View Full Version : What is the Real Minima

Sweet Surrender
19th May 2004, 06:37
I have just got home after my sim check ride and an interesting discussion on minimum altitudes.

The discussion centered on whether you can descend below the minima on an ILS approach. That is do you initiate your missed approach at the minimum altitude and therefore end up below the minima or do you commence your missed approach at some higher altitude so that you do not descend below the minima?

I have no doubt about what it should be but I am lacking a book reference to legitimise my stand.

Anybody out there able to help.


19th May 2004, 07:28
The missed approach maneuver following an ILS approach to minima is frequent cause for a descent below the published minima...and yes it is indeed allowed, as the decision to go-around is taken at minima (or at least it should be)...note that this is not allowed on non-precision approaches.
In older 707 aircraft, have actually just touched the main gear on go-around, such was the rather long spool-up time required with straight pipe engines.:ooh: :oh:

19th May 2004, 08:39
Not to mention a go-around from a CATIII app with a decision height of say 14 ft...

The Stooge
19th May 2004, 11:51
If you look at an ILS chart it will say DH or RA. DH meaning decision height. Where as a non precision chart will say MDA meaning Minimum Decent Altitude. That is one must not go below.

Capt Claret
19th May 2004, 11:55
Sweet Surrender

For an ILS the minima is annotated D A(H), whereas for an NPA the minima is annotated MD A(H).

From Jeppesen Introduction, Chart Glossary, come the following definitions. I've used the ICAO definition rather than the US one.

Decision Altitude/Height A specified altitude or height (A/H) 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.

Minimum Descent Altitude/Height MD A(H) A specified altitude or height in a non precision approach or circling approach below which descent may not be made without visual reference

The germaine differences between the two definitions is that for a precision approach, one initiates a missed approach if not visual at the DA(H), whereas with an NPA, one may not descent below the MDA unless visual.

If the missed approach is not initiated until reaching the DA(H) then inertia will carry the aircraft below the D A(H) and this is taken into account when the procedure is designed. Reality is that (in poor weather) the decision is not made until the DH, then the appropriate attitude and power changes are made, with human lag, so, the DA can't be a not-below limit.

19th May 2004, 20:08
By the time you assess the situ at "decision Height",prompt selection of "Go-Around"(CAT2/3)will have allowed the a/c to 'sag'below the said DH..This is acceptable to the Reg'authorities provided one is trained to the 'prompt'evaluation.
The became controversial with the advent of 'Managed/Vnav'Approaches,and towards this end(the 'sag' situ)several Companies 'add' to the MDA for abnormals(Like eng/out,heavy and of course your usual temp 'difference ).The additives varied from +50-100'........Thats why the GEAR comes up last with the posative rate o'climb...:ok:

20th May 2004, 01:15
Incase you are interested, the height loss margins for Cat I precision approach DA's (using baro alt) are as follows:
Cat A 130ft
Cat B 142ft
Cat C 150ft
Cat D 161ft

Sweet Surrender
21st May 2004, 02:06
Thanks everyone for the replies. That answers my question in spades!


23rd May 2004, 05:22
The NPA MDA definition has been changed a little by JAROps from the old UK defintion. It seems that JAR accept that dip will occur below thw MDA during the go around, I think they've come to realise that flying 250T of large jet level to see if you can acquire the runway and then continuing to land is a farcical proposition. JAROps, seems to me, almost removes the DA/MDA distinction.

23rd May 2004, 07:31
Interesting topic!

reynoldsno1: Did you find those height margins in ICAO doc 8168 vol 2?

Bst rgds

23rd May 2004, 08:19
JAROps, seems to me, almost removes the DA/MDA distinction.
And takes them further away from ICAO. Makes me wonder why anyone bothers to refer to ICAO at all. About the only time that Pans Ops allows "sink thru" in a NPA is with a VNAV approach.

Did you find those height margins in ICAO doc 8168 vol 2?
I think that you'll also find the height loss margins in Volume 1.

Miles Magister
23rd May 2004, 09:23

AN honest question, not a criticism.

What is it which leads you to think descending below MDH is acceptable under JAR-OPS? The only bit I could find is in JAR-OPS 1 Appendix 1 to 1.430 which is pasted below;

(2) Minimum Descent Height. An
operator must ensure that the minimum descent height for a non-precision approach is not lower
than either:
(i) The OCH/OCL for the category of aeroplane; or
(ii) The system minimum.

(3) Visual Reference. A pilot may not continue an approach below MDA/MDH unless at
least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:
(i) Elements of the approach light system;
(ii) The threshold;
(iii) The threshold markings;
(iv) The threshold lights;
(v) The threshold identification lights;
(vi) The visual glide slope indicator;
(vii) The touchdown zone or touchdown zone markings;
(viii) The touchdown zone lights;
(ix) Runway edge lights; or
(x) Other visual references accepted by the Authority.

Para (3) would lead me to believe we can not descend below MDH but I am open to other interpretations.

IEM 1.430 which refers to circling approaches also states repeatedly that you must not fly below MDA/H but is too long to paste here.


23rd May 2004, 22:45
The HL figures are from PANS-OPS. The same figures are applied in the current (and proposed) criteria for baro-VNAV approaches ...

24th May 2004, 03:58
This discussion reminds me of an issue that I fear is mostly overlooked regarding MDA's, DA's and missed approaches.

Here we are talking about how much of a big deal it is whether or not the momentum of your aircraft will take you below the MDA if you initiate a go-around at rather than above the published MDA.

That's fine. But who here has thought about this. You arrive at the MDA. You're "Visual", so you continue the approach below MDA toward the landing runway (I'm thinking of a runway aligned approach here). Then, for whatever reason, you decide to discontinue the approach, you press TOGA and head skyward again.

Reasons for the late Go-Around / Aborted Landing?

1) Becoming visual but subsequently losing visual reference again due to cloud drifting across the flight path.

2) Flying into ever increasing precipitation that makes it increasingly difficult to see through the windscreen.

3) Windshear and/or turbulence. (Not necesarily asociated with microburst/thunderstorms).

4) Previous landing aircraft too slow vacating the runway.

5) Previous departing aircraft Rejecting Take-Off and remaining on runway.

6) Animals on runway.

7) Crosswind component above Operations Manual "limits"

So here you are, going around from a position not that far off the deck. Makes the previous concerns about MDA vs RA rather academic! You have now possibly progressed 1 or 2 nm beyond the Missed Approach Point, at an altitude 100 ft, 200 ft, 500 ft below MDA. What the @#$%$ are you going to do now?!?! Have you even thought about what you'd do if or WHEN this happens to you? :uhoh:

I have some ideas but will expand on them later.

I think this worth starting another thread. If you care to respond to this post, please do so there.


24th May 2004, 07:33

the less precise your navaids, the higher you have to go-around because the greater can be your position error in relation to the rwy and the obstacles nearby.

But if you saw the rwy, re-assessed your position and continued the approach, your old minima make no sense anymore.

You know exactly where you are and you go-around because you can't see rather than because of the danger of hitting some obstacle due to position uncertainty....

In same cases you can even touch the runway during the go-around, right? But what took you there were either your eyes or a very precise approach system.

If English was my language I'd brobably be more clear....

Rgds, maddog