Radar altimeter requirement for CAT I, II, III

[Cecco]

I recently was safety pilot on a citationjet (CAT I certified) and it struck me that it had no radar altimeter. I gather that a radar altimeter is therefore only
mandatory for CAT II and CATIII certified aircraft?

Cheers

Cecco

[aterpster]

Cecco:

I recently was safety pilot on a citationjet (CAT I certified) and it struck me that it had no radar altimeter. I gather that a radar altimeter is therefore only mandatory for CAT II and CATIII certified aircraft?

Mostly the case although the RAs on an autoland aircraft are required for even a VFR autoland.

Only on (most) CAT II approaches can the RA be used to determine DA instead of the baro altimeters, because where "RA" is stated on the CAT II chart the elevation of the CAT II DA point has been surveyed.

[safetypee]

Rad Alt would not normally be required for a Cat 1 aircraft, although I would expect one to be fitted for EGPWS/TAWS (an essential safety aid). Or does (your) TAWS use geometric altitude?

The equipment requirements for All Weather Operations (Cat 2-3) are in CS-AWO. (http://www.easa.eu.int/agency-measures/docs/agency-decisions/2003/2003-006-RM/decision_ED_2003_06_RM.pdf). For the US, AC 29 is similar, but not identical.
Operational (national) approvals might vary these requirements, but recent European tightening of the rules should align GA all weather ops with commercial ops; however the US might differ significantly with their FAR 121 / 135 operations.
Also the new generation of technology may be challenging some to the existing rules.
Crosscheck with EU OPS 1, but that might only add to the confusion !

[FE Hoppy]

Roughly for EASA

Cat I - not required
Cat II - One required
Cat III - Two required

[max nightstop]

The RAs are required for a VFR auto land because they tell the autopilot when to flare!

[aterpster]

max:

The RAs are required for a VFR auto land because they tell the autopilot when to flare!

As I stated in Post #2. The autoland system doesn't know about weather. :)

[de facto]

I remember reading that RA are also used by the TCAs system....

[Check Airman]

Correct. TCAS uses the RA input to inhibit RA's below a certain height.

[max nightstop]

Aterpster,

My reply has nothing to do with weather. My point is that without RA the autopilot doesn't know where the ground is! I was simply expanding on your point that RSS are required for VFR autoland by saying why they were required.