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MDA
Apologies if this has been discussed before.
Does anybody out there believe that it is permissible to go below MDA when not visual and conducting a missed approach from a NPA? I am aware that many operators add an amount (around 50') to the published MDA and treat it as a DH. In this case the aircraft will not go below the actual MDA. I am also aware of at least one operator that states in its ops manual that you can initiate the missed approach at MDA. This clearly results in the aircraft going below it. Just to clarify this question relates only to a "traditional" NPA not a LNAV/VNAV approach with a DA published. An authoritative reference showing the acceptability of this practice would be appreciated. |
A traditional NPA (some of them still have that option) has a step down procedure. In this case you level off and continue to the MAP, where you decide to go around or not. In this case you would not loose ALT, so you won't go below your MDA. With these CDAs of nowadays it gives you the possibility to shoot a NPA faster, easier and with less noise. Only problem is, that you will not level of towards the MAP. When you initiate the go around you will indeed go below the MDA.
As it stands for MINIMUM DECENT ALT, you can not bust it unless you continue with the approach. The 50' is a safe buffer. |
When you initiate the go around you will indeed go below the MDA. If you procedures are such that you intend to fly level to the MAP you also should not descend below MDA without the required visual reference. This has previously been extensively discussed on Pprune - suggest you do a search! |
That's why the 50' bobby. Read before you reply!
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zonnair, yes apologies - you did say that - it's all down to semantics - have a great day.
Believe the a/c allowance depends on a/c type also. |
A recent change has allowed operators to remove, or not add, the 50' buffer if carrying out a continuous decent final approach on a NPA.
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I have flown for airlines that didn't specify 50' addition for a constant descent approach.
The MDA is the MDA, and the tolerance specified was +100', and -0' ... it was left up to the pilot's skill to meet that tolerance, rather than specify any particular figure. |
MDA on NPA's is slowly being replaced by DA at a lot of EASA member-state airports, sometimes resulting in slightly higher minima's.
In some cases, 50' makes the difference of becoming visual or doing a G/A. This is why I am not a fan of VNAV NPA approaches combined with an MDA in marginal weather. Use the V/S and put the MDA in the MCP. Keep slightly low in order to ensure level flight at the Decision Point. (Decision Point is the point where on a normal ILS, you would end up at minima considering a standard glide angle of 3°). If no contact, go around. (and NO there is NO requirement to fly level up to the MDA, this is a big misunderstanding by some collegues). HOWEVER, and this I must emphasise: FOLLOW your COMPANY SOP. This is a personal note and in my present company VNAV NPA's are not done. |
No you can't
MDA means Minimum Descent Altitude. You cannot descend below this altitude unless adequate visual reference is achieved.
DA means Decision Altitude. This is the altitude that the decision is made to go around. You will descend slightly below this altitude in a go around. In my company, when conducting a NPA there is an auto call-out of "Hundred above" when there is 100 feet to go to the MDA. We count to two seconds and if not visual, push the thrust levers forward and go around. From the call of "Hundred above" and two seconds thereafter, we ensure that at no time we descend below the MDA. |
Keep slightly low in order to ensure level flight at the Decision Point. (Decision Point is the point where on a normal ILS, you would end up at minima considering a standard glide angle of 3°). If no contact, go around. (and NO there is NO requirement to fly level up to the MDA, |
On charts, the movement is towards MDA for non-precision approaches, DA for precision approaches.
You have to add your momentary descent to the MDA to get your DA. When you initiate the go around you will indeed go below the MDA. |
Actually MDA is the "old fashioned" way and there is a movement towards DA for all approaches including NPAs. As far as i know it is even a requirement in EASA-land to convert to DA for NPAs.
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If you note the Jeppeson charts, many have just recently been converted to an MDA. ICAO RNP 9905 uses MDA, while FAA 8260.52 uses a DA.
It is not due to precision, but multi-variant performance considerations. With RNP baro-vnav, the approach can be anywhere from 2.5 to 3.1 degrees on a 3 degree approach, hence an MDA... RNP transition to GBAS final, uses of course, a DA. |
When conducting approach other than ILS, there is no need to add 50ft to MDA as long as a CDA is being used. This is relatively new proc at my current company. We were doing it 15 years ago at my previous company.
7 |
What do you mean there is no need to add 50 ft to MDA?
If the MDA is 650, what is your DA min? |
With an approach MDA of 650 we set 650. This means that the A/C will dip below MDA if a missed approach is flown.
7 |
That is completely wrong. You can not go below the MDA on a missed approach.
SKYbrary - Minimum Descent Altitude/Height |
I assure you that it is sop at our airline. It is in black and white in our OMA.
7 |
Sorry to hear about that, but you are violating the rules of you descend below the MDA on the missed approach.
You have no assurance of obstacle/terrain clearances. |
Nope, they're not. EU-OPS says so.
A fairly recent change, like us, allows it only if your are carrying out a CDFA. The actual plates show a DA(H) as opposed to MDA(H). |
FlightPath is correct ,not allowed below MDA. If the chart has been constructed to show a DA then it should have an allowance so as not to go below MDA in the event of a GA
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The plates shows DA if it is a precision approach. Non-precision approach shows MDA. DA assumes a 50 foot momentary descent, if yours is greater, than you must adjust your min.
If the plate shows MDA, you cannot go below that for a missed approach. |
I am 100% confident that we are not breaking the rules. This is clearly laid out in our SOP. I work for one of the largest airlines in the world and am sure that such a policy would not exist if there was any doubt as to its legality.
In fact we do have assurance of obstacle/terrain clearance in the same way we have clearance when conducting ILS approaches. I can see that you and others here are highly qualified and have much to offer to the community. I respect you. Please extend me the same courtesy. We are not violating the rules. The combined total experience in our airline is somewhere on the area of 42,000,000 flight hours. Trust me we have the issue covered. |
I am just as equally certain that you cannot go below the MDA on a missed approach.
This is a very common misconception regarding MDA. Why have an MDA if you can go below it? |
That's the point. Our, specific, charts show all NPA's with a DA(H).
So, therefore, we do not need to add 50' to the nonexistant MDA(H) Para (b)(2). The use of the MDH as a DH is introduced as a consequence of the prohibition of level flight at MDH (or multiple step-downs during non-precision approaches) when using the CDFA technique. |
That is not the point, this was the post I was responding to...
With an approach MDA of 650 we set 650. This means that the A/C will dip below MDA if a missed approach is flown. |
Keep up at the back!
CDFA has been mandated by EU-OPS and, therefore, the requirement for an MDA has been removed. If you can't or don't currently fly a CDFA there is, I believe, a three year transition period but at the end of that time you will be expected to use CDFAs on every NPA. That will remove your requirement for an MDA and you will be flying to a DA(H). It's not an MDA anymore, it is a DA. It doesn't matter if you go below it carrying out a go around. Have you heard of "Lower Than Standard" and "Other Than CAT 2" approach ops? That means that I can fly an ILS to a minima below the conventional limit. That means I can fly a CAT 1 ILS in 400m RVR instead of 550m. But I suppose it's wrong to do that too. It's all part of EU OPS Subpart E. Have a look! |
I design procedures, including RNP to GBAS finals that have no minimums.
Of course there are DA approaches, but what happens if there is no ILS? Its an MDA approach. The question was specific to MDA, not DA, and I gave a specific answer. |
If there's no ILS you fly a NPA with a CDFA to a DA.
Whether that DA is now the old MDA or the old MDA+50' depends on how the minima is derived, not my department but...every NPA that I do, daily, has used the old MDA figure as a DA, in accordance with current regulations. |
Think I'll divert - this is getting too complicated for mortals like me!
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If you used the MDA as a DA, then it was wrong. If the MDA stated 650, and you used 650 as the DA for a NPA, it was wrong.
Also, currently ICAO 9905 RNP procedures are non-precision, and the RNP procedures use CDA to an MDA... |
Sorry OBN but that is simply not the case.
From the horses mouth - Subpart E: Para (b)(2). The use of the MDH as a DH is introduced as a consequence of the prohibition of level flight at MDH (or multiple step-downs during non-precision approaches) when using the CDFA technique. |
Perhaps rather than cut/paste a quite, you should go to the document.
The ref you make is an explanation of the terminology where MDH as a DH is used. (note introduced)This was to harmonize with the FAA and now DA is DH.... However, this is the MDH and DH, which is in the process of being transitioned to. This MDH/DH is a DA, but it is NOT the MDA. The DA or DH take into account the momentary descent for the CAT aircraft, MDA does not. page 254/57 SUBPART E ALL WEATHER OPERATIONS OPS 1.430 Aerodrome operating minima — General note in the minima Tables... Note 7: The MDH mentioned in Table 4a, 4b, 4c and 4d refers to the initial calculation of MDH. When selecting the associated RVR, there is no need to take account of a rounding up to the nearest ten feet, which may be done for operational purposes, e.g. conversion to MDA. http://eur-lex.europa.eu/LexUriServ/...01:0238:EN:PDF |
JAA document NPA-OPS 41
While this is paraphrasing, this MDA vs DA is of concern.... The application of the CDFA technique requires all NPA operations, to be flown with a decision altitude/height (DA/H). When determining the applicable DA/H, the operator must take account of the missed approach point (MAPt) and the minimum descent altitude/height (MDA/H). While it is quite clear that a missed approach must be initiated not later than at the MAPt, the question of MDA/H is a different matter. Quite a few operators use MDA/H as the DA/H with no height add-on; in fact this is the case with a majority of the largest European operators. This modus operandi has raised concern that the unavoidable height loss below the MDA/H during a go-around might introduce a safety risk, even if the height loss can be minimised by the use of appropriate operational procedures (call-outs, high degree of on-speed/on-path discipline, training) The matter of using the MDA/H as a DA/H is progressing in the ICAO OCP and OPSP. Until there exists a final result of the work in ICAO, it must be left to the discretion of each Authority to make decisions on the matter. Since the benefits of the CDFA technique are generally acknowledged, the decision is typically whether to require an add-on to the MDA/H to ensure that the height loss does not lead to flight below the related MDA/H during a go-around, based on formal or other reasons. http://www.jaa.nl/publications/npas/NPA-OPS%2041.pdf |
OBN, that document you quote from is over four years old. However, Subpart E has now incorporated some of the proposals.
You seem to have conveniently missed this bit. Issues related to DA/H and MDA/H The application of the CDFA technique requires all NPA operations, to be flown with a decision altitude/height (DA/H). When determining the applicable DA/H, the operator must take account of the missed approach point (MAPt) and the minimum descent altitude/height (MDA/H). While it is quite clear that a missed approach must be initiated not later than at the MAPt, the question of MDA/H is a different matter. Quite a few operators use MDA/H as the DA/H with no height add-on; in fact this is the case with a majority of the largest European operators. This modus operandi has raised concern that the unavoidable height loss below the MDA/H during a go-around might introduce a safety risk, even if the height loss can be minimised by the use of appropriate operational procedures (call-outs, high degree of on-speed/on-path discipline, training). In order to evaluate the safety of the use of MDA/H as DA/H, the AWOSG has compared the obstacle protection for this type of approach with the obstacle protection for ‘traditionally’ flown non-precision approaches as well as with the protection for approaches with vertical guidance (APV) using the criteria contained within ICAO PANS OPS. The AWOSG is convinced that using the MDA/H as a DA/H offers adequate obstacle protection. Another comparison between the CDFA technique and the ‘traditionally’ flown non-precision approaches indicates that the latter involve several safety traps, such as: • Early descent with a prolonged flight close to obstacles; • Multiple step-downs possibly inside the FAF; • An approach which is, by definition, destabilised; • Temptation to make a late and steep descent from MDA/H towards the threshold; • Risk of descending early from the MDA/H; While there are no records of accidents related to the use of the CDFA technique during approach operations, there are several accidents attributable to the risks listed above. The DA(H) figure is derived from the OCA(H) for the associated procedure plus any buffer decided by the operator or the authority. MDA(H) is the lowest altitude (height) for the level portion of an approach flown using the traditional dive-and-drive technique, not the CDFA technique. Use of MDA(H) would undermine the philosophy of the CDFA Use of the CDFA technique is considered as a significant safety improvement. (Ref to ALARP). It is also important to note that the NPA does not suggest anything that is not already in widespread use by European operators |
The "4 year old document" is where your quote
Para (b)(2). The use of the MDH as a DH is introduced as a consequence of the prohibition of level flight at MDH (or multiple step-downs during non-precision approaches) when using the CDFA technique. I agree that it has been incorporated, in ways that you may not understand. I also understand that the calculations associated with determination of MDH have been revised to account for momentary descent given the propensity for "Quite a few operators use MDA/H as the DA/H with no height add-on; in fact this is the case with a majority of the largest European operators." "The DA(H) figure is derived from the OCA(H) for the associated procedure plus any buffer decided by the operator or the authority. MDA(H) is the lowest altitude (height) for the level portion of an approach flown using the traditional dive-and-drive technique, not the CDFA technique. Use of MDA(H) would undermine the philosophy of the CDFA" Just how do you think we calculate Obstacle Clearance? And I will let you guess...."plus any buffer decided by the operator or the authority." what that buffer is. dont worry, we have you covered... check out this thread...http://www.pprune.org/tech-log/45774...ml#post6597656 |
Actually you're wrong, my quote came from the EU OPS document which is currently in use, right now, based on the proposals in the document that you quoted from.
MDA(H) is the lowest altitude (height) for the level portion of an approach flown using the traditional dive-and-drive technique I know I'm repeating myself, again but what don't you understand about this? The AWOSG is convinced that using the MDA/H as a DA/H offers adequate obstacle protection. Use of the CDFA technique is considered as a significant safety improvement. For example, at an airport I used to frequent. VOR/DME approach with an MDA of 530'. We used to have a DA of 580', 50' buffer you see. We now operate the very same approach, completely legally, to a DA of 530' using a CDFA. I have yet to be charged with flying illegally or dangerously. Even though I once flew that approach to that minima with a CAA inspector on the jumpseat. End of story. |
I understand the issues completely, and may have designed many of the procedures you may be using.
As noted, even if the AWOSG is convinced using the MDA without a momentary descent is okay.... Unfortunately, the criteria and procedure design parameters do not support this. (if there is an event, good luck, as I am sure that when your operation decided to use the MDA as a DA, you did a re-evaluation of the terrain and obstacles to support this decision) Precision and non-precision approaches have a base terrain and obstacle evaluation, that assume the controlling obstacle, add the ROC, and that is how the MDA is calculated, but what you fail to realize, is that the areas that are evaluated are very small considering the procedures. http://operationsbasednavigation.com.../P773dsurf.jpg note the area of analysis for the missed on this approach... I look at it this way...the minimums are the bare minimum to where the procedure would be illegal... if I designed the procedure that didnt meet the minimums, it would be illegal per the criteria, and not approved... would you want the procedures designed to not meet the minimums? they are after all MINIMUMS...one can always be more than the minimum...but less? So basically, you are stating that you just dont care what the basis for that MDA was, you feel that you can drive below that altitude at your leisure. There are many places in the world where I would NOT suggest trying this. |
LSM, just because it is legal does not make it safe. Most regulators in the world at the moment are struggling to have the experience to make informed decisions on such matters. We have been watching the European proposition to convert MDA's to DA's for a while now, and to be honest I don't agree with it.
So I'll leave the decision on this up to you. PANS-OPS says that in the final approach segment I must give you 246ft of obstacle clearance. The MDA is derived by giving you at least 246ft of obstacle clearance from all obstacles. If you convert the MDA to DA with no additional allowances then basically you have (246ft - height loss) clearance from obstacles. For a CAT D aircraft height loss is 161ft (PANS-OPS published height loss) As you can see adding 50ft to a minima to turn it into a DA doesn't seem quite adequate does it?? a) If you want 246ft clearance from obstacles (IAW PANS-OPS) I strongly suggest you don't descend below MDA. b) If you are happy with down to 85ft of obstacle clearance, then punch on through the MDA at hearts content. Which one do you think is right? But do all the procedure designers in the world a favour....if you choose option (b), and the DA was simply converted from an MDA, when you hit something, don't point the finger at us and accuse us of not protecting you. Sorry if this seems harsh, but there are alot of people running around at the moment claiming to know what they are talking about. When there is smoking hole in the ground, they all dissappear. |
Our Part A states that 50' must be added to the MDA to account for the height loss in a go around on a CDF NPA. And it is referenced as OPS 1.430 2
Unless of course it is a baro-VNAV where we use the charted DA |
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