Help settle an argument about DA/MDA

[Geoffersincornwall]

I have a colleague who believes that the number in brackets after DA on a Jeppesen approach plate is the ACTUAL rad alt reading at DA/MDA. I say it is the height above the runway reference (i.e. QFE equivalent).

I say you cannot use Rad Height as a guide to DA on a Cat 1 ILS. He says you can.

Who is right?

G.

[dingo9]

I vote it's the QFE. Unless I've been wrong for the last 15 yrs.

[Pete O'Tewbe]

From PANSOPS Vol 1 (my underlining):

Decision altitude (DA) or decision height (DH). A specified altitude or height in a 3D instrument approach operation at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.

Note 1.— Decision altitude (DA) is referenced to mean sea level and decision height (DH) is referenced to the threshold elevation.

And

Minimum descent altitude (MDA) or minimum descent height (MDH). A specified altitude or height in a 2D instrument approach operation or circling approach operation below which descent must not be made without the required visual reference.

Note 1.— Minimum descent altitude (MDA) is referenced to mean sea level and minimum descent height (MDH) is referenced to the aerodrome elevation or to the threshold elevation if that is more than 2 m (7 ft) below the aerodrome elevation. A minimum descent height for a circling approach is referenced to the aerodrome elevation.

Also from PANSOPS:

5.5 DETERMINATION OF DECISION ALTITUDE (DA) OR DECISION HEIGHT (DH)

5.5.1 In addition to the physical characteristics of the ILS/MLS/GBAS installation, the procedures specialist considers obstacles both in the approach and in the missed approach areas in the calculation of the OCA/H for a procedure. The calculated OCA/H is the height of the highest approach obstacle or equivalent missed approach obstacle, plus an aircraft category related allowance (see 5.5.8).

5.5.2 In assessing these obstacles, the operational variables of the aircraft category, approach coupling, category of operation and missed approach climb performance are considered. The OCA/H values, as appropriate, are
promulgated on the instrument approach chart for those categories of aircraft for which the procedure is designed. OCA/H values are based on the standard conditions (among others) listed in the sub-paragraphs that follow.

5.5.2.1 Aircraft dimensions: See Table I-4-5-1.

5.5.2.2 ILS:

a) Category I flown with pressure altimeter;
b) Category II flown with radio altimeter and flight director;
c) [..]

In my opinion, you are correct.

[matelo99]

We use the QFE height (the one in brackets) to set our radalt warnings. NHP will set 100' and HP will set the QFE DH. We use the QNH DH for working out the actual DHs.

[[email protected]]

Rad Alts should be set to provide ground proximity warning only - not DH/MDH/DA/MDA - those are all based on pressure settings.

Basic stuff - your colleague needs rebriefing Geoffers.

if your airfield is on top of a hill and you rely on your radalt - it might go off just as you hit the rising ground ahead of you!

[DOUBLE BOGEY]

Crab surely depends how the BUG aural warnings are designed in the individual aircraft. In the EC225 with the latest VMS software, the DA/MDA bug is set to the relevant value on the approach chart and DH bug is set some margin below the relevant DH/MDH on the chart. This provides visual warnings when the minima is achieved (via the DA/MDA bug) and an aural warning at the margin chosen below the minima, at the height set on the DH/MDH bug.

The logic is the crew can converse at minima without the aural warning sounding at the same time. However if they miss the achievement of minima through some distraction the aural warning will alert them that they are BELOW the minima. Of course if the approach continues normally the aural will sound anyway so its not perfect but better than average I would say.

[Geoffersincornwall]

DB

You seem to be agreeing that the DA figure in brackets is the relevant height, and that the 225 systems bear this out.

My argument is that the figure in brackets is in effect a conversion of the DA (bar alt) and therefore may NOT be the ACTUAL height displayed by RAD ALT when DA is reached.

Is it your contention that one could (i know you wouldn't) ignore the bar alt and judge DA using only Rad Ht indications? i.e.. the rad ht and bar alt indications would display DA and DH at the same point in space?

According to me this is a no no! Rad Ht should never figure in DEFINING DA/DH during a conventional Class 1 ILS. Or for that matter figure in the definition of MDA/MDH.

Surely people must realise that the terrain is not uniformly flat enough on the approaches to many airfields where the DA/MDA is outside the airfield perimeter (LEEDS, BRISTOL, SAO PAULO, etc.)

G.

[diginagain]

Originally Posted by Geoffersincornwall

Surely people must realise that the terrain is not uniformly flat enough on the approaches to many airfields where the DA/MDA is outside the airfield perimeter (LEEDS, BRISTOL, SAO PAULO, etc.)

But conversely it will be (relatively) flat on approaches to installations in the middle of a large expanse of water.

[tistisnot]

There appear to be differing thoughts both side of the pond. I considered for simplification Decision Altitude DA to be used only for Cat I ILS using baralt. Decision Height is used for Cat II and III ILS using radalt.

FAA TERPS: DH Decision Height. The height, specified in mean sea level (MSL), above the highest runway elevation in the touchdown zone at which a missed approach must be initiated if the required visual reference has not been established. This term is used only in procedures where an electronic glide slope provides the reference for descent, as in an instrument landing system (ILS) or precision approach radar (PAR).

FAA TERPS: Radio Altimeter Height (RA). An indication of the vertical distance between a point on the nominal glidepath at DA and the terrain directly beneath this point.

Decision Altitude normally 200 ft though may rise due to increase of glideslope angle for obstacle separation. If you wish to include DH with DA then add THR elevation? Decision Height depends on type of ops, ac performance etc.

There seems to be no MDH defined for onshore FAA ...... even though we might use it for offshore ARA being a non-precision approach using radalt?

[[email protected]]

DB - I see what you are saying, on the N3 we set a DA bug on the altimeter tape to the minima we are flying to and use the DH bug on the rad alt tape for ground proximity. The audio warning is only from the rad alt (DH) bug.

We make the decision approaching DA and go around without descending below that. This is a slight difference from what the military do where the decision is made at DA (DH if on QFE) and the go around can go below that figure.

Our standard settings for rad alt are 200' for non-precision and 100' for precision approach.

As I understand it, the figures in brackets are for those that fly the approach on QFE rather than QNH - we do both depending on whether it is a civil or military airfield.

tistisnot - completely agree except that a height ie DH must be based on the airfield datum - if it is referenced to msl it is an altitude and becomes a DA.

QNH for DA/MDA, QFE for DH/MDH.

[Geoffersincornwall]

DH ?

So can we wrap up the first argument before I start the next

The figure in brackets on a Jeppesen plate is NOT the rad height you expect to see indicated on your radalt when you get to DA on a Cat 1 ILS but IS the height of the DA when referenced to the runway threshold.

Now a new argument

Crab - if you are making your go-around decision BEFORE the DA then I suspect you may be the only one that is? We teach that the decision is made AT DEcision Altitude.

Descent below the DA is permitted unlike the old days (in the military) when we did GCAs down to Minimum Descent Height and were not supposed to go below that height. We did precision approaches based on QFE in those days.

Can someone please pitch in and either agree or disagree, thanks.

G.

[OvertHawk]

I'm with you Geoff

The number in brackets is the QFE conversion of the DA, NOT the RADALT value. We bug the RADALT to that value but the cues for the DH are taken from the BAR-ALT - The RADALT is secondary for terrain warning.

Similarly - I'm with you on "Decision Altitude". It's the Altitude at which you decide and you can drop below it after you've made a decision to go around.
OH

[Non-PC Plod]

DA is the height at which the decision is made, and the go-around is initiated if the visual references have not been acquired. However, MDA still exists for non-precision approaches, especially in US. There are fewer depicted these days on Jeppesen charts, because on most NPAs, step downs and MDAs have been replaced with CDFAs and Das, which are better for noise abatement, fuel consumption etc for commercial fixed-wing operators. It is still an area which is poorly understood as to what we can and cannot do in a helicopter for NPAs. e.g. can you level off at the depicted DA and continue to the MAPt in the traditional manner? Do you have to add a figure to the published DA to level off at if you are going to do this, and where do you get this figure from? etc

[[email protected]]

Geoffers - our guys go to Bond for training and they say that is the way it is done on a civil IR - not allowed below DA on the go around plus 50' minus 0 so I don't know if they have got that wrong.

I agree with you that the whole point of a DA is that you make your decision - it is the way I operated in the Military for 32 years and how the mil still operate.

[John Eacott]

Originally Posted by Non-PC Plod

D Do you have to add a figure to the published DA to level off at if you are going to do this, and where do you get this figure from? etc

+50ft, if there is no published PEC for the aircraft.

Geoff, I quite agree that the RadAlt is purely secondary reference: it is unlikely to be giving a useable value at DA, given the vast variations of height around (and outside the boundaries of) the airport. There are plenty of approaches in Australia where there is a significant fall in the land surrounding the approaches to the runways.

[Pete O'Tewbe]

I refer you to the PANSOPS definitions of DA/DH and MDA/MDH above.

There is nothing in the definition of DA that prevents descent below DA if the required visual references have not been acquired provided the MAP has been initiated at (or above) DA.

In contrast, however, one may not descend below MDA if the required visual references have not been acquired.

As I understand it, the figures in brackets are for those that fly the approach on QFE rather than QNH - we do both depending on whether it is a civil or military airfield.

I agree.

our guys go to Bond for training and they say that is the way it is done on a civil IR - not allowed below DA on the go around plus 50' minus 0 so I don't know if they have got that wrong.

There may be some confusion here. The -0ft +50ft window is the tolerance for the initiation of the MAP for the purpose of testing and checking. I want to see the candidate initiating the MAP (assuming he is not visual) at or between DA and DA +50 ft. Provided he has done that, I am content for him to descend below DA. Initiation of the MAP below DA or above DA +50 ft is, however, a fail item.

Paragraph 11 to Appendix 7 to Annex 1 to Commission Regulation (EU) No 1178/2011 refers as does the FE Handbook and Standards Documents 1(H), 14(H) and 24(H).

[212man]

the figures in brackets are for those that fly the approach on QFE rather than QNH

It's probably more to do with cross-referencing against the cloud base/ceiling

[jayteeto]

OMG, this is basic stuff. Simply put, not all ac have a rad alt. it is the equivalent minima if you have qfe set. The end

[John Eacott]

Originally Posted by Pete O'Tewbe

There may be some confusion here. The -0ft +50ft window is the tolerance for the initiation of the MAP for the purpose of testing and checking. I want to see the candidate initiating the MAP (assuming he is not visual) at or between DA and DA +50 ft. Provided he has done that, I am content for him to descend below DA. Initiation of the MAP below DA or above DA +50 ft is, however, a fail item.

Unless there has been a major sea change, the +50ft is a requirement if the aircraft does not have a published PEC in the flight manual. From a previous discussion on this:

The 50' addition to an ILS DA relates to Aerodrome Operating Minima, and is described in Jepp Terminal 2.6.2 (page AU-17). The requirement is that operators establish operating minima for each aerodrome, and particularly that operators apply aircraft Pressure Error Correction (should be a table in your Aircraft Flight Manual) or in lieu of this, add 50' to the published DA.

[DOUBLE BOGEY]

Jayteeto think you hit the nail on the head!

DA if QNH and the equivalent DH (in brackets on the plate) if you fly QFE.

Most UK IFR Operations use QNH.

The procedure for setting bugs is necessarily type specific and in my view, as an optimum, should provide a clear visual indication to the crew(s) that the minima is achieved AND an aural warning that the minima has been breached AND the margin between both the indication and the warning should be sufficient to prevent the aural warning interfering with the MCC calls at the applicable minima. We have this capability on the latest 225 software but for other types the bug setting procedures may involve compromise against the optimum.

In the 225 we use the DH value (in brackets on the plate) as the reference from which we set the aural warning some margin below that value.