MDA Go around

[737OPR]

Gentleman,

Quick question, probably asked before but still.

At my airline we fly ( as most european carriers do) our non precision approaches stabilized ie constant decent rate ( or lately VNAV) to MDA, if we don't see the runway at MDA, go around. The thing is, a DH compared to MDA. When flying an ILS a DH is just that, A decision height and you are allowed to fly under it during the GA procedure, the OCA accounts for that.

However, a MDA is really that, a minimum descent altitude, and you are not allowed to fly under it. However if you initiate the GA at the MDA, you are bound to dip a little below it.

Lately, we have changed to boeing procedures and copied there books letter for letter. This change also included that we are now allowed to fly VNAV approaches. The wording in our books is: at minimums initiate the GA if the runway is not in sight.

Wouldn't you be breaking the law if during a NP app you descent below the MDA during the GA?

I'm sure other company use the VNAV/VS app as well. How do your companies deal with this issue? Probably nitpicking but still.

Thanks

[Capt Chambo]

For non-precision approaches flown this way most operators will add 50' to their MDA(H) and then call it a DH. Then if you initiate the GA at he DH you should not go below the MDA(H).

[737OPR]

Thanks Capt Chambo,

My question was the result of reading a US forum and that particular question was asked with that precise answer given.

However, at my airline we don't do that. Leaves me a little mystified because it is a reputable airline and I can't image I'm the first with that question.

I guess my next move is to send an email to flight tech on this but I'm a little afraid of asking a dumb question so that's why I wanted to get some feedback on here.

Thanks once again.

[safetypee]

737OPR, not a dumb question at all.
The ICAO PANS OPS definiton of MDA does not allow descent below that altitude; however not everyone follows PANS OPS thus what the US does may not apply to JAR-OPS. There are differences in the obstacle clearances provided by PANS OPS and TERPS. (FAA allows descent below MDA for VNAV ?)
With the advent of a stabilised approach / constant angle approach, and VNAV, JAA have debated (still debating?) the overall safety case for using MDA as a ‘DA’.
The anti CFIT advantages of stabilization etc are judged to outweigh the very small increased risk in descending below MDA during a go-around; one study has shown that there is no change to the overall risk in an operation when all aspects are considered.
It is possible that JAA OPS guidance material has been issued, even included in the stabilised approach criteria, but I do not have a copy of JARs to hand. If I recall correctly, the issue was left open so that each operator could decide on a procedure (add 50ft or not) in conjunction with their National Aviation Authority.
Thus your question should be put to flight ops management as to what the policy and procedure is; – and have they discussed it with their NAA.

[411A]

Well now, it depends on how you fly the NPA, the speeds used, and the configuration standard.
For example, in the Lockheed L1011, non-precision approaches as normally flown at Vref+20, with the approach flap setting...22 degrees.
Runway in sight, and descent required for landing, landing flaps are selected (33 degrees) and the speed reduced toward Vref plus any additives.
This is for both straight in and circling approaches.
Using this technique, the aeroplane can be flown right down to MDA, and if the runway is not seen, go-around thrust is applied, a climb commenced, and the landing gear is retracted.
No need to reset the flaps, as they are already at the go-around setting...22 degrees.
The problem arrises when you have slow spooling engines, and landing flaps are selected prior to having the runway in sight.
In these cases, a descent is possible below MDA while going around, IF not done properly.

In short, the 50 foot additive to MDA is for folks who don't know how to do non-precission approaches properly in the first place.:}

[gb777]

Some good reading in a previous thread:

http://www.pprune.org/forums/showthread.php?threadid=108259

gb

[212man]

411A, could you explain (in simple terms) how it is possible to initiate a missed approach AT MDA, from a stabilised descent, and then not continue to descend below the MDA in the process?

737OPR, if you search for 'NPA OPS-41' that will give some useful background info too.

[411A]

411A, could you explain (in simple terms) how it is possible to initiate a missed approach AT MDA, from a stabilised descent, and then not continue to descend below the MDA in the process?



212man, you might be presuming that I might fly a missed approach when reaching MDA, but in fact this is not my particular style.
I would prefer to have a 'fly level' segment.
This provides both an ample opportunity to look for the runway (especially useful for circling approaches), the engines are nicely spooled-up, and the flaps are still at the approach (maneuvering) setting, thereby giving a reasonable assurance that a go-around can be performed without descending through the MDA.

I have found over the years that this works the best for me, and indeed when I began flying 4-engine heavy jet aeroplanes, this is how it was taught.
Over the years however some operators have decided that, for their particular operation, the continous descent profile is better suited.
I generally combine the two, IE, the continuous descent profile is used for step down fixes, however a missed approach is not started until reaching the missed approach point...not necessarily when reaching MDA.

[Der_dk.]

Hi 737OPR

As I understand your post, your company SOP´s says you shall fly the non precision approaches as CANPA. That rules out flying as 411A suggests.
Are you sure that your Approach charts are not published with a factor of f. ex 50 ft added to the original MDA thereby giving you a "new" MDA you can actually use as a "DH".
Our charts from EAG have a factor included in the MDA for non precision approaches.

Regards

DER

[sudden Winds]

converting your MDA into a DA involves a few things...In the cases where the MDA is low enough say 300 ft AGL or so, then that´s no big deal, you´re already too close by the time you reach your MDA during the constant descent (this point´s called VDP right?) and there´s little space for level offs. You have to make a decision at that point rather than waiting to see whether what ´s dead ahead looks like a rwy or not.

But if the MDA is rather high (450 ft plus) then things get a bit more complicated....1) a VDP at such distance is at least 1.5 nm from the threshold, wich means you definitely have some more margin to level off and try to find visual references that allow you to land with normal rates of descent, normal maneuvers, in the TDZ, etc etc.
2) published minimum visibility for the aircraft category could be and certainly sometimes IS lower than the distance between the VDP and the rwy threshold. This means that if visibility is right at minimums, the rwy won´t be visible at the VDP !!!! now you solve this problem 2 ways, either you raise visibility requirements, or accept a level flight segment at the MDA, at least until you see it and then make a decision whether to land or not based on what you see. If a stabilized approach can still be flown, even if it´s a little steeper, then there´s no problem, but if someone is going to say that a go around is mandatory at the VDP then many airports visibility minimums WILL have to go up, otherwise you´ll be legal to depart to an airport knowing that you won´t see the rwy at the VDP, that in the end becomes an ammended missed approach point...
Regards,
SW.

[BOAC]

Not forgetting you do not need to see 'the runway' at VDP.

[reynoldsno1]

Baro-VNAV procedure design includes a height loss margin, and the minima are available as a DA/H. CDA approaches without VNAV guidance coded into the database do not qualify.
Adding an arbitary 50' to an MDA is just that - arbitary. The PANS OPS height loss margins for a pressure altimeter are as follows: Cat A 130', Cat B 142'. Cat C 150', Cat D 161'

[411A]

Adding an arbitary 50' to an MDA is just that - arbitary. The PANS OPS height loss margins for a pressure altimeter are as follows: Cat A 130', Cat B 142'. Cat C 150', Cat D 161'

Indeed so.

I suspect this 50 foot business was propagated by BA some years ago, and like a London fog, has crept ever so slowly into other regions.
Completing an 1179 sim session about ten years ago, the UKCAA inspector thought the 50 foot idea was the greatest thing since sliced bread, until I pointed out the PansOps data stated above.
For this, he had no ready answer.:ugh:

[212man]

I think you'll find there were more interested parties than just BA, involved:

http://www.flightsafety.org/alar/alar_bn7-2-nonprecision.pdf

[xetroV]

If the company uses the published MDA as a DA, then most likely a suitable margin for height-loss during go-around (between the moment where the go-around is initiated and the point where a positive rate-of-climb is achieved) has already been factored in the published mimima on their approach charts. Whether or not that is the case here depends on the contract between the company and their charting supplier.

[737OPR]

Thanks for all the quick and informed answers. At the next available opportunity I will bring it up at the office.

Greetings

[VMK]

im new and confuse. im very new. MDA/MAPt... whats the diffrence?

[hawk37]

The PANS OPS height loss margins for a pressure altimeter are as follows: Cat A 130', Cat B 142'. Cat C 150', Cat D 161'
Reynoldsno1 and 411A, is the minimum obstacle clearance on a 200 foot HAT ILS something like 90 feet? In which case, how does the "height loss margins" indicated above fit into this equation?

[safetypee]

suddenWinds caution with the interchange of MDA and DA. Whilst this is OK procedurally, the definition of each and the resultant obstacle clearances may not be the same.
The problem of the visual minima not correlating with the required vertical flight path at MDA was a factor in the Avro RJ CFIT accident at Zurich, i.e. when on a nominal 3 deg path the runway could not be seen in the procedure visibility minima of 2000m; for the 900ft MDA, 4k vis was required. The Zurich procedure was modified and the principle was to be applied universally. However, I do not have evidence of any action, thus operators are recommended to publish their own (increased) visibility minima to provide a reasonable chance of seeing the runway at MDA.
Don’t kid yourself that a slightly steeper approach will be OK, the situation at Zurich resulted in a flight path of 6 deg as opposed to one nearer 3 deg. (Info from Presentation ‘Reducing CFIT Risk’ FSF European Aviation Safety Seminar circa 2003).

A pedantic point (maybe), but VDP should not be inferred as being at MDA. On a non precision approach (NPA), you have little or no control of the point along the approach path when you reach MDA, thus the range to the runway is unknown (thus use time to MAP). Some NPAs use DME or allow RNAV, but these are not a substitute for a precision approach (and fraught with opportunities for error).
This point is the main reason why altitude range-checks (VS/timing) are an essential series of checks during NPAs, and why the altitude component must be checked first. Altitude is the dominant limit (if you fly to a distance from a runway without checking altitude, then there is no protection from terrain Celebrating TAWS ‘Saves’: But lessons still to be learnt. (www.icao.int/fsix/_Library%5CTAWS%20Saves%20plus%20add.pdf) ).

VKM MDA is an altitude which provides the required obstacle clearance during a non precision approach. The MAP is normally a location along the approach track.
It has been established that level flight at MDA increases the risk of operation thus the concept of flying a stabilized approach was introduced which also includes commencing a GA when at or just above MDA (CANPA). (GA is also flown from the MAP if it is reached before MDA.) Unless directed otherwise the approach track should be maintained and the MDA position should be over-flown during the GA.

hawk37 my understanding is that the PANS OPS height loss values are used in procedure design, which I believe looks at absolute values; whereas the JAA discussion on height loss during GA from MDA related to the collision risk and thus a ‘50ft height loss’ not an absolute value – more of an alleviating margin.

[extreme P]

From the FAA. Canada is the same.
50' would be added to minimums for a single engine approach, over weight landing or certain equipment failures.
C. When the use of VNAV path guidance is incorporated into the approved training program and a VNAV path is used to fly eligible procedures with a DA(H), a slight momentary descent below the published DA(H) is considered to be acceptable while arresting the descent during the initiation of a missed approach which has been initiated at or above the specified DA(H).