Adjusting the ILS minima

[Fonz121]

Question; On conducting an altimeter check at the OM you find the altimeter is reading 100ft high (as an example, OM height should be 3000 on slope and your altimeter reads 3100 with GS centred), are you allowed to adjust the minima? In this case raise it by 100ft.

I've been asked this in a formal situation and when given the OM scenario and asked what my actions would be I said I would revert to the LOC approach.

When pushed further as to what else I could do apart from that I said I would conduct the missed approach.

Once again I was asked if there were any other options apart from reverting to the LOC and conducting the missed approach. The only other thing I could think of was to raise the minima but I was hesitant in this answer as I have never seen it written in black and white that you can actually do this.

Based on the wording that if there is any "unexplained discrepancy" at the OM check then the ILS has to be abandoned, I wouldn't have thought this would be allowed.

Thoughts?

[havick]

would the GS not be considered suspect if the GS is centred, and whilst fixed over the OM there is a discrepancy in the check height (ie the error cannot be explained)

[Fonz121]

Well yeah that was my thought, but I was getting the impression the questioner was looking for another option. ie raising the minima.

I have heard this use to be allowed a few years back before my time by I can't find any reference to it these days.

[havick]

edited.. I just re-read your original post.

[Angle of Attack]

The danger is if the altimeter reads high actually at the OM height check. Used to adjust the minima up if it read high and dont change it if it read low. Although I notice with the AIP now it just says discontinue if there is an unexplained error, even I didnt know that must have changed, need to do some more research but seems like reading that you cant amend the DA up anymore..:ooh:

[Angle of Attack]

It has been changed here is a link I found quoting the AIP circa 2002

AIP states (in ENR 1.5 para 7.3.1) that
On final approach the aircraft altimeter indication must be noted at the fix or facility. If the aircraft altimeter indicates a higher altitude than the check altitude, the difference between the two altitudes must be added to the approach minima. If the aircraft altimeter indicates a lower altitude than the check altitude, corrective action is not required.

So I guess you can't do that anymore! You learn something everyday! :)

[Fonz121]

Thanks AOA,

That's what I was looking for. Good to know.

And to the gentleman who posted before more or less calling me a retard, telling me to do some "simple" research and finishing off with a few of these ones:ugh::ugh:, thankyou for removing your post.

[FRQ Charlie Bravo]

Thanks Fonz for posting that question and AOA for the research. I reckon I would have gotten that one wrong in an interview, I probably would have stumbled for a bit and eventually muttered LLZ but not before looking the fool.

Actually, come to think of it do you trust the glideslope or the altimeter? Fine if you've got two of each and can rule out the odd one out against three which agree but suppose the NAV2 is still on VOR (having just hunted for the lead radial) or perhaps you are PVT in a single-engine AC and only have one altimeter and one NAV.

So are we to assume then that it is best to conduct a Missed Approach or switch to LLZ? They both sound prudent but obviously one is safer than the other (assuming it's otherwise ops normal).

That makes me think of another question, what if you are conducting the ILS and the glideslope fails whilst you are below LLZ minima, can you just regain LLZ minima or must you accept that you are now outside of tolerances and go around? (Even if during the go-around you find yourself 'magically' on track and within tolerances for the LLZ.)
FRQ CB

[havick]

AOA has got it..

Fonz, either your C/T was checking to see if you knew yourself or he didn't know the reference had changed him/herself..

[Chimbu chuckles]

If the aircraft altimeter indicates a higher altitude than the check altitude, the difference between the two altitudes must be added to the approach minima

While I accept that is what it says in the AIP its horse****.

What you guys are discussing is the difference between Indicated Altitude and True Altitude

The formula to correct for this is;

TA = IA +/- 4' per Deg of ISA Devn(Ht/1000). 'HT' in the formula refers to Height above the Temperature datum (the airfield)

Or there are correction tables available - but probably not in the Oz AIP:hmm:

You should correct for TA for ALL IAL altitudes, LSALTs etc when temperature is significantly COLDER than ISA. No need when temp is warmer than ISA...which is what they are alluding to in that AIP reference.

Example: You're conducting an ILS at a MSL airport with 4000' MSA and its ISA - 20. So -5C

To correct the MSA;

TA = IA -(4x20)(4)

So if you descended to 4000' IA you would actually be at 3680' TA...leaving you only 680' above the highest obstacle. Therefore you would descend to 4320' IA and you would be 4000' TA therefore preserving your 1000' MSA protection.

If the platform altitude was 2000' you would correct that in the same way adding TA=IA-(4x20)(2) 160'. An IA of 2000 = TA 1840' - So platform should be raised to 2160'.

If the GS check height is 1000' you correct that the same way TA = IA -(4x20)(1) 80' BUT now you're following the G/S so you are passing over a geographical point following a fixed beam so with the G/S centered you expect the IA to say 1080' (your TA is 1000') and that is PERFECTLY correct and proper.

If the minima is 200' you correct in the same way adding (-4x20)(.2) - 16'. Now you're descending towards the ground and MUST be visual at 200' - if you descend to an uncorrected IA of 200' you will actually be at 184'...so you add 16' and at an indicated altitude of 216' your TA will be 200'. If visual land if not Go Around.

As you can see the magnitude of the ISA Devn altimeter error reduces the closer you get to the height of the temperature datum - the airfield. If you just add the difference at the OM you may not get visual and end up flying a missed approach for no good reason.

Now you guys do the correction of all the above for ISA + 20 and you will soon see why there is no need to correct IA in the ISA + case.

[DirectAnywhere]

There is a copy of a table and a nice little explanation and worked example here:

http://www.ce560xl.com/files/Cold_Wx_Altitude_Correction.pdf

Generally speaking, it's not of huge consequence in Australia or when flying an ILS as the error reduces as you approach the minima.

It is particularly relevant, however, for an FMC generated VNAV approach where the altitude constraints are built in to the FMC. If these are not corrected for cold conditions you may be several hundred feet lower than you think early in the approach.

Where it may also be highly relevant, but not for most Australian pilots:

ATC Minimum Vectoring Altitude (MVA) may not be increased due to cold temperatures. In Canada, MVAs are increased during cold weather operations. The FAA does not provide the same service. The flight crew should consider this if the aircraft is being vectored at MVA in extreme cold weather.

[alphacentauri]

There have been some temp corrn tables produced in the latest DAP amendment (126). Page DAP1-1 and DAP2-2/3 refer. It is my understanding that these tables are the precursor to an AIP amendment to follow in June requiring temp corrn to be applied when forecast aerodrome temps are below ISA -5.....thats all I know. Guess we will have to wait until June for the comprehensive guidance to be published.

In the meantime, you might want to familiarise yourself with the tables.

[404 Titan]

DirectAnywhere
ATC Minimum Vectoring Altitude (MVA) may not be increased due to cold temperatures. In Canada, MVAs are increased during cold weather operations. The FAA does not provide the same service. The flight crew should consider this if the aircraft is being vectored at MVA in extreme cold weather.
That is actually incorrect. All ATC MVA clearances in the US are corrected for low temp operations. We operate about 63 pax services plus about 30-40 freighter services a week to the US and our Ops manual is quite specific about it.
Radar vectoring altitudes assigned by ATC are temperature compensated and require no corrective action by flight crew member.

[DirectAnywhere]

News to me! Thanks.:O

[Metro man]

All these complicated calculations are very impressive and highly theoretical, I think the correction is there for the benefit of general aviation pilots flying tired old Barons and Navajos with wonky instruments. I remember flying these sort of aircraft and more often than not there were inaccuracies in the panel.

Modern jet transports with central air data computers and highly skilled maintenance are a different matter.;)

[Chimbu chuckles]

Metro man I explained it fully so people would understand it fully - and it is JUST as relevant to 'Modern jet transports with central air data computers' - I fly 777s and when I am crossing the Austrian Alps etc in winter I amend the Flight plan MORAs using that formula and a handy ATIS.

I do get weird looks when I do so:ugh:

There is not an aircraft on the planet that corrects for ISA deviation altimeter errors.

Its all about the height of the 'column of air' the aircraft is sitting 'on'. When its cold the air is denser than when its hot.

You may not think it has have any real effect for MOST Baron/Chieftain pilots - what about those operating in Tas/Vic/SA in winter?

On the other hand how can you do a meaningful G/S check at the OM in ISA+20 unless you have done a quick calc to see what it should say.

Sailing past the OM at say 1500' and noting the altimeter says 1400' and simply mouthing "OM 1500', Checked" when it DOESN'T say 1500' and you have no freaking idea why doesn't sound smart to me:rolleyes:

[Capt Fathom]

Sailing past the OM at 1500 and noting the altimeter says 1400.

As would happen at ISA+20!

[Angle of Attack]

Well regardless of all these calculations the high at OM add to DA/low at OM disregard is still safetywise safe. If its undereading you will hit the DA at a point higher up on the GS beam, if its overeading you will hit the DA at a lower point on the beam, takes care of all altimeter/ISA/temperature errors all at once. Dont know why they changed it actually, there was always scope for temperature corrections in the AIP.

These questions are useful actually as they highlight things you thought you knew then look at the books and realise the Airservices NAZIS have changed it years ago...:ok:

[404 Titan]

Metro man

I think you are CX. If you are I suggest you re-read Ops Manual (Part A), Sections:
8.1.1.4 Low Temperature Altimetry,
8.1.1.4.1 Low Temperature Correction to Minima,
8.1.1.4.2 Low Temperature Correction to Procedural and Enroute Altitudes.

[sleeve of wizard]

Chuck, do you also correct for the pressure difference? QNH below 1013.2 (29.92 in HG) will require a correction. Taking your Austrian Alps example, during the winter very low QNH's seen with the winter storms.:cool:

[ANCIENT]

Very Dangerous quoting old AIP reference. The current date for ENR 1.5 - 32 para 7 is 27 AUG 09
Currently if there is an unexplained discrepancy at the check point then an immediate go around is called for.
So if temperature has been taken into account and all is as calculated then continue.
And remember a correction calculation must be made for every check height,
GS intercept-OM/DME check and minima.

[404 Titan]

sleeve of wizard

I’m not sure what you are getting at? If you are in the terminal area you should be on QNH, not standard. If, as I think you are alluding to, you should correct the “Minimum Route Altitude” for high wind speeds which can cause the barometric altimeters to over read because of a possible non-standard atmospheric pressure, i.e. on the lee side of the mountains, then I agree with you. Most airlines should have correction tables for this in their ops manual.

If though you are suggesting while being radar vectored in a TMA, ie MVA that you need to make a correction simply because the QNH is different to standard then I don’t agree. If you are descending to an MRA after say a depress then you would be descending on an area QNH, not standard. Wind correction would be required if in my companies case the winds are greater than 30 kts at MRA.

[anothertwit]

as this is the GA section i can only assume the original question was regarding the good old "trying to trip you up" questions used by interviewers and on renewals.

as i understand it, if there is an unexplained discrepancy you have two options.

1. continue with the LLZ app

2. conduct a missed app.

and FRQ CB as to your question ie if your below LLZ minima you only have one choice, missed app!

[Metro man]

The subject has been discussed numerous times in the past as this thread from the year 2000 shows

http://www.pprune.org/tech-log/10870-ils-app-outer-marker-glide-slope-altimeter-checked.html

The Flight Safety Foundation also have a publication related to this

http://flightsafety.org/files/alar_bn3-1-altimeter.pdf

Altitude/Temperature correction charts show only a small correction if the temperature is above -20C and the airfield below 700' elevation. -30C and 5000' gives an 820' error

Barometric altimeters and manual flying obviously have their limitations, hence Rad Alt and autopilot for CAT 3.

as i understand it, if there is an unexplained discrepancy you have two options.

1. continue with the LLZ app


Some airlines won't allow this below a certain altitude as there isn't sufficient time to adjust, a go around is mandatory

[FRQ Charlie Bravo]

Digging up this old thread after a considerable amount of research (PPRuNe, AIP, textbooks, YouTube etc) I thought I should point out a new link to an old video about NZ60 into Faleolo.

New Zealand Flight NZ60 A Free Lesson

This video, whilst not exactly about Temp corrections, is about altitude cross checking and for that reason is entirely relevant to the topic at hand (i.e. an "unexplained discrepancy").

FRQ CB

PS For some reason this video keep getting deleted despite the fact that it shows ANZ in a good light... maybe because people keep editing it and make the airline look bad. Not much of an issue now that YouTube don't have the old 10.5 minute limit.

[stable approach]

Get the Snowtam app for I-phone - makes the temp correction very easy!

[muffman]

Sensational video, thanks for posting it.

:D

[havick]

Great training video!

I wonder if there would have been a very different outcome if they were in IMC to the minimas (ie. not seeing the lights - trigger mechanism).

** Note Armchair expert hat on.

[Checkboard]

as i understand it, if there is an unexplained discrepancy you have two options.

1. continue with the LLZ app

2. conduct a missed app.

You can't continue with the LLZ approach. :ugh:

Think about the reason behind the rule. If you have an unexplained error at the check altitude (so we are not talking about temperature corrections) then you MUST suspect your altimeter is mis-set (or has a large error). You thus would be pretty stupid to continue to descend IFR to ANY minima (such as a LLZ minima) knowing that your altimeter is mis-set. :ugh:

The AIP rule is there to get you to go-around, identify and fix the problem (confirm the QNH, inquire as to the serviceability of the Glide Slope, inquire as to whether an aircraft was taxiing past - ie. it wasn't protected, cross-check altimeter/static sources etc etc) - and once it is fixed, you may attempt another approach (of whatever type).

[Oktas8]

Regarding temperature corrections.

You can get used to the topic, by always considering "hot day or cold day?" when flying an ILS. Get used to the altimeter under- or over-reading and you'll develop a feel for what looks right on any given day. This will help greatly when one day you have a 50' error at the FAP because it's 40 degrees at sea level.

On another note. Why do Australian pilots like to check height only at a marker beacon, notwithstanding Ancient's advice? DME (or GPS in lieu of DME) is both more accurate and more flexible.