ACAS: Absolute or Relative
 

Questions:

Should the ACAS display be set to indicate the absolute or relative altitude readout of other aircraft?

What are the pros and cons of each?

I'll stick with relative...
 

...because then I don't have to do any maths.

PM

Is there a form of ACAS where you actually have a choice? The only type I've ever had, TCAS, has TA only, TA/RA and Off as the only avaiable settings, traffic then shown in relative terms.

Some units have the "Look Up/Down" function which shows traffic aoutside the normal window, and it was a feature I liked when i had it.

Some units have a FL switch which shows temporarily the absolute flightlevel of any traffic on display and some very rare units actually have the choice to permanently switch between absolute and relative. However as far as i know the default setting is relative on nearly all boxes.

For the use as an anti collision aid i would think relative is the most usefull setting, at least it is for me.

Most units that have abs will revert to rel after 15 seconds. On the e-jets this is true and also if you are in abs and you have a TA or RA it reverts to rel automatically. I guess that means rel is the mode of choice!