Tolerances on the ILS

[Julian]

Thought this would be the best place to post this question.

I hold an FAA IR and I am intending to convert in the future to the JAA IR via the 15 hours, etc route. I understand that the ILS is required to be flown to 1/2 scale deflection rather as in the FAA test where you can carry on as long as you don't go full scale.

Whilst I don't have a problem flying it 1/2 FSD my question was what is the thinking behind this? As long as you dont peg the needles then you are maintaining your situational awareness on the loc/GS and therefore be able to carry on - once you peg it then you have lost it! The only thing I could think of was it makes sure that you are in a more stabalised approach down the ILS.

I would be interested in views or the 'offical' answer if anyone knows it.

[Genome_Junkie]

When I did my IR at PAT I had a sim instructor, Steve.

He said to me: What are the tolerances?
I said: ehm +/- 50ft or so etc etc.
He said: And what are my tolerances?
I said: 0?
He said: Right. Theres a good boy. And now you fly it that way.

I passed.

Stick to what it should be: On the dot. And it is do-able.


Cheers

Arnie

[Julian]

G-J, Yep I understand that and thats what I aim for when I shoot an ILS. You would be pretty nuts to try for anything but nailing the needles!

BUT the question was why the difference? Why go missed/fail on 1/2 FSD?

[Tinstaafl]

When I did my FAA ATP conversion the practical test standard for the ATP - essentially anL IR test - specified a quarter scale tolerance. Other countries also use a specified tolerance eg Oz now uses 1/2 scale but a number of years ago it was 2 dots

Julian, I suspect the tolerance specification is to provide a known probability of deviation outside 1) the tolerance specified, and 2) the display area of the instrument.

If the a/c is flown at full scale deflection the likelyhood of flying outside the display capabilities is greater than if you are nearly always contained within the defined parameter eg 1/2 scale etc.

Once you're at the edge of the instrument how can you know if you've gone any further outside?

Flight safety is calculated on the basis of probabilities. Procedures, equipment & training (read: accuracy level to which the pilot is expected to perform) are designed so that the interaction of all them constrains the chance of an accident occuring to within the specified probability level for that part of the flight.

Constraining the tolerance limits that the pilot must be capable of achieving is one factor that contains the safety level to within the defined probability for that segment of flight.

[Stan Evil]

I think that the half scale comes from an ICAO PANS-OPS limit defining the 'safe area' for the approach but don't have a copy of the document to hand. Half-scale is a JAA requirement - not just CAA.

[Genome_Junkie]

Tinstaafl

Very good. Makes sense.

Thanks!


Arnie

[TomBola]

Julian,
I fail to see how failing to keep the needles centred 'maintains your situational awareness on the loc/GS'!
The ILS is designed to be flown with the aircraft on the glideslope and localiser. In the UK, for your IR you are required to fly with the needles centred, with an occasional excursion to a maximum of half-scale deflection being accepted. If you fly the whole ILS with the needles only just inside half scale deflection you will be failed for not attempting to correct back onto the GS or localiser. By the time you finish your IR course you are expected to have achieved a level of competence which will enable you to achieve this because you should be very current and have been doing a lot of instrument flying in a fairly short period. This should ensure that if in future you only achieve the minimum currency required to keep your rating valid, you should still be capable of safely flying an ILS approach for real should the need arise. It's like everything esle in aviation really.

[eyeinthesky]

If you want any convincing, next time you fly an ILS in VMC try flying to full scale below the glideslope (whilst still keeping the needle alive) and then have a look when you get to 1/2 mile from touchdown at how close the ground is!

And anyway, if you can fly it and keep it at less than full scale deflection (otherwise you 'peg the needles'), why can't you fly it to within half scale? It's the same process, only within narrower margins.

Do you attach the same "almost there" criteria to the rest of your flying (approach speeds, W&B, level keeping etc)?? :rolleyes:

Edited because the ground will always be the same distance away at DH! You'll just reach DH earlier than normal if you're below the GS.

[Julian]

Tombola,

The point of my question was why the difference which Tinstaafl has put forward a reasonable answer.

My point about the needles is if you let them wander too far you will lose then and lose the Loc/GS. How do you know where you are on it if they are pegged? You dont so there is the answe to your question.

I dont disagree with having a diff test standards I was asking why.
And before you ask yes I can fly one :D

[Rotorbike]

FAA Instrument Practical Test Standards (ftp://av-info.faa.gov/data/practicalteststandard/faa-s-8081-4c.pdf)

Page 40 Item 11

Allows, while on the final approach segment, no more than
three-quarter-scale deflection of either the localizer or glide
slope indications, and maintains the specified airspeed
within 10 knots.

Not that far from JAA standard really :)

[TomBola]

Julian,
Au contraire - you don't seem to understand the ILS, the ONLY time you know where you are on the GS/loc is when the needles are centred. You are then precisely on the localiser and glideslope, which is the ideal place to be when you arrive at your DH/DA in minimal weather conditions if you want to be ale to effect a safe landing. If you are off to one side or high/low you only have an approximate idea of where you are, based on your distance from the transmitter (and very little idea at all if the DME is u/s as it's difficult to factor in the 1:60 rule then). It is also less likely to lead to overcontrolling. I can only say again that if you fly at half scale deflection on either localiser or glideslope, and make no effort to correct back within a reasonable time (say 2-3 seconds) you will fail your IRT. The approach is not 'required to be flown to 1/2 scale deflection' - it is required to be flown with the needles centralised, with a MAXIMUM occasional deflection to no more than half scale.

[soggyboxers]

Julian,
Have to agree with Tom on this. Consider if the instrument on which you are relying to fly your ILS is on the outer limits of its tolerance, you are going to be a long way from the runway when you arrive at your DH.
I also remember the 'wicked' old CAFU instructor who conducted my IRE course flying an entire ILS half scale deflection off the centreline and doing nothing about it until a few seconds before we arrived at DH, explaining that the approach he had just flown was unacceptable because he had not attempted to fly with the needles in the centre and had made no attempt to recover on to the localiser.

[YouNeverStopLearning]

Just for the record ladets and lads, half scale deflection IS 2 dots.

AND I have just remembered - HSI !!!!!! HSI's only have 2 dots. Or am i wrong?

[Julian]

Soggy, totally agree with what you are saying - dont know if I am reading posts wrong but seems people think I am advocating flying at full tolerance without correcting??? I fully agree with what you are saying about correcting so lets put that one to bed straight away. Intersting enough the FAA doc given states to 3/4 FSD and does not make any ascertion about correcting - merely dont drop below the DH - WEIRD!!!!

Tom, the point I am making about knowing where you are on the ILS meant in relation to Above/Below - Left/Right of the GS/Loc. If you want to know EXACTLY where are you WILL need DME (assuming no fancy GPS) as everything else is just guesswork otherwise.

I was taken right down to 50' on my IRT by the examiner to see if I could hold it. And yes I passed :D

Thanks for the discussion chaps!

[soggyboxers]

Julian,
Congratulations on passing your IRT.

YNSL
Half scale deflection is half of whatever you happen to have on your instrument, be it ADI, HSI or whatever - and no, most HSIs I have flown don't have only 2 dots. Just as a matter of interest, should you elect to use a FD for your IRR (if you have one of course) you will be assessed to half the normal limits!
:D

[Tinstaafl]

On older type VOR/ILS displays 1/2 scale = 2.5 dots. ie there are 5 dots L & R and up & down.

In essence the Oz move from 2 dots to half scale very slightly relaxed the tolerance requirement.