Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

TCAS - Absolute Altitude

Tech Log The very best in practical technical discussion on the web

TCAS - Absolute Altitude

Old 8th Sep 2015, 06:04
  #1 (permalink)  
Thread Starter
Join Date: Apr 2005
Location: Abroad
Posts: 7
Likes: 0
Received 0 Likes on 0 Posts
TCAS - Absolute Altitude

My aircraft has a Rockwell Collins TCAS II 4000 controlled by an RTU 4210. On the RTU there are two altitude choices of "REL" and "ABS".

I cannot find anywhere a reference as to what these two settings mean.

I am happy that REL is intruder altitude relative to my altitude and ABS shows intruder pressure altitude however I need a reference.
Private Parts is offline  
Old 8th Sep 2015, 07:04
  #2 (permalink)  
Join Date: Jan 2009
Location: Cab of a Freight Train
Posts: 1,214
Received 117 Likes on 61 Posts
This Pilots Guide to a Bendix King TCAS II system, page 38, describes the difference. Generally speaking, the system defaults to Relative Altitude, but if you press Absolute, it'll show you the threat's actual altitude for X seconds, before reverting to Relative Altitude, which is what you really want, afterall.
KRviator is offline  
Old 9th Sep 2015, 14:49
  #3 (permalink)  
Join Date: Oct 2007
Location: UK
Posts: 27
Likes: 0
Received 0 Likes on 0 Posts
See FAA Advisory Circular 20-151a, "Airworthiness Approval of TCAS II, Versions 7.0 & 7.1 and Associated Mode S Transponders"


2-7. Pilot Control. A pilot control for the TCAS equipment shall be provided...
c. The following optional controls may be provided:...
(4) Selection of actual flight level (FL) or relative altitude of traffic.

fellman is offline  
Old 13th Apr 2016, 20:56
  #4 (permalink)  
Join Date: Apr 2016
Location: Maryland
Posts: 1
Likes: 0
Received 0 Likes on 0 Posts
TCAS Absolute Altitude is Absolute Pressure Altitude

Absolute Pressure Altitude is confusing because TCAS manual writers drop the pressure, and land short with Absolute Altitude.

This is confusing because Absolute Altitude means height above the ground to the FAA.

A TCAS standards document, called DO-185B says basically, Altitude is... absolute pressure altitude... 29.92 in. hg.

Any time TCAS panels or TCAS manuals say "absolute," add pressure. Absolute pressure altitude.
matt.wing is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.