Inbound
Topic Author
Posts: 614
Joined: Sat Sep 15, 2001 7:59 am

Attitude Indicator Pointer

Sun Nov 16, 2003 6:33 am

why is it in smaller airplanes with mechanical Attitude Director Indicators, for example in light single and twin prop planes that the Roll pointer goes in the direction of the turn, while on Electronic Attitude Director Indicators, the pointer goes to the opposite side?

What I mean is, in a cessna 172 for instance, if you turn left or right, the pointer goes to the respected side, but in Dash8-300Q or 777 etc. , the pointer goes left when the plane turns right and vice versa.

any particular reason for this difference?
Maintain own separation with terrain!
 
shaun3000
Posts: 439
Joined: Thu Mar 14, 2002 4:10 pm

RE: Attitude Indicator Pointer

Sun Nov 16, 2003 1:06 pm

can you post a picture to demonstrate this? I'm fairly certain the attitude indicators turn the same way in every aircraft...
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Attitude Indicator Pointer

Mon Nov 17, 2003 12:12 am

...except a few that are on British registered aircraft.
These use 'ground pointers' rather than 'sky pointers' to indicate bank angle.
 
rthrbeflying86
Posts: 241
Joined: Sun Oct 12, 2003 12:31 am

RE: Attitude Indicator Pointer

Mon Nov 17, 2003 12:54 am

I think it has to do with what is actually turning. On a Cessna, which is what I usually fly, a miniature airplane turns to the right or the left along with the actual airplane, and the artificial horizon is fixed. From my limited experience with 777's (cough...flightsim....cough) I think the horizon does the turning and the plane stays fixed. I'm not sure though, so don't come flaming if I'm wrong, just correct me.
I'd rather be flying.
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Attitude Indicator Pointer

Mon Nov 17, 2003 7:51 am

Inbound,
you're probably looking at the turn rate indicator in the small aircraft and confusing it for an attitude indicator? Both have (at times) small aircraft symbols moving around, but they are different animals.

Could someone with more time dig up a few pics of turn rate indicators?

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
pilottim747
Posts: 1577
Joined: Tue Jul 17, 2001 2:34 pm

RE: Attitude Indicator Pointer

Mon Nov 17, 2003 12:28 pm

Rthrbeflying86 has got it right.

Large planes with LCD screens will normally have both types. The backup attitude indicator will have a fixed miniture airplane while the LCDs will have a fixed artificial horizon.

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Photo © Basic T



pilottim747
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FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Attitude Indicator Pointer

Tue Nov 18, 2003 12:10 am

Pilottim,
I'm afraid not. Both on the EFIS attitude displays (PFDs) and on the mechanical backup horizon gyro, the horizon tilts/rises/sinks while the aircraft symbol remains fixed to the aircraft.

The a.net search function appears to be down due to server workload, but I scrounged up a picture of a turn coordinator as found in many GA aircraft. This will indeed bank in the direction of the turn, but indicates turn rate rather than attitude. In other words, it says little to nothing about the bank attitude of the aircraft and nothing about the pitch attitude. I also found a site with a few (simulator) shots of EFIS PFDs.

On a sidenote, I've seen russian AIs which worked differently. Except for the colouring the upper half of the ball brown and the lower half blue, there are those where the pitch indication is a blue/brown ribbon display (actually a wheel segment rather than a ribbon, but you get the picture) and the bank indicator a needle (vertical when level) on top of this. Furthermore, I've seen HUDs where the bank angle of the aircraft was indicated through the bank angle of an aircraft symbol, relative to the aircraft frame of reference. Very confusing to those of us used to "normal" artificial horizons!

Cheers,
Fred

[Edited 2003-11-17 16:12:56]
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Inbound
Topic Author
Posts: 614
Joined: Sat Sep 15, 2001 7:59 am

RE: Attitude Indicator Pointer

Tue Nov 18, 2003 12:42 pm

I searched and searched and only came up with the following pics as suitable support for my question...

here are a couple pictures of mechanical attitude indicators in a Piper Warrior and Twin Commanche respectively.

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Photo © Florian Sindermann
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Photo © Nils Alegren


Both planes are turning to the left where you can see the little plane on the attitude indicator with the wings tilted to the left, and the pointer showing a bank angle of 10 degrees in the comanche and 20 degrees in the warrior.
The pointer (orange in comanche, clear in warrior) points to the same side as the turn and gives the angle for that respective side.


Now here are a couple EFIS pics.

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Click here for bigger photo!

Photo © Nicholas Osborne
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Photo © Felix Harte


Here you'll see both planes turning right, but the pointer goes to the left...meaning...in the 737, the plane is going right, but the pointer points to approximately 25 degrees on the left side.
and likewise, the Cheyenne is turning right, but the pointer is showing approimately 17 degrees on the left side.

In other words, with an EFIS, you more or less have to read the angle of turn on the opposite side of the turn.
I hope you guys understand my question now because I think I just fried my brain trying to explain it...  Nuts
I'm going to have a drink now.

Maintain own separation with terrain!
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Attitude Indicator Pointer

Tue Nov 18, 2003 5:05 pm

why is it in smaller airplanes with mechanical Attitude Director Indicators, for example in light single and twin prop planes that the Roll pointer goes in the direction of the turn, while on Electronic Attitude Director Indicators, the pointer goes to the opposite side?

The difference is not "small plane" vs. "large plane." The difference is mechanical vs. electronic display capabilities.

In a mechanical ADI, the Angle-of-Bank [AOB] pointer is oriented (fixed) relative to the display's glass panel (i.e. the cockpit). IOW, the pointer always points to the top of the cockpit/plane and the horizon moves behind it.
The physical limitation of the mechanical ADI is that nothing can be placed on the horizon "gyro" since it will move out of sight with any up/down pitch.

An electronic display has no moving parts, therefore no physical limitation on how information is displayed. These are just computer generated images and how you program the computer controls how information is displayed. IOW, an AOB oriented to the real world horizon will be in view at all times.

So why the change you ask? With physical limitations removed, instrument designers are free to apply more human-factors engineering to how an instrument displays information. IOW, what's the best way to display information so the pilot sees and knows what he is seeing almost instinctively.

In electronic ADI's the AOB pointer is always pointing toward the real world "up" [pilots already know where the top of their cockpit is]. For most normal flight operations that makes little difference [heck, most planes I fly have both and I never noticed the difference until this question was asked and I did a little research]. However; during extreme unusual attitudes where the horizon line may be out of view, the pilot is seeking exactly that information... where is UP in the real world. Therefore, the better human-factors display is one where the ADI shows the pilot where real-world "up" is at all times and that is the display where AOB is fixed to the horizon [moves with the horizon] which is only possible with electronic instumentation.

Instrument designers have gone one step further by programing the electronic ADI to enlarge/enhance the AOB pointer into what is known as a "sky pointer." Usually large, sometimes flashing arrows pointing "up." AA's HUD adds even more symbology all showing where the horizon is (or "where is up").

Hope this helps.
*NO CARRIER* -- A Naval Aviator's worst nightmare!

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