Autopilot / ILS?

[Thunderbird8]

Can anyone give some top tips on intercepting an ILS, the Glideslope, flight planning (it's relation to intercepting an ILS), and managing the autopilot?

I have learned some useful things this weekend which has opened up a whole new world (I just generally fly the Cessna IFR and VFR at the moment) but extra ideas are always good.

Thanks guys.

[NikeHerk67]

Can anyone give some top tips on intercepting an ILS, the Glideslope, flight planning (it's relation to intercepting an ILS), and managing the autopilot?

I have learned some useful things this weekend which has opened up a whole new world (I just generally fly the Cessna IFR and VFR at the moment) but extra ideas are always good.

Thanks guys.

Whether using the autopilot or when hand flying to access an ILS there are two basic rules. First approach the localizer, the beam that points to the runway, BELOW the glide-slope. The second rule of thumb is to approach the ILS localizer at or about a 15 to 30 degree (no more than a 45 degree) angle from the RW heading, (azimuth).

Think of the glide-slope in these approximate terms. At 10 nm distance from the AP the glide slope will be approximately 3000 feet AGL (above ground level). Thus at 5 miles it will be 1500' AGL. If you catch the localizer at 1000 feet AGL 5 miles out from the RW in the C172 you will be well below the glide-slope. Your speed will be from 70 to 90 knots, slowing to 70 knots IAS once you acquire the glide-slope.

Keep in mind, if you're flying jets like the B737-400 for instance, increase the speed, distance, and altitude AGL accordingly. With the 737 you'll want to acquire the localizer at 150 to 170 knots at 10 nm out from the RW at around 2000 to 2500 feet AGL slowing to approximately 140 knots once you acquire the glide-slope.

Also note that these rules of thumb apply to rather flat approaches. Once you get good with these there are some mountain approaches that have steeper glide-slopes. Most glide-slopes have approximately a 3 degree slope with flat ground approach.

I hope this helps

[Thunderbird8]

It does indeed, many thanks.

One further question I have is that it seems that sometimes when getting fairly close to the runway (whilst flying down the beam) the aircraft just climbs sharply or (more likely) drops like a stone, is this because the ILS stops working short of the runway?

[alastairmonk]

Hi Duncan,

I can recommend this website for learning the basics about instrumental navigation and ILS procedure:

http://www.navfltsm.addr.com/

Although parts of the site are now a bit old, the basic principles haven't changed.

In answer to your last question, getting a completely automated landing using ILS is actually quite rare. For such "autoland" situations the autopilot and ILS both have to be near perfect, and in many cases either the autopilot isn't good enough or the scenery file for the ILS has minor faults. In some cases the ILS is offset and would not allow an autoland in real life.

For most ILS landing you may need to disconnect the autopilot at the "Decision Height" (around 200-500ft AGL) and fly the final few seconds manually. If you've set things up right with the aircraft fully configured for landing and the approach speed and descent rate correct, you will hardly notice the difference, other than not seeing the sudden change that you referred to.

Alastair

[Thunderbird8]

Thanks.

I am finding it hard to judge using the flaps on an ILS approach, if I am flying purely visually it's a lot easier, but as my current mode of FS 'training' involved learning the autopilot and ILS approaches I need to master this. Any tips here, is the autopilot in the Cessna likely to compensate adequately on the glideslope for flap deployment?

ALSO: I sometimes find myself allowing the autopilot to fly me down the glideslope (well before decision height) and suddenly I will dive into the ground for no reason.

[Majormonty]

Silly question probably but if Im say 20 miles out from the airport how will i know that i have intercepted the localizer?

[Thunderbird8]

Silly question probably but if Im say 20 miles out from the airport how will i know that i have intercepted the localizer?

The way I was shown is that if you are on AP and have the 'approach' button pushed, the heading light will go out and the plane will turn of it's own accord away from your designated heading (towards the runway). Same principle with the altitude setting and the glideslope.

[alastairmonk]

Silly question probably but if Im say 20 miles out from the airport how will i know that i have intercepted the localizer?

This is difficult to visualise without a map, but essentially the ILS is a cone with its apex at the end of the runway and heading away from the runway at (typically) a 3-degree angle. Thus, if you're flying towards the runway on the correct heading and at an altitude where, at some point, you'll intercept the beam from below, at some point you will "enter" the beam.

When this happens the vertical and horizontal guide bars in your VOR (assuming that it has been correctly tuned) will come alive. If you've been flying the correct heading the vertical guide bar (indicating sideways alignment, the Localizer) will be centred and the horizontal guide bar (indicating vertical alignment, the Glideslope) should indicate that the centre of the beam is above you (again, assuming that you've intercepted the beam from below.)

If you continue to fly at the same altitude the horizontal guide bar will slowly descend until, when its centred, you are exactly in the middle of the cone. Typically, when this horizontal guide bar is "one notch" above centre, you will switch from Heading Hold/Nav + Alt Hold to APP + Alt Hold. When the plane reaches the exact middle of the beam the AP should cancel the Alt Hold and descend, following the middle of the beam.

It stands to reason that before you reach this point the plane should be appropriately configured for landing. Lowering the flaps and gear will affect the performance of the plane, and when the AP takes over the descent you don't want to cause any sudden altitude changes, so (generally) once the VOR/HSI has come alive with the ILS beam you should start to lower the flaps and gear in stages to avoid such sudden attitude changes.

Thus, at 20 miles from the airport the ideal position will be 20 miles from touchdown on the correct approach heading and altitude. In real life this may not be possible, and you may have to approach at an angle to the landing heading and at a higher altitude. In order to enter the beam correctly you will need to know where you "should be" in order to "slot into" the beam at the correct height and heading. The easiest way to do this is to look at an approach chart for the airport in some form. In real life things are made easier for you because often this "ideal entry point" is marked by a waypoint, so that when you intercept the waypoint at a specific height and direction you will enter the cone. Proper approach charts will detail approach routes, marking a series of waypoints to be followed and the correct altitudes to be at when passing them. Taken all together the "system" works to put you into the correct position for landing approach.

Alastair

[Thunderbird8]

Alistair,

I know I didn't ask that specific question but just to say thanks, your input here is appreciated.

[Clipper24/7]

Most is already said here but,.. some visuals to demonstrate how the localizer and glideslope works:

http://img138.imageshack.us/img138/8439/locrangesg5.jpg

http://img133.imageshack.us/img133/8112/gprangesl7.jpg

Also if you want to fly the ILS manually without the use of the Autopilot and/or fly an approach with ILS out or no ILS:

To keep you on a three degree gp you need to start final descend take
AGL altitude diveded by 300 will give distance from threshold
example: 2000ft : 300 = 6.66nm

To figure oout what descent rate you need to maintain:
Juat add a zero to your Indicated speed and divede by two.
example: 120KIAS 1200 : 2 = 600fpm

[jsr418]

Im hoping you can help me with the Captain Sim 757 ILS approach. I have no problem flying any of the default fsx planes with an ILS approach. When I attempt to turn on the APR button on the 757, the plane immediately turns away from the runway. I am using the same method that I would for trying to land the default jets and have no clue where I am missing something. Thanks in advance.

[tigisfat]

Im hoping you can help me with the Captain Sim 757 ILS approach. I have no problem flying any of the default fsx planes with an ILS approach. When I attempt to turn on the APR button on the 757, the plane immediately turns away from the runway. I am using the same method that I would for trying to land the default jets and have no clue where I am missing something. Thanks in advance.


A few questions that must be asked:

Do you have the correct frequency for the ILS dialed into your NAV1 radio?

Is the NAV/GPS switch in the NAV position?

Is the CRS selector at the right heading (this won't fix it, but it helps)?

Are you intercepting the localizer and then the glideslope?

Are you intercepting the glideslope from below?


All of these are the correct conditions. Numbers one and two are the most likely to net you results.