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ILS Flashcards

1
Q

What is an ILS and what components does it comprise of?

A

Instrument Landing system

3 components:
Localiser (lateral/azimuth readings from the centre line)
Glide path (vertical distance)
Distance info (how far from the threshold, usually paired with DME for
this)

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2
Q

What is the freq band for localisers?

A

Terminal VOR

108 to 112 MHz (odd no.s)

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3
Q

What freq band is used for a glide path?

A

UHF

330.95 to 334.7Mhz

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4
Q

Freq for marker beacons?

A

Transmits on a carrier wave on 75mHz

VHF

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5
Q

Which 3 displays can be used to indicate position of A/C to localiser and glide path?

A

CDI/OBI
HSI
PFD

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6
Q

What is a localiser?

A

An frangible antenna 300m on the non approach end of the runway sends out 2 radio signals known collectively as a localiser beam. Which provides azimuth guidance.

The antenna is made of dipoles and reflector elements. It stands 20m wide and 3m high.

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7
Q

What are the localiser lobes?

A

As the localiser sends out 2 laser beams, 1 is at 90Hz and the other is 150Hz.

These are used to advise A/C position relative to the centreline. So if more 90Hz than 150Hz, the A/C is to the left of the runway and so must amend right. This difference in Hz is called difference in depth of modulation (DDM).

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8
Q

What is the difference in depth modulation?

A

This is where the two signals, left and right lobes freq are compared, and the difference between the two is called difference in depth modulation (DDM).

If there is no difference then the 2 signals freq depth should be the same i.e. Zero.

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9
Q

What is reverse sensing?

A

This is where the A/C picks up on the wrong signal freq/lobe and turns the opposite direction to correct A/C position, but because the wrong signal has been picked up, this correction is therefore wrong.

As the wrong correction is made, often the A/C is moving in the opposite way in which it should and thus its called reverse sensing.

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10
Q

Why is there a localiser coverage?

A

This is a guaranteed protected zone where there is no interference from other lateral lobes/signals/obstructions (i.e. Mountain)

This is to protect the A/C from accidents due to picking up the wrong indications to crashing into high terrain.

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11
Q

What is the localiser coverage area?

A

+/- 10˚ out to 25nm on either side of the centre line

OR

+/- 35˚ out to 17nm on either side of the centre line

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12
Q

What is a LOC BC?

A

Localiser back beam.

This is where there is only 1 localiser transmitting through a runway, as usually there are 2, one on either end of the threshold. LOC BC are not found in the UK.

LOC BC are a mirrored signal from the main beam, thus any equipment not set up for LOC BC will receive the reverse sensed of the data. LOC BC cannot give glide path info if the A/C does not have the correct equip on board. Thus if you don’t have the correct equip then you may still be able to carry out a non precision approach.

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13
Q

What is a glide slope?

A

Aka glide path.
This is where 2 radio waves are transmitted to give vertical info.
The glide path antenna is usually located 120m to the side of the runway, 300m from the threshold.
The overlap of the lobes of the beam is the glidepath.
Like the localiser it also has a 90Hz and 150Hz lobe.
This time the 90Hz indicates too high and 150Hz is… Slow you’re descent rate (i.e. too low).

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14
Q

What is the glide path centre point?

A

This is the angle that a glide path cannot exceed, which is normally 3˚ above the horizon. Though there will be some between 2.5˚ to 4˚.

This is set for obstacle clearance purposes, and EU regs define any glide path > 4.5˚ is a steep approach.

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15
Q

What are false glide paths?

A

This is where you capture the wrong beam for you’re approach/descent.

The 1st false glide path appears at 2x true glide path so with a 3˚ glide path the 1st false glide path is at 6˚, then 9˚
As there is no intersection below the true glide path, false glide paths only occur above the true glide path.

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16
Q

False glide path reverse sensing is?

A

This is where you’ve picked up on a false glide path and thus follow the wrong corrections to descend onto the runway. E.g. If you were on the correct glide path and the indications are to increase ROD. However if a false glide path, the instructions are to decrease your ROD, thus making the A/C too high on approach. In these cases it is safest to go around.

As there are no intersects below your true glide path, it is best to try pick the correct glide path by intercepting it from below.

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17
Q

Why is there a glide path coverage?

A

To offer a protected zone where A/C can be free from interference of glide path signals.

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18
Q

What is the glide path coverage?

A

Laterally:
+/- 8˚ either side of centreline out to 10nm

Vertically:
Is the function of the glidepath angle (ø):

From 0.45 x ø to 1.75 x ø

19
Q

ILS approach: Range to touchdown, why is it required?

A

To ensure that you are descending at a safe ROD, a comparison must be made range to touchdown. This can be done using DME/P, however older procedures require the use of marker beacons.

20
Q

What are marker beacons?

A

These beacons are used to calc you’re ROD with range to touchdown to ensure a safe ROD.

These beacons are used in the absence of DME equipment/procedures.

21
Q

At what freq and how do marker beacons transmit?

A

75mHz and transmitted 6,000ft vertically in a fan shape.

22
Q

How do you use marker beacons?

A

As the A/C passes through a beam, an audio noise and flashing light indicate on the display.

As noise is a set of pulse strings very similar to morse code and can be used to identify which beacon has been passed. The differing pulse strings also coincide with a different colour of light on the indicator.

23
Q

How is an outer marker ident?

A

OM:

Range to touchdown: 3.5 -6nm
Colour: Blue
Audio freq: 400Hz (low pitch)
Ident: 2 dashes/sec

24
Q

How is a middle marker ident?

A

MM:

Range to touchdown: 0.5-0.6nm
Colour: Amber
Audio freq: 1300Hz (med pitch)
Ident: Alternating dots/dashes @ 2 dots and 6 dashes/sec

25
Q

How is an inner marker ident?

A 
IM:
Range to touchdown: <0.25nm
Colour: White
Audio freq: 3,000Hz (high pitch)
Ident: 6 dots/sec
26
Q

NDBs are sometimes placed with marker beacons, at which point are they placed?

A

NDB type LTR beacons are sometimes placed at MM and OM.

If there is a LTR beacon at MM and OM these are now known as LMM and LOM respectively.

27
Q

The presentation on a CDI/HSI for a glideslope indicate..?

This is the horizontal bar

A

Each dot deflection = 0.14˚
Full scale deflection = 0.70˚

To continue with ILS approach YOU MUST remain within 1/2 scale deflection i.e. 0.7˚. Failure to do so implies you MUST go around ie. must carry out missed approach procedure (MAP)

28
Q

CDI/HSI during an ILS approach becomes ….. More sensitive than used with…..

A

4x more sensitive than when it’s used with VOR

So on a CDI/OBI (the vertical bar) each dot represents 0.5˚ (as 2˚÷4º) and full scale deflection is now at +/- 2.5˚

29
Q

What is the formula for ROD?

A

ROD = ø x groundspeed (nm/min) x 100.

If descent gradient rather than angle is given, memo 3˚ approx 5%

30
Q

On an EFIS display, deflection on approach represents:

A

Max deflection +/- 2.5
2 dot display, each dot = 1.25˚

As range to touchdown becomes closer, dots become squares and

Max glide path (GP) deviation +/- 0.7˚
1 square rep 0.5˚

31
Q

What is the formula for passing height used in ILS approach?

A

Passing height = ø x range from touchdown (nm) x 100

Add 50’ (screen height) if EASA Q states crossing threshold

32
Q

During an approach if you increase speed and reduce ROD, what will happen?

A

Overshoot

33
Q

During an approach if you decrease speed and increase ROD, what will happen?

A

Undershoot

34
Q

ILS group equip self monitors. What are the localiser parameters?

A

If fault is detected, localiser will cease transmission and sends failure flag to ATC.

Self monitor will take action when:

CAT I: +/- 35’
CAT II: +/- 25’
CAT III: +/- 10’

E.g. If failure at 12’ the localiser will revert to CAT II transmission until >35’, then cease transmission.

35
Q

ILS group equip self monitors. What are the glide scope parameters?

A
  • If the glide slope changes by 0.075ø

* reduction in power > 50%

36
Q

What are the ILS perf categories?

A

Provides ILS guidance

CAT I: 200’ above threshold
(this is just coincidence that inner markers are also located there)

CAT II: 50’ above threshold down to 100’ or less

CAT III: Runway elevation/down to and along SFC of runway (with correct equipment)

37
Q

What is FM immunity?

A

Apart from the usual scalloping and interference cause by hangars, mountains, static interference, because ILS use FM signals, there is often interference from local radio stations.

To allow the A/C equipment to filter out these stations, equipment capable of FM immunity must be used.

38
Q

During LVP, where aerodromes with certification for LVP, CAT II and CAT III approaches and T/O with RVR <400m are protected, how?

A

2 areas are declared:

Critical area - an area of defined dimensions, published in AIP around the glideslope and localiser is ring fenced and vehicles and A/C are excluded from this area during ILS procedures.

Sensitive area - an area beyond the critical area where parking and movement of vehicles and A/C are strictly controlled.

39
Q

In an ILS system concerning the glidepath principle of operation, the DDM …?

(Increases/decreases above/below glide path)

A

Increase with angular displacement above, and

Increase with angular displacement below the glidepath.

40
Q

ILS localisers which are associated with normal angle glidepath transmitters in the UK provide coverage out to a range of …… within the centre line.

A

1.25nm and 2.10˚

41
Q

For a 3˚ glidepath, what is the glidescope coverage?

A

1.35˚ to 5.25˚ horizontally
1500’ vertically
10nm out from the centreline

42
Q

Which ILS component(s) are protected from interference by the FM community filters in NAV equipment?

A

Localiser only.

Protected by local FM radio stations nearby

43
Q

ILS glide transmitters use which assigned freq band?

A

UHF

329.3 to 335MHz

44
Q

Scalloping causes….

A

Rapid indicator changes from side to side of the intended approach path which can not be followed by the A/C

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