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

I sometimes use negative figures during the maintenance test so I don't have to wind the altimeter all the way up, but I guess it's not the reason behind it.

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.

i understand the reason behind having it deselected, ie -20 when not using rad alt minimums, i was curious why it went to -20 though, instead off having an 'off' position or -1??

Maybe for when you land in Death Valley, CA where it's 250 feet BELOW Sea Level.:)

The indication being discussed is based on height above terrain (as measured by radio altimeters), not height above sea level (barometric-based).

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

My understanding of the -20 setting is that it is used during a Cat IIIb No DH Autoland.

Setting -20 will not give you the Auto call-outs which do not apply to a No DH approach.

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!

:ok:


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.

NSEU

I guess you didn't notice my smiley face in my post regarding the -20 maybe for Death Valley.:)

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 (???).

Rgds.
NSEU

These are the callouts I was referring to - should have expressed myself a little better.

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...