ILS Scalloping

[2close]

Recently, flying an ILS by the numbers with the cross-hairs holding the LOC and GP centre-lines very nicely (for once! :rolleyes:) I was quite surprised to see the LOC CDI shoot off to the right side to nigh on max displacement (indicating a sudden, severe drift to the left - very surprising considering the wind was coming from the left!).

The CDI slowly drifted back into the centre without any correcting action required by myself and was probably a case of Receiver / Display error or temporary ILS Beam Scalloping - I'd be tempted to go for the latter.

If I had tried to "recapture the LOC" by turning right (with a left wind) I would have drifted wide of the beam to the right and then, when the beam re-stabilised itself the aircraft would have been right of centre-line and the CDI would have drifted off to the left, possibly outside half-scale deflection.

The thought that springs to mind was that, if this had been an examinee on an IRT, would he have failed the test through no fault of his own? After all, IR examinees are not expected to recognise scalloping and I have not come across anyone who teaches IA drills when a LOC CDI shoots off to one side unexpectedly apart from treating it as a 'lost' LOC.

Any thoughts. :confused:

[DFC]

Test or no test - if the indication exceeds half scale you commence a missed approach.

If the student experienced what you describe and completed a safe missed approach then I would regard the situation as well handled.

If the student did as you did (remember that they are IMC) and assumed that they could continue then it most definitely would be a fail.

To try and captureb the LOC while descending from a position outside the tollerance allowed would also be a fail.

Same thing applies if a flag appears for any length of time - missed approach.

It would be a harsh examiner who would fail a candidate who noticed that they were about to exceed half scale deflection and initiated a missed approach. Of course being unable to fly an ILS is something different!!

[Gulfstreamaviator]

If the beams are interupted for any reason, that will happen.

Cat 2 and better have protection to prevent interuption to the beams.

Cat 1 has no such protection

Training approaches have absolutly no protection.

Any movement near the TX aerials, will cause sudden deflection.

Suggestion: Monitor the RT, and if you hear an aircraft ceared for TO, on your ILS Runway, be ready for that sudden deflection.

I certainly would not fail someone who announced" preparing for possible ILS interuption" It shows professional understanding.
However if a coupled approach, then it is quite possible that the interuption is rapid, and that the deflection so short, that the smoothing circuits in the FMS, will (at a reasonable distance from TD), will ignore the problem, but it might not. SO always keep hand on control, and one on power leaver.

glf

[hueyshuffle]

Interesting thread. Quite interesting to hear the 'old salts' (read: people who fly IFR) talking through a solution.

How close would you have to be to the LLZ before the 'smoothing circuits' would not be able to cope with full deflection for a second or two??

[Gulfstreamaviator]

I can not answer the question, in all honesty, because my answer would not be based on documented facts, and then I would be shot down.

In general the ILS system is proteced from a certain range to TD, lets say 15 miles.
There will almost cetainly be slight bends, in the LOC/GS due to reflections, (fixed), that introduce the normal scallops. If you fly down an ILS and also monitor visually, you will see slight deviations. More so on the old installations.

If you intercept, at a high speed, and a large angle then the FD or AP can not couple without passing thru, and then capturing from the other side, after perhaps one or two wanders.

Te sensitivity of the ILS increase as you get closer, the beams get narrower, and most modern FMS will adjust their sensitivity accordingly.

I would think that unless the deflection was accompanied by a loss of signal flag, then perhaps 0.5 / 1.0 second would be a reasonable figure.

I know this does not answer the Q M Huey posed, but it set the scene for an expert......

glf, (the non expert).

[DFC]

Training approaches have absolutly no protection.


In what country is that applicable?

In most European countries, an IFR flight cleared for an ILS approach will have the normal CAT1 protections. It is only when we want to practice CAT2 / 3 in CAT1 or better conditions that we have to inform ATC so that the protections can be put in place or we are informed that it is not possible.

An aircraft departing on the same runway will not cause a large deflection to the ILS. This is the normal situation for many aerodromes withg single runway ops - one lands - one departs - one lands. If the departing aircraft caused a significant deflection in the ILS then departures would not be permitted with an arrival being inside something like 4nm or the OM before the departure was past the LOC antenna. That would really slow things down at single runway busy airports.

Places like Gatwick put about 5nm between a string of 737's on approach and get one away in each of the gaps. Never had a problem with the ILS in such cases.

If you read the original post, you will see that it was not a simple flick of the pointer as can happen when a vehicle crosses in front of the antenna or an aircraft crosses at the end close to the antenna, the post said that the pointer exceeded half scale and remained there for some time.

That requires a missed approach.........and if one remembers that one will live loing enough to be correctly referred to as an old salt!!


Quite interesting to hear the 'old salts' (read: people who fly IFR) talking through a solution.

How close would you have to be to the LLZ before the 'smoothing circuits' would not be able to cope with full deflection for a second or two??


However if a coupled approach, then it is quite possible that the interuption is rapid, and that the deflection so short, that the smoothing circuits in the FMS, will (at a reasonable distance from TD), will ignore the problem, but it might not.

FMS flying an ILS approach???

Are you not confusing the FMS with the flight director / autopilot????

Most aircraft I am familiar with will use "APP" to fly the ILS - with the flight director coupled to the ILS. The pilot (or autopilot) follows the flight director.

The FMS is not (must not be) used for the ILS approach portion of the procedure when it is an ILS or LOC. It can get you established and can be sued to fly the missed approach - (LNAV) but can not fly the ILS.

If you use the FMS to establish the aircraft on the LOC centerline then it will anticipate the turn based on GS and will make a perfect turn onto the LOC centerline. If on the otherhand one arms APP and uses heading to fly towards the LOC then at too high a speed you will indeed shoot through the centerline and the flight director will often cause some S turns to establish. This is clearly poor pilot technique and can be at least a discussion point if not a repeat in the sim.

So to make the situation clear, no matter if you are using the Flight Director and Autopilot in a Gulfstream or Palm and her 5 friends in a PA28, attempting to continue an ILS approach when the indications are beyond the half scale deflection permitted or a flag is showing for any length of time will be a failure.

If you (or the flight director) manage some crazy S turns in a Gulfstream and keep the pointer centred - ask yourself just what the stabilised approach criteria are before choosing not to make a missed approach. :D

[2close]

Apologies if I have confused this issue with a lack of clarity in my original post.

If you read the original post, you will see that it was not a simple flick of the pointer as can happen when a vehicle crosses in front of the antenna or an aircraft crosses at the end close to the antenna, the post said that the pointer exceeded half scale and remained there for some time.


I didn't actually state that; my statement was:

"I was quite surprised to see the LOC CDI shoot off to the right side to nigh on max displacement........

The CDI slowly drifted back into the centre without any correcting action required by myself and was probably a case of Receiver / Display error or temporary ILS Beam Scalloping - I'd be tempted to go for the latter."

To clarify:

It was an IR(A) Training Flight - upgrading from IMCR - in VMC, the aircraft was No.1 with no other aircaft departing, as I recall about 3 - 4 NM from TD (but well above DH) and on LOC and GS when the CDI jumped out to the right, prompting an immediate, "Whoa, what's happened there?" from myself, whereupon the CDI immediately but slowly (in comparison to the leap to the right) drifted back to the centre, without any correcting action being required. Yes, maybe I should have immediately flown the MAP but the trend of the CDI was visibly back towards centre and it was seconds not minutes until the CDI was back within 1/2 scale deflection.

The Instructor was visual with the runway and afterwards, during debrief, confirmed position on centre line throughout the approach.

I telephoned ATC and reported the incident and was informed that similar occurrences had been reported on 09 but to the best of his knowledge this was the first that had been reported on 27.

Hope that clears up any ambiguities. :)

[DFC]

A few seconds can be a very long time when flying among obstacles. Unfortunately, you never included the most important piece of information that is relevant to the case - for how long was the indication showing more than half scale deflection and if there was any flag. It is the slowly returning to centre bit that is most significant and indicates that since the aircraft was on the centerline the displacement was for an over long period of time.

People get lulled into a false sense of security when training / flying in the UK where most places with ILS have little in the way of obstacles close to final approach.

However, the important thing is that if you were qualified and alone in IMC making an ILS approach between two very close large obstacles what would you do after the initial "Whoa, what's happened there?" and more importantly how comfortable would you feel descending with the pointer off centre?

Even in a training scenario, ask yourself what benefit is to be gained from pretending to follow an ILS but actually doing nothing to track the pointer? A missed approach is a good exercise.

I would recomend that you write up the problem in the tech log and have it checked - the next pilot may not be in the same conditions. At least then you can properly eliminate the aircraft systems from the issue.