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67

Short answer: ILS is rather sensitive to interference and not all electronic devices take much precaution in avoiding the generation of interference. The pilot wants to be sure that the readings he's getting on the localizer and glideslope are accurate, since he can't actually see the runway to verify the final approach path visually. Longer answer: ILS is ...


39

ILS approaches were in common use in 1970 when I got my instrument rating. The normal (Cat I) ceiling minimum was 200 feet. So, yes, a 707 would have been able to land with a 300 foot cloud ceiling in the 70s. I checked Wikipedia and and found the history paragraph below: Tests of the ILS system began in 1929 in the United States.[14] A basic system, ...


29

The beam strength decreases as you move away from it's own centreline, so is it actually that the entire modulated signal strength decreases which when de-modulated is effectively a difference in amplitude modulation depth? Your question is very relevant, it can arise naturally if you happen to look at web pages using half correct descriptions (for ...


25

The ILS works using two components, a localizer and a glideslope. The frequencies for the localizer are between 108.1-111.95 MHz and the glide slope between 329.15-335.0 MHz. These frequencies are the carrier waves that the modulation you mention takes place upon. A pilot is only concerned with the localizer frequency as the navigation equipment knows the ...


20

The answer is "Maybe". An ILS is actually comprised of two signals - a localizer (which lines you up with the runway) and a glideslope (which guides your vertical descent). If the glideslope is out of service (as it was when KAL801 was flying its approach) you fly a localizer-only approach, which requires visual contact with the runway at a higher altitude. ...


20

The ILS is precise enough to allow landings in essentially zero visibility, including those performed by autoland. The ground based bits The vertical range is 1.4° (0.7° above glideslope, 0.7° below) and the horizontal varies, but is a maximum of 6° wide. What this means in terms of how many feet or meters off the centerline an aircraft is varies ...


20

The glideslope system is an analog system, and as such, it's subject to aliasing, resonances, heterodyning, and eight other technical terms I pulled out of my ass. Maybe a diagram will help: false glide slopes In plain English, above the "real" glideslope, there are false glide slopes caused by your equipment locking onto the wrong phase of the signals. ...


20

Assuming that you're asking about intercepting the glideslope from above rather than the localizer, the answer is that it is definitely NOT recommended. There are at least two significant problems if you do this. First, is the fact that at high vertical angles there can be false glideslopes. Looking at the diagram below, if you're coming at the glideslope ...


19

A monitored approach is a special kind of instrument approach involving added verbal call outs and increased monitoring of the airplane and is typically conducted when weather is below a certain threshold. For example, a crew may be required to fly a monitored approach if the weather is below 3/4 mile vis and the captain has less than 100 hours in type, or ...


19

You can read this related question if you want to learn more about the interference between electronic devices and airplanes. An answer there links to a very good document written by NASA on the topic. Long story short: Electronic devices are complicated. Airliners are complicated. Therefore, we can not predict exactly what the interference between them ...


18

Having worked as a software engineer on the lateral guidance subsystem of the FMCS (Flight Management and Control System) for the Airbus A310 about 30 years ago I found @reirab's answer fascinating. I can fill in some gaps as to how the information from the different systems is used and why the ILS information is particularly critical. On the A310 3 sets ...


17

A point shown in parentheses like this is called a Computer Navigation Fix (CNF). If it didn't fall right at the end of the runway, it would be marked with a small X. It's defined in the legend on page 39 (page 41 of the PDF) of the Terminal Procedure Publication User's Guide. These points are only used to define the navigation track in the flight computer. ...


15

The two ILS approaches on parallel runways will have a different ILS Localizer frequency. The pilots will select the correct approach from the charts and then either manually tune in the correct frequency or (for more modern aircraft) select the approach in the flight management computer, which will then automatically tune the correct frequencies. Before ...


14

These markings designate the hold-short point during different runway operations. Runway hold-short points are marked by white text on a red background. They designate a point which any aircraft must not cross without an explicit clearance from ATC. Note that in the below picture, there are two markings: Cat II or III ILS is used during low visibility ...


13

That image is a basic representation of a course deviation indicator, as mentioned by Steve H. An actual gauge would look like this: Image Source In your image, it shows that the aircraft needs to correct to the left and a little upwards, so that it will be on the glideslope as shown here.


13

At least in the U.S., ILS is popular because it is popular. It's relatively cheap to install and operate, the receivers share technology with VOR, and it is already installed everywhere. Momentum (and cost of replacement) has a lot to do with the failure and success of navigation systems. MLS requires new hardware on the ground and in aircraft. The FAA ...


13

Yes, since the ILS is just a set of radio signals emitted and received by aircraft, there can be more than one aircraft established on localizer and glidepath. The ILS does not lock or tune onto one single aircraft, it continuously broadcasts the localizer and glidepath signal. Since the localizer and glidepath antennas are located at the end of runway for ...


13

The decision to disconnect the autopilot is made by the pilot and can be made at any point as long as the operational limitations of the aircraft are satisfied (according to the type of ILS). The pilot also has to satisfy the applicable regulations and company SOPs (as @Sami already pointed out). Note that this depends on the pilot decision too- the pilot ...


12

Airports don't typically close because of visibility. In fact, ATC won't stop a pilot from flying an approach just because the weather is below minimums. There are a couple of reasons for this: The determining factor for landing is flight visibility, which may be different from the ground visibility that ATC has access to. Under US Part 91 rules, a pilot ...


12

Nontowered airports absolutely can have ILS approaches, and it's fairly common, though of course not as common as at towered fields. Two examples in my neck of the woods are KIKK (Kankakee, IL) and KMTO (Mattoon, IL). Also, at many towered airports, the tower is not 24/7 and closes at night. If such an airport has an ILS, it's still available after the ...


12

Because many copter autopilots are capable of flying a Cat I ILS to 200AGL, and then descending on autopilot into a hover at a pre-determined altitude above the runway. The pilot can then descend down to the runway surface without any forward visibility. The radar altimeter is used to judge distance to the ground. Because of the unique capabilities of ...


12

Visual approaches can be conducted from any point around the airport where the runway is in sight, e.g. if you are approaching from the north of the airport, you can be vectored to a position which is closer to the airport and be cleared for a visual approach from a position where you can turn visually and reach the runway threshold. ILS approaches begin at ...


11

That is the course deviation indicator, or CDI.


11

There is no provision in the ILS system to provide for an auto-taxi or runway vacate. IIIc is just zero-zero and roll-out control, not auto-vacate as the linked AvWeb article seems to suggest. Its a logical extension of the FAA definition, however auto-taxi is not part of the CAT IIIc definition or system requirements. The only requirement is to get the ...


11

Instrument landing system - ILS - is a very common precision approach system used in airports around the world. An ILS consists of two ground antennas and an airborne received in the aircraft. One of the ground antennas, known as a localiser, transmits a narrow beam along the runway, giving lateral guidance to aircraft approaching the runway. The other ...


10

ILS is can be very precise, but there are many factors that may compromise the precision. When all these factors are avoided, automatic landing with no visibility is reliably possible. The ILS installations are classified in three categories: CAT I: Category 1 allows for decision height (DH) no less than 200 ft (height, so above ground level). ILS creates ...


10

ICAO and FAA CAT III definitions A CAT III operation is a precision approach at lower than CAT II minima. Sub-categories are listed below. A category III A approach is a precision instrument approach and landing with no decision height or a decision height lower than 100ft (30m) and a runway visual range not less than 700ft (200m). A category III B ...


10

The relevant question is not whether or not the ILS system is used at all, but when the pilot switches from ILS to visual. At the beginning of an ILS approach, the ILS signals function as a navigation aid, helping to make sure that the aircraft is lined up correctly with the runway from a longer distance out than the runway itself would be visible clearly ...


10

From "Smart Cockpit" B777 Automatic Flight (PDF!) AFDS Status Annunciation The following AFDS status annunciations are displayed just above the PFD attitude display: • FLT DIR (the flight director is ON and the autopilots are not engaged) • A/P (the autopilots are engaged) • LAND 3 (three autopilots are engaged and operating normally for an ...


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