Commercial HUD question

[Mark_Hazeldine]

Hi, can some of you airline pilots who use HUDs un-confuse me please!

In military jets, the pilot is seated centrally and the HUD behaves quite like a normal PFD, yes?

Well i was thinking, in an airliner the captain is sat to the left of the axis of rotation (bank). So when you bank left, the portion of horizon viewable through the HUD surely rises relative to the HUD itself. Does the HUD's artifical horizon line rotate about the centre of the HUD (and thus come out of alignment with the real horizon) or does it rise with the real horizon? Does this then give a false pitch indication?

I'm curious to know how this works!

Thanks, Mark

[ft]

The neat thing about a HUD is that the image in it is effectively overlaid on the horizon. You have a virtual image at an infinite (for all practical purposes, a given distance in reality) distance. This means that as you move your head around, the image will stay fixed relative to the surrounding scenery. This is the feature which enables the flight path vector to actually show where the aircraft is headed, and also enables military HUDs to keep other aircraft boxed regardless of how the pilot positions his head.

If you think about it, banking does not change the viewline straight ahead for either pilot. Even if it would, it wouldn’t be a problem for the reasons given above. The marginal vertical displacement in a bank is not really a factor at all.

Regards,
Fred

[Mark_Hazeldine]

Not sure i really understand why this doesn't happen in real planes. Here are two images from MS flightsim that illustrate the phenomenon i'm talking about. The first one shows acentrally mounted HUD ala fighter jet style.
The second shows one slightly offset to the left like in an airliner.
The second one would give the pilot a false sense of where the real horizon is in IMC and would confuse a pilot in clear weather who'd be looking at 2 different horizons.

http://img.photobucket.com/albums/v505/Mark_Hazeldine/hud1.jpg

http://img.photobucket.com/albums/v505/Mark_Hazeldine/hud2.jpg

[ICT_SLB]

Mark,
The lower view is just wrong - the Horizon line is conformal with the actual horizon unless you're up high (say > FL250) where you can see the line slightly above but parallel to the actual visible horizon. At a low level such as shown, it definitely would be conformal.

The only effect you might see with a commercial HUD is that the centerline ("Boresight") of the display is slightly offset to the left by the same amount that actual HUD is physically displaced. This can be a bit distracting when taxying but doesn't appear to be of any consequence when landing especially in CAT III conditions.

[Mark_Hazeldine]

I guess it has something to do with the fact that the horizon is physically a very long distance away compare to the HUD and a slight displacement doesn't change much. But surely the HUD itself phyiscally rises and sinks as the plane banks? Does this not give some error?

As the horizon sinks in the HUD (like in the 2nd picture), does the artificial horizon sink AND also the "bird" that shows your pitch attitude? Or does it appear still very much like the 1st picture in which real and artificial horizons seem to rotate around the central point of the HUD (providing you're at 0 degrees pitch)?

[ft]

The symbology on the HUD is not fixed in place relative to the HUD. It follows your head around.

If you raise your seat so that the real horizon is seen through the upper part of the hud reflector, this is where you will see the virtual horizon. Lower your seat so that the real horizon is seen through the lower part of the HUD reflector, and the position relative to the HUD reflector glass where you see the virtual horizon will move down with you. No matter where you place your head, the virtual horizon will remain superimposed on the real one*.

Perhaps it is easier to picture if you imagine a HUD with a simple gunsight, at all times indicating the direction in which the nose of the aircraft is pointed. With a fixed physical sight, the indication would only be true when you had your eyes aligned with the sight. For this purpose, iron sights consist of a ring and a bead. When they line up in your view, you have your eyes in the right position for the ring to tell you where the nose is pointing.

With a sight on a HUD, the image of the gunsight on the HUD will always be in the direction of the nose of the aircraft, as measured from your eyes. If you move your head to the side enough, the gunsight will slide off the edge of the HUD. Other than that, you can keep your head wherever you want and you will still get a correct sight reference.

Think of it as drawing a giant image on the outside world, at an infinite distance. This is effectively what a HUD does, although the infinite distance is achieved through optics rather than a very long brush. :)

Is that scary example of how a HUD does not work an actual screenshot from FS or is it a Photoshop job?

Cheers,
Fred

*) In practise, what you see can be thought of as an edge-on view of a plane through your current position and normal to a line from your current position to the center of the earth. This means that at higher altitudes, the virtual horizon will be above the actual horizon due to the curvature of the earth, as described by ICT_SLB.

[Groundloop]

"Well i was thinking, in an airliner the captain is sat to the left of the axis of rotation (bank). "

But the captain is only 1 to 1.5 metres left of the axis of rotation and the horizon is hundreds or even thousands of metres away, so the very small offset would have a neglible effect on what can be actually seen.

Also the lower Sim view is a totally false representation of what it is actually like as the HUD is still in front of the pilot, therefore what the captain will see is the top view.

[ICT_SLB]

As I said in my previous post - I agree with Groundloop - the lower one is just wrong. Commercial (Holographic mirror) HUDs look like the top one.

One thing you can't appreciate is that the actual point where you can see the display is relatively small and known as the "eyebox" - about 3 inches high by only about 6 inches wide. You have to get into the correct Eye Reference Point for the cockpit (often shown by sight balls that you look at out of the corner of your eye not right at) for the illusion to work. There is not the extreme angle of view as on a gunsight. This ERP is very important for CAT III as a small difference in ERP height makes a big difference in Runway Visual Range. (Sorry I don't have the books at home to prove this).

I don't know if you've looked but try searching on Rockwell-Collins & Flight Dynamics - they should have a lot of pictures of actual HUD displays.