From: zwork@starfighter.den.mmc.com (Michael Corvin)
Subject: Re: HUD question
Keywords: HUD, Head Up Display
Organization: Martin Marietta Astronautics Group
Date: Thu, 5 Nov 1992 20:54:23 GMT

In article <7895@transfer.stratus.com>, spenny@sw.stratus.com (Steve Pennypacker) writes:
|> In a few recent posts in one of the rec.aviation subgroups, there has been
|> some discussion about HUDs (Head-Up Displays) for GA.  One of them mentioned 
|> that a HUD should focus on infinity.  That makes sense if you're flying VFR 
|> with your eyes focused outside the airplane.  But how about a HUD used for IFR
|> (I know we don't have them yet in GA... but we will, and I'm curious)?  
|> Assuming the HUD doesn't completely eliminate the need to periodically scan
|> the panel, you are going to have to periodically focus your eyes inside the 
|> cockpit (ie- at much less than infinity).  In this case, for continuity it 
|> seems to be desirable to focus the HUD at the same distance as the panel, so 
|> the eyes never need to refocus.  But... the attitude display should probably 
|> be focused on infinity for the most natural view possible. 
|> 
|> So, given these apparently diverging needs, what's the philosophy on where
|> a HUD should focus?

The HUD is intended to provide instrument information while allowing the pilot
to view the outside scene.  When 'heads down', i.e. flying on instruments alone,
one would view equivalent information on the conventional instruments or a
HDD (Heads Down Display) on the panel rather than using the HUD.  I think
that having to refocus to infinity when viewing the HUD is actually a benefit.
Specifically, during the transition to visual on an IFR approach one would
transition to using HUD info before breaking out and having to aquire the runway
visually.  As the runway appears (throught the murk, and it better be before
decision height!) it will come into view in clear relation to the HUD symbology
with the pilot's eyes already focused properly.  The HUD displays being tested
on airliners (and an available option on the latest Airbus, I believe) are
primarily used for approach.

In any case, regularly transitioning between a HUD (focused at infinity) and the
panel would be just the same as all us VFR types do while keeping up our panel
scan!

Although I doubt it will be a feature of airliner or GA HUDs, military ones
in tactical aircraft are sometimes used to display imagery obtained from low light
or IR sensors.  These images provide a 'daylight' like view of the outside scene
that permit the pilot to use 'visual' tactics.  In this case, the HUD clearly
must be focused at infinity so that the imagery properly melds with any
natural view of the same scene.

Just my $0.02...

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Michael Corvin 	      PP-ASEL, PP-G		zwork@starfighter.den.mmc.com
just another spaced rocket scientist at    Martin Marietta Astronautics Group
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===============    My views, not Martin Marietta's   ========================
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Newsgroups: rec.aviation.misc,rec.aviation