ILS course set ... risks!

[Sax412]

After a many years flying Bell 412 and flying now A 109 E and S, I found out a very strange thing to me.
Flying the A109 E/S, during an ILS approach if I change the main course set the helicopter follows the new course set getting out of the right loc / path. I found out that even the AW 139 works the same way but I don't understand why.
The "old" Bell 412 like many many others helicopters and aircrafts, as far as I know, works differently.
If you tune an ILS frequency the FD recognizes it in a way that does not allow the helicopter to get off the right path once it's been locked.
So I guess why modern helicopters work differently ?
Thanks a lot to everybody,
Sax.

[Sax412]

Is this a difficult question or just that I have not explained it properly?
Cheers,
Sax

[The Nr Fairy]

No, you've just not waited long enough for an answer . . .

[Heliharm]

The S76B I have been flying had the same thing as you explained. I do not know either why it is like this. It seems safer to me if the helicopter does not follow the course needle when you turn it during an ILS. Hope there is someone who can explain it.

[HeliComparator]

I don't know about the 109 but I do know about the 225 and perhaps the answers are similar.

On an "very old-fashioned" helicopter without a coupled autopilot, the pilot interpreted the ILS indications and was hopefully aware of such things as where the wind was, what the final approach track was, how far down the approach he was, and how the seat of his pants was doing.

On an "old-fashioned" helicopter with coupled autopilot (B412 etc), the analogue electronic autopilot interprets the ILS data and is reactive - ie the autopilot carries on until localiser starts to move out, then makes a heading change to correct that and bring it back in. Ditto for glidepath. This type is fine if its smooth air with no great wind gradient, but does not cope well if its rough especially near the bottom.

Modern helicopters such as the 225 and probably the 109 have digital (software-driven) autopilots that use the same info as the pilot - ie instant wind (by comparing gps track, heading, TAS and groundspeed), final approach track from the course setting, how far down the approach it is from the ILS DME (if there is one), and how the seats of its pants is feeling from accelerometer data.

Thus on intercepting the ILS Loc it knows that if the final approach track is x and the wind is y then the ballpark heading should be z. It doesn't have to wait for a deviation from GS or LOC to make a correction because the accelerometer data is used to predict the flight path - the autopilot then makes control inputs to maintain the flight path before a deviation occurs. That results in a comfortable flight path with ILS needles bang in the middle, even if the daft pilot decides to slow down near the bottom of the approach etc.

It also knows how narrow the ILS cone is getting by ILS DME and adjusts its control responses accordingly, though it can cope without this with slight degredation in performance.

On the 225, the FM tells us that its important to set the correct final approach track on the course bar, and if for some reason the DME is not at the threshold, to use the DME HOLD function to let the system know. I have not tried to change the course during the ILS, but my feeling is that if I did so, there would be an initial heading change because would think its "ballpark heading" was now different, but it would soon react to the LOC deviation and come back onto track, though perhaps flying with a fixed offset on the loc.

Hope that goes some way to explain.

HC

[Sax412]

HC your answer has been the best I've heard about this topic in the last three months of research. Thank you very very much. It all makes sense to me but my question now is: don't you think that engineers could have programmed new helicopters software differently such in a way to add precision to the approach without trading it with flight safety somehow?
.....


P.S. To my first "repliers" I apologize for not having waited enough to get an answer

[HeliComparator]

don't you think that engineers could have programmed new helicopters software differently such in a way to add precision to the approach without trading it with flight safety somehow?


Well its reasonable that the system has to know the final approach track. I suppose that the system can derive it once its been on the loc for a while, but there is no radiated information from the ILS to allow it to get it right at the start. One way to deal with the problem would be to lock the course bar (or the information derived therefrom) once coupled on the loc, but that runs the risk that the pilot set the wrong value to start with and is now unable to change it. I will try to remember to try it on the 225 - if there is no change to the heading I will suspect that the software ignores changes once coupled, but I know its important to set it correctly from the start.

Although you say "flight safety", surely the system designers could expect the pilot to take some responsibility to make the correct settings. The days when all the flight controls are locked out once the pilot has started the approach are not yet with us!

HC

[Sax412]

I suppose that the system can derive it once its been on the loc for a while, but there is no radiated information from the ILS to allow it to get it right at the start.

Maybe I'm missing something about how the B412 and similars work so.
How can it stay stable on the right course in spite of a completely different/changed course set? Could not this " way of working " be used as well in modern helos?

[HeliComparator]

Sax

Not really - the 412 system is pretty crude (I am guessing) as it doesn't have a concept of final approach track, it just makes a heading changes in response to localiser deviation and rate of deviation change. I have never flown one but I would suspect that if there was a big change of wind near the bottom of the ILS, or turbulence, or pilot selected change of airspeed, that it would not work very well - at least it could be exploring the limits of the ILS needles! That is how the 4-axis AS332L autopilot behaves anyway, and the L2 is not that much better.

With the modern autopilot, even when the crosswind component near the bottom changes substantially, there is no localiser deviation as the system is maintaining the final approach track by changing the heading before a loc deviation occurs.

HC

[Sax412]

... So I suppose that a mixed system, old + new, would be not so easy to figure out... ok my friend, thanks a lot again...
I'll be wearing my "donkey hat" before going to sleep tonight! :)

[MarcK]

The autopilot input has two components. First, the error signal from the localizer. Second, the error signal from the course datum. It doesn't know absolute course, only the difference between desired course and current heading. It corrects against the heading error to zero the localizer error and error rate. If you change the course datum, you make an abrupt change in the heading error signal and things have to re-stabilize.

[helopat]

Sorry to disagree, but I also thought that flying an ILS was (supposed to be) independent of the course bar setting. I mean, flying an ILS approach you are flying BEAMS, not courses. The needles should be telling you whether you are on the LLZ and Glideslope beams, not whether you are on a particular radial. Having not flown an ILS in the 109 (yet) I haven't had an opportunity to play, but I suspect its not a matter of the aircraft being off track, but rather the way the software is written that links the course needle position has something to do with your position on the LLZ.

In the Seahawk, for example, when you're established on the localiser and on glideslope (ie. needles centred) you can twirl the CDI bar knob around to your hearts content and you keep right on going down the slot. The CDI bar is purely for orientation. Even if the bar is off set, say, 45 degrees, the bars will still stay centred as long as you're on the LLZ.

IN any case, something to be aware of...thanks for passing along the information. Once I've had a chance to 'experiment', I'll get back on.

HP

[MarcK]

The autopilot in NAV mode is independent of OBS or CDI inputs (on an ILS). It only looks at the error signal, which won't change when changing the OBS. It responds to HDG course input, measuring the error between dialed-in course and current heading.

[Gomer Pylot]

The 412 handles wind and airspeed changes very well. I've never seen the needles move out of the circle, even with rather large wind changes. The flight director reacts to changes very quickly, and doesn't allow the needles to move very far.

[Thridle Op Des]

Hi HC,
FYI (interest) the Boeing and Airbus derive the ILS course from database so it is not a requirement to set the course, its simply presented when the ILS push button is selected. It is possible to overwrite the course in the Rad Nav page of the FMS, but the Airbus for one starts complaining about the course overwrite if it is not the same as the database, I'm sure the Boeing will do the same but I don't have the info.

Not saying that this is the best system necessarily, but it is interesting that there is still this lag in system integrity monitoring between rotary and fixedwing world.

Regards
TOD

[HeliComparator]

Hi TOD

Sounds like a good system, but I think the rw world is lagging behind in its FMS technology generally. Not having flown large FW but I presume that the FMS has a much more integral part to play in the aircraft systems. Presumably you have 2 boxes in case of failure etc? Even the latest RW cockpits seem to have 1 FMS as standard.

To indicate how far behind we are, I am currently battling with our FMS database provider (and have been for over a year) to get all the approaches and transitions into the database for the Aberdeen area. At the start, there were no approaches in for many of the airfields North of Edinburgh, and even now we only have some of the TRANSs (intermediate approaches) in the system even at airfields with no radar (so procedural approach). Even at Aberdeen we only have the final approach track in the system, with no TRANSs.

One day we will catch up (but then you will have moved on!)

HC

[Sax412]

HC,
after having verified how A109's FD works, I'm sorry to say that your explanation doesn't fit this helo.
A109 FD is fully analogic just as the B412's one.
It doesn't compute any particular data from GPS or FMS when installed.
So ... the question remains ...

[Thridle Op Des]

Triple Air Data/Inertial Reference units.
Dual 12 channel GPS which update the IRS position.
Dual Multi Mode Receivers which update the IRS if the GPS fail - using DME/DME, VOR/DME, VOR/VOR, and even ILS LOC. Radio Navaids are automatically tuned to match the route and departure/arrival procedures - a fully integrated system.
All the transitions we would ever need including DME arcs, and now starting to include engine out procedures in performance demanding areas. Database integrity checking systems to ensure the validity of all the routes.
Managed Non-Precision Approaches (both latererally and vertically), soon to get Precision Approaches with GPS/RNAV (approx Cat 1 minima)
With the A380 we will be getting a vertical profile display on the Navigation Display to ensure Terrain Clearance and all the vertical constraints are met.
We still can't hover though - unless there is a 145kt headwind at 500'. We still need 4000 hectares of terrain devoted to our activities.
As you say; it is very disappointing to see how far the rotary world lags behind the fixed wing world, especially how the same FMS suppliers are used between the two sides of the divide (Thales and Honeywell for example). I know money is the big influence but it is sad to see how new helicopters are released with big fanfare, boasting about their modern flight decks, but are essentially on a par with the Boeing 757 when it was first operational.
One thing I like from the 225 - the HSI indication with the ovoid compass rose.
One thing I am staggered about - the CFIRE is still an individual annunciator on the sub panel - not a ECAM-ECIAS warning.
Regards
TOD

Apologies for the partial thread creep, but I think it is worth comparing the differing safety systems!

[Wizzard]

Well somebody has to say it!

Dial up the correct inbound course
Establish on the Localiser
DON'T FIDDLE!



Easy:8

[Hippolite]

Thridle Op Des and HC

Maybe we should all get together for a 30 year reunion in 2010

We could split a boiled potato or enjoy some "fag ash" bacon

What do you think?

Hippo

[HeliComparator]

TOD

Well we have triple ADC and triple AHRS but after that the similarity ends! I guess weight and space is an issue for RW. Actually there is an interesting question...You say you have triple ADC and interial ref but are they triple identical systems? We have two identical AHRS systems and one different. So far we have had no single AHRS failures but a couple of double ones where the two identical systems decided to crash at the same time (software issue associated with magnetic anomaly on a particular platform). So I am glad that we don't have 3 identical systems!

Hip - yeah, why not! Which of the colonies are you in these days - convict land or cowboy land?

HC

[Thridle Op Des]

Sounds good Hip, I'm hoping we can all get together! HC I'm PMing you with a little little extract of the Flight Crew Training Manual FYI

TOD

[Hippolite]

TOD and HC

We will have to make the effort to locate the whole cast, lost are SW KM and CC, you may know where they are but I don't.

HC, I am back in the former, got tired of chewing baccie and wearing a big hat.
3 AHRS huh? Do you have any platform info in the FMS?

[zebedee]

So, what happens if you "fiddle" with the course bar and stick your arm out the window at the same time?
Zeb

[Sax412]

The problem, of course, is not fiddling somehow the course set, but selecting the wrong one . . . that's it.

[HeliComparator]

Do you have any platform info in the FMS?

Just the position of the platforms. We also have the company routes (HMRs etc). Its the customised part of the database - same contents as for the other fleets with Trimble 2101

HC

[212man]

Referring to the original question: does the aircraft eventually regain the localiser, once it has departed, or does it remain off course?

HC, is your FMS of FW origin, and if so, is there no opportunity to subscribe to a larger database? We use the Universal UNS-1E, in the S-92, with a worldwide airport database, including all SIDS, STARS and Approaches (including the transitions and Missed Approaches). It even allows us to fly the fully Managed LNAV/VNAV NPA approaches TOD mentions, allowing CDFA stabilised procedures, rather than 'dive and drive'.

TOD, our FMS sets the course too, but only for NPAs, not for ILS.

The S-92 coupled ILS function seems carefully programmed to closely resemble a badly hand flown ILS, with even a 6 kt crosswind capable of generating in excess of half scale localiser deflection close to minima (flashing yellow diamond: just what you want in IMC)! :ugh: Some changes required before looking at Cat II I would say!

[HeliComparator]

212 - The FMS is a CMA3000 made by Canadian Marconi, though later aircraft will have the CMA9000 which is a later version of the same thing.

From their web page "The CMA-3000 is a compact flight management / radio management system well suited for fixed and rotary wing aircraft requiring performance, but having space and weight constraints." whatever that means!

We supposedly have a worldwide (European at least) database and it seems to have everything in it for all airfields except those north of Edinburgh. Duh! Of course the contents of the database are not the fault of CMC but there seem to be very few providers of customised databases in the vicinity and our current provider is unresponsive.

You say your UNS can do NPA approaches - does it have TSO 145 GPS receiver? If only TSO 129 I don't think you would be allowed to do NPA approaches with gps as the sole aid in most of the world (except USA).

...with even a 6 kt crosswind capable of generating in excess of half scale localiser deflection close to minima...

225 is amazing in that area - one night I was flying training ILSs into EGPD RW16 with wind at 2000' 75kts at 60 deg right of final approach track (coming over the hills) with ATC man giving real-time surface wind callouts as they like to do. It went like this "....50 kts.....5 kts....50 kts..." etc - kind of gusty! We had to slow to 130 just to reduce turbulence so that we could see the instruments and avoid physical damage with parts of the cockpit, but ILS needles remained dead centre all the way. Afterwards Eurocopter man said they had decided to always paint the needles dead centre regardless, but I think he was joking!

Afterwards ATC man was heard to pass comment that whilst the 737s had been exploring the ILS envelope, the 225 came in right down the middle (they monitor with radar)

That's what an inertial autopilot does for you and personally I would not trade that for a gadget that sets the final approach track for me!

HC

[Lenticular]

Very surprised about the poor performance with the S92 on the ILS. Like the EC225, the EC155 is very accurate even in challanging cross winds.

[Sax412]

212man
Referring to the original question: does the aircraft eventually regain the localiser, once it has departed, or does it remain off course?
yes, unfortunately it remains off course !

[OEI and Still Flying]

Hi Sax412
Slightly off the subject but I am interested to hear your opinions on the Grand in turbulence with regard to the autopilot being able to handle it. Like you I have transitioned from the power to the grand.
Flying along at 145/55 is great until you find that a small level of turbulence will pitch and role the machine to VNE unless you intervene.
On the power I find that the machine would hold heading and altitude but on the Grand it’s up and down 100/150 feet.

[212man]

Lenticular, not as surprised as me! As you say, the 155 is on rails come rain or shine.

[Sax412]

OEI and Still Flying . . .

. . . I haven't experienced such a kind of problem so far but the Grand is, generally speaking, a little bit more sensitive though.

[212man]

HC,
"You say your UNS can do NPA approaches - does it have TSO 145 GPS receiver? If only TSO 129 I don't think you would be allowed to do NPA approaches with gps as the sole aid in most of the world (except USA)."

No, it's TSO-129, but the point is that you don't use it as the sole aid: you MUST have the raw data available to monitor (see photos of the P-ILS in the other S-92) thread.

[malabo]

With the Garmin 530 installations I've seen on 412 and 76, you just keep programing whatever approach you want (NDB, VOR, ILS, RNAV) on the GPS and let it drive the track bar on the HSI to wherever it wants (does a better job of the correct inbound than the pilot). It also tunes the correct ILS frequency into the Nav receiver. Once on final for an ILS you simply switch the OBS source to VOR/LOC and leave the trackbar where it is. The old 530 is only TSO 129 A1, but you still fly RNAV only approaches with it as long as they exist in the database.

Looked at the S92 panel pictures but can't figure out what is meant by "raw data to monitor". You mean like a wet compass?

I see Malaysia and Brunei are a little behind the RNAV curve, but you could certainly file IFR to Singapore (Changi) and fly the RNAV approach with only the Garmin. My take on the TSO 145/6a is that in the US you can file destination and alternate using a single GPS receiver that meets that TSO, but with the older non-WAAS units under 129, either destination or alternate needs at least one ground-based navaid approach.

Nothing but pilot stubborness to keep you from flying any NPA in the GPS database coupled to the FMS, and simply monitoring the underlying navaids. Fixed-wing guys been doing it for years, and I've noticed the helicopter side catching on now too.

Anybody in the GOM flying GPS Approaches with the 92? I doubt if the North Sea or SE Asia is, except as overlays.

Malabo

[HeliComparator]

212 thanks, that clarifies it.

malabo - what is meant by "must have the raw data available" is exactly what you mean by "monitoring the underlying navaids.

I think we are on the same wavelength, just using different words!

I know that Australia now allows NPA approaches with gps as the sole aid but it must be tso145/6

HC

[212man]

Malabo, it's as HC says: Raw Data is the information coming straight from the ground based navaid, through the onboard receiver, and appearing on your display, either using the bearing pointers or CDI. It would exclude Flight Director processed information, even if the FD was coupled to a ground navaid.

TSO-145 also allows LPV (Localiser Precision, Vertical guidance) approaches, as well as being sole aid. Having said that, despite only being TSO-129'd, the UNS-1 uses angular scaling inside the final approach (rather than fixed distance scaling) fix when used for an NPA: hence the expression 'Pseudo-ILS' that Sikorsky use.

[ShyTorque]

OEI, please check your PM. :)

[Bomber ARIS]

HC
To fly GPS NPA in Australia only requires that the box be TSO 129.

[HeliComparator]

Bomber, I guess you are sort of right but I did say "gps as the sole aid" - Oz AIP Gen section 1.5 para 8.5.5.4, item 4 in the requirements list for NPA says

"If a TSO-C129 or a C129a receiver is used, an alternative instrument approach utilising ground based navigation aids must be available"

In other words, whilst it has to be available, it doesn't say it has to be monitored. Perhaps it says that somewhere else, but then again maybe not! I would have thought it was good airmanship to monitor the ground-based navaids anyway.

And of course item 3 says that you cannot use the GPS approach to satisfy the requirements for an alternate unless you have 145/6

HC

[ukv1145]

Just to come back to the original post, the autopilot must have a course reference, consider that if you are positioned left of the runway the needles will indicate fly right whatever your heading (ie even if flying away from the runway. The course bar is used in the autopilot as a reference, if you are flying with a crosswind then there will be a permanent 'course error' equating to your track angle - this error is washed out and ignored after a set period of time, however if the course bar is moved then the error signal is reinjected. Some very old systems may not reintroduce this signal and this could be what you are seeing, but essentially once the qdm is set the course arrow should not be moved (not sure why you would want to anyway).

regards

Jase