ILS course set ... risks!
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ILS course set ... risks!
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.
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.
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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.
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
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
Last edited by HeliComparator; 31st May 2007 at 18:15.
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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?
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P.S. To my first "repliers" I apologize for not having waited enough to get an answer
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P.S. To my first "repliers" I apologize for not having waited enough to get an answer
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?
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
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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.
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?
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
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
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... 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!
I'll be wearing my "donkey hat" before going to sleep tonight!
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.
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
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
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.
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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.
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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
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
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
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
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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 ...
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 ...
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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!
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!
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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
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