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LAND 3 Below 1500'

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Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
Hello,

I was practicing the last-minute runway change maneuver as per Britjet's excellent video and I noticed that even when the autopilot intercepts the new localizer and glideslope below 1500' (you can start the maneuver at 2000', but it could be less than 1500' before you are established on the new ILS), it still goes to LAND 3.

Can anyone verify that it's supposed to do this? I was under the impression that if it had not locked onto a localizer and glideslope prior to 1500', where it does its auto-checks and isolates the busses, you would get a message: NO AUTO LAND.

I tried an ILS where I intercepted the glideslope from level at 1300' and it still goes to LAND 3 as soon as the glideslope captures.

Thanks.

Jon D.

[ADDENDUM]:

After scouring pprune.org, I found a post that may shed more light on my question, but I'm unable to verify if it is really from Boeing.

Quote
From a letter from Boeing in response to a question about latest allowable AP engagement for Autoland:

"Provided that an ILS that meets the autoland criteria, has been tuned, the airplane is being manually flown on the published ILS path in trim and on speed, and APP has been selected prior to 1500 feet AGL, selecting the autopilot prior to 600 feet AGL (which is the lowest altitude for LAND 2 or LAND 3 to be annunciated) should satisfy the logic for autoland. The time required for the autoland system to perform a self-test and confirm correct operation will vary depending on conditions. We recommend that the crew engage the autopilot as soon as possible in order to satisfy approach logic.
Note: The autoland status must be confirmed by the crew at 500 feet AGL.
Note: Late autopilot engagement is not a recommended practice."
« Last edit by emerydc8 on Mon, 13 Jul 2015 07:31:02 +0000. »
Member
Registered: Dec 2011
Posts: 497
Location: Portugal
There had to be some reason...

I actually think that Boeing uses PSX to match their logic... Whenever something weird is discovered, the programmers at Seattle check PSX for the correct behavior, and Oops! this was not right - followed by the necessary corrective actions.

;)
_______________
Jose Monteiro
Moderator
Registered: May 2009
Posts: 5140
Hello,

if I recall correctly, the RA must be above 700 feet. That's also the limit in PSX.

Has Boeing reduced it to 600 feet -- for the 747-400?


Cheers,

|-|ardy
Member
Registered: Jun 2015
Posts: 103
Location: Tucson, AZ USA
I didn't even know you could engage APP below 1500' and still have it satisfy the autotests required to get to LAND 3.

I searched the web to see if I could find more from Boeing on this, but no joy. It sounds like you are on top of this, Hardy, so I'll consider my question answered -- you can engage APP below 1500' and still get LAND 3 (at least down to about 700' RA). Thanks!

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