Just curious: color of the heading bug?

[Will]

This is not a PSX question; this is a Boeing question.

Boeing's philosophy seems to be to use magenta to indicate things being commanded.

I've always thought that the heading bug should be magenta in HDG SEL or HDG HOLD, but it should be green in LNAV, because in that mode, the bug it's more of an advisory than a command. I know we all obsessively match the heading bug to the heading being flown in LNAV, but if we didn't, then you'd see the magenta heading bug referring to a heading that the aircraft isn't being commanded to fly. Thus the magenta color doesn't make sense to me.

Any thoughts about this?

[Hardy Heinlin]

Interesting idea.

The same should then be applied to the altitude bug perhaps? Make it green when ALT HOLD is engaged?


|-|ardy

[Jeroen Hoppenbrouwers]

ALT HOLD still is a pilot command; VNAV ALT would be a better example, I think. Although the magenta altitude bug obviously indicates the clearance limit, and two bugs are confusing.

Green only should be present when the bug represents actual target data. So if LNAV would itself use the bug to indicate where it wants the nose to point, and move it around by itself, green would be appropriate. Handy also because a push on HDG then directly has the current heading and you can twist it to where you want to go without accidentally turning the wrong way. I think Fokker planes have that logic, don't know about Airbus or Embraer.

If LNAV would not move the bug, it probably would be better to make it disappear altogether. And only reappear, in magenta, when you twist the knob and it would appear exactly where LNAV left it. Also when pushing HDG HOLD. Appear right there, stay there.

[Hardy Heinlin]

I mean when ALT HOLD is holding 8000 and the bug is at 6000, the commanded altitude is 8000 rather than 6000.

Similarly, when HDG HOLD is holding 080 and the bug is at 060, the commanded heading is 080 rather than 060.

[Jeroen Hoppenbrouwers]

Ah that way. Yes.
Apparently the thinking here is that the bug is always magenta no matter what it is doing, which for visual reference likely is a good thing. Two bugs would be worse. And in the end, we come back to the old rule: the flight mode annunciator holds the truth. Everything else (LEDs, bugs, colours, ...) is secondary.

[Hardy Heinlin]

The 744 actually has one green/magenta change – on the thrust command.

When VNAV controls the thrust, the EICAS thrust reference line is magenta instead of green. But, as we know, there are further exceptions within the VNAV mode: When the A/T is disconnected in cruise, the line is green. Or in descent the line is green as well, except when anti-ice is activated; then it turns magenta and the line moves down to high idle. So, in that area the designers decided to implement a color change – for some reason.

[IefCooreman]

Boeing's philosophy seems to be to use magenta to indicate things being commanded.

The Boeing wording is: "Magenta is used for target values".
 
"Commanded" is related to APFD, that is ie FMA.

The heading bug is the target of the heading selector on the MCP. The target of the FMC is the track depicted on the MAP. The target of the altitude is connected to the MCP altitude window. On the VSI, the target is connected to the MCP V/S window.

To reach a target, it is up to you as a pilot, to make the good "commands" ;-). Using a certain V/S target doesn't mean you are going to reach your altitude target. The depicted targets are correct, you are not commanding correctly.