Mil rotary IR precision approach tolerances

[drugsdontwork]

JSP 550 says that for an IRT you have a tolerance on MDH/A of +100/-0ft. It does not however mention any such tolerance for DH. I always thought that the plus 100ft/-0ft applied to precision approaches too? Certainly Lasors section E has +50ft/-0ft for both MDH and DH. Seems a little harsh to have tolerances everywhere else but not on a DH?

Thoughts appreciated.

[HEDP]

Minimum Descent Height (or Altitude) is just that therefore you cannot go below the minimum but must aim to make the lowest height you can to give best probability of success.

Decision Height is a height at which you make a decision whilst still descending therefore you will go below the decision height until re-establishing the climb. Hence there is not a +/- figure placed on it. If there was a +/- figure then by definition it would become an MDH(A).

[drugsdontwork]

I realize you go below a DH. What I question is why there is not a +100/-0ft tolerance to commence the MAP. The civvies have a +50/-0ft for both precision and non precision approaches. Why don't we, or is it badly phrased in 550?

[HEDP]

I guess in simplistic terms that if you had a +50 tolerance on a decision height that you would never reach the point at which you make the decision or am I missing something.

Maybe the issue is with civilian interpretation of the procedure given that it is a military only procedure.

[Pub User]

.......procedure given that it is a military only procedure

I think you'll find that civvies fly precision approaches too!

[moggiee]

We call them ILS approaches and they come in CAT 1, 2 and 3 varieties!

Not rotary specific, but hopefully relevant:

MDA has a -0' tolerance because you are expected to not go below but also to fly level at the MDA until reaching (at latest) the Missed Approach Point at which time you begin the Missed Approach Procedure. As the MDA is (roughly) defined as "the altitude below which you do not continue the approach unless in possession of the required visual references for landing" it must have a -0' tolerance. Most fixed wing FTOs will make use of the +100' above tolerance by telling students to fly MDA + 50 which effectively gives you a much more usable +/- 50' tolerance.

DA, of course, does not have the "not below" restriction as it is only the point at which the decision to go-around is made. However, the go-around should be initiated immediately, in effect giving the DA a "-0' " tolerance. A typical jet airliner will lose something like 30' during the go-around as it overcomes its downward momentum and starts to climb. You may, of course, initiate the go-around any time you desire if above DA (if for example it becomes unstable, you suffer a system failure, experience windshear etc). A helicopter will also have that downward momentum, just less of it due to its slower approach speed and lower RoD.

With regard to the MDA and the "fly level at MDA to the Missed App Point (MAPt) and then commence the go-around and Missed Approach Procedure (MAP)", you will find that very few (if any) UK airlines actually attempt to fly level at MDA and their SOPs will actually mandate an immediate go-around and MAP if not visual. To ensure that they do not bust MDA they will add a buffer onto the MDA and create a company mandated non-precision Decision Altitude. BA do this, for example, even going so far as to print their non-precision DAs in their performance manual. Their SOP then allows them to use the same decision-making and handling procedures for both precision and non-precision approaches and it works nicely.

Eg: NDB has an MDA of 500'. Airline adds 50' to that creating a 550' DA. Crew fly a stabilised (not stepped) approach and if not visual at DA (550') initiate an immediate go-around and MAP. During the go-around the aeroplane will descend about 30', meaning that it never breaks the 500' MDA and all the rules are complied with - and more to the point complied with safely.

It has to be a stabilised/continuous descent so that the aeroplane is flown in much the same way as for an ILS and does not have a high rate of descent at DA (which would screw up the maths). This means that the maths/physics of the go-around work out the same as on the precision approach.

Reflecting this procedure, Jeppesen (and I assume Aerad) have now started to publish Decision Altitudes on many of their non-precision approach plates, rather than MDAs as previously. In order to use it as a DA, you MUST fly a stabilised (continous) descent and must commence the go-around immediately upon reaching DA if not visual. If you plan to step the approach or fly level at the bottom of it then the minimum becomes an MDA and you must not go below it. If the minima are printed on the plate as an MDA then that it how it is treated and you must NOT go below under any circumstances (unless visual).

If you think about it, once you've leveled at MDA in a jet airliner you will no longer be able to land the aeroplane safely because you will be above the visual glidepath and any attempt to land will lead to an unstable approach, which is strictly verboten in the SOP.

Hope that helps.

[moggiee]

I guess in simplistic terms that if you had a +50 tolerance on a decision height that you would never reach the point at which you make the decision or am I missing something.
Yes you are because a go-around decision can be made at ANY time between the start of the approach and DA so you can not put an upper limit on it. The +100'/-0' tolerance on the non-precision approach is not for the decision making but is for the altitude keeping on the "fly level" portion between MDA and the Missed Approach Point.

Almost no-one uses DH, by the way, DA or MDA are the almost-universal standard. You usually can't set QFE at Nairobi or Calgary, for example, and so it's safer to always use DA (QNH) and back it up with monitoring of the radar altimeter.

[kharmael]

Moggiee,

Most RAF Airfields use QFE and therefore have a DH. They also have more than one type of precision approach, ILS and PAR.

CDFA (Continuous Descent Final Approach) on non-precisions has been standard (at least on ME fleet) for a while now where as you said, you add the 50ft to the MDA and it becomes a DA, then you fly a 3 degree slope fully configured.

Good explanation on the reason for DA/H tolerances. :ok:

[drugsdontwork]

So for a civvy instrument rating test, if the pilot commences the go around to within plus 50 minus zero on a precision approach he/she has achieved the published requirement (lasors section E). For a military pilot the lack of a published tolerance implies he/she MUST hit the DH (conditions of light/nil turbulence etc). Why the difference? And many mil pilots I talk to think there is a +100/-0ft tolerance for a DH on an IRT (or once was).

[HEDP]

Sorry,

With regards military only I was referring to the hardware installation and not the flying of the procedure.

Please note that we are discussing the limits for the succesful passing of an IRT using this equipment and not how the decision heights or altitudes are calculated or applied in practice.

HEDP

[moggiee]

So for a civvy instrument rating test, if the pilot commences the go around to within plus 50 minus zero on a precision approach he/she has achieved the published requirement (lasors section E). For a military pilot the lack of a published tolerance implies he/she MUST hit the DH (conditions of light/nil turbulence etc). Why the difference? And many mil pilots I talk to think there is a +100/-0ft tolerance for a DH on an IRT (or once was).LASORS section E doesn't actually give tolerances for IR tests, just IMC tests - however, assuming that the same tolerances are applied then yes, the candiate will be expected to make the decision to go around at between DA and DA+50'.

As for JSP550 - I don't have access to that so can't tell you what it says. Perhaps you should ask an RAF/RN/AAC IR examiner what he expects? Civilian examiners have a handbook telling them how to conduct the test - do the military examiners have a similar handbook? (I never asked when I was in the service, just flew as well as I could on the tests).

[drugsdontwork]

I have consulted several mil IRE's from different rotary fleets and got different answers/opinions. JSP550 has the limits outlined already with mention of +100/-0ft for MDH/A but nothing for DH/DA. There is a line to say that further limits can be put in for specific force orders; predictably there is nothing in my own force orders.

[HEDP]

As a military IRE I would expect that 100 feet before DH(A) I would give the 100 feet to go warning and at DH(A) the handling pilot would announce DH(A) and I would then direct either a Go Around or Visual to Land whereupon either I take control to land or the handling pilot carries out the pre-briefed instructions prior to commencement of the approach.

Simples!:)

[drugsdontwork]

As an IRE myself I would expect do the same! However a civvy could start his missed approach 50' above his precision approach minima and pass his test. There appears to be no tolerance for the mil pilot. On every other aspect of the mil IRT there are guideline height limits: +/-100ft general flying (standfast master green) and +100/-0ft for a non precision approach. Does this mean that technically there is no margin for error whatsoever? Seems a little harsh?

[Easy Street]

In a Tornado IRT there are 2 precision approach test points, one PAR and one ILS. I don't have a copy of the test schedule in front of me, but from memory the wording is something like "safely complete approach to minima and correctly initiate missed approach procedure". This obviously implies that you must fly all the way down to DH/DA and not bug out early for any of the reasons mentioned previously. I can't recall any stated tolerance for the attainment of DH/DA: it is down to the examiner's judgement.

All of the other tolerances listed in JSP 550 relate to parameters that the subject must maintain for a period of time (e.g. constant heights, speeds) and flying along at MDA/MDH is one such. However, initiating a precision MAP at DH/DA is an "instantaneous" event and as such I guess they didn't think it necessary to specify a tolerance. Assuming the approach is flown safely to minima there should be no reason to initiate the MAP before DH/DA, and certainly no reason to continue beyond it. Which (kind of ) sounds like an informal +/- 0ft, left to the examiner's judgement! So the rules are not as prescriptive as the civilian equivalent but the practice is similar. I would certainly expect to fail an IRT for initiating the MAP below DA/DH.

[drugsdontwork]

Logical and sensible Easy Street, and I agree. I am uneasy with no margin for error though. In fact, 550 makes no mention at all of limits for a precision approach, plus OR minus height. By that logic, on an IRT, I could call 'decide' below my DH as there is no -0ft 'clause' stated as there is for a non precision approach. The only other height guidance is the 'generic' +/-100ft. The civvie IRT guidance is much clearer, stating a tolerance of +50/-0ft to commence an overshoot for BOTH precision and non-precision approaches.

[HEDP]

Easy,

Agreed, it is a point at which a decision is made, however accepts that during the commencement of the MAP that you will transitionally go below the decision height.

I would interpret this as a less stringent criterion to the civilian equivalent practice and that the civilian practice leads to less chance of success for the procedure.

I suspect that not many civilians would be conversant with a DH(A) procedure therefore there has been some element of read across from MDH.

[moggiee]

I suspect that not many civilians would be conversant with a DH(A) procedure therefore there has been some element of read across from MDH.
Could you run that one by me again? We use DA all the time and there is no difference between procedures for DA and DH other than the altimeter setting. The vast majority of instrument approaches will be flown to a Decision Altitude and rather fewer to MDA. Non-precision approaches are probably most commonly flown when training for the IR test!

Civilian DA/DH also allows for the transient "below minima whilst converting from descent to climb" state - as I mentioned in an earlier post. As such, it's just like the way the military do it (no big surprise there).

As an ex-RAF AT man now a civilian instructor I can tell you that there is very little difference between the two systems. A civilian IR examiner will expect a candidate to make a decision at DA but will allow them a tolerance of +50'/-0' feet on that - so in practice there is very little difference between the CAA and the MoD here.

drugsdon'twork - you can't call "Decide" below DA because the definition of DA precludes it! It is the "lowest altitude at which a decision to land/go-around must be made" or words to that effect. So even if tolerances are not specified the very definition of DA/DH sets a -0' limit.

Not wishing to be rude to anyone here but........this is really a non-issue. The tolerance is -0' and +50' (civilian) and, it would appear, -0' and examiner's discretion (military). It should be perfectly feasible on any precision approach to work within a -0'/+30' band.

[drugsdontwork]

I agree it is not a big issue but disagree that its a non issue. The number of different opinions on this is surprising. A military IR test currently places huge emphasis on instructor discretion, but in this case needlessly so. For a white rating you patently do NOT fly to the lowest permitted altitude/height once you add in your allowances. So can I bong a white rating DH on my IR test on the basis that I am above the procedure minima? Well, depending on your interpretation of 550, yes. But as we all know we do not operate that way.

Agreed that something like +30/-0ft is a reasonable tolerance, even better +50/-0. I just think it should perhaps be formalized in 550 to make an instrument rating test less open to individual IR examiner interpretation, and hence a little more objective, clear and sensibly aligned with civil procedures.

Rant over. I'm off for a beer.

[Tourist]

ddw

I don't think you understand DH at all.

MDH is tricky on your test, you have to be near it but definately not below it all the way to the MAP.

DH you continue on down and as you casually pass through it you make a decision. No tricky level flying, not even a specified ROD, in fact very little for the examiner to pick you up on.

Dead simple.

[HEDP]

Moggie,

Just for my understanding, bear with me please!

How many civilian airfields are equipped with PAR, I haven't found many, hence my query regarding whether there would be widespread civilian use of a precision radar approach.

I assume, and it is a big assumption as i am not procedurally trained, that ILS and GPS approaches are both non-precision therefore DA(H) does not apply and therefore MDH(A) does. Am I wrong?

If I am right then does that not mean that there would not be widespread use of DH(A) by civilians?

[drugsdontwork]

Tourist,

Thanks for clearing that one up for me. Sorted. Should have come to you first!!

[Tourist]

HEDP

You are incorrect in your assumption. ILS and GPS are precision approaches. Where did you get the "radar" bit from?

DDW
Whilst I recognise your sarcasm, quite frankly, yes!

[HEDP]

The platform I operate is not cleared for ILS or GPS approaches. Hence I am answering from a radar approach perspective.

GPS approaches are practiced but emergency only and have to have a non-precision minima applied.

[drugsdontwork]

Tourist, rest assured you are my first port of future call.

I have a reasonable grasp of the proceduresof an IRT though. My point is merely that the civvies seem do it better on this occasion, because in terms of precision approaches JSP 550 arguably leaves itself open to interpretation, Lasors section E does not.

Simples.

[moggiee]

Tourist: spot on with DH/MDH - operating to a DH/DA is a piece of cake and really doesn't need a lot of thought. An MDH/MDA is much more tricky if it involves the "fly level to the MAPt" section. That's why most of the UK airlines modify the MDA into a DA (as I posted earlier).

DDW: LASORS Section E still leaves a grey area because when I looked at it yesterday it only gave limits for an IMC test, not the (more stringent) IR test. Without seeing a copy iof the UK CAA examiners handbook, you are left to assume that the tolerances are the same.

HEDP: the only civilian airport at which I've done a PAR approach was Keflavik, Iceland (PAR provided by the US Navy). Their talkdown was less accurate than the VOR/DME (non-precision) approach that I'd just flown.

Civilian pilots are very unlikely to do a PAR approach but the staple diet is the ILS which is a precision approach, of course - and therefore the use of DA is almost universal. That's why I say that precision approaches are the staple diet of the civilian world.

[Talk Reaction]

Yeah DDW,

I also think you're confusing yourself into a point that doesn't exist.

The 200' (or 150' IIRC for rotary approaches) absolute minima is the lowest DH that can be used. That and the figures on the plates takes into account the descent post discussion. I'm sure someone will correct me but civvies can have much lower limits therefore it's safe to assume we're nowhere near the actual surveyed minima for an approach. So your concerns about busting a minima are unfounded.

I reckon you should worry about other things that your IR candidate is doing as I've yet to meet a pilot who couldn't make a decision at DH (I have however had a student who made he wrong decision - it turns out that basic studes are seldom still IMC at 200 feet and he'd kinda learned "200ft - decision - land" by rote, and promptly went quiet whilst attempting to land sans seeing.......)

HEDP - Are you really a Mil IRE and don't know what is a precision and non-precision approach, you might not have the kit to do it but that is a shocking lapse in knowledge - I bet your ground cat is a piece of........

[HEDP]

Talk Reaction,

A bit barbed there mate. You can only IRE on the kit fitted, hence no procedural rating therefore no ILS. I fully know what constitutes a precision or non-precision radar approach thanks. Never done an ILS, if I ever do then I guess then would be the time to know the minima and approach.

No lapse involved, not fitted, not trained, no need to be!

HEDP

[drugsdontwork]

I'm not confused actually but my point is not concerning approach minima or differences between mil/civvie etc. My point merely relates to JSP550 and the fact that the height limits for a mil instrument rating test are laid out oddly, stating "+/-100ft (but +100/-0ft on an MDH)" or very similar. Nothing more, crucially, for DH. So if a candidate on a white IRT flys a precision approach and calls 'decide' 50ft below HIS minima (but above the procedure minima) has he failed? Yes according to most mil pilots I speak to, no according to others as he is within the +/-100ft stipulated in JSP550 for IRT height limits.

And yes I know civvie rules are different but if one would care to look in Lasors section E and indeed JAR 2.210 then one would find that it is clearly laid down: +50ft/-0ft is the window to commence an overshoot for BOTH precision and non-precision approaches if you wish to gain an IMC rating or an IR. Much clearer.

Anyway, id much rather have a drink and a chat rather than engage in barbed exchanges for nil gain so cheers and good night all!

[Paul Chocks]

I'm a civvy pilot and I've done a PAR (at Lyneham for a cloudbreak - bloody stupid idea though considering I had a coupled autopilot that could have done the ILS for me!)

[HEDP]

DDW,

A white rating adds 200' to the published minima for the pilot concerned therefore he flies to minima +200' with a tolerance of +100/-0 therefore a fail if he calls at 50' below the minima calculated with his rating added in.

[drugsdontwork]

HEDP,
Show me where the tolerance is laid down for a precision approach! The +100/-0ft is mentioned ONLY for MDH (non precision) not DH (precision). My entire problem with this is that JSP550 does not specify a tolerance for precision approaches, only non precision. There appears to be confusion between accepted practice and what the book actually says in present form.

[HEDP]

The precision minima requires a decision at DH not 50 feet below it hence fail. Having taken the decision at (or before) DH you may transiently go below DH whilst re-establishing the climb.

Your entire problem seems to be that the precision minima requires a precise decision at DH, if you had a tolerance then it would not be a precise procedure with a precise decision as is required of a lower height during the go around.

You are quite correct in that a precise procedure requires a precise decision. Introduce tolerance then it becomes non-precise.

IMHO:)

[drugsdontwork]

Erm...a couple of posts ago you mentioned a tolerance of +100/-0 yourself when we were talking about a precision approach.

If precision is to be just that and tolerance-free why do civvies have a +50/-0 tolerance?

And anyway I don't mind zero tolerance, it just needs to be stated so that Examiners can act accordingly. Yes, examiner discretion is good and proper but the lack of guidance here requires sorting out. The overiding mil regs document has guidance that currently splits opinion among our pilots and that is not ideal when the subject is instrument rating tests when consistency and uniformity are crucial.

I'm really done now: back to SAR H thread!!

[Paul Chocks]

Although this doesn't answer any of your burning questions re what is the mil tolerance, the following link may be of interest as it gives an insight into how one mil to Civ examinee and examiner saw things on one particular occasion:

http://www.pprune.org/rotorheads/391831-some-clarity-required-decision-altitudes.html?highlight=IR

[moggiee]

You can only IRE on the kit fitted, hence no procedural rating therefore no ILS.

You need to separate "Procedural' from "ILS" because you don't have to have a procedural rating to fly an ILS approach.

I had an amber, non-procedural IR as a student on the JP3 in the early 80s - it just meant that I had to be vectored onto the ILS rather than self positioning (procedural). I got a procedural rating on the Jetstream because that had enough kit for me to be able to find my way around without help from ATC.

You do seem a bit confused because at one point you said to me that you assumed that an ILS was "not a precision approach" and at another point you tell Talk Reaction that you "fully know what constitues a precision or non-precision approach".

I assume that your "if you introduce a tolerance it becomes imprecise" comment was intended to be tongue in cheek - but of course it's not correct! A precision approach DOES have a tolerance because no-one can fly the localiser and glideslope 100% accurately and, depending on how fast you speak, your "decide" call will not be exactly at DA/DH - but it must not be late. The term "precision" means that you get guidance in both azimuth and elevation, whereas non-precision approaches only give azimuth guidance and require you to sort our your own elevation.

Hence PAR and ILS are precision approaches, but NDB/DME, VOR/DME or SRA (for example) are non-precision approaches.

DDW: you don't need to set a -0' tolerance for a DA/DH because the definition says that they are the LOWEST Altitude/Height at which you make the decision. Whilst JSP550 may not say -0' the definition makes it clear.

The upper limit is set because the examiner doesn't want to see you break off the approach early - he needs to see you go to/almost to DA/DH because that's what you will have to do in marginal conditions.

[drugsdontwork]

Moggiee,

Incorrect. Mil AIP definition of DH makes no mention of lowest (unlike MDH). Therefore I refer back to JSP 550 which gives a height tolerance of +/-100 with the only caveat specified for an MDH. Ergo, call decide up to 100ft below your minima (assuming still above procedure minima) and pass your IRT.

I am of course playing devils advocate but it illustrates the ambiguity. What 550 needs is a line saying call "decide" not below your DH minima. Ideally, to make the test objective, a plus height tolerance should be inserted too to minimize unnecessary examiner discretion.

I'm actually boring myself now with all this!