navagation headache
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navagation headache
Hi! <img border="0" title="" alt="[Confused]" src="confused.gif" /> all you pros out there. .studying for navs at the moment from home,no rush at the present(with little/no jobs about). .Have read over, and over, and over again INS and i'm still not getting my head around it, anyone out there can remember how they got around this horrible little instrument, with all the little instruments attatched, just before i go completely insane,any constructive advise would be appreciated!
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I vaguely seem to remember that once I'd got my head around how the thing aligned it all seemed to click into place. You'll probably find lots of eager beavers on the Wannabees forum who can help you with this.
I don't know what's in the ATPL syllabus, but I'll give you my simplistic Engineer's approach to this.. .. .(1) Take a lump of metal. Hold it in some mounts. .(2) Whenever you move the lump of metal, there will be stresses in the mounts.. .(3) Strain gauge the mounts, feed the output into a computer.. .(4) The computer calculates the accelerations in the lump of metal, from the strain gauge output.. .(5) Going back to high school physics, speed = acceleration x time. So, the computer can work out how fast the lump of metal is going (assuming it started off still).. .(6) Ditto, distance = speed x time, so the computer can work out not just how fast the lump of metal is going, but where it has got to (assuming that it knew where you were in the first place).. .. .That's the basic principle, but to really work, you need.... .. .(1) Strain gauges in 3-dimensions, because the aeroplane moves in 3 dimensions.. .(2) Gyroscopes and damping to keep the metal from swinging about and messing up the strain gauge readings.. .(3) The computer also needs, to know where you really are, to compensate for the fact that the earth wobbles in a sine wave of about 3/4nm at 78 minutes (this is called the Schuler Cycle).. .(4) Incredibly accurate measurement devices.. .(5) Some quite difficult sums to work from three accelerations, to a 3d position in space.. .. .The modern versions use what's called a "Ring Laser Gyro", which does the same thing but uses a laser, travelling around a loop. Because the speed of light is fixed, a very accurate measurement of the time it takes the laser to travel around the loop does the same thing.. .. .You also have a personal built-in INS, which is the canals in your inner ear, it's what gives you a sense of balance - although your brain isn't quite clever enough to work out your position from this, just which way up you are (most of the time !).. .. .Hope this helps,. .. .G
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I think that has probably made things worse !! I also had a hard time coming to grips with all of this - don't know how I managed to pass instruments first time - still haven't really got a clue.. .Good luck !!!
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What is it you can't get your head around more specifically?. .. .I'm still struggling with just how on earth a RLG really works. It seems much of the avionics community has just accepted it as "black box magic" as well but I'm the curious kind.. .. .To make matters even worse, there are different kinds. Half the explanations I find either simplify without saying just where and how or mix the various kinds freely, adding to the confusion. Didn't even find the answers in course material from the manufacturers. :/. .. .Cheers,. . /ft
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Well the RLG is easy if you can understand the regular gyro INS. Basically you have one laser ring for each dimension ( as I think the conventional system has one gyro for each dimension). Basically you point a laser in each direction around the glass triangle and measure how long each beam of light takes to get back to the start. If you rotate the triangle about its plane then one receiver will move closer to its light source, and the other receiver will move further away from it's light source. This creates a difference in the time it takes each laser to travel the circuit, which is manifested in an apparent frequency change of the laser light. By some clever maths you can thus determine how much the triangle has rotated by and then do all the usual calculations to determine aircraft position.
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That's the simple explanation I can find every now and then. I want a bit more detail - how is the phase shift determined, for starters, and how can it be integrated with such high precision to give the rotation? There has to be a few tricks to it.. .. .That only works for one kind of RLG too, forgot the specific term but it's the one with the laser outside of the resonant cavity. The other kind where the laser crystal is actually within the cavity supposedly creates a standing wave in there which is star fixed. Understanding a laser gets me a long way towards being able to figure that one out, but I'd still like the details. If for nothing else, the to confirm my homebrewed understanding.. .. .Once you have the standing wave, one photodetector can count the number of nodes as the RLG is rotated about its sensitive axis and with another one an odd number of quarter wawelenghts away, you can determine the direction of rotation. But... (now it's starting to sound familiar, innit <img border="0" title="" alt="[Smile]" src="smile.gif" /> ) ...DETAILS?!. .. .Then there's phase locking, exactly how the oscillator prevents phase locking... etc etc etc.. .. .Cheers,. . /ft
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No noose,. .DON'T PANIC!!!!!!!!!!!!!!!!!!!!!!!!!!!!. .I aint too good a book learnin but a real good copilot can help you thru this. U have been good to your c/p havent U? If not , in that case U should PANIC!!!!!!!!!!!!!!!!!!!!!!. . <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />
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thank you. .for your replies,genghis the engineer i know where UR coming from and i agree it is slightly off the mark.a friend told me of a mnemonic for the INS and if is going to get me through the exams i will use this, i suppose if u know the basics then this will suffice.( i'm still waiting on my pal producing the mnemonic!)
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Get hold of the FMC guide. There are 2 of them and either will do. Transair have them or you could borrow and read up the opening chapters. It's one of the best no nonsense guides to IRS and will answer most of your questions.. .I had a mate who passed Nav/Instruments and never understood what the gyros(real ones as in tristar vintage) actually did. He thought that the gyros calculated some kind of position fix and had answered all his questions on that basis. the gyros(INS) or ring laser gyros(IRS) simply keep the platform level so that the accelerometers can faithfully determine which way it moved and how much for how long. the INS system had an actual dinner plate affair with gimbals and christ knows what. the IRS does it mathematically. So, having adjusted the output of the accelerometers mathematically to simulate being a stable platform, the system number crunches all that wonderful sin/cos/lat stuff that you spent hours plotting on a distorted photocopy of an Icelandic chart, and comes up a version of your position.
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(a) Don't lose sleep over any of this;. .. .(b) You'll never need to know how it works when you fly aeroplanes which have them;. .. .(c) Really, it's only in the syllabus because it used to be, (and it helps keeps some dunderheads out, so best it's left there);. .. .(d) Don't buy from Transair - save money and buy direct from my chum Bill at <a href="http://www.fmcguide.com" target="_blank">Bill Bulfer's page</a> (type in <a href="http://www.fmcguide.com" target="_blank">www.fmcguide.com</a> if the hyperlink doesn't work). Shopkeepers who own jets mark up far too much.
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Interesting replies. However, the original question highlights the somewhat ridiculous situation we are in with exams at the moment. Why on earth do you need to know how a RLG works or how the gyros are connected? If they stop working you will not be able to troubleshoot or fix them! All you really need to know is that you put your start position into an INS and it does the rest. You just have to check from time to time using other navaids that the system is accurate.
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eyeinthesky,. .. .I might agree with you, except that whilst we retain some genuinely difficult topics for study and examination, we help to maintain the minimum academic standard within the profession. There's already far too much de-skilling in our industry, and by ensuring that those who join it are relatively intelligent, we may work against this to a degree.. .. .For my part, I would happily throw away INS and replace it with Latin grammar, pure mathematics, or whatever you like, just to ensure that the study is hard and you need to be bright to pass.. .. .I would also introduce yearly examinations to check on the level of retained knowledge of basic subjects such as Meteorology and Air Law, (and this would be a cash cow too, of course!!), but doubt anyone would be with me on this. Pilots are never tested on anything but their flight deck skills in manoeuvreing and managing an aircraft - but their knowledge should be deeper than can be apparent from this.. .. .Once you're into the airlines, of course, it gets worse, as the manufacturers insist on publishing 'need to know' manuals which leave lots of important stuff out - despite which, IRE/TRE types still ask questions on material which is not published (thus the success of the various 'cockpit notes' which give more informaton than the manufacturers do).. .. .Pilots are happy to count themselves in with lawyers and doctors, as 'professional' people. However, a pilot might spend only one year 'studying' before joining an airline where he or she will spend two months 'training', and then be qualified. A doctor will train for five years or more, a lawyer, just as long. Where is the parity?. .. .If you're going through the examination 'mill', work at it, and if you're good enough you'll get through. Bear in mind that you'll be competing for jobs with all sorts of others, and the fewer of them there are, the better your chances.. .. .I still wouldn't let my children do it, though, and if I could move into another job which pays as well, I would do so - like a shot.. .. .(I'm one of the old-fashioned people who regard spelling as quite important too - so, Nunos, it's navigation with an 'i').. . . . <small>[ 09 March 2002, 14:12: Message edited by: NorthernSky ]</small>
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There's no way you can measure time difference round the 2 laser light paths. The laser light path in each direction is about 15 cm. Light travels at 300 million metres per second. Any timing device is doomed to failure.. .. .No, you've missed the point. Why do they use lasers? Because laser is COHERENT light, ie, single frequency light - completely unlike the light in a sunbeam or from a light-bulb, which contains the whole spectrum from the infra-red through to the ultra-violet.. .. .If it's single-frequency, it's also single wave-length - that's the whole point. When 2 pure sine waves of light (which is what you've got with laser light) arrive together at a destination, they make a static interference pattern. When they're slightly out of phase - which they are if the path length is different - then the interference pattern moves across the photo-electric cell with respect to time. It is that movement which is detected - as a change in heading, pitch, or roll.. .. .Simple, isn't it? - if it's explained properly.
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oxford,. .you're both saying the same thing. The phase difference depends on the time in transit which, with constant speed, depends on the distance travelled. For a time measuring device doomed to fail, I'd say it's doing rather well!. .. .I'm beginning to realise that my quest for knowledge will have to go on elsewhere though. That explanation is simple indeed and that's the problem - it's not detailed. <img border="0" title="" alt="[Frown]" src="frown.gif" /> . .. .Cheers,. . /ft
Couldn't resist this sidenote (please pardon the thread drift).. .. . NorthernSky says: </font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica">Pilots are never tested on anything but their flight deck skills in manoeuvreing and managing an aircraft - . . </font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">Obviously NorthernSky has never attended recurrent training at my airline (or most other airlines I know of, for that matter)!
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Nunos,. .. .The word is 'grammar', with two 'a's. <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" /> . .. .McD,. .. .Care to re-read my post and explain how your airline goes beyond testing 'flight deck skills in manoeuvreing and managing an aircraft'? What IS the genuine content of your OPC/LPC beyond tick items and LOFT?. .. .How often do you check that your pilots understand RVSM, TCAS operating rules, Instrument Flight Rules as applied in all states into which you operate, correct RTF phraseology, current AIC content, etc etc (I might try to write a full list...)?. .. .I've been doing recurrent training for some years now, and know of other operators' syllabus content, none of which cover many areas which I would specify for ongoing training and testing.. .. .However, I am talking of 're-skilling' a profession in decline, and I understand that some will feel defensive when challenged.. . . . <small>[ 09 March 2002, 23:45: Message edited by: NorthernSky ]</small>
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Nunos: If you're going to be rude, then please try and address it to the right area of sky!. .. .Having said that, Northern Sky and I are on the same frequency with regard to spelling and grammar. I am sure I am not alone in thinking that the standard of these is declining rapidly, not helped by the craze in abbreviated spelling brought about by text messaging. If you cannot spell and punctuate properly, seem not to care about it and then are rude to those who point it out, then it displays a certain lack of willingness to take criticism and learn from mistakes when they are identified. Doesn't really display ideal pilot training tendencies, does it?. .. .By the way, plurals do NOT have an apostrophe!. . . .Northern Sky: Generally I agree that to understand a subject is better than trying to learn all the answers, but I think some of the subjects are a little too far off the line.. . . . <small>[ 10 March 2002, 09:04: Message edited by: eyeinthesky ]</small>