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JWMuller

The ILS and the Course select

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Well, me saying about 3000 feet was a really just part of a 'rough guide' for the OP on this thread, i.e. having a go at an ILS approach from quite a long way out so that he'd have time to suss everything out without feeling rushed; so it was something which I knew would work and not have the guy doing what my instructor used to call a 'Spitfire turn' in order to line up! As with anything like this, reading the manuals is the way to go if we want to do things 'properly'. so here's a bit more about that kind of thing..

Obviously a glideslope signal which extends outward and upward from the end of a runway could be intercepted at pretty much any altitude, so long as you were in that signal's path so the signal was being received, and the equipment could make use of it.

According to the book figures, glideslope capture on the 737 will occur at 2/5 dot deviation from the the glideslope signal's centre and (according to Boeing) the glideslope can be captured from above or below if it is within that 2/5 dot parameter. Most people consider it to be more comfortable in terms of G loading to capture from below, since the autothrottle only has to reduce a bit to increase the descent rate to stay in the slope, as opposed to having to pitch over into a dive of considerably more than 3.5 degrees if the slope is below you. When glideslope capture occurs, the APP push button extinguishes, the light extinguishing is what makes some people think it's not working and is one of those 'gotchas' when you're not used to things, but it is working in spite of that light going out. You'll see GA annunciated to indicate that is so, unless you do one of the following... pressing TOGA switches, disengaging the AP, retuning the Nav radios, or turning off both FDs.

Now the bit you're interested in: As noted, you can capture a glideslope from quite high altitude and a long way out, and whilst that does mean you've got plenty of time to get the aircraft stabilised, it also means that you'll be flying a long slow approach with the gear and flaps down, which will require more thrust and use a lot more fuel than necessary, and ATC won't exactly be in love with you if you fly a 15 mile final at 140 knots lol.

So, this is the bit you need to know: If you are above 1,500 radar altitude, a faulty/invalid ILS signal will not cause the autopilot to disengage, but the FD needles will retract if the signal is invalid, so you should monitor those when capturing an ILS from above 1,500 feet to make sure that doesn't happen. when you get below 1,500 feet RA, the second autopilot kicks in, you'll see 'flare' annunciated, and go around mode is armed (but you won't see that annunciated) and in CS mode, you can't override autopilot steering. At that point if you mess with pretty much any setting on the autopilot, the autopilot will disengage because it is in 'full on I'm trying to do an ILS approach for you' mode. Note too that below 800 feet RA, some modes are inhibited for ILS capture, so don't do it too late. :cool:


Alan Bradbury

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54 minutes ago, Chock said:

According to the book figures, glideslope capture on the 737 will occur at 2/5 dot deviation from the the glideslope signal's centre and (according to Boeing) the glideslope can be captured from above or below if it is within that 2/5 dot parameter. Most people consider it to be more comfortable in terms of G loading to capture from below, since the autothrottle only has to reduce a bit to increase the descent rate to stay in the slope, as opposed to having to pitch over into a dive of considerably more than 3.5 degrees if the slope is below you.

Hmm -- I'm not familiar with the 737 so I may well be talking nonsense, but in the Airbus, 767 and the Jumbo the aircraft will very much not shove the nose down to capture the GS if you are above it -- you have to already be descending at a sufficient rate to be closing on the glide and therefore as the diamond moves up in to the centre you get GS annunciated and the aircraft actually gently reduces the rate of descent to stay on the glide (given that you would probably have been coming down at something in the region of 1500 fpm to capture from above, at least using the Airbus "glide from above" procedure -- that will then naturally have to reduce to somewhere in the region of 700-800fpm).

There are of course good reasons to be circumspect about capturing from above, however -- notably the potential for a rushed/high energy approach, and the risk of capturing a false glideslope at 6 or 9 degrees.

Of course, in a truly ideal world one has managed one's descent in order to capture from neither above nor below -- just gently decelerating towards Vref with the thrust at idle and the glideslope diamond centred, with no level flight segment at all from top of drop :cool:.


Simon Kelsey

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If you are just ginning up the intercept on your own, use the rule of 3 to estimate how far from the airport you should be in order to intercept the glideslope from underneath like you should. Take your height above the ground and multiply by 3.  That is at least how far you need to be when you intercept. eg 4*3=12 You should be at least 12 miles away from the runway if you intercept at 4000'agl.

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20 hours ago, jcpg said:

Hi  Chock,

Just one question, I thought you catch the ILS frequency at about 2000/2100 ft. I've been doing this for some time. It has worked for me. I read your reason for catching it at 3000ft, as the signal might bounce up. I thought 3000ft a bit too high. At that height coun't the plane go into a neg G because of the extra +-1000ft?  Just trying to do it the right way.   Please correct me?

Thanks for the info.

John Goncalves

Hmm... Try 2500, Thats the altitude that the FSX default ATC gives you to stay on


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11 hours ago, Chock said:

Well, me saying about 3000 feet was a really just part of a 'rough guide' for the OP on this thread, i.e. having a go at an ILS approach from quite a long way out so that he'd have time to suss everything out without feeling rushed; so it was something which I knew would work and not have the guy doing what my instructor used to call a 'Spitfire turn' in order to line up! As with anything like this, reading the manuals is the way to go if we want to do things 'properly'. so here's a bit more about that kind of thing..

Obviously a glideslope signal which extends outward and upward from the end of a runway could be intercepted at pretty much any altitude, so long as you were in that signal's path so the signal was being received, and the equipment could make use of it.

According to the book figures, glideslope capture on the 737 will occur at 2/5 dot deviation from the the glideslope signal's centre and (according to Boeing) the glideslope can be captured from above or below if it is within that 2/5 dot parameter. Most people consider it to be more comfortable in terms of G loading to capture from below, since the autothrottle only has to reduce a bit to increase the descent rate to stay in the slope, as opposed to having to pitch over into a dive of considerably more than 3.5 degrees if the slope is below you. When glideslope capture occurs, the APP push button extinguishes, the light extinguishing is what makes some people think it's not working and is one of those 'gotchas' when you're not used to things, but it is working in spite of that light going out. You'll see GA annunciated to indicate that is so, unless you do one of the following... pressing TOGA switches, disengaging the AP, retuning the Nav radios, or turning off both FDs.

Now the bit you're interested in: As noted, you can capture a glideslope from quite high altitude and a long way out, and whilst that does mean you've got plenty of time to get the aircraft stabilised, it also means that you'll be flying a long slow approach with the gear and flaps down, which will require more thrust and use a lot more fuel than necessary, and ATC won't exactly be in love with you if you fly a 15 mile final at 140 knots lol.

So, this is the bit you need to know: If you are above 1,500 radar altitude, a faulty/invalid ILS signal will not cause the autopilot to disengage, but the FD needles will retract if the signal is invalid, so you should monitor those when capturing an ILS from above 1,500 feet to make sure that doesn't happen. when you get below 1,500 feet RA, the second autopilot kicks in, you'll see 'flare' annunciated, and go around mode is armed (but you won't see that annunciated) and in CS mode, you can't override autopilot steering. At that point if you mess with pretty much any setting on the autopilot, the autopilot will disengage because it is in 'full on I'm trying to do an ILS approach for you' mode. Note too that below 800 feet RA, some modes are inhibited for ILS capture, so don't do it too late. :cool:

What is the Second Autopilot for? I didnt know there was two??!!

Thanks


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The autopilot in modern airliners is important, so there isn't just one autopilot, but two (sometimes there are three), the buttons for them are over on the right side of the Mode Control Panel and usually labeled something like: CMD A and CMD B or CMD 1 and CMD 2 (CMD is short for Command). The reason there are two, is because you don't want to be sitting there having the autopilot fly an automatic approach and have it conk out at a critical moment in the proceedings, so there are two, just in case that occurs.

All commercial aircraft have a number of back up systems, for example, the artificial horizon instrument on a typical Boeing airliner can be powered from any one of a total of five different electrical sources and can be displayed on lots of different primary flight displays too (should one of them fail). But even if all those fail, there is a separate battery-powered artificial horizon as well; it's the small one to the left of centre in the 737 cockpit and is called the standby artificial horizon, (you can hear it running even when the power is off because it works off its own dedicated battery). This is because, like the autopilot, the artificial horizon if a critically important instrument, so Boeing wanted to be sure pilots would always have one available.


Alan Bradbury

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On 28/03/2017 at 5:08 PM, Chock said:

The autopilot in modern airliners is important, so there isn't just one autopilot, but two (sometimes there are three), the buttons for them are over on the right side of the Mode Control Panel and usually labeled something like: CMD A and CMD B or CMD 1 and CMD 2 (CMD is short for Command). The reason there are two, is because you don't want to be sitting there having the autopilot fly an automatic approach and have it conk out at a critical moment in the proceedings, so there are two, just in case that occurs.

All commercial aircraft have a number of back up systems, for example, the artificial horizon instrument on a typical Boeing airliner can be powered from any one of a total of five different electrical sources and can be displayed on lots of different primary flight displays too (should one of them fail). But even if all those fail, there is a separate battery-powered artificial horizon as well; it's the small one to the left of centre in the 737 cockpit and is called the standby artificial horizon, (you can hear it running even when the power is off because it works off its own dedicated battery). This is because, like the autopilot, the artificial horizon if a critically important instrument, so Boeing wanted to be sure pilots would always have one available.

Aha, so its just a backup autopilot?


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Basically, yes.


Alan Bradbury

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On 3/30/2017 at 6:10 PM, Chock said:

Basically, yes.

Thanks


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On 3/15/2017 at 2:25 AM, Chock said:

Basically, you put the radio frequency of a ground based navigation aid into the Navigation radio (Nav 1), then choose a course heading, and select the appropriate mode on the autopilot, usually NAV (typically the selector on the autopilot will be labelled with switches or buttons which say things like NAV, or VOR, but you will also see labels such as APP, LOC, BACK COURSE etc, more on this below). When you fly into range of th nav aid which ou have tuned into on your Navigation radio (usually the range for a VOR beacon is about 30-50 miles, depending on terrain etc), the autopilot with steer you onto that course heading which you put into the course selector, going directly toward the navigation aid. In this way, you can navigate to where you want to go, by flying from one navigation beacon to the next and then the next and so on.

For an instrument landing, you would put the exact magnetic heading of the runway into the Course selector (keep in mind that runway numbers are either rounded up or rounded down to the nearest five degrees when they are named, so Runway 24 might not necessarily be on a magnetic heading of exactly 240 Degrees, it could be on for example, 237 Degrees, so you would have to put that exact heading into the course selector).

Then you would select LOC (localiser) on the autopilot and fly to within about 10-15 miles of the runway on a heading of no more than about 30 degrees off the actual runway heading (this is known as intercepting the localiser). When your aeroplane, gets within about a 50 degree cone of the runway's localiser radio signal, the autopilot will line you up with the runway and turn to the exact course heading to be coming straight at the runway, lined up exactly with it. When you got within about ten miles of the end of the runway, you would select APP (approach) on the autopilot, and the autopilot would then 'fly down' a second radio beam which is fired out from the end of the runway at an angle of about 3 degrees so when the autopilot keeps you in that beam, it will take you on a gradual descent down to the runway so you can land. Not all runways have an Instrument Landing System, and even some with an ILS only have a localiser signal and not an approach signal, in which case the autopilot can line you up with the runway, but you have to fly the descent yourself.

Normally, when doing an automated landing, you put the correct course into both Navigation radios (Nav 1 and Nav 2) and you would use more than one autopilot (these are usually labeled CMD 1 and CMD 2, and you'd press both of them when you selected APP). This is so that if something goes wrong with one of your autopilot systems, it will still work okay.

There are lots of tutorials out there which will show you how to do all that properly (look on youtube for something like ILS tutorial and you will certainly find one). It is worth doing that for a few reasons if you want to learn it all properly, for example; you always fly into an approach radio beam from below (typically at about 3,000 feet above ground when you fly into the beam). This is because the radio signal from the Instrument Landing System transmitter can bounce off terrain and occasionally it will send false echo radio signals above where the true radio beacon's beam is, so you never come at it from above. Also, if you come at it from above, the aeroplane would have to go into negative G in order to capture the approach beam, which would be uncomfortable for the passengers on an airliner.

Older airliners usually have the Navigation radios on the centre pedestal, in between the pilot's seats. You put the frequency into the bit which is labelled STB (standby), and then click the transfer button to make that frequency the active one which you are tuned to. Note that there will be four radios, two of them are COM frequency radios (the ones you talk to ATC on) two are the NAV frequencies, which are the ones you use for autopilot navigation. More modern airliners usually tune the Nav radios automatically when you select a runway on the Flight Management Computer's display unit (i.e. the thing which looks like a big pocket calculator, of which there are a couple in the 737, one for each pilot, but they are slaved together, so it doesn't matter which one you use).

For those airports that don't have ILS, such as Princess Juliana International Airport, Seletar Airport Singapore, how do pilots align their planes to the runway?

Edited by Billcoke

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26 minutes ago, Billcoke said:

For those airport that don't have ILS, such as Princess Juliana International Airport, Seletar Airport Singapore, how do pilots align their planes to the runway?

The most common method for approaches where there is no ILS, is to use a VOR/DME Approach. VOR beacons can be tuned into, then you can select a course to or from that beacon which an autopilot can lock onto if need be. In other words, if I am landing at an airport with a runway on heading 09 degrees, and there is a VOR/DME beacon on that airport, if I tune it in and select a course heading of 090, I can use my avionics to line me up on the correct heading for an approach to that runway although I will need to look at the charts for that approach to ensure I am clear of terrain etc.

Airports such as TNCM (Princess Juliana ) publish VOR/DME charts to assist with the correct course to fly and the correct altitudes to be at to fly a VOR/DME Approach. You can see some of those here.

A VOR/DME Approach is classed as a non precision approach because it does not provide any automatic altitude glide slope guidance, instead, you read off the distance to the VOR beacon on your instruments, and follow the listed altitude recommendations on the approach chart to provide you with the required information to fly a suitable descent profile.

Unlike with an ILS, where the autopilot can lock onto and fly the glideslope, a VOR/DME Approach is therefore only capable of providing you with automatic guidance to align with the runway, and in some cases it won't even do that since the VOR beacon is not always perfectly in alignment with the runway, so a VOR/DME approach might align you perhaps two or three degrees off the actual runway's alignment. This can be another reason why they are called non precision approaches.

Because of this, the MDA (minimum descent altitude, aka the 'decision height') for a non precision approach is typically higher than for a full ILS approach, and it is unlikely to ever be less than 250 feet AGL.

What is critical in a non precision approach, is to have your altimeter set to the local field elevation, then you pay attention to what the radar altimeter is reporting, and you of course also need avionics which accurately measure the distance to a beacon. You judge your height along the descent toward the runway by aiming to be at specific altitudes at specific distances from the VOR beacon, these are marked on the approach chart. This is why the beacons used for such approaches have DME (distance measuring equipment) built into the signal they put out, whereby your nav radios measure the ping time for the radio signal and then calculate the distance from the beacon based on that.

In good weather you can forgo the chart guidance and use the PAPI/VASI lights of the runway to assist you in getting onto the correct descent profile too of course, but in bad weather, you are reliant on flying distance/height positions accurately until you reach the decision height, and if you don't see the runway by then, you go around for another try, if you don't make it on the next go, that's when you consider heading to your alternate airport, so sufficient fuel to divert is another important consideration when flying to airports where a non-precision approach is expected.

Edited by Chock

Alan Bradbury

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36 minutes ago, Chock said:

 

The most common method for approaches where there is no ILS, is to use a VOR/DME Approach. VOR beacons can be tuned into, then you can select a course to or from that beacon which an autopilot can lock onto if need be. In other words, if I am landing at an airport with a runway on heading 09 degrees, and there is a VOR/DME beacon on that airport, if I tune it in and select a course heading of 090, I can use my avionics to line me up on the correct heading for an approach to that runway although I will need to look at the charts for that approach to ensure I am clear of terrain etc.

Airports such as TNCM (Princess Juliana ) publish VOR/DME charts to assist with the correct course to fly and the correct altitudes to be at to fly a VOR/DME Approach. You can see some of those here.

A VOR/DME Approach is classed as a non precision approach because it does not provide any automatic altitude glide slope guidance, instead, you read off the distance to the VOR beacon on your instruments, and follow the listed altitude recommendations on the approach chart to provide you with the required information to fly a suitable descent profile.

Unlike with an ILS, where the autopilot can lock onto and fly the glideslope, a VOR/DME Approach is therefore only capable of providing you with automatic guidance to align with the runway, and in some cases it won't even do that since the VOR beacon is not always perfectly in alignment with the runway, so a VOR/DME approach might align you perhaps two or three degrees off the actual runway's alignment. This can be another reason why they are called non precision approaches.

Because of this, the MDA (minimum descent altitude, aka the 'decision height') for a non precision approach is typically higher than for a full ILS approach, and it is unlikely to ever be less than 250 feet AGL.

What is critical in a non precision approach, is to have your altimeter set to the local field elevation, then you pay attention to what the radar altimeter is reporting, and you of course also need avionics which accurately measure the distance to a beacon. You judge your height along the descent toward the runway by aiming to be at specific altitudes at specific distances from the VOR beacon, these are marked on the approach chart. This is why the beacons used for such approaches have DME (distance measuring equipment) built into the signal they put out, whereby your nav radios measure the ping time for the radio signal and then calculate the distance from the beacon based on that.

In good weather you can forgo the chart guidance and use the PAPI/VASI lights of the runway to assist you in getting onto the correct descent profile too of course, but in bad weather, you are reliant on flying distance/height positions accurately until you reach the decision height, and if you don't see the runway by then, you go around for another try, if you don't make it on the next go, that's when you consider heading to your alternate airport, so sufficient fuel to divert is another important consideration when flying to airports where a non-precision approach is expected.

Thanks for the detail write-up, give me some time to digest the material

 

BTW, why does US has a higher transition altitude than Europe??

US has a 18,000 ft transition altitude while Europe has transition altitude of 3000-6000 ft. When flying in US airspace, given the high altitude at 18,000ft,  wouldn't approach be more difficult , due to steeper descent?

Edited by Billcoke

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I should think it is to do with terrain clearance and the amount of local airports there are. The continental US has quite a lot of high mountains, in central Europe the mountains are typically a lot lower, it's only until you start getting into eastern Europe/Russia when the mountains start getting really high. It won't affect the descent since any flight plan descends based on range to the destination and not because of the transition altitude. All the transition altitude actually affects is when you switch your altimeter from local pressure to standard pressure. Since there are a lot of airfields in the US, not switching to something other than local pressure setting could potentially cause problems too, so not having that switch occur until things are up at high cruise levels probably makes things simpler and therefore safer.


Alan Bradbury

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13 hours ago, Chock said:

I should think it is to do with terrain clearance and the amount of local airports there are. The continental US has quite a lot of high mountains, in central Europe the mountains are typically a lot lower, it's only until you start getting into eastern Europe/Russia when the mountains start getting really high. It won't affect the descent since any flight plan descends based on range to the destination and not because of the transition altitude. All the transition altitude actually affects is when you switch your altimeter from local pressure to standard pressure. Since there are a lot of airfields in the US, not switching to something other than local pressure setting could potentially cause problems too, so not having that switch occur until things are up at high cruise levels probably makes things simpler and therefore safer.

Thanks Chock for the clarification.

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15 hours ago, Chock said:

I should think it is to do with terrain clearance and the amount of local airports there are. The continental US has quite a lot of high mountains, in central Europe the mountains are typically a lot lower, it's only until you start getting into eastern Europe/Russia when the mountains start getting really high. It won't affect the descent since any flight plan descends based on range to the destination and not because of the transition altitude. All the transition altitude actually affects is when you switch your altimeter from local pressure to standard pressure. Since there are a lot of airfields in the US, not switching to something other than local pressure setting could potentially cause problems too, so not having that switch occur until things are up at high cruise levels probably makes things simpler and therefore safer.

When a plane flies from US to Europe, change from 18,000ft to 3,000 ft transition level, does the standard pressure need to re-calibrated? But the plane has only two possible altimeter settings, 

- Altimeter set at local QNH
- Altimeter set at standard pressure 1013hPa

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