Help settle an argument about DA/MDA
 

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. :E

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

From PANSOPS Vol 1 (my underlining):

And

Also from PANSOPS:

In my opinion, you are correct.

Help settle an argument about DA/MDA
 

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.

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!

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.

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.

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

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?

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.

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 :E

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.

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

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

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.

+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.

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.

I agree.

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).

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

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

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:

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.

DB
 

...... but you can only use that alerting system overwater - right! To attempt to use it over land would be to compromise your attention on the REAL reference for DA - the BARALT.

As Crab said earlier, the Radalt can only give you info about the terrain directly beneath the aircraft. At 200 feet on an ILS to an (overland) airport your are roughly 0.65 nm from the touch down point. Unless I am very much mistaken That would put many DA's outside the airport boundary where terrain could be a valley or contain buildings.

G.

Geoffrers, no we use it over the land but remember PAN-OPS approach design criteria does not allow for significant terrain or obstacles on the FAT. a combination of descent, coupled with the margin described should preclude inappropriate aural warnings.

It works fine generally but I guess you could argue that a ship on a non-CDFA non precision approach could experience an aural warning if they descend to MDA very early but in EU-OPS land most of us are not doing this any more and following the the CDFA concept.

DB

Pete - you said that you are content for a candidate to go below DA, is that the official line for IR?

It's perfectly acceptable to descend below DA during a go-around, and physics dictates it's inevitable when the go-around commences at DA. However, over the years there has been a divergence in policy between the CAA Flight Ops departments with regard to testing, with the RW section requiring the go-around to be initiated at DA to DA+50ft and the FW section simply stating DA - like what airlines actually do in the real world! You can see the difference if you look at Standards Docs 24(A) and 24(H). So, in the testing arena, you will see candidates being conservatively coached, and then flying the go-around within 50 ft of the DA, which at lower speeds may result in the aircraft not actually descending below the DA at all, leading to the impression that this is the desired outcome. It then becomes a cyclical myth......

Whilst factually correct, most Jeppesen users globally are FW operators, and their pilots probably wouldn't know what QFE was if it dropped on their heads! Hence, my suggestion that knowing what the DH and MDH are is far more relevant to the cloud ceilng/base, rather than a quaint little habit that the British are fond of!

Approach 8-17 explains it rather nicely.

That number has nothing to do with RADALTs....your friend is dead wrong.





http://ww1.jeppesen.com/documents/av...ry-legends.pdf

Whilst PEC may indeed apply, I'm not sure that this is what is being discussed here. If you are applying a PEC of, say, 50 ft, what was a DA of 400 ft now becomes 450 ft. In any event, PEC does not apply to NPAs with a published MDA, at least in the UK.

Yes, provided the MAP has been initiated within the tolerances previously stated.

Well, throwing a cat in with these pigeons, the DA is actually derived from the DH and not the other way around. To be precise, the number in brackets is the absolute minima for a particular system at that location. In other words, a plain old vanilla Cat I ILS has a system minima of 200ft providing there are no funny obstacles in the relevant approach/climb-out segments.

So, we all sit there and fly to a system minima with this minima being converted to an altitude (normally handily done by the chart editor in annotating a figure outside the brackets) using the threshold elevation as the number to add on to the minima. We then add our various safety factors which are often prescribed in company ops manuals.

PS. Anyone adding a number to DA/DH in order that you don't descend below this figure on a go-around clearly doesn't understand procedure design and should hand the reigns to someone who does.

Pete - thanks, 212man has neatly explained how this difference in interpretation has come about.

How is it that there is no standardisation between the CAA flight ops departments?

DB et al
 

I'm troubled by the notion that all ILS approaches universally arrive over flat terrain. I'm not sure of the relative details of places like Bristol and Leeds or especially places like Sao Paulo Congonhas but they certainly would appear to be problematic when it comes to Rad Ht at DA.

On an ILS the Da of 200 ft is about 0.65nm from the touchdown reference point.

If I recall the reference point for the GS of a CDFA is the TCH (Threshold Crossing Height), not the touchdown reference point. This puts the arrival at MDA much further back and therefore even more at risk from terrain effects/buildings etc.

G.

Geoffers, you are right about TCH being the reference for CDFA. Just bear in mind that helos are not REQUIRED (as FW are) to do CDFA. This reinforces your point about MDA being further out.

NOTE: Jeppesen approach charts use the abbrevia- tion DA(H). The decision altitude “DA” is referenced to mean sea level (MSL) and the parenthetical decision height (DH) is referenced to the TDZE or threshold elevation. A DA(H) of 1440ft (200ft is a Decision Alti- tude of 1440ft and a Decision Height of 200ft.

Troubled by what? Rad alt height at DA is a complete irrelevance! If you fly a Cat 1 ILS in your Trinidad Tobago (small SE aeroplane....), you will have no idea what the rad alt height is, and have no need to!

212
 

As I said - I'm troubled by DB's assertion that it's OK to use a Rad Alt triggered warning that DH has been achieved when the terrain beneath may be anything other than on a level with runway threshold. I was troubled by his assertion that the terrain on short finals will be so close to runway threshold level that it makes no difference.

I can't see how this can be when we are talking about a tolerance of just 50 feet when commencing a GA.

There seems to be one school of thought that says "Rad Alts and their associated bugs and lights and whistles" are a reasonable substitute for the BarAlt. Please tell me it's not so.

G. :ugh:

Gospel according to the FAA (from the Instrument Procedures Handbook). Your aviation authority may vary.

Minimum Descent Altitude (MDA), Decision
Altitude (DA), And Decision Height (DH)
MDA—the lowest altitude, expressed in feet MSL, to which
descent is authorized on final approach or during circle-toland
maneuvering in execution of a standard instrument
approach procedure (SIAP) where no electronic glideslope
is provided.
DA—a specified altitude in the precision approach at
which a missed approach must be initiated if the required
visual reference to continue the approach has not been
established.
DH—with respect to the operation of aircraft, means the
height at which a decision must be made during an ILS, MLS,
or PAR IAP to either continue the approach or to execute a
missed approach.
CAT II and III approach DHs are referenced to AGL and
measured with a radio altimeter.
The height above touchdown (HAT) for a CAT I precision
approach is normally 200 feet above touchdown zone
elevation (TDZE). When a HAT of 250 feet or higher is
published, it may be the result of the signal-in-space
coverage, or there may be penetrations of either the final
or missed approach obstacle clearance surfaces (OCSs).
If there are OCS penetrations, the pilot has no indication
on the approach chart where the obstacles are located. It
is important for pilots to brief the MDA, DA, or DH so that
there is no ambiguity as to what minimums are being used.
These altitudes can be restricted by many factors. Approach
category, inoperative equipment in the aircraft or on the
ground, crew qualifications, and company authorizations
are all examples of issues that may limit or change the
height of a published MDA, DA, or DH.
For many air carriers, OpSpecs may be the limiting factor
for some types of approaches. NDB and circling approaches
are two common examples where the OpSpecs minimum
listed altitudes may be more restrictive than the published
minimums. Many Part 121 and 135 operators are restricted
from conducting circling approaches below 1,000 feet
MDA and 3 SM visibility by Part C of their OpSpecs,
and many have specific visibility criteria listed for NDB
approaches that exceed visibilities published for the
approach (commonly 2 SM). In these cases, flight crews
must determine which is the more restrictive of the two
and comply with those minimums.
In some cases, flight crew qualifications can be the limiting
factor for the MDA, DA, or DH for an instrument approach.
There are many CAT II and III approach procedures
authorized at airports throughout the United States, but
RNP AR restricts their use to pilots who have received
specific training, and aircraft that are equipped and
authorized to conduct those approaches. Other rules
pertaining to flight crew qualifications can also determine
the lowest usable MDA, DA, or DH for a specific approach.
14 CFR Part 121, section 121.652, 14 CFR Part 125, section
125.379, and 14 CFR Part 135, section 135.225 require
that some PICs, with limited experience in the aircraft
they are operating, increase the approach minimums and
visibility by 100 feet and one- half mile respectively. Rules
for these “high-minimums” pilots are usually derived from
a combination of federal regulations and the company’s
OpSpecs. There are many factors that can determine the
actual minimums that can be used for a specific approach.
All of them must be considered by pilots during the
preflight and approach planning phases, discussed, and
briefed appropriately.
Pilots are cautioned to fully understand and abide by
the guidelines set forth in 91.175(c) regarding proper
identification of the runway and runway environment when
electing to continue any approach beyond the published
DA/DH or MDA.
It is imperative to recognize that any delay in making a
decision to execute the Missed Approach Procedure at
the DA/DH or MDA/Missed Approach Point will put the
aircrew at risk of impacting any obstructions that may be
penetrating the visual obstacle clearance surface
The visual segment of an IAP begins at DA or MDA and
continues to the runway. There are two means of operating
in the visual segment, one is by using natural vision under
14 CFR Part 91, section 91.175 (c) and the other is by using
an Enhanced Flight Vision System under 14 CFR Part 91,
section 91.175 (l).

It's not so!!!

DB is somewhat complicating the answer to your question by discussing a RadAlt based warning process. Even in AHTS, all onshore instrument approaches are done with reference to BarAlt and BarAlt alone. (Note that an offshore ARA MDH IS RadAlt based.)

The RadAlt setting is there to alert the crew if the are getting too close to terra firma when IMC, at a point where EGPWS warnings won't be generated as you will be in the landing configuration.

:ok:

Geoffers, if you are descending on an ILS with a DH state minima at 200 feet, during the entire descent on the correct glideslope you should never, ever be closer to the surface than 200 feet MSD or indeed the published DH if above the state minima.

To be crystal clear, I have never stated it is OK to use DH when on a QNH approach. It is however highly recommended use the DH as a safety reference especially in a modern cockpit where both the BARALT and the RADALT bugs can be set independently and manipulated to provide visual indication that the minima has been achieved AND aural warning if the minima is breached.

Therefore, and this is the entire point, whatever the DA, which reflects threshold elevation essentially, setting the RADALT bug slightly below the published DH (the figure in brackets) is the safety backstop to prevent a whoopsie due to poor glide path accuracy or god forbid, a wrong QNH.

212 man, to suggest that the RADALT does not matter during a QNH approach is crazy given that the ONLY thing that matters is where the ground is as you get closer to it.

As I already stated, for a non precision approach not following a CDFA, if you dive immediately to the minima once on the FAT, it is more than likely an aural warning will sound unless the entire FAT is at the same elevation as the threshold.

To be clear, all QNH approaches I have flown use the BARALT DA/MDA to achieve minima. The RADALT is the safety backstop and in my cockpit is always referenced in accordance with the DH on the plate and the bug is set 30 feet below that reference. OM states, aural warning sounds and still IMC, immediate GA. This is sensible, safe and works.

DB

212 man, I can't agree with your comments re STDS 24H. The DA +50 feet -0 feet s the tolerance for the test.

If I see an IR candidate make DA+ 50 feet and declare minima it's a pass but debrief point to ensure he has simply applied the safety catch for his IR and does not in fact believe adding 50 feet is a normal procedure.

It's alway been the same, a descent below minima IMC before declaring a GA is a fail. Declaring a GA within 50 feet of the minima and subsequently descending below the minima is acceptable provided the positive GA manoeuvre has been applied.

In my experience and in defence of CAA Flt Ops I have never been subjected to anything different in 25 years.

So DB, to round things up......
 

..... You will never observe a rad alt indication of less than 200 feet on a Class One ILS but - (my assertion) - you may see a rad alt indication MORE THAN 200 feet at DA. Therefore a rad alt (if you are blessed with one) is a good device for telling you where the ground is right now but a useless device when deciding where the DA/H is or where the ground is up ahead.

I agree that the validity of the rad alt indications will increase as you approach the threshold after DA/H but at that vital moment when DA/H is achieved the rad alt information is purely incidental as a dip in the terrain may mean that it shows 250 feet at DA/H rather than 200 feet..

This takes me back to my original post - the number in brackets is NOT the height indicated by the rad alt when DA/H is reached.

Thanks to all who have contributed - your posts have blown away some myths and some cobwebs and I am grateful for your expertise.

G. :ok:

Just to summarise then, instrument approaches (onshore) are flown by reference to the pressure altimeters only - on QNH or QFE as you wish. Most helicopters have at least two altimeters that allow for cross checking, thus virtually eliminating the possibility of instrument error. Most helicopters have a single radalt (maybe two displays) and thus no means of cross checking a complex electronicky gizmo. An undetectable fault could occur.

However, GIGO rules, and it is possible to miss-set the altimeters. Preparing for an ILS I once had ATC pass me a QNH that was 10mb out. I questioned it (it was unfeasible bearing in mind the QNH at the departure aerodrome and the pressure gradient (or lack of). ATC repeated the incorrect QNH. It was only when I queried a second time pretty much telling them it MUST be wrong, that they apologised and gave the correct QNH. To protect against this sort of error the radalt bug can be set to something below the DH or MDH so that a warning will be issued before the helicopter actually flies into the ground. This is what DB is referring to. It it a good technique for an ILS or CDFA non-precision approach. Yes it can give a false warning on a non-CDFA non-precision approach over high ground (depending on how far below DH you set the bug). But a lack of perfection doesn't mean it is generally a bad idea, and of course these days CDFA should be the norm.

JT2 got it right. People seem to be confusing company policy for use of the radalt with the legal requirements.

I'm surprised there is so much uncertainty about this. There is no requirement for an IFR equipped aircraft to carry a radalt. It isn't taken into consideration as far as the procedure design is concerned; back when most approaches were designed, radalts weren't in widesepread use and there certainly has been no change to the way the minima are published, at least not over the last thirty eight years or so that I've been flying for a living.

Despite what company policy requires, it needs to be borne in mind that radalts can suffer "unlocks" for a variety of reasons and should only be used for supplementary information during letdowns to airfields.