View Full Version : TCAS - Absolute Altitude

Private Parts
8th Sep 2015, 07:04
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.

8th Sep 2015, 08:04
This (http://www.bendixking.com/HWL/media/Pilot-Guides/006-08499-0000_7.pdf) 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.

9th Sep 2015, 15:49
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.

13th Apr 2016, 21:56
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.