AIP ..ILS - Also confused

[The Hedge]

AIP 1.5- 32 7.3


Altimeter Checks:

The final aproach contains a fix (FAP) at which the glide path/altimeter realationship should be verified. If the check indicates an unexplained discrepancy, the ILS approach should be discontinued. The following should also be complied with:

a. To maintain obstacle clearance,both LLZ and GS should be maintained within half scale deflection (or equiv. on expanded scale

b. If at any time during the approach, after the FAP, the LLZ or glideslope indicates full scale deflection, a missed approach should be commenced.

c. For ops where the temperature on the ground is colder than -15c the DA/H should be adjusted for true altitude.

However....

AIP 1.5.-7 1.10

A missed approach must be executed if:....

A: during the final segment of an instrument approach, the aircraft is not maintained within the applicable navigation tolerence for the aid in use

(b) seems to contradict (A)



What the...?

[Zhaadum]

Dont lose any sleep over this. It is just another of many examples of the often contradictory AIP/CAO/CAR bull**** we have in OZ.

Stay within tolerance 1/2 scale and you will be ok mate! Easy peasy if you ask me.

Cheers,

[radar o'reilly]

Zhaadum is right.

Remember the discrepancy in case it affects you in an exam, but set your own standards in the aircraft. If I am losing the ILS by more than ˝ scale deflection but still within the operation of the guage, I'm going missed approach.

Simple as that.

[Bula]

hahaha... should we bring more of these into the equation???


The best one would have to be at the check height how they changed from telling you what you should do to telling you that if you think there is a noticible descrepency you should conduct a missed appraoch.


I wonder... if they have chaged to reg from 1/2 scale to full scale............ does that mean the obstacle clearance splays for the ILS have changed aswell?

anyways if you cant hold it half scale................. move your hand to the other stick... either way you should find the correct glide path.

happy hunting

[ITCZ]

Chaps,

All good advice, but did it answer the question that was asked?

AIP 1.5- 32 7.3 (Jepp Terminal 2.7.3.1) Altimeter Checks is the only AIP/Jepp reference to an actual tolerance in the IAL/missed approach.

The only other time navaid tolerances are mentioned in the AIP or Jepp books is in relation to (1) Aircraft Off-Track in CTA - Advice to ATC (Jepp ATC 5.4.6.2) and (2) Avoiding CTA (Jepp ATC 5.5.2).

These sections are often remembered as the 'tolerances' for other stages of IF track keeping.

They are pretty close, but not definitive.

The correct reference for tracking tolerances in instrument approaches comes from CAO 40.2.1 Appendix I, the Instrument Rating Test.

3 FLIGHT TOLERANCES

3.1 Flight within the tolerances specified is necessary for the applicant to be judged proficient in the required flying manoeuvres. There shall be no sustained errors in excess of the specified tolerances.

[snip by ITCZ]

3.5 Navigation aid tolerances:

[snip by ITCZ]

(c) ILS and LLZ ± 1/2 scale deflection (and equivalent on expanded scale); able to land from minimum altitude without undue manoeuvring;

You could argue that a tracking excursion outside half-scale deflection, that has been noted and a correction applied, is allowed and executing a missed approach is not mandatory. Exceed full scale in any way, shape, or form, and you must discontinue.

Your ops manual (and your personal standards) might apply tighter tolerances.

But you asked what the rulebooks said!

[kavu]

my comment would be to use two check heights so you can verify the g/s.

the dme distance that you capture the slope.

ie. 3000ft approx 9.2 DME (all depending on the approach plate, I used AA ILS/DME 23L)

and then another height along the way. this will verify the slope.

by using two heights the error is reduced and is a straight line to compare.

hope this helps.

[DownDraught]

I found this too to be ambiguous The Hedge. No where could I found the exact meaning of a "unexplained discrepancy". So where/when is this limit reached? In IFR terms the best I could find/reason is that it is somewhere around 60 - 75 feet. I self taught myself the IFR theory, and from the material i used (an outdated Bob Tait study book) and by searching the docs I couldn't find an answer, the reason I say 60-75ft is because of two things.

1. All practice questions under this margin where allowed, all those over where not.

2. It is the upper allowable instument tolerance that must be checked pre flight.

To actually *know* the correct margin/meaning would be great?

What would the powers at be say in a court of law I wonder?

"You where on G/S with an altimeter reading 100 ft below that indicated on the chart, doesn't this constitute a unexplained discrepancy Mr Pilot"

[The Hedge]

Thanks for all the replies thus far.

It is widely accepted practice that half scale deflection is the maximum tolerance, however there are no hard and fast rules it would seem. It is up to the individual to set the standard.

A bit crap, considering all the junk in the AIP/CAOs but no specifics except to where ITCZ has pointed out.

[I Fly]

The Outer Marker or DME Altimeter check is simply to make sure your Altimeter reads correctly. If you had a 220' minima and the Altimeter over read by 220' you would get a surprise on breaking visual at minima. The unexplained error comes from the question "am I on Glide Slope, above or below it? If you are 1/2 scale above you would be at a greater height and that would be an explained error. Now, you only have to quickly mentally work out "I am 0.0X degrees high at X distance, using the 1 in 60 rule, that makes me XX feet high. The check height + XX is what I had on the altimeter, therefore my altimeter is correct" Crunch, o s**t wrong calculation.
When you consider that in ISA the first millibar drop occurs at 16' and the difference between 100 000' and 200 000' is also 1 millibar, then we can see that the altimeter is a bit of a rubber band when it comes to measuring height. Normally that does not matter because the aircraft next to you is also measuring with the same rubber band. However when you approach the Ground it is much more 'fixed' and therefore much more caution is required. The airlines use Radar Altimeter height and even that varies with the aircraft's pitch angle.
As far as 1/2 scale or full scale goes on Glide slope or Localizer, if you are 1/2 scale you can still 'see' where you are. Once you have full scale, how would you know whether you are 3 degrees off or 30?

[ITCZ]

I Fly...

That is not the reason for the OM/DME altimeter check.

The combination of localizer (azimuth), glideslope (elevation) and OM/DME (distance) intersect each other at virtually 90 degrees and when you are at that X,Y,Z intersection you are in a very small box, a much smaller 'circular area probable' box than even a Differential GPS can put you in. Certainly smaller than IFR altimeter tolerances.

There are two reasons for the check.

1. For you to ensure that you are on the correct lobe for glideslope guidance. Your NAV equipment can possibly capture a 'false' lobe above or below the 3 degree path that you want to be on. This is also the reason that you were (probably) taught to intercept the glideslope 'from below.'

An example of a false lobe is if you have ever had a false localizer capture, where you are say on an arc to intercept the localizer, you haven't passed the lead bearing yet the localizer bar comes alive. Something I've seen about every one in three times intercepting the Cairns 33 LLZ/DME from the southwestern arc.

2. To ensure that in "ISA minus lots" conditions, the sort of cold conditions that we don't see much of in Oz, that you do not descend below the Cat I Decision Altitude.

Presuming you are on-slope at the OM, if your altimeter indicates a higher altitude than the OM check altitude, you should add the difference to your DA, to prevent you going below the Cat 1 minima.

The airlines do not use Radar Altimeter height in Oz, that is a Cat 2 ILS procedure.

The then-CAA used to publish pilot notes on the ILS, VOR and NDB. Worth a read.

Your comments about altimeters as 'rubber bands' are true, but a little bizarre in the context of this forum. Not too many readers here are cruising above 100,000'. A standard altimeter is considered good enough for instrument approaches close to the ground where Airservices charts round figures to 5', it is good enough for VFR/IFR separation of 500' up to FL200, and only starts to run out of puff if you want to use RVSM airspace at or above FL290 when you start being separated from other IFR by 2000'.

-----------------------------

I disagree with the other comments such as "the rules are confusing and contradictory, therefore I will apply my own standard."

Granted, we don't/didn't get paid super salaries in GA, but the courts and the regulator don't give two hoots about that.

The rules are in the books, and the books are out there to be read. Not as easy to read as a Picture magazine, but there they are and the presumption is that they be read by us.

It is also presumed that our personal standards are built on a good knowledge of the rules, not as a reaction to them.

Now, let me give you an example and see what you guys think.... what would you REALLY do here?

Your flight started out as a nice little jaunt. Low cloud at your destination, but stable conditions indicated in the forecasts on your briefing. Your knowledge of the route and the destination gives you no cause to doubt that this will be a routine journey.

You have a maximum pax load, and therefore are carrying just the fuel required by your calculations plus a small margin, you could even nick off to a nearby military base if something prevents you from landing at your planned destination at the last minute. In other words, you have fuel to meet the requirements as per your valid brief, plus a little bit of margin.

OK, as you pass your last point of safe diversion to an off track field, you get a call from ATC and they inform you that conditions at your destination have deteriorated significantly. The conditions are currently fluctuation at or about the minima, and are expected to worsen and eventually sock in. The nearby military field is already at minima and will also shortly be unavailable.

After saying "Fire truck" a few times and double checking ATIS's, AERIS and FlightWatch for all possibles, you realise you don't have enough fuel to divert anywhere in the clear.

You do see though that if you proceed, you will have one good shot at the runway before it really craps in.

You fly the ILS, and man its a rough old night. Gusting winds, the whole lot.

About halfway down, due to a bit of terrain or whatever, the crosswind that you were holding off a bucketload of drift for, disappears. You shoot out the 'windward' side of the localizer, you pass half scale deflection.

Now, according to the 'personal standards' advocates, they would go around.

But if they knew from the books what they were allowed to do, such as exceed a half scale deflection so long as the error is not sustained and does not exceed full scale deflection, they would know in the heat of that moment that they could make a heading change, get back on the localizer and continue the approach to a reasonable chance of getting visual at or before the DA instead of going around and rooting their last chance, WHEN THERE WAS NO REQUIREMENT TO DO SO.

[I Fly]

ITCZ, both of your reasons given are quite correct, but they are not the only reasons. What would your altimeter read at the outer marker if you had set the QNH 10 HPA? If you were on the GS it would read 300' high but still in that small area defined by the outer marker. Would you assume you were on the wrong GS lobe and execute an overshoot or would you add the 300' to the DA? Would you check your QNH setting?
If you crossed the outer marker only 50' high, you would not be on the wrong lobe and it probably would not be an incorrect altimeter setting. Regardless of whether it is super cold or hot you must add the 50' to the DA because the error can't be explained.
The booklet you refer to does actually not say much. There are 2 references to the Markers.
4.4—FAN MARKER LIGHTS
The fan marker receiver is usually equipped with three lights—a white light which is associated with airways marker beacons and the two others (blue and amber) are associated exclusively with the ILS. These lights are mounted on the pilot's instrument panel adjacent to the cross pointer indicator. Although all fan markers are on the same carrier frequency (75 megacycles), the different lights are designed to operate independently, depending on the modulation frequency (tone) of the signal being received. A 400-cycle band-pass filter is included in the blue light circuitry and a 1300-cycle filter for the amber light.
When the aircraft is over the outer marker (400-cycle tone) the blue light flashes in synchronism with the coded signal being received (2 dashes per second),
When the aircraft is over the middle marker (1300-cycle tone) the amber light is similarly energised (alternate dashes and dots).
The fan markers serve as a definite radio fix to mark the progress of the aircraft down the approach path. The pilot normally utilises them to monitor the alignment of the glide path by comparing the altitudes at which each marker is actually crossed, on the glide path, with the crossing altitudes specified on the instrument approach chart for the facility. (See specimen approach chart at the back of this publication.)
Figure 1 illustrates the function of the cross pointer indicator and the marker lights related to the aircraft position on the ILS.
And
6 -Points to Observe When Flying the ILS
1. Carefully study the appropriate ILS approach chart before committing the aircraft to an ILS holding pattern or approach.
2. Ensure that equipment indications are normal and that flag alarms are not visible before committing the aircraft to holding or final approach.
3. Identify the ILS aurally and select the marker beacon aural switch for aural identification before commencing final approach.
4. Check the aircraft altitude through the marker beacons with the altitudes given on the ILS approach chart. This will provide a check of the glide path alignment.
The AIP states that false courses may exist or course reversal may occur outside the sector 35 degrees either side of the localizer course. (nothing said about the glide slope and in any case 35 degrees would be a loooong way off and should be apparent if the pilot has some situational awareness).
Appendix 1 of CAO 40.2.1 states the requirements for a FLIGHT TEST. The AIP states the requirements if you fly a 'none flight test ILS'. (there is also a difference for when you can commence descent on a NDB approach).
I realise that we do not fly above 100 000' but in ISA when the altimeter indicates you are at 30', you are actually only at 16' (remember we do not use an altimeter to measure an obstacle's height). The 500' separation between VFR and IFR still works because both aircraft use the same 'rubber band' and we are still well apart. At the DA we are around 1/2 that height above the ground but the ground or an obstacle does not use a 'rubber band' to measure its height.
As for your example "what would you do"?
The TAF at the 'nearby' military field would probably read pretty much the same as the destination and REQUIRE fuel to fly to a SUITABLE alternate. Therefore the problem should not arise in the first place. If I were on long haul and got my TAF 14 hours ago, I would get an update BEFORE I pass the last point of safe diversion.

[DownDraught]

Mmmmm

ITCZ, in your example, why wouldn't you declare an emergency????

As for the alt v gs issue

"glide path/altimeter realationship"

Is it not a check for both, and simply if it is not within tolerances, then you must discontinue the approach, and one would think you would find out why???

"The final aproach contains a fix (FAP) at which the glide path/altimeter realationship should be verified. If the check indicates an unexplained discrepancy, the ILS approach should be discontinued."

[turbinejunkie]

Hi all,

I have wondered about that changed wording regarding the GS / OM altimeter check in the Jepps.

Why have we gone from (in the regs I mean) adding to our decision altitude any difference noted at the check height fix (assuming on glide-slope at the time and identifying an overreading altimeter)

to

"If the check indicates an unexplained discrepancy, the ILS approach should be discontinued"?

Is this simply more CASA arse-covering stuff to avoid being sued in incident / accident from someone stuffing up their ILS or is there a good reason for the changed wording that I'm missing?

In practice, would CASA still view it as legit to add to DA any discrepancy identified from the check height? If so, why the change?

Depending on your point of view, the new wording seems more ambiguous and open to disregard than previously.

Could someone in the know please enlighten me on that?


TJ

p.s.

[FlareArmed]

I may be able to shed some light.

Taking Canberra for example, on a typical cold winter morning of -5C, the altimeter over reads by about 100' at the outer marker, but only 20' at the minima. If the temperature was -15C, the error at the outer marker would be about 150' and, at the minima, 25'.

On a typical Canberra morning, if you were to follow the old AIP procedure, you would unnecessarily add 100' to the minima.

The new procedure, IMHO, is a gross error check that guards against such things as false glideslope captures, having the incorrect chart or having the incorrect QNH set (eg. 10 mb/300' error). I don't believe it was ever intended to have the OM check height as a tool for fine tuning the minima.

For more information about altimeter errors at cold temperatures, look here.

[turbinejunkie]

FlareArmed,

sorry about the delay in getting back to you but I have been away from PPrune-land for a couple of weeks.

Appreciate your response and thanks.

TJ