Radio Altimeter vs. Barometric Altimeter

[dapilot]

Wouldn't it be a good idea when descending on an ILS approach, to use your "radio altimeter" and use the "DH" to judge when you arrive at your landing minimums instead of using your "barometric altimeter" and "DA?" I would think the RA would be much more accurate then all the errors associated with the barometric altimeter, especially during pressure changes such as when t-storms start and end, frontal passages and along frontal lines. Barometric altimeters are subject to +/- 75 foot errors which could mean the difference between landing and deciding to go missed !

Just a thought - would like to hear others' opinions

 

[Snapshot]

RA is great but has some terrain based limitations.
Can't have a large elevation change (ditch/road/valley) right before the runway.

Anywhere there is scaffolding/trusswork supporting the approach lights (SEA, PIT, CRW etc..) it's a red flag.

In SEA, RA is not authorized for CATII.
http://204.108.4.16/d-tpp/0708/00582I16CC2.PDF

 

[dapilot]

hmm i agree with you but what about airports where it is level in front of the runway, or where there are CAT 2 and 3 approaches? why not use RA for the CAT 1 approach on the approaches which also have CAT 2 and 3?

 

[SSDD]

Nothing stops you from doing that as a backup to your altimeter, but you'll be surprised at how irregular terrain that appears flat, can be.

 

[Snapshot]

I was researching/editing while you were responding. By all means, RA is great. Just make sure it's authorized, be safe and have fun.
Always a good idea to cross-check everything you are able to, though.

 

[dapilot]

I was researching/editing while you were responding. By all means, RA is great. Just make sure it's authorized, be safe and have fun.
Always a good idea to cross-check everything you are able to, though.

what do you mean "authorized?" (besides CAT 2 and 3) Where would it say it is or is not authorized? (for CAT 1) You have any examples of this on any approach plate? All approach plates show a "DH" and a "DA".... so why couldn't I use a "DH"? (playing devils advocate)

 

[Snapshot]

If it says DH, use it.

If it says "RA NA", as in the example posted above, it's not authorized.

I couldn't find any other examples online. Jepps don't put the stuff out there for free and it's been about 10 years since I've held an NOS plate.

If it's not authorized, it should say in the profile view, in the Approach Minimums, as a note in the corner of the plan view, or possibly on the back of the 10-9. My plates are at work, so I can't really research it.

 

[xjcaptain]

Short answer....DH is HAT (height above touchdown zone elevation not height AGL). RA only applies when the jepps show "RA" as the minimums (CAT II or III) These have been flight tested and surveyed to show actual RA. When at DH the RA that you show can vary quite a bit from the HAT that you would expect, some show quite a bit above 200' others show less than 200'. Like old Chris Columbus and others found out way back....the world ain't flat!

 

[MauleSkinner]

what do you mean "authorized?" (besides CAT 2 and 3) Where would it say it is or is not authorized? (for CAT 1) You have any examples of this on any approach plate? All approach plates show a "DH" and a "DA".... so why couldn't I use a "DH"? (playing devils advocate)

If you've got a radar altimeter that can show DH on a Cat I approach, go for it.

I've never seen one, though. All of the ones I've flown only show your height above the terrain underneath you, not your height above terrain half a mile in front of you.

Fly safe!

David

 

[igneousy2]

Radar is more difficult to use because of the scalloping due to the terrain. You might hit your DH 2-3 times on the radar prior to the actual Decision Height".

"approaching minimums! oh wait not yet...now! approaching miniums!! no no...oh yeah there it is minimums minimums.

Additionally as someone else has pointed out - the DH published is a HAT not an AGL.

At the end of the day I think it comes down to the fact that you can probably use radar safely a lot of the time if you know the approach is a flat field, not a lot of uneven terrain or buildings on short-final and the airport is not on/in a plateu/valley like Billings, MT or SEA-TAC. However, an aircrew with no information about any of the above mentioned needs to be able to operate into the airport with the same procedure all the time.

A CAT-I approach does not require the precision of a radar-altimeter so it is not flight tested utilizing one - so if you do it, you (and your pax) become the flight-test pilot. If they designed the approaches to utilize the precision of a Radar Altimeter then you would be required to have an RA installed in your 172 before you could fly IFR and AOPA would go ape s*hit.

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