Use of Radio Altimeters

[connection fee]

Use of Radio Altimeters ??

Is it fine to use your RAD ALT readout for MDA / DA?
Any Jepp references to this would also be handy?

Cheers, CF

[Transition Layer]

Radalt only used for Cat 2 or Cat 3 ILS.

Otherwise use MDA (i.e. based on QNH) for all non-precision approaches and Cat 1 ILS.

There was a thread in Tech Log about it not that long ago.

DH v MDA

Cheers,
TL

[flyby_kiwi]

In your a/c flight manual there will be a form detailing the standard of your avionics. Here its called a CAA form 2129 (or something like that) Normal NAV/COM is rated 1 so it can be used IFR, the radar alt (in a/c I have flown) has been something like a 3 which means it can only be used as a supplementry system.
Best thing to do Id say..... On approach set to just under MDA/MDH so if it goes off you know youve dropped your height - make your decisions off the altimeter.

[Pierre Argh]

A radalt gives you height above ground in the immediate vacinity of the aricraft, does it not?

At the airfield where I work there are significant drop offs within 2nms of both ends of the runway... using Radalt for DH there on final stages of an instrument approach would give a false indication of height above the airfield. Your "normal" rate of descent combined with a rapidly reducing height above (rising)ground, would I suggest be impossible to interpret. You should use QFE if you want your height above runway.

Pressure altimeter has, IMHO, to be your prime datum with radalt as a warning device. Using radalt as you describe is toooo risky!!!

[404 Titan]

Pierre Argh

A radalt gives you height above ground in the immediate vacinity of the aricraft, does it not?

Yep, infact right under the aircraft.

Pressure altimeter has, IMHO, to be your prime datum with radalt as a warning device. Using radalt as you describe is toooo risky!!!

On a Cat I ILS pressure datum is the only reference that can be used to determine a DA. On a Cat II or III ILS the Radalt is the only reference that can be used to determine a DH. In determining the DH the designers of the procedure have taken into account the terrain on the approach and the exact height the aircraft will be above it at the minima.

[Stayinalive]

One way to flunk an instrument renewal is to set Rad Alt to DH without telling the examiner it's for backup. Rather set it to 300 ft above minimums as a rough reminder of approaching minimums to MDH on baro alt. Let him know.
St Johns Newfoundland Canada has published Rad Alt approaches due to level ground on approach end of certain runways and lower published mimimums than standard. Fog being the reason there. Be careful.....

[Pierre Argh]

404 Titan... why are PPRuNer's so rude?...

I think I'm right in saying your radalt will not give you the height of ground directly beneath your aircraft if you're in a turn...

I bow to your apparently superior knowledge on Cat II & III ILS procedures; (although having completed a course on Instrument Approach Procedure Design many years ago the facts you alude to regarding terrain being considered in a procedure don't come flooding back to me... and would add we have to be careful to consider the similar but different systems in use around the world before making blanket statements on an Intl forum such as this?)

However, what is probably significant is that, on a Cat II or III ILS approach the lower procedure minima will almost invariably mean that the aircraft is over the airfield (if not over the runway) at DH? Not the same with a Cat I or Non-Px approach... where the pilot may find themselves a mile or more from the end of the runway at minima/MAP ?

I think, however we are saying basically the same thing, and don't feel the original post was from a Cat II or III capable pilot... but presume from the GA fraternity (I stand to be corrected... Connection Fee?), in which case I stick basically by my original statement with the addition... unless you have knowledge that special procedures are in force?

[404 Titan]

Pierre Argh

404 Titan... why are PPRuNer's so rude?...

I not sure where that came from. What did I say that offended??

I think I'm right in saying your radalt will not give you the height of ground directly beneath your aircraft if you're in a turn...

This is true. In most aircraft the Radalt is alive below 2500” AGL. Ignoring undulating terrain on the approach this will be at just over 8 nm from touchdown. In most places in the world that I fly I am on the localizer by this distance with no more manoeuvring required except to capture the GS. So in essence the aircraft will be wings level to touch down ignoring turbulence etc that may be encountered. On a CAT II ILS the auto pilot can be used and on a CAT III ILS must be used for the landing.

(although having completed a course on Instrument Approach Procedure Design many years ago the facts you alude to regarding terrain being considered in a procedure don't come flooding back to me... and would add we have to be careful to consider the similar but different systems in use around the world before making blanket statements on an Intl forum such as this?)

There are small differences in procedure design all over the world but I don’t know any that done take terrain into account in their design. Infact I would be worried if they didn’t. After all isn’t that the reason why they are there in the first place? To keep you safely clear of Cumulus Granitas while in IMC until the minima.

Like you I am aware that they were talking about a CAT I ILS. I was making the point what the Radalt is there for. It isn’t there to determine the DA for a CAT I ILS or a MDA on a non precision approach.

[Centaurus]

Set the radio altimeter to max warning height before top of descent especially at night. If no warning height just note when it first indicates 2500 agl. Useful as a poor man's GPWS especially if a big yellow light (if installed) suddenly comes on telling you that you are currently 2500 ft above SOMETHING that can kill you. We used to set it to full scale warning in the 737-200 soon after take off.

Descending into Honiara from Brisbane at night the warning light would flash on as we passed over the big mountains in the centre of Guadacanal. Very useful situational awareness lesson.
Regardless of arguments re use in NPA or ILS, the instrument can be a life saver providing you keep it in your instrument scan and pick up it's first movement from top of the scale.

[Pierre Argh]

Titan 404...

Although you are right to say Radalts generally kicks in at 2500ft AGL, the obstacle clearance given by most, if not all, IAP criteria during the initial stages of a procedure is 1000ft. So, may I respectfully remind you that, a pilot may pick up Radalt indications whilst still manoeuvring long before establishing on the LOC or even thinking about GS interecept.

Readers should not get get hung up on the ILS issue (where the debate on DH is, as we have seen, slightly more complex if considering the finer aspects of high-grade ILS approach systems)

If you reread the original post the question related to Non-Px approaches too... I think I am safe to say, the only way a Radalt can be used for Minimum Descent Altitude is if you happen to be flying a Seaplane... altitude is, of course, vertical distance above sea-level not above ground.





NOT

[Sqwark2000]

Numerous airports in NZ have approaches with the missed approach point/MDA over the sea but the airport up to 3nm away on elevated terra firma.

Example: NZWR missed approach point is 1 DME which is over sea but the runway is on a elevated penninsula 133ft amsl MDA by night is 900ft qnh but 767 is set on the DH (agl)

All descents to MDA should be based on the altimeter (unless you are flying a Cat II or III ils, of which there are none in NZ).

S2K

[404 Titan]

Pierre Argh

Although you are right to say Radalts generally kicks in at 2500ft AGL, the obstacle clearance given by most, if not all, IAP criteria during the initial stages of a procedure is 1000ft. So, may I respectfully remind you that, a pilot may pick up Radalt indications whilst still manoeuvring long before establishing on the LOC or even thinking about GS interecept.

Have a re-read of what I wrote. While I concede that I didn’t say “Arrival”, only approach, I think one can construed I meant both when I said “Ignoring undulating terrain”. I will leave it at that because we are in agreement that Radalt isn’t to be used in determining DA or MDA. Both are referenced on a baro altimeter to sea level. If one wants to use it as a reminder they are getting close to MDA, this is fine, as long as it isn’t a distraction at the minima causing you to stuff the missed approach up if it was required.

[jarjar]

Altimeter setting is used for MDA/DA (QNH), Rad ALT in Australia used for back-up only(situational awareness). Every company has different procedures, however this is what I do. On visual and NPA's I set the OCA for the aircraft category, on the precision approaches i set the DA. After take -off i reset rad alt to 1000ft

[Centaurus]

Jarjar. Why only 1000 ft? Does that height have any specific significance? If early warning of terrain below is the desired outcome, then surely setting the radio altimeter to it's highest altitude 2500 ft (?) would be the way to go?

[Pierre Argh]

Sorry guys... am I just being a thick POM? Aside from the ocassional post pointing out that there are airfields at or v.close to sea level (I've worked at some too)... at most airfields, how can you set DA on a Radalt and expect to get a meaningful alert?

[Wizofoz]

Pierre,

DA- Field elevation = Ceiling, e.g the Height AGL you will be at the DA or MDA. If you set this on the RA, you WILL get a "Meaningful Warning" although, as has been stated, it will be reference to the ground underneath you, not the threshold elevation.

HOWEVER, the actual, legal definition of DA and MDA is that they are an ALTITUDE, which is a height above MSL as determined by an Altimeter set to QNH.

Cat II and III approaches have DHs (Descision HEIGHTS) which are heights above the ground as determined by a radio altimeter. As such, the groung beneath the aircraft at the DH point is surveyed and taken into account when determining DH.

[Pierre Argh]

Wizofoz... thank you for the explanation, but I think you missed that I once qualified as an Instrument Approach Designer. It is not the ILS that concerns me?... Under the IAP with which I am familiar, a pilot can descend to MDA on a Non-Px approach once they have passed the FAF, which may be 5nms or more from the MAP?... and the procedure minima is based on applying the Required Obstacle clearance to the elevation of the highest obstacle within the Final Approach Sector. But if the ground drops away in the approach below airfield elevation, a pilot using Radalt may be persuaded to descend below a safe level based on height AGL rather than height AAL?

Radalt is designed to stop CFIT, not to facilitate IFR landings, and its use on an Instrument Approach is, IMHO, dangerous other than, as many have said in this thread, as a safety device and for specific use on a CAT II or III ILS. Although there has been much, interesting, debate; I think this is the concensus of opinion?

[Wizofoz]

No question that it is not only unsafe but illeagal to use RAD ALT as a basis for MDA or DA.

The only point being made is that Rad Alt is a useful tool as it accuratley tells you your current height AGL and can be used to increase Situational Awarness near the ground. For instance, if there was a large difference between your altitude and the Rad Alt height that should corespond, it could be an indication of am altimetry error such as an erroneous sub-scale setting.

As you have said, consensous reached!!

[jarjar]

Centaurus,
In response to your question:1/ Its my company standard ops, 2/ picture a scenario where you are on descent into an aerodrome with reasonable terrain(and its associated peaks and troughs) as you fly over a peak, rad alt goes off, (if it is within 2500ft), then as you fly over the trough, rad alt is lost. then comes the next peak, rad alt goes off again and so forth. By setting rad alt DA to 1000ft it only gives you notice when you need it. Remember enroute LSALT, MSA's etc give 1000ft terrain clearance.

[CopterD]

DA = Decision ALTITUDE
MDA= Minimum Descent ALTITUDE

For these two you have to use your BARALT

DH = Decision HEIGHT --> OK to use RADALT as a backup (DA minus/plus RWY elevation) but your BARALT remains primary with your DA.

CopterD