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Flashcards in Machmeter/Terrestrial magnetism Deck (58):

Why do you need a machmeter when have an ASI?

Both the machmeter and ASI can be used for navigational and manoeuvring purposes however the machmeter is more reliable as it gives you TAS. TAS is more useful than IAS due to it is more accurate and reliable in calculating where you are, how long it takes to get from A to B etc. When you have MN you don't need to work out TAS, but if flying at IAS will need to calc TAS.


What 2 capsules are in the Machmeter and how do they function?

There is an aneroid (altitude) capsule to measure Static Px and an airspeed indicator to measure dynamic px.


WHat is the formula for calculating MN?



What is the formula for LSS?

LSS = 39 (square root) K


Why is the MN different at different places?

Because the only thing that changes in LSS is temp. As MN refers to temp, as MN = TAS/LSS.

Therefore different MN at different places.


What is another way of looking at the LSS; what is it a ratio of?

LSS = dynamic px/ static px

Therefore there is an aneroid (altitude) capsule (static px) and an airspeed capsule (dynamic px).


What does the machmeter not measure?

The machmeter instrument itself doesn't measure temperature. The MN doesn't use temp, only LSS relies on temp for measurement/calculations.


What errors does the machmeter suffer?

IMP only - instrument (hysteresis), manoeuvring and position error.

Machmeter doesn't suffer from the altimeter and ASI errors also cause it cancels each other out.


LSS = M?

LSS = M1, so if go twice the speed of sounds it's M2 and so on...

A/C don't usually fly at M0.75-M0.85 and don't fly at M1 as strain on structure. Problem with flying at M0.85 is that somewhere on the A/C airflow will be close to M0.95 and don't want to be at M1.


What is the acronym for CAS, TAS, MN?

Chicken, Tikka Masala.


What is the relationship between CAS, TAS and MN during an isotherm layer?

If CAS is constant TM both increase. If TAS is constant then MN is constant. If TAS is increasing/decreasing, MN also increasing/decreasing.


What is the relationship of CAS, TAS and MN in an inversion layer?

At constant CAS, MN & TAS increase but TAS increases at a greater rate.

MN & TAS is reversed. TAS will increase or decrease at a greater rate.


If flying from cold to warm air at constant Mach what will happen to TAS?

TAS will increase.


If flying from cold to warm air on constant CAS, what happens to TAS?

TAS increases.


What happens to TAS when flying from warm to cold air on constant Mach or constant CAS?

TAS decreases.


What happens if a machmeter's pitot tube is blocked during a climb?

The machmeter will over read.



What happens to the machmeter when the static port is blocked during a descent?

The machmeter will over read in the descent.



Which is the most simple form of compass and why do we use it?

DRC and use as standby as does not require A/C (pwr).


Why is the DRC called the DRC?

Because the sensors and the reading(s) are taken directly from the compass itself as compared to remote reading which is measured direction with computer getting feedback from wires.


As the DRC doesn't use A/C, how is it powered?

By earth's magnetic field.

These magnetic fields are magnetic lines of flux.


Dip has 2 components, what are they?

Horizontal (H) component and vertical (z) component.


Where do we suffer max angle of dip?

At the poles.


Why is the H component important?

Because H component is what aligns a DG/compass.


Where is the DG most efficient?

At the magnetic equator as there is no H component at the poles.


Where is the DG least effective?

At the poles because there is no H component at the pole(s).


What line show areas of equal dip at a specific (˚M) latitude?

Isoclinic line (equal incline)


What line indicates equal areas of zero dip?

Aclinic line

(No incline)


How do you measure variation?

Variation is the difference between ˚M and ˚T and you're ref point is ˚T.

So if you're standing on a Meridian and ˚M is West of you're position then it is a westerly variation. Conversely if ˚M is east of you're position then ˚M is easterly in variation.


What is an isogonal line?

Line joining areas of equal variation.

Can find this on a map, which will indicate the variation from ˚T


What do you call a line joining areas of zero variation?

Agonic line.


What is the max variation you can have?



What causes deviation?

Electrical and electromagnetic fields of computers and instruments on the aircraft. The A/C structure itself will cause deviation.

Deviation errors are different for different HDGs.


How is a compass swing carried out?

On a circular compass swinging base.


How do you remove deviation?

Deviation cannot be removed but you can reduce it.

After compass swinging there will be a compass (residual deviation) card next to the compass to advise of deviation error(s).


What 3 things must a DRC have?



What is horizontality?

As way of reducing the effects of the Z component and increase H component, horizontality.


How do you improve horizontality?

With a pendulous suspension compass card.

Moving the pivot point to the top of the magnet and the weight of the compass is at the bottom.


What is sensitivity?

The longer the magnet, the better the sensitivity, the more accurate the reading. However if space is restricted a long magnet is not ideal. Therefore to make a magnet more sensitive, thus more accurate, you put in more magnets.


What is aperiodicity?

The compass must not be prone to oscillating. If disturbed the compass must settle quickly and this is done with the oil based liquid already in the compass. The liquid also helps with sensitivity as it's lubricating.


When is the directional acceleration error the greatest?

When accelerating E or W as the magnet is aligned N-S.

Thus turn away from N or S causes an acceleration error.


When is latitude acceleration error at it's most minimum?

At the magnetic equator as there is no dip here.

Therefore max error at the poles.


When will you not have compass reading error during accelerating or deceleration?

When there is no dip, so when you are at the equator (latitude error) and when you are accelerating or decelerating flying north or south.


You're in the N. Hemis accelerating east, what does the compass indicate?

Apparent turn to the N and an indicated HDG <90˚

Compass under reads.


You're in the N Hemis and you accelerate west, what does the compass indicate?

An apparent turn to the north, indicated heading >270˚

Compass over reads.


You're in the S Hemis and you're decelerating east, what does the compass indicate?

An apparent turn south, indicated HDG >90˚

Compass over reads.


You're in the N Hemis and you're decelerating west, what does the compass indicate?

An apparent turn south, HDG <270˚



In a turn, where does the max compass error occur?

Max at the poles due to dip.

Remember dip is what displaces the COG below the pivot point on a compass. So where dip is greatest, COG displacement is the greatest thus greatest compass error at the poles.


Which direction are you turning when you have the least compass error?

Turning either E or W.

This is the opposite to accelerating.


In the N Hemis, if you are turning left through N, what happens to the compass reading?

Compass is sluggish and slower than the aircraft. So if you want to turn at the correct HDG you need to under shoot.


Inertia is what makes the compass lag behind the A/C. Thus the compass is sluggish.


If you are in the S Hemis and you are turning through south, what will the compass read?

Opposite of NUNOS so....

S hemis through south you will need to under shoot i.e. Roll out earlier to get the correct HDG.

Roll outs are usually 20˚-30˚ earlier (or later) for the opposite scenario. The opposite scenario also indicates that the compass is lively.


Why does liquid swirl affect compass readings?

Because when during a turn the liquid is also displaced, even after the A/C has stopped turning the liquid will take more time to settle. Due to this, liquid swirl adds to the error readings.


What does liquid swirl do to a compass error reading if you are in the N. Hemis and the A/C is turning through north?

NUN: the error increases while

N--OS: the error decreases


Flying at high speed at high altitude, the difference between RAM air temp and static air temp is:

Due to adiabatic warming


The formula to calc TAT is:

SAT + ram rise in temp due to adiabatic compression


TAT is always higher or lower that SAT and depending on....?

TAT always higher than SAT depending on CAS


A/C air temp thermometers are shielded to protect them from:

Solar radiation.


The magnetic moment of a magnet:

Is the product of pole strength and effective length.


The slow change in the earth's movement of the magnetic variation is know as .... Change and is caused by.....

Secular, westerly movement of the magnetic pole.