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. :E

[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]

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]

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]

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.

[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]

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 :ok:

[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]

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.

[Geoffersincornwall]

...... 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.

[DOUBLE BOGEY]

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

[[email protected]]

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

[212man]

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......

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.

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!

[Boudreaux Bob]

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).

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/aviation/business/ifr-paper-services/glossary-legends.pdf

[Pete O'Tewbe]

Unless there has been a major sea change, the +50 is a requirement if the aircraft does not have a published PEC in the flight manual.

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.

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

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

[Cows getting bigger]

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.

[[email protected]]

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?

[Geoffersincornwall]

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.

[keithl]

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.

[serf]

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.

[212man]

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.

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!

[Geoffersincornwall]

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:

[EN48]

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).

[Variable Load]

Please tell me it's not so.

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:

[DOUBLE BOGEY]

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

[DOUBLE BOGEY]

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.

[Geoffersincornwall]

..... 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:

[HeliComparator]

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.

[ShyTorque]

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

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.

[Cows getting bigger]

DB, you are incorrect. I can think of a number of scenarios where the ground on the approach is at a higher elevation than the threshold thus presenting a radalt indication of less than 200ft. The ICAO standard for MSD in the final approach segment involves a complex calculation which generally employs specialist modelling software, but the absolute minimum obstacle clearance on a precision approach within the final approach segment would be in the order of 130ft. ICAO DOC 8168 Vol II, Part II, Ch 1 refers.

PS. I concur with the rest. :)

[[email protected]]

As I understand the procedure construction - a non CDFA approach will have a step-fix to protect you from obstacles intruding into the approach plane so that it is safe to descend immediately to MDA/MDH unless a step fix is there.

Setting the radalt to anything other than ground proximity warnings (100' and 200' for precision and non-precision approach) adds complexity to the process (more calculations required) and is a further distraction if you are slightly tardy in your initiation of the GA - the last thing you want in a high workload situation is the radalt audio bleating.

I find it difficult to understand why the need to initiate GA above your DH/DA is now accepted as best practice for IR - it makes a mockery of the whole idea of Decision height/altitude as you won't fly to your minima - that minima which has been carefully constructed to permit flight to DA/H, then make a decision - then initiate the GA. If you start the GA at 50' above the minima you probably won't get to your actual minima so why have it in the first place.

It rather smacks of something that has been intellectualised rather than kept in its intended simple state.

[John Eacott]

I appreciate that the vast majority of responses here relate to the UK, but Boudreaux Bob has already given the Jepps decode which categorically rebuts the use of the rad alt, and my references are to my 'home ground' of Australia. As already mentioned, many ILS runways here have the surrounding land significantly below the threshold height thus rendering a radalt reading unreliable at DA/MDA.

This is still an International Forum!

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.

Pete O'Tewbe, the use of PEC relates to whether one is published for the aircrat in use: the +50ft is applicable in Oz for aircraft that do not have a published PEC. It is derived from the practicality that a published PEC will be of a low value, so 50ft is a reasonable impost on the lag of an baralt during a precision approach. If there is a PEC then you should apply that.

Have you ever checked whether your aircraft has an approved PEC, and the ramifications if it doesn't?

[DOUBLE BOGEY]

CGB the MSD you refer to alows for PEC aircraft position error, equipment tolerance and the GA manoeuvre.

John Eacott are you seriously suggesting that flying the GS at the very few international airports that have ILS in Oz you will be less than 200 feet MSD during the approach! I know its hard and trendy to accept greater risks down under but really!

You guys can hypothesise all you like. For me I will execute an immediate GA if my RADALT tells my I am at 170 feet MSD during an ILS where the state minima is 200 feet.

For all the usual heros out there that want to be as low as you can and think you can stay safe...good luck.


DB

[John Eacott]

CGB the MSD you refer to alows for PEC aircraft position error, equipment tolerance and the GA manoeuvre.

John Eacott are you seriously suggesting that flying the GS at the very few international airports that have ILS in Oz you will be less than 200 feet MSD during the approach! I know its hard and trendy to accept greater risks down under but really!

You guys can hypothesise all you like. For me I will execute an immediate GA if my RADALT tells my I am at 170 feet MSD during an ILS where the state minima is 200 feet.

For all the usual heros out there that want to be as low as you can and think you can stay safe...good luck.


DB

I'm not at all sure how you have devolved such a hypothetical from what I said. I have tried to point out (along with cows getting bigger) that using the rad alt will not give an accurate height above the airfield datum nor the threshold when used (as seems to be the inference here) as a warning at the 200ft point on the GS. The land under the aircraft at that time may be lower (as in the the case with many ILS airports here, not just international) or higher, and therefore you will not get a 200ft radalt warning at that point.

I have not said that you will be below 200ft at the appropriate point of the approach. Methinks you do protest too much and not read what is written, including Boudreaux Bob's link to the Jepperson decode which firmly rebuts the use of the radalt. In the same way that you misread my explanation of the +50ft penalty when a PEC is not available :rolleyes:

[DOUBLE BOGEY]

John, reap what you sow! I have never advocated using DH to achieve the minima on a QNH approach! It is you who needs to read posts more carefully.

Ther is no ILS in the world with a state minima of 200 feet height above the threshold that would put you below that height, anywhere on the approach provided you are on the glideslope. That is made clear in pans-ops.

However, you are permitted some glideslope error at DA, accepting this at DA you may well be below 200 feet height above the threshold.

In modern helicopters fitted with a reasonable AP, coupled to the G/S such errors are eradicated.

All I advocate is that if the helicopter is fitted with a RADALT it is poor airmanship not to use it as the safety device it is clearly intended to be. To effect this it needs to be set below the listed DH minima for the approach, giving a margin to reduce the possibilities of the aural warning before the DA minima is reached. This is the design concept behind the AIRBUS bug system in the EC225/EC175 and it works well in practice.

The ILS systems in Oz do not have significant obstacles in the final approach. If they did the state minima for that approach would be greater than 200 feet DH CAT 1.

[Cows getting bigger]

DB, you are incorrect. PANS OPS make no such assertion regarding 200ft MSD prior to DH if on GP. Take a look at:

Northolt RW25 - TDZ 124, 0.55nm (3.5 deg GP) 131ft
Gloucestershire RW27 - TDZ 87ft, 0.55nm (3.5 Deg GP) 102ft
There's many more where your radalt at DH will be less than baro alt (QFE). There are a few more exotic ones where the scenario appears at greater range.

Use of Radalt in IAP design is also mentioned in 8168. Personally, I set radalt to 50ft on an IAP but that's because I'm normally calibrating the ILS and 50ft is a good point to start roll-out guidance on a Cat III. :ooh:

We digress, I see where you are coming from and would support the use of the radalt bug to increase SA.

[John Eacott]

John, reap what you sow! I have never advocated using DH to achieve the minima on a QNH approach! It is you who needs to read posts more carefully.

Ther is no ILS in the world with a state minima of 200 feet height above the threshold that would put you below that height, anywhere on the approach provided you are on the glideslope. That is made clear in pans-ops.

However, you are permitted some glideslope error at DA, accepting this at DA you may well be below 200 feet height above the threshold.

In modern helicopters fitted with a reasonable AP, coupled to the G/S such errors are eradicated.

All I advocate is that if the helicopter is fitted with a RADALT it is poor airmanship not to use it as the safety device it is clearly intended to be. To effect this it needs to be set below the listed DH minima for the approach, giving a margin to reduce the possibilities of the aural warning before the DA minima is reached. This is the design concept behind the AIRBUS bug system in the EC225/EC175 and it works well in practice.

The ILS systems in Oz do not have significant obstacles in the final approach. If they did the state minima for that approach would be greater than 200 feet DH CAT 1.

Round and round we go. Obviously you are seeing something in my posts which I am not.

I will try again. Using (as I believe that you advocated) the radalt bug set at 200ft to indicate that you are at or below the DA of 200ft can be erroneous. Many ILS approaches that I am familiar with come in over low ground (eg valleys) such that the glideslope height AGL is significantly greater than 200ft, even though the aircraft is on glideslope and is at 200ft above the airfield reference (be it threshold height or airfield datum).

Therefore setting 200ft (as you advocate) when the actual height AGL may be more like 250 - 300ft at that point serves no purpose. Since most ILS (and all NDA) precision approaches are designed to cross the threshold at 50ft, the ideal use for a radalt setting would be more beneficial if using a known reference point rather than a spurious and inaccurate one. My posts have been trying to make the point that the surrounding land is lower than the threshold, I see you are more concerned with land being higher: you are quite correct that this would create a higher DH, but that is not the point I am trying to raise.

If all the approaches that you are familiar with have surrounding surfaces level with the airfield, then this concept may be a difficult one to understand. But making silly remarks about Australian procedures is not a good way of discussing the issue, and I note that you have gone quiet on the PEC concept?

[Pete O'Tewbe]

Pete O'Tewbe, the use of PEC relates to whether one is published for the aircrat in use: the +50ft is applicable in Oz for aircraft that do not have a published PEC. It is derived from the practicality that a published PEC will be of a low value, so 50ft is a reasonable impost on the lag of an baralt during a precision approach. If there is a PEC then you should apply that.

I have not disagreed with anything you have said with regard to PEC. Except that with regard to the DA -0ft +50 ft discussed earlier, it is nothing to do with PEC.

Have you ever checked whether your aircraft has an approved PEC, and the ramifications if it doesn't?

Yes, and it is 21ft.

Thank you for your concern.

[John Eacott]

P O'T,

Is there a similar requirement in Europe/UK for application of PEC to the DA? It is everything to do with DA here since it must be applied (or 50ft in lieu) to an ILS, which I believed to be the topic under discussion.

[Pete O'Tewbe]

From AIP AERODROME AD 1.1 AERODROME/HELIPORT AVAILABILITY:

4.6 Altimeter Error

4.6.1 When calculating Decision Height (DH), account must be taken of the errors of indicated height which occur when the aircraft is in the approach configuration. Details of the Pressure Error Correction (PEC) should be available from the aircraft Flight Manual or handbook. In the absence of this information a PEC of +50 ft has been found to be suitable for a wide range of light aircraft and should be used. This addition of 50 ft need only be applied to DH. The required RVR should be calculated prior to applying the
PEC.

4.6.2 The use of a radio altimeter is only applicable to approved Category 2 and Category 3 operations. For an aircraft flying a Category 1 or non-precision IAP, DH/Minimum Descent Height (MDH) is indicated on the pressure altimeter. At DH/MDH any readings from a radio altimeter may be unreliable because of the large area of terrain providing return signals to the instrument.

since it must be applied (or 50ft in lieu) to an ILS, which I believed to be the topic under discussion.

Again, I have not disagreed with you.

Presumably, you are now going to introduce temperature corrections into the debate?

[John Eacott]

Presumably, you are now going to introduce temperature corrections into the debate?

Not at all: you just did ;)

[Pete O'Tewbe]

Touché!....

[DOUBLE BOGEY]

Hi John. I have not advocated setting the RADALT to the DH. It is set usually 30 feet below that value with the minima set on the DA bug. We have the ability to set both bugs but the aural warning is only activated on reaching the DH bug. Hence the safety backstop.

[DOUBLE BOGEY]

CGB - the values you quote refer to height at the TDZ (touchdown zone), I have not checked but I suspect this is the TCH (Threshold crossing height). This is much further on than the Decision Altitude, which, acknowledging errors that may be present, is a minimum 200 feet height. If obstacles or higher ground are present this DH is modified accordingly.

I am no expert on approach design but the TDZ does not makes sense. Can you elaborate what this means?

[Boudreaux Bob]

As this is an International Forum, the concept of Temperature Correction for very cold temperatures is not as esoteric as some might think.

Come up to Deadhorse in Alaska some Winter or any other location at either end of the World covered in White for Months on end.

If you think RadAlt readings matter during an ILS approach, just consider having a Sling Load hanging under the aircraft while doing the approach.

How does that figure into your concepts being put forth here?

[EN48]

From this link: Flying - US going back to TDZE on approach charts (http://www.euroga.org/forums/flying/1708-us-going-back-to-tdze-on-approach-charts)

"For international harmonization, US approach charts have been migrating to removing the TDZE altitude from approach plates and moving towards THRE as new approaches were designed and older ones were periodically updated. To date, over 4000 approaches have undergone this change, this is about 20% of the total.

The TDZE (TouchDown Zone Elevation) is defined as the highest elevation in the first 3000 feet of the runway starting at the threshold. The THRE is defined as the Threshold Elevation. Anytime the THRE is the highest point in the TDZE, they are the same. However, when the highest point is further down the runway, TDZE can be higher than THRE. If the runway meets standards, this can be as much as a 20 foot difference. In some rare cases where the runway is provided a variance, it can be more.

The THRE is not considered an operational altitude as one does not land on the threshold, but inside the TDZE. In US regulations, THRE is not mentioned, but TDZE is. In particular, on an approach (not Cat II or III) where the approach light system being in sight is used as the sole means of permitting descent below the DA/MDA, US regulations specify that they may only be used for the continued descent to an altitude 100 feet above the TDZE without meeting other criteria. With only THRE shown on the approach chart, this does not provide the data necessary to comply with the regulation."

[Cows getting bigger]

DB, I'll try to keep this simple (not that I'm accusing you of being simple).

AS EN48 says, the TDZ is the first 3000ft of an instrument runway and the notional touchdown point is found within this area. An ILS GP is set up such that an aircraft is guided down to a point in space that we would commonly understand as coincident with the TCH (in other words, a point about 50ft above the runway threshold). Beyond this point, the ILS GP signal is nonsense (it's actually pretty useless below about 120ft) but the notional GP is extrapolated to a point on the runway which is about one quarter distance into the TDZ (let's say 800ft). The ILS DME is then normally adjusted to give a zero reading at this point (try it next time you fly an approach - check the DME reading over the threshold and you will note is is normally a smidgen over 0.1nm)

The system minima is designed around the highest centreline elevation of the runway within the TDZ but the elevation of this point may not be coincident with the DME zeroed notional touchdown point. Indeed, in the most extreme cases, the highest TDZ elevation may be 2000ft further down the runway than the notional touchdown point; you could be short changed!!

What does this all mean?

It means that an ILS GP 'origin' is assumed to be a point some 800ft within the TDZ but the system minima is based upon the TDZE which will often be different to the elevation of the GP origin - the runway threshold plays no part. However, if we keep things simple and assume that TDZ, threshold and GP 'origin' are all the same elevation, we will find that a 3 degree GP crosses the threshold at about 50ft and will be 200ft at 0.65nm from the GP origin (about 0.52nm from the runway threshold). The two examples I gave, albeit 3.5 deg GP, demonstrate a higher ground elevation as an aircraft passes system minima and your radalt will, at this point, read less than 200ft (there are also buildings at these locations further reducing the radalt indication). I've personally flown one example (not in Europe) where the radalt was less than height above TDZE by nearly 100ft. London City also gives some interesting indications.

In simple terms, DAs/DHs aren't normally increased due to obstacles that occur on final approach prior to system minima as any such obstacle would probably demand an increased GP angle. Increased DAs/DHs are far more common due to problems in the missed approach segment which for a precision approach commences at system minima, i.e. the 200ft/0.65nm point.

I think I've rambled on; sorry. :bored:

[Geoffersincornwall]

An excellent contribution and one that will earn me a bottle of whisky from my erroneous colleague. He's also a great friend so will have to manage the situation carefully. :) :}

[EN48]

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?

You are right. The number in parentheses on a Jepp chart is HAT (Height Above Touchdown Zone Elevation) See elsewhere in this thread for Touchdown Zone Elevation (TDZE). Radar altimeter is not a permitted means (and not an appropriate means) of determining DA on a CAT I ILS. (A CAT I ILS could potentially be designed to use radar altitude, but they are not, in part, because this would require a radar altimeter.) And on a non precision approach you may be bumping along at the MDA for some distance before reaching the MAP; the radar alt may be all over the place. On Cat II & III approaches radar altimeter is used and "RA" is shown in the minimums section of the chart. This is according to U.S. TERPS. Lots going on here; a link if you want to dig in: http://www.faa.gov/documentLibrary/media/Order/8260.3B_Chgs_1-26_rev.pdf Some differences may apply in other jurisdictions.

I found it useful to study TERPS carefully early in my instrument flying. Recommend that all instrument pilots at least give it (or the equivalent where you fly) some study, however, far more detail than you will need to keep in mind. What I found most useful is an understanding of the obstacle clearance provided (usually not much!) in critical phases of flight.

[EN48]

DAs/DHs aren't normally increased due to obstacles that occur on final approach prior to system minima as any such obstacle would probably demand an increased GP angle. Increased DAs/DHs are far more common due to problems in the missed approach segment which for a precision approach commences at system minima, i.e. the 200ft/0.65nm point.


Maybe I am not correctly understanding this statement, but ... the TERPS criteria allow relatively little change in GP angle in most cases. More likely that the DA would be increased as needed to assure required obstacle clearance and stay close to a 3 deg GP angle. Agree on the MA segment. One reason for increased DA is to meet the standard minimum climb gradient on the MA segment. Some airports have multiple precision approaches to the same runway with different DA's depending on the climb gradient the acft is capable of.

[Cows getting bigger]

EN48, think of it this way.

Let's assume the ideal GP is in the 3.0deg territory. Whichever way you look at things, you will be at 200ft at 0.65nm, regardless of obstacles. Increasing DA will only shift the MAP further away from the runway (you'll still be following a 3.0deg GP) and consequently, the only reason you would want to do this would be due to an obstacle beyond/below the 200ft point. For any IAP design, the MAP segment takes primacy as it requires a climb gradient of 2.5% which is a more shallow angle than the GP (about 5%) and consequently is more susceptible to obstacles. (OK, there's a level bit of the MAP segment after MAP and before commencing climb but, for ease, the numbers still favour the more shallow gradient)

PS. Good comment on pilots understanding TERPS (other versions are available ;) ).

[Geoffersincornwall]

My colleague has read all contributions to the thread and we have made peace over the subject of rad alts and Class 1 ILS approaches. I have to make a small correction to my original post and point out that he did not say you must use the radar altimeter readings for a DH reference.

In view of the huge amount learnt from all your submissions a truce is called and the only whisky involved will be a shared glass of single malt - after work of course.

G. :ok:

IMHO this is when Prune works best - not a single swear word or raised voice, a little bit of sarcasm here and there but otherwise an excellent discussion with a little hint of humour and a great deal of enlightenment

[DOUBLE BOGEY]

HI CGB and others. Firstly thank you for the comprehensive explanations you have provided and it all makes sense to me.

In Europe the Jeppesen plates generally include a TCH ( threshold Crossing Height) based on the theoretical GP angle and the point the G/S impacts the runway. As the optimal TDZ is normally 300m from the threshold the 1:60 rule, for 3 degrees, puts the G/s 50 feet above the threshold. Consequently most 3 degree slopes, without any GA infringements quote a TCH between 45 and 55 feet.

As the aircraft descends along the G/S the crew make the decision as the BARALT approaches/reaches the DA minima. On the 3 degree path this theoretical point is 1200m approximately from the optimum TDZ point.

Therefore, unless the ground is significantly higher than the threshold, out to about 1km from the threshold, which would be highly irregular, the practice of setting a backstop DH 30 feet below the approach DH minima, works very well.

I have never operated in the US or much in very mountainous regions so this may not work everywhere. However, my natural sense of safety/justice would be seriously challenged if I thought at a theoretically correct DA (no flight path error) I was significantly lower to the surface than the procedures published DH.

In any event, getting lower than 170 feet from the surface IMC during the approach is probably close enough for me and I would really want to be going around at that point (accepting I will continue to descend below 170 feet momentarily in the go-around action). I may be a little naive in this respect but at a 500 fpm ROD I am a little less than 15 seconds from impacting the surface.

Interesting thread though that does challenge the concepts we all carry in our heads.

[EN48]

Increasing DA will only shift the MAP further away from the runway (you'll still be following a 3.0deg GP) and consequently, the only reason you would want to do this would be due to an obstacle beyond/below the 200ft point.

Got it! :ok:

[212man]

My colleague has read all contributions to the thread and we have made peace over the subject of rad alts and Class 1 ILS approaches. I have to make a small correction to my original post and point out that he did not say you must use the radar altimeter readings for a DH reference

Of course, it doesn't help much that more than one manufacturer actually labels the rad-alt bug as 'DH' ...........:E

[Geoffersincornwall]

Yes..... it makes me want to spit feathers ?@*&!***

Like having three automation related buttons labelled 'NAV' and critical power supply switches located next to each other and given potentially ambiguous names. (Main???? Master ?????) - non English speakers beware and to top it all an 'LNAV' button that delivers 'FMS' and a 'NAV' button that delivers 'VOR'.

I think the design team must have been out to lunch that day :{

G.:ugh:

[[email protected]]

And I thought ergonomics was supposed to be science rather than something made up on the back of a fag packet!

Engineers should not be allowed to design cockpits!