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# ATPL theory questions

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# ATPL theory questions

5th Aug 2012, 15:17

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How does an INS/IRS work?

The general principle of all inertia navigation systems is that the system measures the aircraft's inertia movement from an initial position as a great circle track direction and distance to continuously determine it's up-to-date position.
The compnents of an INS are
1 Accelerometers
2 Gyroscopes
3 Position Computer

The aircraft moves in three dimensions, but the navigation equipment is only interested in acceleration in the horizontal plane. Therefore, the key to the whole INS arrangement is the accelerometers.

With this point;
"The aircraft moves in three dimensions, but the navigation equipment is only interested in acceleration in the horizontal plane. Therefore, the key to the whole INS arrangement is the accelerometers."
How does the aircraft measure if accelerating when climbing etc it is only interested in acceleration in the horizontal plane?
5th Aug 2012, 17:07

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Quite simply, it does measure acceleration in all three axes, the navigational output to the pilot is in two.

GF
5th Aug 2012, 17:19

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Basically, like galaxy flyer said. INS uses stabilised platform which should be always parallel to earth's surface - so that the accelerometers only measure horizontal movement. IRS is much better, because it uses mathematical calculation to correct output from the accelerometer by using the attitude information - it's easier to do a little calculation than to have a mechanically stabilised platform completely parelel to the earth's surface for the entire time of flight.
5th Aug 2012, 18:11

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Thanks guys
5th Aug 2012, 19:31

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Schuler loop

What it is;
"Schuler loop i only common to a stable-platform INS that has been programmed to remain horizontal as the aircraft moves around the surface of the earth. The error exists at the first accelerometer level, i.e., acceleration, which can be passed up through the integration to affect velocity and distance, resulting in a distance error during the schuler loop cycle, but the error returns to zero at the end of the cycle."
Can anyone come up with a simpler explanation of schuler loop, i've tried trusty google but still finding schuler loop hard to understand
Thanks
5th Aug 2012, 20:12

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http://www.pprune.org/tech-log/11907...er-effect.html

Have a read of this if you haven't already, it may help.
5th Aug 2012, 20:29
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The earth wobbles, at a frequency of about 1 wobble per 78 minutes.

An INS doesn't naturally wobble, so therefore it appears to wobble relative to the earth.

This wobble is called the Schuler cycle.

A Schuler loop is a resonant circuit set to the period and amplitude, but in antiphase, to the Schuler cycle.

Adding the output of the Schuler loop to the INS output makes the output correct, relative to the surface of the earth.

G
6th Aug 2012, 10:07

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But note that a strapdown IRS keeps a mathematical equivalent of the stable plaform and reacts to setup errors or gravitational errors in the same way as a stable platform INS - with bounded position errors at the Schuler frequency

It is not true to say an IRS does not display Schuler frequency errors

Dick

Last edited by Dick Whittingham; 6th Aug 2012 at 10:08.
6th Aug 2012, 10:14

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Akafrank07

Whatever referece notes you are using, I suggest you get a better set.

Schuler loop i only common to a stable-platform INS that has been programmed to remain horizontal as the aircraft moves around the surface of the earth
That statement is not true

Dick
6th Aug 2012, 11:12

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A lot of rubbish has been said about the schuler oscillation... Much of it found in pilot literature...

It is actually very simple.
If you have a platform that is horizontal when you are in London, it will not be horizontal anymore when you reach Australia, if you keep it in a fixed position (relative to space).
And gyros provide fixed positions relative to space only. So we need some sort of mechanism that will tie the platform relative to the round earth.

In INS systems this is done by measuring acceleration and thereby the speed that we move with.
So if you are moving with XX kts direct south, a tie-mechanism will tilt the platform xx deg/hour so it is kept horizontal with regards to the earth.

So far so good... The problem is then, that this tie mechanism has no clue weather an acceleration is actually caused by the airplane moving, or because the platform is not horizontal.
If the airplane is parked, and the platform for some reason is not horizontal, you will pick up an acceleration due to earth gravity.. This will be interpreted as if the plane was actually moving, and the tie-mechanism will start tilting the platform. The tilting will continue until the platform passes the horizontal, and starts tilting the other way. This is then interpreted as a deceleration, and eventually an acceleration in the opposite direction, causing the tie-mechanism to now tilt the platform back again...And we then have an oscillation....
The frequency of this is quite slow (84 minutes), and unless you dampen this the INS/IRS wold be worthless.
Schuler was a German engineer who claimed that INS/IRS system would be impossible to build, because an accelerometer, is unable to figure out if it is measuring actual acceleration or "false" acceleration due to a tilted platform. He actually published a science paper about this :-D

Last edited by lasseb; 6th Aug 2012 at 11:14.
6th Aug 2012, 11:53

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lasseb

If you "dampen" the response to errors due to incorrect levelling how do you allow the primary function to continue un-damped?
6th Aug 2012, 12:28

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You are not dampening a response, and it is not incorrect leveling. The platform will always oscillate. That's the nature of a feedback circuit. The idea is just to subtract a known (error) oscillation from the incoming signal.

What you are doing is deducting a very low amplitude/low frequency signal from the input. This signal is (or rather should) correspond to the oscillation of the platform. The signal is generated internally in the INS/IRS computer, based on a math-model of the system.

Measuring raw input from the platform when an aircraft is parked would tell you that the aircraft is moving forth and back (both N/S and E/W) with an 84 minutes oscillation period. Basically it would tell you that you where driving around in a circle ;-). The INS/IRS computer has a model of this signal internally and deducts it from the input signal. If the modeled signal is a perfect match to the actual oscillation of the platform, you'll have a perfect INS/IRS, that tells you that you are actually parked when you are parked ;-)

This compensating circuit is working both in platform and strap-down units. It's just easier to imagine using a platform. but the accelerometer errors exists in both scenarios.

To add confusion, some models of the INS actually fed the compensating signal to the platform leveling system in stead of deducting it from the input. This will cause the platform to hold still even in parked scenarious. I guess that computers where to slow these days to handle all the calculations, so the more that where done mechanically the better.
Both approaches will work.

EDIT:
The best explanation out there I have found is a NASA document describing low orbiting satellites. They have an initial paragraph on schuler. Very good stuff..

I am apparently to stupid to figure out the link to the PDF, but search for "SCHULER PERIOD IN LEO SATELLITES ", and you will get it.

Last edited by lasseb; 6th Aug 2012 at 12:33. Reason: Added link
6th Aug 2012, 13:43

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lasseb

Continuing, for simplicity, with the stable platform INS, if in ALIGN the levelling is carried out without error and there is no gravity element being sensed by the N/S and E/W accelerometers, why would the platform oscillate?

I think you are saying that if there is a leveling error and the platform is oscillating then the response is memorised, carried forward into the NAV regime and removed from all subsequent navigation computations. Do I read you correctly?

Edit. I think I see that you are assuming a rough initial alignment leading to error which is then computed out or, in older systems, fed back to the platform control. Am I getting warmer?

Last edited by Dick Whittingham; 6th Aug 2012 at 13:54.
6th Aug 2012, 15:46

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Hi Dick
This is actually harder to describe en words than I thought :-D.

What I'm trying to emphasize here is that the schuler error signal is not read from the platform, or memorized, or anything, it is strictly calculated/generated in the computer (based on a math model) and deducted from the incoming signals from the accelerometers almost at the very start of the calculations.

So the signal going to the integrators is = (SENSOR_DATA - SCHULER_DATA_MODEL.)

Regarding initial oscillation you are kind of correct. In theory if the platform is perfectly aligned no error should ever be present, and the platform should never oscillate. There are 2 problems with this. The first is that there is no such thing as perfect, and even small noise levels in the signal wiring will introduce this error. Also mathematical noise in the computer will do this, since we are not dealing with infinite decimals.

Secondly, because we have introduced the schuler compensations, that will actually make the platform oscillate if starts with no oscillation :-D.

Consider the following:
Lets say that the platform is perfectly aligned, dead-level, perfect wires, perfect computer, no oscillation.
The signal input to the integrators, are now the signal from the accelerometers - (which is zero), but deducted for the current schuler period value. So if the platform is perfect level, the input to the integrators would give us the 84 min oscillation signal (or rather the inverted value). This would then lead to the integrators giving us speed, which leads to the tie-mechanism trying to tilt the platform, and then we have the oscillation...

You could say, that because we have introduced the schuler compensation, the platform must oscillate all the time,
If we where only flying around in a 200NM radius (or so) from home base all the time, we could skip the entire schuler compensation mechanism, and use the same tie-mechanism as an attitude indicator. We would then only need to compensate for heading, and it would be much easier.

Hope this answers the question :-)

Last edited by lasseb; 6th Aug 2012 at 15:53.
6th Aug 2012, 16:59

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Lars

Yes, it's a word thing. I now think I understand what you say, and I am happy to find that I have not been wildly wrong, Thanks a lot
7th Aug 2012, 00:25

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Thank a lot of the explanations - genesis the engineer Dick and Lasseb, i now understand it a bit better now, but obviously a quite complicated subject! Dick and Lasseb you two guys know your shit.
N.B. Dick i got this from 'ace the tech pilot interview' i understand there is meant to be a few mistake from this book, but sadly it seems to be what the airlines are using, as a couple of my friends that are now with the airline studied this book religiously though for my own sake and others reading trends i am trying to find out where these mistake are in the book, thanks for your help Dick.

Last edited by akafrank07; 7th Aug 2012 at 00:29.
17th Aug 2012, 09:25

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Hey guys,

What is the vertical separation minimum below 30,000 ft?
a. 500 ft.
b. 1,000 ft.
c. 2,000 ft.
d. It depends whether or not RVSM is applied.

The correct answer is; b. 1,000 ft, and the reference states:

The vertical separation minimum (VSM) is:
• Within designated airspace (subject to RAN agreement (RVSM)), a nominal 300m (1,000 ft) below FL 410 or a higher level where so prescribed for use under specified conditions, and a nominal 600m (2,000 ft) at or above this level and; and
• Within all other airspace: a nominal 300m (1,000 ft) below FL 290 and a nominal 600m (2,000 ft) at or above this level.

This however doesn’t appear to coincide with the flight levels stated in the ICAO tables (the semi circular rule), in which case you can have an east bound VFR flight on FL 35, a west bound VFR flight on FL 45 and a west bound IFR flight on FL 40, giving a vertical separation of 500 ft.

Am I missing something here?
17th Aug 2012, 09:46

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Anders,

what you are missing is that VFR flights are only seperated in airspace classes A through C (cf. ICAO Annex 10, Appendix 4). Apart from this, you will seldomly find IFR traffic at FL 40 in uncontrolled airspace. Firstly, FL 40 would only exist in a hypothetical country having a transition altitude which is low enough, and secondly such a low flight level would mostly be in uncontrolled airspace.

Edit: Sorry, the part after 'secondly' is rubbish. Forget it, please.

Last edited by hvogt; 17th Aug 2012 at 09:55.
17th Aug 2012, 10:06

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Ok, so basically it’s not considered “separation” outside of controlled airspace?

Those flight levels were just examples, the principle would still stand, I think, if flight levels 135, 145 and 140 were used instead.

Thanks for prompt response.
27th Sep 2012, 09:14

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Hey guys, I have a question for you.

Look at this TAF for Zurich airport
TAF LSZH 211322 22018G35KT 9999 SCT012 BKN030 BECMG 1315 25025G45KT TEMPO 1720 4000 +SHRA BKN025TCU BECMG 2022 25015KT T1815Z T1618Z=

Which of these statements best describes the weather likely to be experienced at 1500 UTC?

B. Meteorological visibility 10 km or more, main cloud base 3000 ft, wind 250°, temperature 18°C

C. Meteorological visibility 10 km or more, main cloud base 1200 ft, gusts up to 45 knots.

The correct answer is supposedly B, whereas C was my selection.

There is the matter of the entries “T1815Z T1618Z” at the end of the TAF which I’m not entirely sure what they mean, but the book states that “TAFs do not contain information on temperature”, and I also ran through the CBT on the subject and couldn’t find anything about it there either, so am I missing something?