Terrain elevation display

[Driver170 42]

Can anyone explain why the terrain elevation display (green) increases and decreases depending if your climbing or descending? But the red elevation doesn't change.

Why does the lower number change? I thought this corresponds to the terrain elevation

[ALF1 15]

The red is above you.

[Driver170 42]

Thats correct but how come the red terrain elevation doesn't change if you climb or descend unlike the bottom green elevation?

[downscc 1,327]

Perhaps you have TERRAIN PEAKS MODE enabled, see PMDG options Introduction pg 77.

[Driver170 42]

Thanks dan i may have the option on will check tomorrow! Do many airlines have this on and do i need it?

[Lashrathius 49]

I don't fully understand your question so I'll do some guessing / assuming...

 

The green will change based on your altitude when you're equipped with "peaks mode", which is an even more enhanced version of the EGPWS. With just the standard EGPWS, your green will not change like how you described. It will simply disappear once you're above a certain altitude above the clearance heights of the mountains / obstacles. With the peaks mode, there are algorithms to "simulate" the aircraft flying at an altitude, thus allowing the display to show the green dot pattern at altitudes where it would otherwise be blank. 

 

The red is part of the standard EGPWS, so it will not be fluctuating according to the peaks mode algorithms. 

 

You can tell whether or not you have peaks mode if there are numbers under the words "TERR" on the left hand corner of the ND. If it says "TERR" and the bottom says "TFC" for TCAS, then you do not have peaks mode. If it says "TERR 000 000" where the 000s are numbers indicating the lowest and highest terrain clearance height, you have peaks mode. 

 

Hope that helps. That's a nutshell summary of the EGPWS, from my real world job working with professional avionics and more specifically the EGPWS. 

 

Cheers,

[Driver170 42]

Great henry answers my query fully. Good job man

So if the lower number says 007 in green that means i have 700ft clearance?

[scandinavian13 5,192]

So if the lower number says 007 in green that means i have 700ft clearance?

 

Pretty sure the lowest number means the lowest altitude being displayed on the ND. So, 007 is 700 feet above sea level.

 

EDIT:

Yep - crazy what you can find with a quick Google search: https://www51.honeywell.com/aero/common/documents/Peaks-Mode-Paper.pdf

[Driver170 42]

So how come that 007 changed when i climbed or descent? Great link cheers

[scandinavian13 5,192]

So how come that 007 changed when i climbed or descent? Great link cheers

 

The filter changed, and/or the predominant terrain feature changed.

[Driver170 42]

True BUT if i fly level it doesn't change? And i'm flying straight and level so its only picking up whats in front

[scandinavian13 5,192]

True BUT if i fly level it doesn't change? And i'm flying straight and level so its only picking up whats in front

 

Is that a question or a statement?

 

If it's changing, it's changing because the aircraft systems have determined that the filter should change. It partially depends on your relative altitude to the obstacles. If you're within 2000 (vertical) feet of the terrain elevation, the lower band will climb/descend with you. There are pictures in the link I posted earlier that show this.

[Driver170 42]

I was just curious why it changed. But i see why it does now. For example if you climb its changing because you have more seperation between you and the lower band?

[Lashrathius 49]

I was just curious why it changed. But i see why it does now. For example if you climb its changing because you have more seperation between you and the lower band?

 

Yes, and also keep in mind that it shows the highest mountain / obstacle in your ND range. If you are climbing, you are most definitely moving, thus if you move into an area with even higher terrain, the numbers will change to reflect the tallest.

 

Example:

 

If the highest mountain near the airport, for example, is 045, and you move foward and see a mountain that is 059, the red will indicate 059 because it is the highest.

Same concept for the lowest.