Approaches

[Felix Saddler]

Hi im no pilot so go easy.
Can someone explain what the difference is between a visual approach, ILS appch, CAT111, CAT11 ans what ever else there is?

Thanks,


FS

[Clandestino]

Some info on ILS here! (http://en.wikipedia.org/wiki/Instrument_Landing_System)

[Spitoon]

A very general answer to give you the basics but there will be detailed situations where it is not the complete answer.

A visual approach is one flown all the way down to the runway by the pilot looking out of the window and judging the approach path and descent rate from what he/she sees - there are lighting aids at many airports to help but it may simply be a case of experience telling the pilot whether the aircraft is too high or too low or whatever. It is fairly obvious that the pilot must be able to see where he/she is - although not necessarily be able to see the runway - to make a visual approach.

An ILS is a common type of instrument approach where the pilot uses the instruments on the aircraft and radio signals from the ground or satellites to position the aircraft on the correct approach. Most instrument approaches are intended to get the aircraft to a point where the pilot can complete the landing visually. The radio aids that are available and the aircraft equipment the instrument determine how accurate the approach can be. The more accurate the aids, the lower you will be able to descend before you complete the landing visually (because you have greater confidence about where the aircraft is relative to the runway and any high ground near the airport). The accuracy is indicated by the category of the approach - a cat I approach will be able to bring an aircraft down to 200ft after which the pilot will land visually, at cat III approach will bring the aircraft down to 50ft or less. A cat III approach will be flown by autopilots and and may be approved for what are often called blind landings where, in some cases, the pilots will not have to see anything before landing - although in this case it's usually too foggy to be able to taxi off the runway. Of course, the poorer the weather that the aircarft is permitted to land in, the more training the crew will get and the more back-up systems are required on the aircraft and on the ground etc.

You will also hear other instrument approaches referred to - e.g. NDB or ADF, VOR and, if you're old like me, you might even remember VDF or QGH approaches. All of these are less accurate than an ILS approach.

There are other types of approach in use or under development like MLS, RNAV and GPS - if you Google these terms you'll find more info but they are less common than ILS or NDB approaches.

Hope this helps a bit.

[Felix Saddler]

Spitoon, thanks for taking the time. Was wondering you mentioned a CAT111 would take you down to 50', would the autopilot ever be turned off at that height as it seems very low? Or would the a/c just land itself?
One last thing, for a visual approach what is programmed into the FMC for the appch? appch fixes or nothing? How is an visual appch dealt with by atc? Vectors to finals?

Thanks again.

Felix

[Spitoon]

No problem Felix, but I'm just a simple controller - I can give you my understanding of how the aircraft is operated but I stand to be corrected by pilots who do that bit for real! The cat III bit is defined by the decision height for the approach (i.e. the height at which the crew must decide whether they can see enough to complete the landing safely and successfully) - below 50 ft, by definition, is cat III. Whether the flight is able to make an approach to cat III limits is determined by many factors including those I mentioned earlier. For a cat III approach I believe all landings will be completed automatically (including keeping the aircraft straight during the roll-out). For a cat II approach I believe that different airlines have different procedures depending on the aircraft capability and equipment certification etc. - as I understand it, some aircraft will be landed automatically whilst others will be flown manually by the pilot for the final portion of the landing from some point at or beyond the decision height. Again there will be many factors that affect what will be permitted and what approaches are available, for example, it may be that the signal indicating the centre of the runway will not be accurate enough for roll-out guidance which, if I recall correctly, means that only cat II can be approved (I would need to dig the books out to be certain so please don't quote me on this one!).

As for what goes into the FMC - I'm sure a pilot will correct me if I'm way off the mark here but for the sort of operation that I think you have in mind the aircraft will be set up for an instrument approach and will only make a visual approach if the conditions are suitable. The fixes used for an instrument approach will still be useful as a guide to the pilot but a visual approach is essentially navigated (officially anyway) by looking out of the window so there is no need for any new waypoints to be entered (and just after going visual is unlikely to be a good time to start entering fixes!).

The ATC side is quite straightforward. In the same way that the aircraft is set up for an instrument approach until a visual approach can be made, ATC will assume that an instrument approach will be flown until the pilot requests a visual approach. I won't go into the details here but ATC may still have to separate an aircraft making a visual approach from other aircraft and if there is no way to achieve this an aircraft may be refused permission to make a visual approach by ATC.

[ABX]

Spitoon, :ok: good on ya mate.

Thanks for a good answer.

Cheers,

ABX

[Felix Saddler]

Thank you very much Spitoon, very much appreciated.

Kind regards,

Felix