Radio / radar altimeter applications on turbo props

[flyboyike]

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]

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]

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]

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]

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.

 

[TheInsider]

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?

When I flew props with RADAR altimeters (no CAT II), for sit. awareness, I would set it to 2000' in mountainous terrain and 1000' in non-mountainous. On approach I would always set it to 50' above DH and 100' above MDA.

 

[yournextFO]

Also, some aircraft use the RA to adjust the autopilot gain when on approach (to prevent excessive A/P corrections as you get closer to the runway.)

The RA can also be used to inhibit the stick shaker and/or pusher near the ground. In fact, there are a surprising number of systems that tie into the RA, though probably more so on a jet than a TP.

 

[Bernoulli]

When I flew props with RADAR altimeters (no CAT II), for sit. awareness, I would set it to 2000' in mountainous terrain and 1000' in non-mountainous. On approach I would always set it to 50' above DH and 100' above MDA.

Good applications of the RA. I like it. Does anyone see any problems with the above applications of the RA if appropriate situational awareness is applied?

 

[Morettis]

Good applications of the RA. I like it. Does anyone see any problems with the above applications of the RA if appropriate situational awareness is applied?

I don't see a lot of point to setting it to 1000/2000 enroute because if you're IFR, as long as you fly the MEAs they guaranty terrain separation, and if you're VFR you should be looking outside and know exactly where the ground is.

Nothing wrong with setting it a little high for the approach to give yourself a warning that you're approaching your minimums and to start leveling off / looking outside.



The bottom line is that the Radar Altimeter isn't a primary instrument, its a supporting instrument. Just like the moving map on the GPS or an RMI, you don't get to do anything special because you have it, but if you use it correctly its one more tool to improve your situational awareness.

 

[trainer8]

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?

TCAS II uses the RA for operations below 1000' AGL while on approach. TCAS logic roles to TA mode only. Typically, if you MEL the RA, you also inop and MEL the TCAS by default.

And, as previously stated, EGPWS logic.

Many GPS's have their own terrain data base info, not requiring RA data.

T8

 

[PeanuckleCRJ]

The RA bug should always be off unless used for a cat ii/iii approach. Otherwise it is wonderful SA.

The only bug on a barometic only approach should be the MDA bug. The RA should never ever be used on a barometric approach.

 

[Tristar]

Not all RA bugs can be parked out of range, and it's a technique whether you use it or not in the ways previously suggested. There is nothing wrong with setting the bug as an additional alert (unless your companies procedures prohibit doing so).

However you are correct, you absolutely cannot use the RA on a baro approach to establish DH or MDA. I may not have made that adequately clear.

 

[av8tor19]

To answer your question, Cat I operations, the DH is a barometric reading and the RA is a backup. CAT II/III, the approach area around the runway (I don't know how far out) is surveyed and leveled. That is why you can use the RA for decision alt. The runway has to be certified as well as the plane. Cat II airplanes also have to have 2 separate autopilots I think.

 

[Sig]

To answer your question, Cat I operations, the DH is a barometric reading and the RA is a backup. CAT II/III, the approach area around the runway (I don't know how far out) is surveyed and leveled. That is why you can use the RA for decision alt. The runway has to be certified as well as the plane. Cat II airplanes also have to have 2 separate autopilots I think.

DING DING!

I (thought) I posted this earlier, alas- I'm an ass.


Try using the RA on a CAT I ILS into BGM, AVP, CRW... it is cute to watch, but absolutely irrelevant.

It helps our NDs who have lost their depth perception to know when to flare the Dash. The ones who don't use RA use the old "saw" technique: Pulling back and forth on the yoke until the plane finally contacts the runway.

Good times.

 

[Mickey]

The Mooney Ovation I used to fly had one.