MDA

[Gulfstreamaviator]

I am aware that JAA insist on 50ft being added to non precision MDA, VOR/NDB approaches, world wide.

Yet the FAA do not require this safety margin, which I am told is to allow for the interia in the level out.

My Question, is this in the new LASORS, or is it a CAA JAA Circular, or where can I find the reference material.

If it applies to NPA, and then this becomes a circle to land, does the circle then have the higher mins.

Lastly, does any other authority apply this rule: UAE, India, China, Russia...for example.

Many thanks, Glf

[eckhard]

Hi Gulfstreamaviator

The 50ft addition to the MDA on an NPA is only applicable if you wish to conduct the NPA using Constant Descent Angle (CDA) techniques. I don't think that the JAA (which doesn't exist anymore BTW) insisted on this. If you wish to employ 'dive and drive' techniques, there is no need to add anything. I think it is true however, that the provisions of EU-OPS and the UK CAA strongly recommend using CDA techniques when conducting NPAs (as do the Flight Safety Foundation).

The MDA remains as published, but the CDA technique requires a Decision Altitude (DA), rather than an MDA.

In other words, when you reach the DA, you either initiate a go-around or continue to land, depending on whether or not you have adequate visual reference. With the CDA technique, you do not level off at the MDA and fly towards the MAPt before initiating a GA. This is similar to the DA employed on an ILS approach.

The reason for the 50ft addition now becomes clear: it is there to enable you to make a decision to GA without descending below the MDA during the manoeuvre.

By contrast, if using the 'dive and drive' technique, you plan to level off at the MDA, so no addition is required.

For a 'circle-to-land', again you are planning to level off at the circling minimum, so you don't need to add anything.

Unfortunately, I don't have the EU-OPS reference to hand. I am basing my response on the practises employed by UK PT operators. I don't think this info would be in LASORS either.

When using this CDA technique, it is important to remember that the GA must be continued straight ahead until passing the MAPt, before any turns may be commenced.

Hope this helps,

Eck

[Capn Bloggs]

With the CDA technique, you do not level off at the MDA and fly towards the MAPt before initiating a GA.
Depends entirely on company SOPs. There is nothing stopping anybody flying a CD NPA, even from a database, levelling at the MDA, proceeding to the Mapt and then executing the missed approach.

The 50ft extra is for those who wish to do the approach like an ILS ie treat the "minimum" as a DA.

[Gulfstreamaviator]

Thanks guys, two opposing views, and not one firm reference to quote....

Sorry, not meaning to be critical.

In a CDA approach, the AP will appreciate the MDA, and commence level out to maintain the MDA, in a Slam dunk, (thus usually NON AP), the pilot will have a lot more inertia to convert, and thus more likely to bust the MDA.

If an ILS APP is permitted to go below the DH, due to intertia, (at 200ft), then what is the problem at 450ft (say), on a NPA.

In the case of a CDA, Assuming response 1, then when can one transition from the MDA+50 to the MDA circle.?????

Again, I seek hard copy reference material, to clarify my position to others.

glf

[galaxy flyer]

Gulfstreamaviator

In the US, RNAV approaches that have LNAV/VNAV minimums usually are charted with a DA; while most VOR or VOR/DME or NDB approaches have MDA with a note "only authorized operators may use VNAV DA (H) in lieu of MDA (H)". Use of a DA, in these cases, are an OpSpec for FAR 121/135 operators and not addressed for FAR 91 operators. The FSDO will probably shrug their shoulders, if a FAR 91 operator asks for a LOA to use DA.

I don't think there is a regulatory guidance requiring adding 50', just that you cannot go below the MDA, so the 50' is a fudge to prevent going below using a VNAV to guide. While some FMS systems will honor the Altitude Selector (ASEL), others will not. Specifically, the Collins system in VGP mode will treat the VNAV PTH just like an ILS GS and NOT level at the selected altitude, in fact, procedure is to set missed approach altitude like on an ILS.

WRT the ILS, the difference is in the obstacle identification slope. A non-precision appproach, by definition, has a hard floor representing the minimum obstacle height. An ILS obstacle clearance floor is surveyed based on a slope underneath the GS, all the way to TCH or a controlling obstacle. If the mins are above 200', watch out for a close-in obstacle penetrating the 1:34 or 1:20 slopes.

To the pilot's eye, I agree that any altitude loss at a VNAV CDA missed approach would be minimal and probably accounted for. But, until surveyed going below the MDA mins is a no-no.

GF

[VinRouge]

http://www.skybrary.aero/bookshelf/books/167.pdf

Lots of Gen here.

and here.
http://www.skybrary.aero/bookshelf/books/811.pdf

[172_driver]

Here is EU-OPS: http://transportstyrelsen.se/Global/Regler/Luftfart/EU-OPS%20Amd%202%20859-2008%20EN%20080820_(OJ%20080920).pdf?epslanguage=sv

Page 57-58 are interesting read. It says:

1) All approaches shall be flown as stabilised approaches (SAp) unless otherwise approved by the Authority for a particular approach to a particular runway.

2) All non-precision approaches shall be flown using the continuous descent final approaches (CDFA) technique unless otherwise approved by the Authority for a particular approach to a particular runway. When calculating the minima in accordance with Appendix 1 (New), the operator shall ensure that the applicable minimum RVR is increased by 200 metres (m) for Cat A/B aeroplanes and by 400 m for Cat C/D aeroplanes for approaches not flown using the CDFA technique, providing that the resulting RVR/CMV value does not exceed 5 000 m

[punk666]

The "dive and drive" technique is more used with piston planes than it is with jets (correct me if im wrong).

The 737 FCTM asks you to add 50' to the MDA for NPA and VNAV/LNAV.
Speaking of the 737 when doing a GA the aircraft will decend at least 30-40' before it will initially climb away so hence the 50' add on to MDA.

During my JAA IR I was told to add 50' on top of the DA for an ILS which I never understood.

[Torque2]

The EUOPS reference that 172 refers to is the latest legislation. Note there is now no requirement to add 50 ft for any reason. The table for calculating minimum RVR from DH refers specifically to a Constant Descent Angle Final Approach and as stated above if you for any reason don't do a CDFA then add the RVR supplements according to your approch category A,B,C or D).
It is different to the previous JAR rules. You can use most methods of finding distance from the threshold to calculate the constant descent angle ie: DME, GPS, IR's so no real reason not to do a CDFA.

Extract from our approach chrat provider Rules and Regs section EUOPS:

7.9.3 Non Precision Approach (NPA) (EU-OPS)
A Non-Precision Approach (NPA) operation is an instrument approach with a Minimum Decision Height or
Decision Height not lower than 250ft and an RVR/CMV of not less than 750m, unless accepted by the
authority.
⇒ Rules and Regulations General Information 7.9.6 System Minima vs. Facilities (EU-OPS Table 3),
7.9.4 Decision Height (DH) and Minimum Descent Height (MDH) for Cat 1, APV and NPA (EU-OPS)
Decision Height (DH)
An operator must ensure that the Decision Height (DH) to be used for an approach is not lower than:
• the minimum height to which the approach aid can be used without the required visual reference; or
• the OCH for the category of aircraft; or
• the published approach procedure decision height where applicable; or
• 200ft for Cat 1 approach operations; or
• the lowest decision height specified in the Aircraft Flight Manual (AFM) or equivalent document, if stated;
• the system minimum
⇒ Rules and Regulations General Information 7.9.6 System Minima vs. Facilities (EU-OPS Table 3)
whichever is higher.

Minimum Descent Height (MDH)
An operator must ensure that the MDH for an approach is not lower than:
• the Obstacle Clearance Height (OCH) for the category of aircraft; or
• the minimum descent height specified in the AFM if stated; or
• the system minimum
⇒ Rules and Regulations General Information 7.9.6 System Minima vs. Facilities (EU-OPS Table 3)
whichever is higher.

As far as EUOPS and circling goes here is an extract from our approach chart provider, Rules and Regs section:

7.10 Circling and Visual Approach (EU-OPS)
7.10.1 Minimum Descent Height (MDH)
The MDH for circling shall be the higher of:
• The published circling OCH for the aircraft category; or
• The DH/MDH of the preceding instrument approach procedure; or
• The minimum circling height indicated on the EU-OPS Table 10.
For more information refer to:
⇒ Rules and Regulations General Information 7.10.6 Minimum VIS and MDH for Circling vs. ACFT Category (EU-OPS Table 10)

7.10.2 Minimum Descent Altitude (MDA)
The MDA for circling shall be calculated by adding the published aerodrome elevation to the MDH, as determined under paragraph "Minimimum Descent Height (MDH)".
For more information refer to: ⇒ Rules and Regulations General Information 7.10.1 Minimum Descent Height (MDH)

These are all extracted from the EUOPS document and do not refer at any stage to having to add 50ft as previously.

[peacekeeper]

I seem to remember on small twins you had to add 50ft to an ILS, this was published in the AIP (Probably the place to look for this one). I think it was due to performance issues on light twins which couldn't maintain a great Climb Gradient with one engine out.

In the company I work for we recently changed our procedures so that we no longer Add 50ft to MDA if it is described on the Jeppesen plate as a DA, I think this will come down to the individual approach (Offset etc) on whether its classed as an MDA or DA.

I think there is probably some good info in the Jeppesen text manual on this if you can get your hands on it.

[compressor stall]

Pressure Error Correction.

[Sciolistes]

During my JAA IR I was told to add 50' on top of the DA for an ILS which I never understood.
Yup PEC as stated above. Altimeter allowable error is +50/-75 (as verified before flight) so the 50 is to prevent a descent below MDA. The PEC is not relevant with an electric altimeter as fitted to airliners and the like.

[Don Coyote]

EU-OPS 1.430 (d)2 details the requirement for continuous descent non-precision approaches, further information can be found in Appendix 1 (old) or (new) to OPS 1.430 depending which your company is using in the Operations Manual for working out approach minima.

Yet more information is available in TGL44,in particular ACJ OPS 1.430, AMC 1.430 and IEM OPS 1.430.

Ultimately you should work out the minima as defined in your operations manuals.

Obviously the above only applies to those operating under an AOC.

[aterpster]

Gulfstreamaviator:

I am aware that JAA insist on 50ft being added to non precision MDA, VOR/NDB approaches, world wide.

Yet the FAA do not require this safety margin, which I am told is to allow for the interia in the level out.

What you don't make clear is whether JAA requires this only when MDA is being treated as a DA. In that case, as some here have said, the FAA indeed does require adding 50 feet.

If, however, a NPA with MDA minimums is flown in the manner envisioned in criteria, then no additive is necessary, either under TERPs or PANS OPS, because of the minimum in minimum descent altitude.

[eckhard]

Capn Bloggs,

Depends entirely on company SOPs. There is nothing stopping anybody flying a CD NPA, even from a database, levelling at the MDA, proceeding to the Mapt and then executing the missed approach.

I agree that there is nothing stopping them, but one has to ask, why would anyone want to do that? With the exception of a circle-to-land, levelling at the MDA from a CD NPA will automatically put you above the desirable approach path to the runway of intended landing.

My understanding is that the idea behind CD NPA is to try and make the execution of these as similar as possible to an ILS. Hence the use of a CDA, a DA and the lack of a level flight segment prior to the MAP. Although I agree that the use of a CD is beneficial, even if you plan to fly level at the MDA, I really can't see the point of this level flight towards the MAPt (unless circling). It seems to invite the pilot to spot the threshold at a later stage than normal and then make a dive towards the runway. We all know what that can lead to.

Regards,
Eck

[aterpster]

exhard:

My understanding is that the idea behind CD NPA is to try and make the execution of these as similar as possible to an ILS. Hence the use of a CDA, a DA and the lack of a level flight segment prior to the MAP. Although I agree that the use of a CD is beneficial, even if you plan to fly level at the MDA, I really can't see the point of this level flight towards the MAPt (unless circling). It seems to invite the pilot to spot the threshold at a later stage than normal and then make a dive towards the runway. We all know what that can lead to.

No approving authority in its right mind would permit treating MDA as a DA and also approve a level off.

[eckhard]

Thanks aterpster, my view as well.

I think we're talking about two things here:

what an Approving Authority might authorise; and

how a crew might interpret an approach chart.

Capn Bloggs made the point that a CDA followed by a level flight segment was not precluded, and I agreed. I then tried to make the point that although allowed, this manouevre would result in a possible steep/deep approach and landing.

In the private world (not under an AOC or Ops Spec), the crew may be free to use different techniques to that employed by the airlines. The point I was making is that the use of a CDA and a DA (=MDA+50) gives the best chance of completing the landing without 'using excessive bank and pitch angles or excessive rates of descent' (which is how I believe some of the regulations are worded).

No approving authority in its right mind would permit treating MDA as a DA and also approve a level off.

The problem is that Private Operators don't usually need permission or approval to do anything, provided the aircraft and crew are properly certified and equipped.

Anyhoo, I think we agree on the fundamentals!

Regards,
Eck

[aterpster]

exhard:
Anyhoo, I think we agree on the fundamentals!

Indeed we do.

In FAA-dom, coverting the MDA to DA has to be on the chart, and is limited to commercial operators that get an FAA blessing.

Part 91 operators cannot obtain this approval, but there is nothing stopping them descending to MDA then trucking on level to the MAP. In a Cessna 182 at KLAX, that would probably work. :)

[galaxy flyer]

Aterpster

As this issue has been discussed many times at recurrent (Global Express), is there an express prohibition on FAR 91 operators using the MDA as DA, IAW the note on the Jepp chart? Where is it?

GF

[aterpster]

galaxy flyer:

As this issue has been discussed many times at recurrent (Global Express), is there an express prohibition on FAR 91 operators using the MDA as DA, IAW the note on the Jepp chart? Where is it?

Most FAA NPAs that have straight-in minimums have the advisory vertical path coded into the procedure's database, RNAV or not (exluding LOC or LDA). Thus, any operator with the requisite avionics can use that vertical path. But, without the stated authorization they cannot treat MDA as DA.

It is a very fine line, indeed. :D

The requirements for the authorization are contained in some obscure advisory circular and, I believe, is limited to qualified commercial operators.

http://tinyurl.com/2cncymj

So, you might ask, why doesn't this IAP simply have a line of LNAV/VNAV minimums with a charted DA? Most likely because the runway's close-in obstacle environment did not pass muster for the FAA glide-path qualification surface (GQS). Also, in this case the descent angle of 3.14 degrees is beyond the limits for Approach Category D.

[bookworm]

My understanding is that the idea behind CD NPA is to try and make the execution of these as similar as possible to an ILS.

The flaw is that it pretends a little too much that the non-precision approach is an ILS , i.e. a precision approach.

If the onboard equipment (e.g. BaroVNAV) allows you to fly to a point in space with high vertical and horizontal precision, then you might as well treat it like a precision approach. And because you dip (because you're treating it like a DH) below the MDH/OCH at that precise point, where there are no obstacles, there's little risk of hitting something even if you're in still cloud.

But if you're using raw data, selecting a rate of descent with mile-spaced DME crosschecks or worse, there's a significant variability in the distance from the runway at which you hit the MDH/OCH, and your dip of 50 ft eats directly into the margin above obstacles on the approach. I would love to see the risk assessment that demonstrates that dipping 50 ft below the OCH in these circumstances is within tolerable risk.

The progression towards CDFA for high-inertia, multicrew commercial air transport with appropriate avionics is clearly a step forward in safety. But the one-size-fits-all attitude of EU-OPS is not.

[aterpster]

bookworm:

But if you're using raw data, selecting a rate of descent with mile-spaced DME crosschecks or worse, there's a significant variability in the distance from the runway at which you hit the MDH/OCH, and your dip of 50 ft eats directly into the margin above obstacles on the approach. I would love to see the risk assessment that demonstrates that dipping 50 ft below the OCH in these circumstances is within tolerable risk.

No matter how much you'd love to see them, they don't exist. :)

[Sciolistes]

Bookworm,

I don't understand your assertion that you would eat into the OCH on a CDA.

[bookworm]

I don't understand your assertion that you would eat into the OCH on a CDA.

If you use the OCH as a DH rather than an MDH, then you will dip below it while making the decision to go around and changing the attitude of the aircraft -- typically 50 ft is considered normal. The OCH may be just 246 ft above obstacles or terrain, so by going below it by 50 ft you're eroding that minimum obstacle clearance (I previously used the word "margin", which in PANS-OPS-speak has a different meaning. I meant MOC.)

[Sciolistes]

Bookworm,

But the use of a CDA is predicated on MDA+50 as the minina, is has to be. Where did OCH=DH come into scope?

[Tee Emm]

Quote: During my JAA IR I was told to add 50' on top of the DA for an ILS which I never understood. Unquote..Yet, in a Cat3A ILS where the DH is 50 feet, do you now have to add another 50 ft to ensure you don't go below 50 ft agl? The Boeing 737 FCTM states it is possible the wheels will touch the runway if an automatic GA is made at the DH of 50 feet. So obviously there is no problem if the wheels do touch on a GA. So it seems a bit superfluous that your JAR testing officer says add 50 ft to the normal ILS DH for an ILS GA. Methinks the testing officer is adding personal opinion rather than hard data?

[Sciolistes]

Already been explained further up Tee Emm, it is trained as standard practice for barometric altimeters give such altimeters have an allowable error of +50'

Of course, such a low DH approach in a 737 (for example) references the radio altimeter.

[eckhard]

Punk666 and others,

During my JAA IR I was told to add 50' on top of the DA for an ILS which I never understood.
Yeah I'm pretty certain we taught the same but I really can't remember the reason behind it. I remember it made perfect sense though when I was told....Bollocks this is going to do my head in now till I find out!!!

The only guess I could make (which I don't think is the real reason for it) is that I think the limitations were +100 -0 so maybe adding 50ft to DA was a safety buffer so the student didn't go below DA before making a decision?...on second thoughts that can't be the reason!!

Anyone able to help?

I think I can answer this one. I spent about 1000hrs training guys and gals for the CAA initial IR test. The criteria for the test required (amongst other things) that the candidate initiate GA from DH -0' +50'.

We used to recommend that candidates THINK about commencing the GA at DH +50' to ensure that they didn't COMMENCE it below DH. Of course, it was (and still is) acceptable to dip slightly below the DH during the GA manoeuvre, provided that the GA was commenced between DH and DH +50'.

PEC had nothing to do with it, as this was a value which was tabulated in the AFM and could be applied if necessary. From memory, the light twins that we used had negligable PEC at SL airfields and approach config/speeds.

For the NPA, the candidate had to level off at MDH and then maintain MDH -0' +100'. For this reason, we recommended adding 50' to the MDH and using this as the 'target'. This gave the candidate + or - 50' to play with while he flew towards the MAPt, tracking the NDB (which by now was behind him/her) on the RBI and working out how long to run to the threshold from the stopwatch which he/she had forgotten to start.

If in doubt, when the examiner asked' 'how long to run?', we used to say, 'just tell 'em 40 seconds. It'll be about right'.

All of this was in the late 1970s and early 1980s, using hand flying (without FD) and using QFE for the approaches.

Also, we used 'dive and drive' for the NPA.

Happy days!

Eck

[bookworm]

But the use of a CDA is predicated on MDA+50 as the minina, is has to be. Where did OCH=DH come into scope?

No, it's not, not with the "New" EU-OPS scheme. See post #10 above.

The EUOPS reference that 172 refers to is the latest legislation. Note there is now no requirement to add 50 ft for any reason.


The Old Appendix split NPAs with an MDH from precision approaches with a DH. The New Appendix does not:

Appendix 1 (New) to OPS 1.430
(b) Category I, APV and non-precision approach operations
...
2. A non-precision approach (NPA) operation is an instrument approach using any of the facilities described in Table 3 (System minima), with a MDH or DH not lower than 250 ft and an RVR/CMV of not less than 750 m, unless accepted by the Authority.
...
4. Decision height (DH). An operator must ensure that the decision height to be used for an approach is not lower than:
(i) the minimum height to which the approach aid can be used without the required visual reference; or
(ii) the OCH for the category of aeroplane; or
(iii) the published approach procedure decision height where applicable; or
(iv) 200 ft for Category I approach operations; or
(v) the system minimum in Table 3; or
(vi) the lowest decision height specified in the Aeroplane Flight Manual (AFM) or equivalent document, if stated; whichever is higher.

While EU-OPS 1.430(d)2 is quite emphatic about the need to fly NPAs as CDFA, it doesn't do a good job of explaining that such CDFA NPAs use a DH, not an MDH. But that is the way it is expected to be.

[PEI_3721]

Aterpster, - risk assessments, #24.
IIRC such an assessment was undertaken by a European research organisation – NLR ??
With fading memory, a conclusion was that was a slightly increased risk in using MDA as DH, i.e. accepting a small transgression below MDA. The risk was quantified and compared predominantly with the risk / benefit of ‘dive and drive’ NPAs (higher risk) vs a continuous descent/DH (reduced risk). I also recall some reference to the ICAO collision risk model.
I did have a copy of the document / presentation – searching my archives after a house move.

The issue was debated by JAA, including AWOSC, who (failing memory) drafted a change to JAR OPS. One aspect was to delegate approval to local authorities. However, these proposals appear to have been lost / dropped / found to be in error, or just not in accord with ICAO, such that they have not been used subsequently.
At the time, as I understood the argument, based on balancing the risks, it was reasonable to equate MDA/DH as an ALAR initiative
and thus I was surprised that this has not been progressed.

One possible concern was that there are differences between the PANS-OPS and TERPS obstacle allowances, which I think still apply in the current debate.
Lest anyone should think that the above is a case for deviating from current regulation / guidelines, don’t. In retirement I’m out of touch with current thoughts and memory fails me.

TM - #28; there is a difference in approach construction with obstacles on an approach (NPA), and a Cat 3 operation with an obstacle free zone. ;)

[Zeffy]

GF

As this issue has been discussed many times at recurrent (Global Express), is there an express prohibition on FAR 91 operators using the MDA as DA, IAW the note on the Jepp chart? Where is it?


The fact that the profile note does not exist on equivalent FAA charts tells you something.

http://i202.photobucket.com/albums/aa92/zeffy_bucket/SANRNAVRwy27.png

If you can bear a visit to the sausage factory, the technical issues related to the approval are a work in progress.
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/afs420/acfipg/open/media/Hist%2004-02-258.pdf

The essence, extracted:

"...It is apparent to AFS-410 that certain technical aspects of this Bulletin are flawed. Specifically, the Bulletin does not provide satisfactory guidance to operators on the method of determining if a visual segment assessment has been made by the FAA in which no penetrations to the 34/1 surface were identified, or the appropriate criteria for industry assessment of the visual segment. In addition, it allows carte blanche application of the operational concept of using the MDA as a DH in cases where the underlying non-precision approach may not be suitable. Finally, the Bulletin provides no authority for Part 91 operators to use this capability..."


Also note that the above FAA chart does not depict the shaded feather in the profile view, indicating that the final is not 34:1 clear.

http://i202.photobucket.com/albums/aa92/zeffy_bucket/ProfileLegeng.png