757/767 Decision Height Selector
Thread Starter
Joined: Jan 2006
Posts: 24
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From: UK
757/767 Decision Height Selector
Hi Guys, just a quick question regarding the ADI DH Selector on 75/767...
What is the thinking behind being able to select a DH down to -20ft
Had to do an autoland recently and the skipper didn't know why either
any suggestions, as neither of us could come up with a reason
757JJ
What is the thinking behind being able to select a DH down to -20ft
Had to do an autoland recently and the skipper didn't know why either
any suggestions, as neither of us could come up with a reason
757JJ
Joined: Mar 2008
Posts: 330
Likes: 0
From: UK
On an approach where there is a DA based on the barometric altimeter rather than a DH based on the radio altimeter, the ADI DH selector being wound down to -20 means we won't get the flashing DH display on the ADI going off when it doesn't correspond to the DA bug on the altimeter. I guess that could lead to confusion i.e. thinking you're at DA when you're not.
With an autoland, where you have a DH based on the radalt, obviously the ADI DH flashing means you're at the DH for the approach, so it's not a problem.
With an autoland, where you have a DH based on the radalt, obviously the ADI DH flashing means you're at the DH for the approach, so it's not a problem.
Joined: Feb 2004
Posts: 1,410
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From: Australia
Maybe for when you land in Death Valley, CA where it's 250 feet BELOW Sea Level
The radio altimeter readout is calibrated to indicate zero the moment the wheels touch down (with bogeys tilted, struts uncompressed and aircraft at average flare angle). On some aircraft, after touchdown, I've seen -10 feet displayed. Perhaps being able to select -20 might prevent activation after touchdown (so no flight crew distraction), even if your struts and/or tyres have deflated (in an abnormal situation)
Rgds.
NSEU
Joined: May 2005
Posts: 187
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From: Standing at P37
My 767 Maint Manual states that when you set the DH below 0ft, the display for DH disappears altogether. This just leaves only the actual RA readout in ft at the top right corner.
I'm not a driver, so can only guess that it gives you the option of removing this piece of information when it's not applicable such as when you are hand flying/landing in VFR conditions!
Don't think so my friend. Callouts ( "Fifty", "Thirty", "Ten" ) are spoken on every approach. Never inhibited. These are generated by the Ground Prox Warning Computer - via inputs from the Radio Altimeters.
I'm not a driver, so can only guess that it gives you the option of removing this piece of information when it's not applicable such as when you are hand flying/landing in VFR conditions!
Setting -20 will not give you the Auto call-outs which do not apply to a No DH approach.
Last edited by Spanner Turner; 5th January 2009 at 11:58.
Joined: Feb 2004
Posts: 1,410
Likes: 37
From: Australia
Quote:
Setting -20 will not give you the Auto call-outs which do not apply to a No DH approach.
Don't think so my friend. Callouts ( "Fifty", "Thirty", "Ten" ) are spoken on every approach. Never inhibited. These are generated by the Ground Prox Warning Computer - via inputs from the Radio Altimeters.
Setting -20 will not give you the Auto call-outs which do not apply to a No DH approach.
Don't think so my friend. Callouts ( "Fifty", "Thirty", "Ten" ) are spoken on every approach. Never inhibited. These are generated by the Ground Prox Warning Computer - via inputs from the Radio Altimeters.
Rgds.
NSEU
Joined: Jul 1999
Posts: 162
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From: Out of a Suitcase
There are often callouts associated with minimums, including "approaching minimums", "plus 100", etc. I assume these would be inhibited with the DH set at -20 (???).
Thread Starter
Joined: Jan 2006
Posts: 24
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From: UK
OKay, there appears to be a bit of confusion and the post is going slightly away from where it was intended.
As far as I understand it:
setting a DH of 1-200' will give all callouts ie 50/40/30 etc etc AND APPROACHING MINIMUMS/ MINIMUMS
setting a DH of 0' will still give you 50/40/30 but no APPROACHING and MINIMUMS, as by definition there is no minima..
Setting any negative value, just removes the bug from the RAD ALT display, and obviously removes the APPROACHING and MINIMUMS call, 50/40/30 still occur though.
My question initially was why have a scale that goes to -20'??? surely an OFF position would make just as much sense...
As far as I understand it:
setting a DH of 1-200' will give all callouts ie 50/40/30 etc etc AND APPROACHING MINIMUMS/ MINIMUMS
setting a DH of 0' will still give you 50/40/30 but no APPROACHING and MINIMUMS, as by definition there is no minima..
Setting any negative value, just removes the bug from the RAD ALT display, and obviously removes the APPROACHING and MINIMUMS call, 50/40/30 still occur though.
My question initially was why have a scale that goes to -20'??? surely an OFF position would make just as much sense...
