Radio / radar altimeter applications on turbo props

Bernoulli

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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

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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

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

jetpig32

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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.
 

crj567

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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

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pt 25 certification maybe?
 

Bernoulli

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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

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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

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

Sig

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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

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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

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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

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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

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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.
 

flyboyike

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I know the main reason for a radio altimeter is for doing Cat II and Cat III aproaches.
So, why did the Connies, Electras and 707s have them long before there was such a thing as a Cat II or III?
 

Morettis

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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?
You put your MDA or DH in the radar altimeter for the approach because that light turning on, or the computer saying minimums is one more thing to help you when you're actually flying to minimums. Its easy to get get busy chasing the needle on a gusty day or looking outside and miss your DH, especially single pilot.
 

Andy Neill

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I know of two stories that illustrate how using the radar altimeter info (or not) saved the day (or doomed the crew).

One was a regional descending to what they thought was glideslope intercept altitude. Beofre getting to it, they got a terrain altert (based on radalt). They took it around and tried again with the same results. They decided to go back to the point of origin when they discovered they were an inch off in the Kollsman.

Then there was the 747 freight outfit that misunderstood a clearance of "2400" to mean "to 400". The altitude alert kept going off as they were running the checklists. The (almost) final words were, "Why is this thing reading under 100?"
 

Bernoulli

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You put your MDA or DH in the radar altimeter for the approach because that light turning on, or the computer saying minimums is one more thing to help you when you're actually flying to minimums. Its easy to get get busy chasing the needle on a gusty day or looking outside and miss your DH, especially single pilot.
But prior to the runway, the ground is not flat. There maybe a hill prior to the runway that is higher than the runway, the runway may be on a plateau, or the ground that seems flat prior to the runway has a bunch of little ol hills... would not all the above give you what appears to be false information? Is it really safe to use the RA for Cat I precision and non precision aproaches?
 

Tristar

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You have to have some situational awareness - am is the terrain hilly, is the airport in a valley or on a mesa, ect. Being aware of what is around you is the first key. That said, if you are anywhere beyond about 3 miles or so from the threshold, and the RA is under 1000 feet, you should be asking some serious questions - as you are getting very close to terrain for your phase of flight. Certainly there are exceptions, a hill on final, ect.

The bottom line goes back to SA - you have to have it or you're likely to get killed regardless of how the aircraft is equipped (another argument for real experience in the cockpit, but that's another story). The Radar Altimeter isn't the end-all be-all of terrain avoidance, but it is a very valuable tool for a competent pilot.

Legally (and professionally), you aren't using RA to establish your DH or MDA - what you are going to use it for is to enhance your overall situational awareness. If the alert is set at 200', then I'd better be on about a half mile final when it goes off, otherwise it may be time to think about going around.

For non-precision approaches, the 121 carriers I'm familiar with round MDA _up_ (always round up) to the next higher 100 feet. The idea being MDA is indeed a minimum you can't go under, so stop a bit shy of it - if the WX is so low that another 50 or 80 feet makes the difference, it's probably too bad to be doing a non-precision approach anyhow (not worth the increased risk). Here there are two schools of though I am aware of: One (if the alert will go high enough), set it to just above the actual MDA, as a warning that you are about to descend below the MDA. Two (my personal preferance), set it to 200' - it provides a final check to insure you aren't too high or low on short final. Again, you should be on about 1/2 mile final at that point.

Again with both of the above goes the first caveat - you have to be aware of what the terrain is where you are. It is as good or bad as the pilot using it.
 
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