9 Instrument Landing System (ILS)

ILS Calculations

When flying an ILS approach it would be sensible to predict the rate of descent required on approaching the glide path, and prudent to have a check on height when established on the glide path. These can be simply achieved by using the 1:60 rule.

Example: An aircraft is at 4 NM from touchdown flying a 3° glide path at a groundspeed of 150 kt. Determine the height the aircraft should be and the rate of descent required.

To determine height by the 1:60 rule:

(ILS) System Landing Instrument 9

	Height	=		Glide Path Angle × Range	× 6076 ft
			60
	This can be simplified to:
	Height	=	Glide Path Angle × Range × 100
	This gives:		Height = 3 × 4 × 100 = 1200 ft

The trigonometric solution, using accurate values gives a height of 1274 ft, so the use of the simple 1:60 formula does underestimate the height. However, we are using this as a check for gross errors.

To determine the rate of descent (ROD) required, using the 1:60 rule:

Find the change of height per NM and then multiply that by the speed in NM/minute:

For a 3° glide path:

ROD =	Glide Path Angle × 1					× 6076 ft ×		Ground Speed	feet per minute
			60					60
=	3	× 100	×	Ground Speed			feet per minute
				60
=	5 × Ground Speed					feet per minute

Hence, for the example the ROD required will be 750 feet per minute (fpm).

As with the height this is an approximation and will slightly underestimate the actual ROD, which works out trigonometrically as 796 fpm.

Note: This is only valid for a 3° glide path. For any other glide path angle, calculate for a 3° glide path then divide by 3 and multiply by the glide path angle (or calculate on your Navigation Computer).

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Instrument Landing System (ILS)		9

ILS Summary

Components and frequencies:

Localizer	VHF - 108 to 111.975 MHz (40 channels). Aerial at upwind end.
Glide path	UHF - frequency paired. Aerial abeam touchdown.
Markers	VHF - 75 MHz. Fan-shaped vertical radiation. OM, MM and IM.
Back beam	From localizer. Non-precision approach.
Locator	Low power NDB at OM.
DME	Freq paired. Possibly in place of markers. Zero-referenced to threshold.
Ident	2 or 3 letters, 7 groups/min. Suppressed when ILS u/s. Continuous tone during
	maintenance.

Markers OM: blue, 2 dashes/s, 400 Hz, 6.5 - 11.1 km

MM:orange, 3 characters per second, alternate dots and dashes, 1300 Hz,

	1050 m
IM:	white, 6 dots/s, 3000 Hz, 75 - 450 m

Ground Localizer within 35 ft (Cat I) at ref datum. GP within 0.075 × glide path angle. monitoring Power within 50%.

Otherwise: Cease radiation, remove ident or lower category.

ILS Coverage	LLZ:	25 NM ± 10°, 17 NM ± 35°.
	GP:	10 NM ± 8°, 0.45 to 1.75 × glide path angle.
Principle of Operation:
Localizer	.
	LH lobe - 90 Hz, RH lobe -150 Hz; DoM increases away from			DDM is zero on
Back course	If approved use for non-precision approach. Reverse readings on CDI. HSI can
	operate in correct sense if front course QDM set.

Glide path Upper lobe - 90 Hz, lower lobe - 150 Hz. DoM increases away from the glide path centre line.

DDM is zero on the glide path centre line.

False GP at multiples of glide path angle. Be aware.

	Ref datum	height of GP over threshold.
	Indicators	CDI:	0.5°/dot; max 2.5° . Reverse indication on back course.
		HSI:	set course selector to front QDM for correct indications.
		GP:	0.14°/dot; max 0.7°. max safe dev - 2.5 dots fly up (0.35°).
	ILS	Facility		Category	I	II	III
	Guidance				≤ 200 ft	≤ 50 ft	0 ft
	Limits
		Operational		Category	I	II	IIIA	IIIB	IIIC
	(EASA)
				DH	≥ 200 ft	≥ 100 ft	< 100 ft	< 50 ft	0 ft
							or 0 ft	or 0 ft
				RVR	≥ 550 m	≥ 300 m	≥ 200 m	≥ 75 m	0 m
	Errors:	Beam bends, scalloping, beam noise,
		restricted vehicle movements during low vis ops,
		check failure flags,
		monitor ident.

Instrument Landing System (ILS) 9

163

9		Instrument Landing System (ILS)
			Range and	Critical area - aircraft and vehicles excluded for all ILS ops,
			Accuracy:	sensitive area - excluded area during low vis ops,
				Cat II/III holds,
				weather, FM broadcasts.
			Approach	Initial, intermediate and final,
			segments:	SSAs,
				landing minima - DH and RVR.

(ILS) System Landing Instrument 9

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			Questions		9
Questions
1.	The coverage of an ILS localizer extends to ............... either side of the on-course line
	out to a range of ............... NM.
	a.	10°, 35
	b.	35°, 10
	c.	35°, 17
	d.	25°, 25
2.	The upper and lower limits of an ILS glide path transmitter having a 3.5° glide slope
	are:
	a.	6.125° - 1.575°
	b.	7.700° - 1.225°
	c.	5.250° - 1.350°		9
	d.	3.850° - 3.150°			Questions
3.	glide path is:
	The minimum angle at which a false glide path is likely to be encountered on a 3°
	a.	6 degrees
	b.	5.35 degrees
	c.	normal glide slope times 1.75
	d.	normal glide slope times 0.70
4.	The visual and aural indications obtained when overflying an ILS middle marker are:
	a.	continuous low pitched dashes with synchronized blue light
	b.	continuous high pitched dots with synchronized amber light
	c.	alternating medium pitch dots and dashes with amber light
	d.	one letter in Morse with synchronized white light
5.	An aircraft carrying out an ILS approach is receiving stronger 150 Hz signals than 90
	Hz signals. The correct actions to be taken to place the aircraft on the centre line and
	on the glide path are to fly:
	a.	DOWN and LEFT
	b.	UP and LEFT
	c.	UP and RIGHT
	d.	DOWN and RIGHT
6.	In elevation the upper and lower limits of an ILS glide path transmitter having a 3.0
	degree glide slope are:
	a.	0.35°	0.70°
	b.	3.00°	at least 6°
	c.	5.25°	1.35°
	d.	10.0°	35.0°

165

9 Questions

7.A category II ILS installation encountered in the UK:

a.provides accurate guidance down to 50’ above the horizontal plane containing the runway threshold

b.has a steep glide path, normally 7.5°

c.provides accurate guidance down to the runway and along the runway after landing

d.has a false glide path that is exactly twice the true glide path angle

8.Which of these ILS indicators shows an aircraft on final approach left of the centre line and at maximum safe deviation below the glide path?

		24	27	30
	21
				33
9	18			0
Questions	OBS			6
	15			3
		12	9

a
	24	27	30
21			33
18			0
15			3
OBS	12	9	6
c

	24	27	30
21			33
18			0
15			3
OBS	12	9	6

b

	24	27	30
21			33
18			0
15			3
OBS	12	9	6

d

9.An aircraft tracking to intercept the ILS localizer inbound on the approach side but outside the published coverage angle:

a.will receive false on-course or reverse sense signals

b.will not normally receive signals

c.will receive signals without coding

d.can expect signals to give correct indications

10.The outer marker of an ILS installation has a visual identification of:

a.alternating dots and dashes on a blue light

b.continuous dots at a rate of 3 per second, blue light

c.continuous dashes at a rate of 2 per second, amber light

d.continuous dashes at a rate of 2 per second, blue light

11.The specified maximum safe fly up indication on a 5 dot CDI is:

a.half full scale needle deflection above the centre line

b.2.5 dots fly up

c.just before full scale deflection

d.1.3 dots fly up

166

			Questions		9
	12.	An aircraft is attempting to use an ILS approach outside the coverage sectors of an
		ICAO standard system:
		a.	from the glide slope needle the captain may be receiving false course and
			reverse sense indications and from the localizer needle intermittent and
			incorrect indications
		b.	the aircraft’s receiver is not detecting any transmissions and the ILS needle OFF
			flags are visible
		c.	from the localizer needle the captain may be receiving false course and
			intermittent indications and from the glide slope needle reverse sense and
			incorrect indications
		d.	from the localizer needle the captain may be receiving false course and reverse
			sense indications and from the glide slope needle intermittent and incorrect
			indications
	13.	The coverage of the ILS glide slope in azimuth is:		9
		a.	± 8° out to 10 NM		Questions
		b.	± 10° out to 8 NM
		c.	± 12° out to 17 NM
		d.	± 35° out to 25 NM
	14.	An aircraft’s Instrument Landing System glide slope and localizer receivers are
		receiving predominant 90 Hz modulated signals. If the aircraft is within the coverage
		of the ILS, QDM of 264°, it is:
		a.	north of the localizer and below the glide slope
		b.	south of the localizer and above the glide slope
		c.	north of the localizer and above the glide slope
		d.	south of the localizer and below the glide slope

167