Rad Alt use

[Bladestrike]

It appears that alot of people set the decision height on their rad alts for approaches even when the DH is predicated on Bar Alt (Cat I or non-precision). Is there some sort of standard on this???

Any input appreciated

[Captain Stable]

Both the company I now work for and my previous one had encountered instances of this, and both have had to stress that DH must be called solely upon Baro Alt.

RadAlts may be used for DH only for Cat II and III approaches.

The reason for this is that, on a Cat I approach (or, indeed, a non-precision approach, there is no guarantee prior to the threshhold that the ground does anything remotely predictable as you close on the runway. You may, therefore, go from 500' Rad to 30' Rad in the blink of an eye.

Hence use of the RadAlt for non-precision approaches is NOT a safe practice.

[Bladestrike]

Thanks for the reply.

It is our company's SOP to set the rad alt to MDA/DH (and we don't have CAT II or higher authorization), and I question the thinking behind it. I believe the intent is to use Bar Alt (obviously) and have the rad alt as a backup, but does that mean that you take the highest indications of the two?

What would you set your rad alt to on a CAT I or NP approach? Something completely unrelated to the MDA/DH?

[Agent86]

Our Company sets 150 ft for Precision approaches and 300 ft for non-precision approaches.

The reasoning for this comes from a recommendation from the Flight Safety Foundation to prevent CFIT in the landing configuration.

Our SOP says if at ANY time you get a "minimums" GPWS call and are IMC carry out an IMMEDIATE go-around.

In Aus there are no ILS minimums below 150 ft AGL or NPA MDAs below 300 ft AGL so if this warning happens and you are IMC

1. You missed the MDA /DH :(
or
2. You are in the wrong place :( :( ie wrong aid selected or wrong plate/ minima being flown

Any one else use this ???

[OzExpat]

The reason for this is that, on a Cat I approach (or, indeed, a non-precision approach, there is no guarantee prior to the threshhold that the ground does anything remotely predictable as you close on the runway. You may, therefore, go from 500' Rad to 30' Rad in the blink of an eye.
What? I would hope that isn't the case because the ILS setup would be HIGHLY suspect! Given that the average GP angle is 3 degrees and that adequate terrain clearance exists at that angle, it stands to reason that the terrain in the vicinity of a Cat. 1 DH can't possibly change by the amount described in the above quote.

I certainly DO use Radalt on a Cat. 1 ILS, but monitor it as a backup for the altimeter indications. I also set 250 FT on a NPA because THIS is the minimum obstacle clearance within 5 NM of the THR (reference Pans Ops), with a fix within that distance. I monitor this to ensure that the MDA doesn't breach the minimum obstacle clearance criteria.

I believe in using everything at my disposal to keep my operation safe.

[Youwererobbed]

If the Perf Manual states a DH it can be used. If it doesn't it can't. Good point about varying terrain and DH..MAN 06L is a good example..where you can't do a Cat2 or Cat1R approach but can do a Cat3b approach (due to the Bollin Valley)...but the Perf Manual should include notes if this is a known problem.
Can't think of any non-precision approaches that have a DH but expect somebody could prove me wrong!

[Captain Stable]

OzEx - nothing to do with the ILS set-up. Imagine a runway on top of a cliff. Half a mile out you could be at 500' agl. Two hundred yards later you could be 20' agl and all of a sudden your decision height is GONE because the pilot was relying on RadAlt rather than Baro for a Cat I approach and didn't read the notes on the bottom of the plate properly.

There is nothing wrong with using RadAlt from about 100' down if you don't get an automated readout, but as a guide only. Setting a RadAlt DH (particularly single-pilot) and relying on it is a very suspect practice. Cat I approaches are by definition by sole reference to Baro altitude.

[Agent86]

Capn Stable,

The opposite can also cause drama. The ILS at Guam in 1996 with the G/S out has high ground on finals with a ****** factor of the VOR/DME being 3.3 dme SHORT of the runway threshold.

In the landing configuration without EGPWS and no major closure rate the GPWS thinks you are going to land. It doesn't know there isn't a runway there :(

If the crew of KAL801 in 1996 had reacted (if they were able to after the long crew day...another link in the chain) to the "minimums" readout (and all the other radalt readouts 500 .....etc) they would probably have missed the ridgeline.

moral of the story. If an approach needs the radalt for minimums. use it. If not set it to something that MAY just save your @ss.

[Bladestrike]

I guess confusion at mins is the concern. If you are at mins bar alt and your rad alt tells you you have another 100 to go, it would be stupid and unprofessional, and downright dangerous, to continue decending, on the other hand, if you are getting your bug light and you still have another 100 to go bar alt???? If you are aware of what the terrain has in store for you and have your act together, the extra information is indeed handy, but as an SOP, playing to the lowest common denominator, I see confusion a possibility at the most stressful time during the approach.

[GlueBall]

Landing on ILS Rwy 01 at Guatemala, the Radar Altimeter altitude jumps from 600 Feet to 100 Feet in one second as you approach the threshold. Radar Altitude is completely useless at D/H.:eek:

[OzExpat]

Just in case anyone thought I was saying that I depend on the Radalt in a Cat 1 approach, perhaps you should re-read my previous post and point it out to me. It isn't there. I DID say, however... "I believe in using everything at my disposal to keep my operation safe."

To demonstrate what I mean by "everything at my disposal"...

Imagine a runway on top of a cliff. Half a mile out you could be at 500' agl. Two hundred yards later you could be 20' agl and all of a sudden your decision height is GONE because the pilot was relying on RadAlt rather than Baro for a Cat I approach and didn't read the notes on the bottom of the plate properly.
First off, again, I haven't advocated reliance on Radalt for a Cat 1 approach. I don't know about anyone else, but I would have taken note of the aerodrome elevation and threshold elevation in my pre-approach briefing. I reckon that this simple safety precaution would resolve this particular situation.


Landing on ILS Rwy 01 at Guatemala, the Radar Altimeter altitude jumps from 600 Feet to 100 Feet in one second as you approach the threshold. Radar Altitude is completely useless at D/H.
See my response to the immediately preceeding quote.


The ILS at Guam in 1996 with the G/S out has high ground on finals with a ****** factor of the VOR/DME being 3.3 dme SHORT of the runway threshold.
I've been there and this did not present a problem to me. Guam has a DME which I can use to cross-check the situation at 3.3 miles from the threshold. If I'm on the right profile, I'd expect to be something over 1000 FT AMSL when passing the VOR - for the moment I can't recall the crossing height there, but a Radalt warning at 3.3 miles from the threshold would not take precedence over the altimeter indication in my mind.


I think the point that needs to be made here is that a certain amount of pilot brain power is necessary. This is applied at the pre-approach briefing (or whatever you choose to call it), and during the approach. This, naturally, includes all the logic steps such as double-checking the altimeter setting, using the DME and setting the Radalt to confirm DH at DA. The time you stop thinking is the time when you're in greatest danger. Hence, as I said at the outset, I always use everything at my disposal to keep my operation safe.

[NW1]

Cat I approaches are by definition by sole reference to Baro altitude.Not true, actually. There are Cat I ILS approaches for which Rad. Alt. DH minima are calculated, surveyed and quoted. My operator's manuals include the comment "...Radio Heights, where provided, should be employed for all autopilot coupled approaches."

The crucial point is, of course, where provided - because of terrain variations (as you quite rightly said) not all approaches will have terrain which makes a RA DH possible for a 200'-ish decision point. But many are, and it is incorrect to imply that a Cat I approach is limited to a Baro decision.

LHR 27R is an example - my performance manual quotes minima of 200' Radio Height and 550m RVR explicitly for a Cat 1 coupled approach.

BRGDS

[Bladestrike]

Thanks for all the great input guys!

Personally I've always set DH/MDA. Our SOP's for helos, apart from the approach; the PF bugs flare height for an autorotation, and the PNF bugs 500 over sea, 1000 over land.

I was asked the question and wasn't sure of the answer I gave. It's a little clearer now..... ;)