Going below the ILS DH in a go-around

[Tee Emm]

Regarding the design of instrument approach procedures for an ILS. What ICAO document (if any) states that the selection of the 200 ft DH on a Cat 1 ILS allows for the aircraft to sink below that DH in the process of transitioning from descent to a climb. Am unable to find anything that discusses this in Australian AIP or equivalent in Jepps. Some operators require 50 or 100 feet added to the chart ILS DA (DH) in order to prevent the aircraft from going below 200ft during the go-around procedure. My understanding being the DH takes the transition from descent to go-around into account.

[MyNameIsIs]

TM,

http://www.casa.gov.au/wcmswr/_assets/main/manuals/regulate/aocm/011r0719.pdf

That is part of the the "Air Operator Certification Manual" for "Other than High Capacity RPT, Initial Issue of AOC".....
Your ops manual or similar document would probably contain a section on Aircraft Landing Minima and Aerodrome Operating Minima etc.



7.19.1.1 Precision Approaches
For precision approaches, the published DA or DH has incorporated, for each category of
aircraft, an allowance for the height that aircraft will descend below the decision altitude or
height, after a pilot has decided to initiate a ‘missed approach’. The actual height loss will
depend, in addition to inertia, on factors such as the skill and training of the pilot, physical
characteristics of the aircraft (distance between the wheels and the glide-path antenna),
and altimeter or other instrument errors.
For Cat I operations involving small- to medium-size aircraft, the addition of altimeter
pressure error should produce an acceptable ALM.

Note:
Altimeter pressure error for various aircraft configurations is commonly given in

the aircraft Flight Manual.

Where altimeter pressure error is not known, the addition of 50 ft to DA is approved by AIP



DAP as an acceptable procedure.



Hope that helps in some way.

[27/09]

My understanding being the DH takes the transition from descent to go-around into account. Correct.

It is a (DA) decision altitude NOT a (MDA) minimum descent altitude.

[neville_nobody]

In some aircraft a CAT IIIB goaround the wheels will touch the runway.

[Cypher]

Try ICAO PANS OPS 2 document 8186, design of instrument approaches.

DH obstacle clearance is a function of type of altimeter used (radio/baro), aircraft speed and aircraft penetration inertia.

Altimeter Tolerance + Aircraft penetration (height loss) = required obstacle clearance. on a ILS where no obstacle penetrates the obstacle assessment surfaces.

[zanzibar]

You cannot go below the minina, full stop, unless visual, of course. From the same document referred to in a previous post - my bolding and underlining.

7.19.1 Aerodrome Operating Minima

Aerodrome Operating Minima are published for each Instrument Approach Procedure and consist of altitude/height minima and visibility minima. The altitude/height minima for precision approaches are known as Decision Altitude (DA) or Decision Height (DH) as derived from a radio altimeter, and minima for non-precision approaches are termed Minimum Descent Altitude (MDA). MDAs can apply to either runway or circling approaches.

Paragraph 1.18 of AIP ENR 1.5 requires operators to consider, and adjust for, the likelihood of each aircraft type being at altitudes below that expected for the declared minima. The adjusted minima are Aircraft Landing Minima (ALM). ALM must never be less than the Aerodrome Operating Minima.



My understanding being the DH takes the transition from descent to go-around into account.

Correct, as long as it is the adjusted DH (now known as the ALM) which prevents your sink taking you through the Aerodrome Operating Minima.


In some aircraft a CAT IIIB goaround the wheels will touch the runway.
Quite understandable because some Cat IIIB operations have no DH. Following is an excerpt from CASA Document EX 36/10 as an example:

8 CAT III B minima for A330-200, B777-300ER, A340-500 and A340-600 aircraft are:
(a) visibility: 75 m RVR TDZ and 75 m RVR MID and 75 m RVR END; and

(b) DH: No DH.

[The Green Goblin]

This question comes up quite often and it depends on who's interpretation (and sometimes you need to agree with the said person for a stamp in the check) as to what is correct.

Most people I have dealt with take a practical perspective of the DH, it's a DH not a restrictive height like an MDA. Quite simply, at the DH you make the decision to commence a missed approach, and if you drift below the DH, it is not of any consequence, provided you meet the climb gradients once established in the missed approach configuration (and don't hit anything in the mean time).

Other schools of thought are as per zanzibars diligent post.

I tend to take the more restrictive view (and agree with zanzibar) and err on the side of conservatism.

[Tee Emm]

You cannot go below the minina, full stop, unless visual, of course.

Interesting. The Boeing 737 Flight crew Training Manual has this to say when discussing an auto-land for Cat 3A which as a 50 ft RA DA: "During an automatic go-around initiated at 50 ft, approximately 30 feet of altitude is lost"

So if you cannot go below the minima unless visual (as you say above in the highlight) then does that mean the revised DA for Cat3A should always be 80 feet RA and not 50 feet RA because you will always go below 50 ft during the go-around?

Furthermore, as the Boeing advice for the 737 is that approximately 30 feet is lost during the transition from descent to climb then does that mean that all Cat 1 ILS DH of 200 ft should be increased by 30 ft?

[zanzibar]

TM, you mention the 737 but the document referred to here is for "Other than High Capacity RPT" so it doesn't stricly apply (unless you've got a 737 to fly privately, ha, ha!!). However, the AIP ENR reference is quite unambiguous and you must take in to account the sink by calculating an ALM. Unfortunately I have no access to 737 documents at present to read fully what they say rather than possibly this isolated reference to the matter. It could be that they are making you aware of the anticipated sink and that you have to make an adjustment (i.e to avoid "being at altitudes below that expected for the declared minima"). You would also need to know what the FAA rules are - maybe they tolerate going below the DH in IMC?

[Capt Fathom]

Paragraph 1.18 of AIP ENR 1.5 requires operators to consider, and adjust for, the likelihood of each aircraft type being at altitudes below that expected for the declared minima.

The ILS DH is NOT a declared Minima! It is a Decision Height!

[Trent 972]

According to SKYbrary (It's wiki stuff, - but content control is from Eurocontrol)
Decision Altitude/Height (http://www.skybrary.aero/index.php/DA)

Missed Approach must be commenced at the DA/H unless the required visual reference has been established. Calculation of the DA/H takes into account that the aircraft will descend below the DA/H during the missed approach.
It's to do with surveyed paths and precision approaches. ie A/C aligned with the runway.
That's how we do it in QF. (and every other operator I've ever worked for)
(MDA for a NPA is a different matter.)

(edit) ref. Zanzi's post about ALM, in the case of an ILS that would include 'Cold Weather Corrections' the final result not being less than the published minima. Nothing to do with the Go-Around procedure.

[Pontius]

Unlike a Decision Altitude (DA) associated with a precision approach, where loss of height during the initial stage of a missed approach is taken into account, obstacle clearance is not assured if descent below the MDA occurs, and pilots need to ensure that descent is arrested prior to reaching the MDA.

This, from CASA, discussing NPAs (CAAP 178 1-(1) ).

I'm afraid I have to disagree with Zanzibar but I will qualify that by stating that I am only talking about RPT airline operations.

Quite simply, at DA you make a decision (hence the term). If you decide to go around then that DA will take into account a descent of, up to, 50' during the manoeuvre (see PansOps, and Terps for the Yanks). PansOps allows for the design of the approach and missed approach procedure to ensure you don't hit bits of terra firma during this stage. Clearly, if bits of the surrounding terrain are going to impact on that design then the DA will be higher than, for instance, 200' for a Cat 1 approach.

I also agree with the comment made by Trent 972. Account does need to be made for temperature extremes on Cat 1 approaches. Obviously, barometric altimeters (including other pressure-based instruments, as supplied by an ADC) are affected by instrument errors when the ICAO standard atmosphere is deviated from and this is very true when looking at the 'dangerous' case of when it is very cold. The airline operators I have flown for have all had a table for calculating the additional altitude to be 'added' to the DA when operating in extreme cold (and, yes, I've done it but only down to -40, so I'm sure there are better stories than mine around). But this does not affect the actual decision altitude, just what is indicated on our instruments and no account needs to be made for descent below DA. Of course, Cat II/III approaches are not affected as they use radio altitude.

MDAs are just that and can not be descended below without the visual requirements being met. Clearly the MAPt allows for clearing the bumpy bits around the airfield but now I'm straying away from the question again :rolleyes:

Having re-read what I've just written I think I could have saved myself the trouble and just typed 'ditto' beneath Trent 972s post (but replaced QF with other abbreviations) :)

[spacemantan]

Apart from what has already been mentioned, another factor to consider is the limited time you spend at the DH.

If you decide to add extra height outside of recommended practices, you could ultimately be cheating yourself 30-50ft. In most cases (infact I would assume 99 out of a 100 in australia), this wouldn't be an issue, but on the off chance that the weather is really socked in around an aerodrome at minima's, it could mean the difference between affecting a landing or conducting the missed approach.

Personally i'd be pretty pissed off if I executed the missed approach a little higher and during the 30-50ft of driftdown, saw that if I had continued to the actual DH I would have been visual.

My understanding of AOM (please correct if i'm wrong) is that contracting states (ie Australia), must require its operators to establish their own minima's not less than those published whilst taking into account a whole range of things including aircraft performance, crew composition/experience etc etc... I don't think it has anything to do with DA being a hard or soft altitude...