Radio / radar altimeter applications on turbo props

[Bernoulli]

I know the main reason for a radio altimeter is for doing Cat II and Cat III aproaches. My question is: why do most twin turbo props that are unable to do a Cat II or Cat III aproach have a radio / radar altimeter? Is it just for a back up for putting in the wrong altimeter setting into the kollsman window... or maybe it's a required instrument for GPWS??? If it is not what I listed above how would you use the radio altimeter on a light twin turbo prop? thanks in advance.

 

[ImbracableCrunk]

I don't know that they can't do Cat II and III, but many aren't equipped or trained. EGPWS still uses RADALT, so it's gotta be on the plane, anyway.

I think many planes display it, but don't really use it.

 

[Flybywire44]

Maybe Insurance, who knows. The ATR at ASA was Cat II before the CRJs.

 

[jetpig32]

After being furloughed from an airline that does CAT III C's, and now flying a Cessna 421 with a RA, I like having one more backup to telling me where the ground is. It is incorporated into my scan. Gives you a good idea if the ground is a little too close.

 

[AverageWhiteGuy]

No Radar Alt. = No Egpws

 

[crj567]

Some planes use it as a safety device to arm systems like reversers, anti-skid, etc...

-Also, a lot of aural warnings are delayed or inhibited below a certain altitude, to prevent distractions.

 

[Kream926]

pt 25 certification maybe?

 

[Bernoulli]

So if you were flying a non cat II / III airplane that had a radio altimiter, and you were flying an approach, would you dial in an altitude like MDA or just keep it on zero and keep it in your scan?

 

[Jar Jar]

So if you were flying a non cat II / III airplane that had a radio altimiter, and you were flying an approach, would you dial in an altitude like MDA or just keep it on zero and keep it in your scan?

PDTs Dash's have radio altimiters, and all the required equipment for CAT II approaches - but the pilots are not trained to do CAT II. We leave the DH/MDA set to zero and the RA is really just additional information.

 

[bigboeings]

CAT II can be done in a turboprop, no problem. Just have to get authorized for it.

 

[Sig]

PDTs Dash's have radio altimiters, and all the required equipment for CAT II approaches - but the pilots are not trained to do CAT II. We leave the DH/MDA set to zero and the RA is really just additional information.

Check your MEL on the RadAlt, and see what happens if it breaks. You lose a lot of nifty whiz-bang stuff, and the cascading MELs get really, really interesting.

First time I ever accepted a plane with that gone had me on the phone to MX with a "WTH?" It's one of those cases where you end up with technically two incompatible MELs... unless you are breaking the RA. The MX procedure is a real eye-opener.

 

[kf4amu]

PDTs Dash's have radio altimiters, and all the required equipment for CAT II approaches - but the pilots are not trained to do CAT II. We leave the DH/MDA set to zero and the RA is really just additional information.

The Beta Lockout system also uses the RA for a 50 foot data point.

 

[Sig]

The Beta Lockout system also uses the RA for a 50 foot data point.

... and to break it per the RA MEL, you have to disable the Beta Lockout Horn. Per the compatibility table, that's like seeing a broken ECU and nosewheel steering (yay!). Not the same in practice, for sure- but dang! Which way is up around here? Are they or are they NOT compatible?

MX Control and I laughed for a few minutes about it. Really weird. Look at the procedure. Imagine an outstation writeup on that one... an hour, maybe longer to accomplish once they show up?

 

[Bernoulli]

So my understanding is, if you are not applying the instrument for a cat II or III you should simply leave it at zero and keep it in the scan for situational awareness. Anyone else have any other cool applications they use it for?

 

[pair_of_pratts]

Anyone else have any other cool applications they use it for?

Calibrating the TCAS and GPS when passing 1000 feet above someone else. Nothing like hearing "RADAR ALTITUDE" or "1000" while cruising along at FL390.