adopted their procedure, and the white box does display if this is done when within 900' of aircraft altitude. However, fortunately our airplanes do not have the chime option enabled.

Hardy Heinlin

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Re: MCP ALT alert logic
« Reply #4 on: Sun, 7 Feb 2021 20:29 »
Thanks. So the alert in PSX is correct. Just my brain dropped out for a moment.