## Wednesday, June 01, 2011

### Small Green Dot

When I was shown this, and had its function described to me, I thought it was an imperfect design, but usable and logical, but that's because I hadn't been listening closely enough to the explanation. I showed it to a computer programmer and he managed too backfit a sort of logic to it, but I don't think he could have done so had I not told him the rules. So here's your turn.

The question is, given that the display above asks the pilot to make control inputs that will centre the green dot exactly on the crosshairs--I'll give you that much--what control inputs does it ask for as depicted? Give your answer in direction and relative magnitude of pitch, roll, yaw and/or speed. If you're not sure what those terms mean (and those who are not probably have a better chance of getting it right), just describe what you think it might be asking the pilot to do, in whatever terms make sense to you. As usual I ask any of you familiar with this particular instrument of guidance to recuse yourselves, or at least hide your knowledge in polite snickering.

I'll give you one more piece of information, although it would be incorrect to call it a 'hint,' and that is that sometimes the dot turns red.

en français:
le point - dot
la réticule - crosshairs
le lacet - yaw
le tangage - pitch
le roulis - roll

Pour mettre le point vert sur le centre de la réticule on doit ajuster le lacet et le tangage, ou peut être le roulis et la vitesse.

Lots of left yaw, a little bit of nose-up pitch. In other words, the crosshairs (presumably fixed to the instrument panel) represent a "window" onto some outside world, and the pilot is required to "move the airplane" to point to the thing that she "sees" out the "window". And since it's a dot, it can't be commanding roll because ... well ... that would be a *line*!

Now tell me how I'm wrong....

A Squared said...

Well, one possibility is that it is commanding you to pitch up slightly, roll left, hold that bank bank angle until dot centers, then roll wings level.

OTOH, if this system is analogous the the flight director in my airplane, which uses a "double cue" presentation instead of the more common single cue "bat wings" it is commanding that you pitch up slightly, roll left until the dot centers, then hold that bank angle until the dot starts to drift right cuing you to level your wings.

It could also be indicating he exact opposite of the first.

Could be any one, or some other I haven't though of.

Sarah said...

Recused. Doubly so now I've read A^2's post.

Mike Kear said...

It all depends whether you're flying the dot or the crosshairs.

If you're flying the dot (i.e. the image represents a pilots artificial horizon) then you want to go down slightly and to the right.

If you're flying the crosshairs (i.e. the image represents a gunsight and the dot a target) then you want to go up slightly and to the left.

We need to know more before we can definitely say what's required.

(By the way, does anyone know how come this blog now prevents posting commments unless you sight up for a blog? I already have plenty of work to do and i dont want the commitment of a blog as well.)

A Squared said...

By the way, does anyone know how come this blog now prevents posting commments unless you sight up for a blog?

I haven't signed up for a blog. Not sure what you're talking about.

A Squared said...

Of course my assumption is that the display is aircraft navigation. It could also be a planview representation of actual vertical sensor axis vs desired sensor axis for equipment installed in the aircraft. IN which case it might be a little pitch up and some roll to the right.

Or pitch down and roll to the left.

Depends on which indicates "target" and "actual", the crosshair or the dot.

Anonymous said...

This represents the centre of gravity of the plane. There's an overweight passenger in 1A, and not enough heavy passengers on the right side or aft of centre. So the pilot needs to use the intercom button to call the Service Director to as them to politely rearrange the salary-payers. :)

JeppsOnFire said...

Warning - major guess: roll coordinated about 50 degrees left and pitch up about 10 degrees.

Eh?

Frank Lee MeiDere said...

You need Windex. That will get rid of the green insect blotch on your window.

Christopher said...

A boot full of left rudder and a slight pitch up.

Aviatrix said...

Awesome, the creative people have started in on the problem.

Louise said...

You need to put more weight on your right foot. The forward/backward doesn't matter. However, no matter how accurate you are the WiiFit wil still tell you that you aren't quite standing correctly (I've had 49.8% vs 50.2% and it still tells me I'm leaning slightly to one side).

Ward said...

Raise the nose and turn to the left. I've only ever done a 2D version - steering a boat to a compass course: the compass card is (more or less) fixed, you have to turn the lubber lines around it.

Anonymous said...

Does the phase of flight also have an impact? At FL390 in a "bus", the rudder pedals are a very effective footrest. The natural "bus" input would be sidestick left a bit and back a little bit until the green spot starts coming o to the crosshair - then let go. I think! Though the actual amount of stick movement is such that just thinking about the input will have the spot moving.

At DH on the approach I would be looking out the window so the little green dot can do as it pleases.

jwenting said...

as an IT person, I have a question here:

are you supposed to steer the dot onto the cross, or the cross onto the dot.
The inputs of course would be diametrically opposite depending on which is expected, yet both are valid potential scenarios.

Ed said...

It wouldn't surprise me too much if you have to fly the dot to the cross-hairs on one axis but vice-versa on the other. That's the sort of "logic" that takes some explanation.

See Wikipedia on Attitude Indicators contrasting Western and Russian-built designs. I'm still muddled about what I'd find "intuitive" having done the four hours "instrument appreciation" required for my UK PPL more than 20 years ago then hardly flown anything with an AI since.

Arthur said...

It looks like a regular house window. The plane is flying from lef to right (showing its green light on the right wing). In order to center the green dot in the window, all the pilot got to do is slow down and loose some altitude.

Just my 2 cents...

John Lennerton said...

No one has addressed the given that the dot may turn red; a speed indicator? Or a mosquito that has already fed?

cockney steve said...

Duh, well I thought it was a plan-view IE-seen from above...so, without any indication as to which way was "front" for the dot, there's no directional information.

Did the question presuppose a working knowledge of the system in question?

I really hate ambiguity in important technical information. (though I must admit that I was quite clear the dot moves with reference to the graticule

Off-topic, i puzzled for years," tyres should be fitted with the red balance-dot OPPOSITE the valve"

-Is that Adjacent to, or diametrically opposite?

Am I just obtuse?

Chris Thompson said...

I think C9 called me fat!

I thought I had this all figured out until I read the other posts. Now I have no idea. It was a nice refresher about how important it is sometimes to jettison what I think I know and be open to what I probably don't know.

Thanks Aviatrix for a fun exercise.

Sarah said...

Awesome, the creative people have started in on the problem.

Ooooh. I am cut, I tell you, cut to the quick!

Aluwings said...

re: The dot sometimes turns red ...

It's like a game of Pacman. While the dot is green you move the controls as previously described. When it turns red, it means that the dot is now coming after you and you must invoke evasive maneuvers. A lot of fun and gives the passengers one heck of a ride!

D.B. said...

It's a traffic conflict alert display. The green dot is Aviatrix's hair coming towards us at Mach 1. Should we dive right and down? Are we trying to hit her? Then left and up.

coreydotcom said...

I decided to skip everyone's comments so I wouldn't be biased. 0 TT in at the commands of an airplane (though I do remember in the early 90's being invited to the cockpit and I think I played with the AP because the captain told me to turn a knob and when I did the whole plane rolled and I was so proud).

Here goes... either:
1) pitch down and roll right until centered.

2) reduce power and roll right until centered.

my guess

DataPilot said...

Ooh, this problem brings out the theorist in me. Fun stuff.

Years ago, when I was a computer science and math major in college, my mother made the mistake of declaring that, "1+1=2." I, her smarty-pants daughter, retorted with, "Not necessarily. It does if you assume that you're doing base-ten integer arithmetic, but not if you assume base-two, or that '+' means the concatenation operator." Mom shot me a dagger-like look to indicate that she was regretting sending me to college.

This window-and-dot problem is similar. There are many possible answers -- with the correctness of your response depending on the base assumptions that you made. Your mind may "see" an artificial horizon in a civilian aircraft, or a bulls-eye in a military fighter, or a bug squished on your windshield, or an airplane flying past your kitchen window. What is actually illustrated is a 2-D doubly bifurcated square with a little green dot. The controls that the pilot should use is determined by your perception of the drawing.

I vote for the fighter pilot point of view. Center the little dot til it turns red and push the fire button!

JetAviator7 said...

Assuming the cross hairs represent some type of flight director (based upon your hints) you would need to have more information to give a precise answer.

However, we can be sure that pitch, yaw and airspeed have something to do with the solution of the problem. We just don't have the context to tell.

I think I saw something like this in my eppesen GFD Instrument/Commercial Textbook JS314520 when I was working on my instrument rating.

coreydotcom said...

almost forgot... "peut-être" not "peut être". Seems like a small difference but its the difference between "maybe" and "could be". but otherwise, nice sentence!

There should be a knob in the middle of two adjacent sides; turning the knobs tilts the table through an arrangement of cords and pulleys. Shown is the pea or ball bearing; not shown are the pits, traps, or obstacles over which one must maneuver the pea in order to win.

ElEmEnOhPee said...

Raise seat one notch; slide forward three notches to obtain proper eyeball position for this aircraft. Green dot turns red if you are seated so far out of range as to cause the landing gear to drag through the approach lights while flying on VASIS approach.

zb said...

Have fun playing PACMAN.

Paul B said...

How about "pull down on the joystick quickly, before your model aircraft, which is currently flying pretty much straight at you, smashes into your nice window, and then into you"?

:-)

Paul B said...

Or perhaps "hard right & down.... it's a sidewinder coming straight at you!".

Hey, could it be a TCAS sort of thing: that's another aircraft coming at you, so you need to just go to the right to avoid it? But surely it would then be red, not green!

In which case, what's the etiquette for overtaking a slower aircraft in Canada? Right, left, above or below? :-)

Sarah said...

I think if the dot turns red you have other problems, but I thought of your job instantly when I saw this picture.

Anyway, it made me laugh. For sure, my job sometimes feels just like chasing a colored dot.