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I notice the atitude indicator displayed in the PFD is either squared-shape or truncated-circle-shaped.

A good example of the square shape is the B747-400, It looks like a square whose corner are a bit rounded:

enter image description here
(Source: airliners.net)

A good example of the circle-shaped is the SAAB2000 or the A340:

enter image description here
(Sources: airliners.net and airliners.net)

They look like a circle whose both right and left sides are truncated to display rules (speed and altitude).

In either case, the information displayed are the same (pitch, bank angle, perhaps flight director,...) and are displayed in a very similar manner (including the colors). Those 2 designs exists for years, so there may not be advantages of one over the other. Why one is preferred over the other for some manufacturer, and the other one for other manufacturer? What criteria are taken into account when choosing this shape?

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  • $\begingroup$ One could imagine the 747's indicator as a larger round display with both the vertical truncation for the speed/altitude tapes, and horizontal truncation because that's all that would fit on the screen. (Though the screen in this case is tall enough for a rounded top.) $\endgroup$ – FreeMan Nov 24 '15 at 13:46
  • $\begingroup$ On many aircraft, the same model can have both according to customer choice! A single PFD computer can generate multiple different displays. It an even be changed after delivery if the customer wants a different display. Why choose one? Commonality with other models in the fleet? Graduation path for FOs to reduce difference training? Customer preference? Legacy? In other words, you can find two 744s with different operators with the same hardware but different displays. $\endgroup$ – Simon Nov 24 '15 at 14:00
  • $\begingroup$ Links are dead now, thus question becomes worthless for future readers. Could you provide updated ones ? Can't find them on airliners. $\endgroup$ – kebs Aug 24 '18 at 14:35
  • $\begingroup$ Just adding that the latest fashion is to use full screen for attitude indicating (sometimes even with terrain model) and overlay the rest of the primary information on top (search for 787 PFD or Garmin EFIS / Garmin Synthetic Vision). $\endgroup$ – Cpt Reynolds Feb 10 at 20:02
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    $\begingroup$ Notice that the circular one follows the inside of the bank angle arc, which means the bank ticks have a higher contrast (against black vs sky) and thus easier to read. $\endgroup$ – StephenS Feb 10 at 23:02
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A circle makes sense because a physical attitude indicator is round. (A physical one has to be round because it consists of a ball moving behind a window.) It makes the screen display look more like the instrument it replaces, which is good for recognition and familiarity, and makes it easier to judge things like bank angles.

A rectangle makes sense because it has to fit on a rectangular screen. You can tile rectangular instruments/displays on a screen efficiently, with no wasted space between them, as your example shows with the tapes. Rounding the corners delineates the edges of the display better than just having abutting rectangles.

The full process by which physical cockpit instruments and glass-cockpit displays are designed and laid out is too long to explain here. The main focus is on making it hard to make mistakes, and after that to make it quick to see the most important information. To support this, each instrument has to be easy to identify and to read correctly. Despite the shape difference, the two designs look very similar. Even if you've never flown any of these aircraft before, you'd immediately recognise it as an attitude indicator, and you wouldn't confuse it with a different instrument.

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Development history, type commonality, and usability engineering.

Let's start with the A340 you have. For commonality it borrows from the A320, which borrows from the A310, which borrows from the A300.

The A300 was a complete circle. When Airbus developed the forward-facing crew cockpit (FFCC) – 2 crew cockpit – an avionics revamp was done by Thomson-CSF (later became Thales Group). The speed-deviation indicator (F and S) was replaced with an airspeed indicator, and to make space for it they cut the sides of the circle. Similar development can be seen for the 737 Classic:

enter image description here
(b737.org.uk)

Quotations and references for the above can be found here: Why does Airbus not display the exact airspeed on the PFD?

For the Boeing 747-400, it was a clean slate design, and they made the best out of the initial big screen – the A300's initial EADI (not a full-fledged PFD) was a small screen. The 747 style carried over to the 777.

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