# ATPL theory questions

Join Date: Jan 2011

Location: England

Posts: 642

This is an example of a question which requires the candidate to do a little bit of deductive work to understand what the author was really asking. The question does not actually say that the compass has been set for any particular latitude, so it is probably best to start by ignoring this possibility. In this case Option A is correct.

If however the compass had been set for some latitude other than zero, then as we move away from the Equator, we would expect the errors to decrease until we reached the latitude for which the compass had been set. The errors would then increase gradually as we moved beyond that latitude. In this case the correct option would be “Decrease then increases”. This is not an option, so we can reasonably deduce that the author of the question did not intend us to employ that interpretation.

This subject was discussed in a thread in the PPL Forum some time ago, and the discussion included how compasses are balanced for different latitudes. Some contributors argued that aircraft compasses are not balanced in this way, but an aircraft compass manufacturer provided the following statement:

The thread becomes rather long and dreary, with much argument about what PPL students should and should not learn, but for those who wish to read it the link is:

http://www.pprune.org/private-flying...ht=blob+solder

If however the compass had been set for some latitude other than zero, then as we move away from the Equator, we would expect the errors to decrease until we reached the latitude for which the compass had been set. The errors would then increase gradually as we moved beyond that latitude. In this case the correct option would be “Decrease then increases”. This is not an option, so we can reasonably deduce that the author of the question did not intend us to employ that interpretation.

This subject was discussed in a thread in the PPL Forum some time ago, and the discussion included how compasses are balanced for different latitudes. Some contributors argued that aircraft compasses are not balanced in this way, but an aircraft compass manufacturer provided the following statement:

I don't know if this will settle your argument, but basically you are both correct. The card assembly is balanced for the vertical component of the earth's flux lines based upon the surveyed strength by NOAA for a given latitude and hemisphere. A small weight is applied to level the display level, non-accelerated flight. The pendular design of the card assembly helps minimize (but not eliminate) the dip errors when turning and accel/decal environments. Hope this helps.

Gil Stone

President

Airpath Instrument Company

Gil Stone

President

Airpath Instrument Company

http://www.pprune.org/private-flying...ht=blob+solder

Join Date: Dec 2015

Location: France

Posts: 433

I suggest we just forget about the usefulness of this question in any serious flying context.. I've flown once with a directional gyro failure. At this time, I had no idea about compass errors but that did not stop me from navigating precisely with it.

I thought that the error was linked to the mass (compensating the dip) lagging behind the axis, isn't it?

I read your post in the linked topic, and you raised a good point : if there was no dip compensating mass, but just a mass well below the C of G, then you're right, the compass will have an "attitude angle" and dip towards the pole, hence the C of G will no longer be on the axis, hence turning/acceleration errors.

However with a non dip compensated compass, these errors should be inferior to those of a dip compensated one ? (this is suggested by Gil Stone's post but rephrased)

And finally Gil Stone tells us that some compasses will use both solutions : pendular design + a supplementary small weight to optimize the compass for a certain latitude ?

Related pic, for the lolz (it is simulation because I could not find it in a real environment)

Emergency/standby compass of an A320

http://www.flightsimlabs.com/index.p...eries-a320-3/#

I thought that the error was linked to the mass (compensating the dip) lagging behind the axis, isn't it?

I read your post in the linked topic, and you raised a good point : if there was no dip compensating mass, but just a mass well below the C of G, then you're right, the compass will have an "attitude angle" and dip towards the pole, hence the C of G will no longer be on the axis, hence turning/acceleration errors.

However with a non dip compensated compass, these errors should be inferior to those of a dip compensated one ? (this is suggested by Gil Stone's post but rephrased)

And finally Gil Stone tells us that some compasses will use both solutions : pendular design + a supplementary small weight to optimize the compass for a certain latitude ?

Related pic, for the lolz (it is simulation because I could not find it in a real environment)

Emergency/standby compass of an A320

http://www.flightsimlabs.com/index.p...eries-a320-3/#

Join Date: Jan 2011

Location: England

Posts: 642

Without any form of dip compensation the compass will suffer the full effects of the dipped lines of magnetic force. This will reduce the accuracy of the compass and further reduce the range of latitudes throughout which the compass would be usable.

Adding the pendulous suspension system will reduce the dipping effect thereby improving both accuracy and usable range of latitudes. But because the dipping is not completely eliminated, the pendulous suspension system introduces the turning and acceleration errors.

Adding a balance weight which is appropriate for a given latitude will eliminate the dipping completely, provided the aircraft remains at the latitude. This will also further reduce the acceleration and turning errors. If the aircraft is moved from that latitude some (reduced) degree of dipping will occur and this in turn will increase the acceleration and turning errors.

So it could be argued that by adding pendulous suspension and balance weights, the compass will achieve the best compromise between accuracy, usable latitude range, acceleration errors and turning errors.

Adding the pendulous suspension system will reduce the dipping effect thereby improving both accuracy and usable range of latitudes. But because the dipping is not completely eliminated, the pendulous suspension system introduces the turning and acceleration errors.

Adding a balance weight which is appropriate for a given latitude will eliminate the dipping completely, provided the aircraft remains at the latitude. This will also further reduce the acceleration and turning errors. If the aircraft is moved from that latitude some (reduced) degree of dipping will occur and this in turn will increase the acceleration and turning errors.

So it could be argued that by adding pendulous suspension and balance weights, the compass will achieve the best compromise between accuracy, usable latitude range, acceleration errors and turning errors.

*Last edited by keith williams; 4th Feb 2017 at 17:28.*

Join Date: Dec 2015

Location: France

Posts: 433

Well, onto your question now.

You can visualise that a perfect gyro will remain in a constant direction. When you move parallel to the direction of this gyro, the earth surface will actually curve under your feet and you will actually be travelling in a plane (not airplane, a mathematical plane in space) formed by the direction of the gyro and a vector that is the projection of the gyro axis on the earth surface

Use the distance measurement tool to visualise it.

I can't personnally tell you more, you have three options :

- Not care and just learn the answer

- Try to think in space, wonder why you couldn't follow a small circle (intersection of a plane with any part of a sphere's surface), use a pencil and orange (or football)

- Wait for a better explanation here

When thinking and experimenting with a pencil and a football, it looks like it's possible to follow a small circle..

You can visualise that a perfect gyro will remain in a constant direction. When you move parallel to the direction of this gyro, the earth surface will actually curve under your feet and you will actually be travelling in a plane (not airplane, a mathematical plane in space) formed by the direction of the gyro and a vector that is the projection of the gyro axis on the earth surface

Use the distance measurement tool to visualise it.

I can't personnally tell you more, you have three options :

- Not care and just learn the answer

- Try to think in space, wonder why you couldn't follow a small circle (intersection of a plane with any part of a sphere's surface), use a pencil and orange (or football)

- Wait for a better explanation here

When thinking and experimenting with a pencil and a football, it looks like it's possible to follow a small circle..

Join Date: Jan 2011

Location: England

Posts: 642

The short answer to this question is that when we fly along with a constant Gyro Heading set, but no Astronomic Precession, the rate at which the Transport Wander changes the direction of flight, is exactly the same as the rate at which a great Circle track would change. So we are flying along a Great circle track.

The rather longer explanation is provided below.

A google search for “Astronomic Precession” reveals an explanation which concerns how the axis of rotation of the Earth describes a circular motion over a period of approximately 26000 years. This is obviously not the interpretation intended by the author of the question.

In this question Astronomic Precession is intended to mean Earth Rate Gyro Wander. It is caused by the facts that the Earth rotates about its spin axis and the Meridians converge towards the Poles. The equation for Earth Rate Wander (ERW) is:

ERW = 15 x Sin Latitude in degrees x time in hours

The number 15 is in this equation because the Earth rotates through 15 degrees of longitude during each hour. So for any given time period the equation can be restated as:

ERW = Change in longitude x Sin Latitude.

The condition of “with no astronomic precession” can be achieved for the purposes of this question by assuming that the Earth has stopped rotating. Or perhaps more realistically by designing a Latitude Nut system which automatically adjusted itself for changes in latitude. Either of these solutions would eliminate Astronomical Precession (ER) but would leave Transport Wander (TW) unchanged. The equation for TW is:

TW = East-West ground speed x time of flight x Sin Latitude.

East-West ground speed x time of flight = Change of longitude, so the equation can be rewritten as:

Great Circles have the following properties:

1. All Great Circle form straight lines on the surface of the Earth and have their

centres at the centre of the Earth.

2. All Great Circles which run in a true North-South direction cross the Parallels of

Latitude at a constant angle of 90 degrees.

3. The Equator is the only Great Circle which runs in a true East-West direction, and

this crosses all of the Meridians at a constant angle of 90 degrees.

4. All other Great Circles cross the Meridians at angles which gradually change as we

move around the circle.

So why is that Great Circles running other than north-south or east-west do not cross the Meridians at a constant angle? The reason of course is that the Meridians are parallel to each other only at the Equator, then converge as latitudes increase towards the Poles. So any straight line (Great Circle) running in a direction other than due north-south or due east-west, must cross successive Meridians at different angles. We can see this if we look at the equations for convergence of the Meridians and the Conversion Angle which defines the direction of the Great Circles.

Convergency = Change in longitude x Sin Latitude

And

Conversion angle = ½ Convergency = ½ Change in longitude x Sin Latitude

The change in direction of a Great Circle track between two points is twice the conversion angle, so:

So we now have:

Equation 1…….Transport Wander = Change in Longitude x Sin Latitude.

Equation 2…….Change in great circle track = Change in Longitude x Sin Latitude

This means that the rate of change of the Great Circle Track between two points is equal to the rate of Transport Wander. So as we fly along with a constant Gyro Heading set, the rate at which the Transport Wander changes the direction of flight, is exactly the same at which a great Circle track would change. So we are flying along a Great circle track.

The rather longer explanation is provided below.

A google search for “Astronomic Precession” reveals an explanation which concerns how the axis of rotation of the Earth describes a circular motion over a period of approximately 26000 years. This is obviously not the interpretation intended by the author of the question.

In this question Astronomic Precession is intended to mean Earth Rate Gyro Wander. It is caused by the facts that the Earth rotates about its spin axis and the Meridians converge towards the Poles. The equation for Earth Rate Wander (ERW) is:

ERW = 15 x Sin Latitude in degrees x time in hours

The number 15 is in this equation because the Earth rotates through 15 degrees of longitude during each hour. So for any given time period the equation can be restated as:

ERW = Change in longitude x Sin Latitude.

The condition of “with no astronomic precession” can be achieved for the purposes of this question by assuming that the Earth has stopped rotating. Or perhaps more realistically by designing a Latitude Nut system which automatically adjusted itself for changes in latitude. Either of these solutions would eliminate Astronomical Precession (ER) but would leave Transport Wander (TW) unchanged. The equation for TW is:

TW = East-West ground speed x time of flight x Sin Latitude.

East-West ground speed x time of flight = Change of longitude, so the equation can be rewritten as:

**TW = Change in longitude x Sin Latitude…………………..Equation 1.**Great Circles have the following properties:

1. All Great Circle form straight lines on the surface of the Earth and have their

centres at the centre of the Earth.

2. All Great Circles which run in a true North-South direction cross the Parallels of

Latitude at a constant angle of 90 degrees.

3. The Equator is the only Great Circle which runs in a true East-West direction, and

this crosses all of the Meridians at a constant angle of 90 degrees.

4. All other Great Circles cross the Meridians at angles which gradually change as we

move around the circle.

So why is that Great Circles running other than north-south or east-west do not cross the Meridians at a constant angle? The reason of course is that the Meridians are parallel to each other only at the Equator, then converge as latitudes increase towards the Poles. So any straight line (Great Circle) running in a direction other than due north-south or due east-west, must cross successive Meridians at different angles. We can see this if we look at the equations for convergence of the Meridians and the Conversion Angle which defines the direction of the Great Circles.

Convergency = Change in longitude x Sin Latitude

And

Conversion angle = ½ Convergency = ½ Change in longitude x Sin Latitude

The change in direction of a Great Circle track between two points is twice the conversion angle, so:

**Great Circle direction change = Change in longitude x Sin Latitude……Equation 2**So we now have:

Equation 1…….Transport Wander = Change in Longitude x Sin Latitude.

Equation 2…….Change in great circle track = Change in Longitude x Sin Latitude

This means that the rate of change of the Great Circle Track between two points is equal to the rate of Transport Wander. So as we fly along with a constant Gyro Heading set, the rate at which the Transport Wander changes the direction of flight, is exactly the same at which a great Circle track would change. So we are flying along a Great circle track.

Join Date: Dec 2015

Location: France

Posts: 433

Could someone tell me the truth about NAT time periods ?

I know the numbers : daytime is Europe to America and is from 1130 to 1900 UTC, crossing 30W

From Paris to 30W there is about 2300km which would be about a three hour flight.

So takeoff from Paris would be between 0830 and 1600 UTC, that is between 930 and 1700 local time.

Maybe a tad earlier when accounting for the winds

So why the f would my book say that westbound flights take off early in the morning ??

When in facts they seem to be taking off all day !

And regarding the eastbound flights, validity is given between 1 and 8 UTC at 30W.

So 3 hours later you're between 4 and 11 UTC in Paris. Which is 5 to 12 local time.

There is no police working at such an early time is paris !

Thanks !

I know the numbers : daytime is Europe to America and is from 1130 to 1900 UTC, crossing 30W

From Paris to 30W there is about 2300km which would be about a three hour flight.

So takeoff from Paris would be between 0830 and 1600 UTC, that is between 930 and 1700 local time.

Maybe a tad earlier when accounting for the winds

So why the f would my book say that westbound flights take off early in the morning ??

When in facts they seem to be taking off all day !

And regarding the eastbound flights, validity is given between 1 and 8 UTC at 30W.

So 3 hours later you're between 4 and 11 UTC in Paris. Which is 5 to 12 local time.

There is no police working at such an early time is paris !

Thanks !

*Last edited by KayPam; 12th Feb 2017 at 21:19.*

Join Date: Jun 2012

Location: -

Posts: 1,176

Indicated airspeed (as read on the on the airspeed indicator) will:

A - increase in tailwind

C - decrease in tailwind

Who the hell writes this question? Is EASA trolling us really hard?!

/rant

A - increase in tailwind

**B - increase in headwind [I answered this]**C - decrease in tailwind

**D - remain unchanged in headwind and tailwind [this is correct]**Who the hell writes this question? Is EASA trolling us really hard?!

/rant

*Last edited by RedBullGaveMeWings; 15th Feb 2017 at 11:44.*

Join Date: Jan 2011

Location: England

Posts: 642

RedBullGaveMeWings.

I suspect that you are reading the question as meaning something like " a sudden increase or decrease in headwind or tailwind" or "The sudden onset of headwind or tailwind". But the question does none of these things.

It simply asks how being in a headwind or tailwind will affect your IAS. What they want you to know is that an aircraft in flight has no way of knowing what the wind is unless of course the wind suddenly changes. In a steady headwind or tail wind, the IAS will be unaffected.

It would of course hav e helped if they had said that the wind was steady.

I suspect that you are reading the question as meaning something like " a sudden increase or decrease in headwind or tailwind" or "The sudden onset of headwind or tailwind". But the question does none of these things.

It simply asks how being in a headwind or tailwind will affect your IAS. What they want you to know is that an aircraft in flight has no way of knowing what the wind is unless of course the wind suddenly changes. In a steady headwind or tail wind, the IAS will be unaffected.

It would of course hav e helped if they had said that the wind was steady.

Join Date: May 1999

Location: Bristol, England

Age: 60

Posts: 1,472

It's not you RedBull, the question is completely unclear. Answer (b) is correct for microburst type scenarios and (d) for steady or slow-changing conditions. In correct English you would also use the indefinite article, for instance "increase in

**a**tailwind". This question was probably written by someone who also does not have English as a first language.Join Date: Dec 2015

Location: France

Posts: 433

So, I am taking my first exams in Toulouse the day after tomorrow.

Air Law, ops proc, communications, and I am consistently scoring 90-100% on aviation exam.

Could I encounter many new questions or not ?

Thanks

Air Law, ops proc, communications, and I am consistently scoring 90-100% on aviation exam.

Could I encounter many new questions or not ?

Thanks

Join Date: Dec 2015

Location: France

Posts: 433

There were indeed questions that I suspect were new.

Nothing terrible.. I was uncertain about some answers but got my 75% in all exams.

Real score might be lost in the French DGAC system forever (in which case the exam lady told me that airlines could do with my school's final tests results, which would be VERY favourable )

Nothing terrible.. I was uncertain about some answers but got my 75% in all exams.

Real score might be lost in the French DGAC system forever (in which case the exam lady told me that airlines could do with my school's final tests results, which would be VERY favourable )

Join Date: Jun 2012

Location: -

Posts: 1,176

I am going through Instruments questions and there's one I really don't understand EASA's reasoning. And I think the difference between inner and outer loop are fairly clear.

4 is pretty clear, but I don't understand what 2 and 3 have to do with the flight path. Altitude is a barometric setting which changes with meteorological conditions from place to place, and we can maintain a speed also during a climb or descend anyway. For example, if a plane slows down on an ILS, RoD decreases and vice-versa to stay on the path to the runway.

In an autopilot system, the flight path modes are:

1) Pitch attitude hold.

2) IAS and Mach speed hold.

3) Altitude hold.

4) Glide intercept and track.

The combination regrouping all the correct statements is:

C) 3

D) 1, 2, 3, 4

1) Pitch attitude hold.

2) IAS and Mach speed hold.

3) Altitude hold.

4) Glide intercept and track.

The combination regrouping all the correct statements is:

**A) 4 [my answer]****B) 2, 3, 4 [corect answer]**C) 3

D) 1, 2, 3, 4

Join Date: Apr 2016

Location: EU

Posts: 156

Have you had a look at www.TheAirlinePilots.com :: View topic - Questions on Autopilot ? That website if great for quickly skimming common questions and topics.

In my opinion, IAS and altitude hold are very clearly properties of the flight path - You're trying to maintain a certain 3dimensional profile.

Holding the pitch attitude says nothing about the 3D flight path, just something about how the aircraft is oriented.

In my opinion, IAS and altitude hold are very clearly properties of the flight path - You're trying to maintain a certain 3dimensional profile.

Holding the pitch attitude says nothing about the 3D flight path, just something about how the aircraft is oriented.