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-   -   MDA (https://www.pprune.org/tech-log/455445-mda.html)

dartman2 23rd Jun 2011 05:41

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.

zonnair 23rd Jun 2011 06:30

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.

fireflybob 23rd Jun 2011 07:19


When you initiate the go around you will indeed go below the MDA.
Well you shouldn't! The aircraft "allowance" (in the case of a continuous descent approach) should be added to the MDA - if not visual at this altitude then you go around.

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!

zonnair 23rd Jun 2011 07:47

That's why the 50' bobby. Read before you reply!

fireflybob 23rd Jun 2011 09:47

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.

Lord Spandex Masher 23rd Jun 2011 10:24

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.

Checkboard 23rd Jun 2011 10:30

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.

despegue 23rd Jun 2011 11:49

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.

Colin Oskopi 25th Jun 2011 14:59

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.

BOAC 25th Jun 2011 15:13


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,
- ?? Can you clarify what you are saying??

FlightPathOBN 26th Jun 2011 00:40

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.
No you do not, never....

Denti 26th Jun 2011 06:40

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.

FlightPathOBN 26th Jun 2011 15:24

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.

airbus757 26th Jul 2011 05:44

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

FlightPathOBN 26th Jul 2011 14:52

What do you mean there is no need to add 50 ft to MDA?

If the MDA is 650, what is your DA min?

airbus757 26th Jul 2011 15:14

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

FlightPathOBN 26th Jul 2011 15:16

That is completely wrong. You can not go below the MDA on a missed approach.

SKYbrary - Minimum Descent Altitude/Height

airbus757 26th Jul 2011 15:20

I assure you that it is sop at our airline. It is in black and white in our OMA.

7

FlightPathOBN 26th Jul 2011 15:33

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.

Lord Spandex Masher 26th Jul 2011 15:42

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).


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