CAT 1 vs 2 vs 3 HOLD What's the difference?

[knish1231]

As the topic states, What's the dfferenc between the different Category Holds?  I see it on all airport diagrams and i can't find the answer on the net.  Thanks for an answer or directions where to look in advance.

Pete

[KSYR-pjr]

As the topic states, What's the dfferenc between the different Category Holds?  I see it on all airport diagrams and i can't find the answer on the net.  Thanks for an answer or directions where to look in advance.

Actually, Category I, II, and III refer to the three categories of the ILS approach, or instrument landing system approach.  The ILS is a precision instrument approach that combines radio signals and ground based lighting to guide an aircraft to the approved runway in low-to-no visibility.

The three categories of ILS's have progressively lower weather minimums, from 200 feet above the ground and 1/2 mile visibility (category I), all the way 0 feet above the runway and 0 visibility (category III C).  Category III ILS approaches have a further breakdown of autoland weather minimums, and are referred to as Category IIIa, IIIb, and IIIc ILS approaches.

What this means is that the pilot of an aircraft coming down an ILS is required to see the runway environment by the minimum altitude listed on the chart for that type of ILS, as well as have the prescribed visibility once the runway environment is in sight.

Typically, general aviation aircraft, such as your typical single or light twin propeller aircraft and their pilots are only certified to fly CAT I ILS approaches.   These approaches only allow an aircraft down to 200 feet above the ground along the approach and have 1/2 mile visibility.  I have read that there is a way for a US-based instrument pilots and their aircraft to get certified to fly CAT II approaches, but I don't recall the details.

Moving on, some commercial airline aircraft are only certified to CAT II approaches, which is 100 feet above the ground and 1,200 feet visibility.  

Many of the airliners flying into reputedly poor visibility airports and those flying into the very large airports are certified to fly an auto-land CAT III approach in absolutely zero visibility, say in heavy fog.  Pretty incredible technology there.

Here is a WIKI article explaining in more detail, all you want to know about ILS approaches:

http://en.wikipedia.org/wiki/Instrument_Landing_System#ILS_categories

Also, a hold is a method to "park" an aircraft in the sky.  The pilot would fly a large "race track" oval one to many turns in order to remain in a specific position in the sky.  Holds are used to hold up arriving aircraft for any number of reasons, for example bad weather, heavy aircraft flow, or any reason ATC needs to slow the arrival of aircraft.  A hold is also a method for performing a course reversal on an instrument approach, but this is above what you were asking.

[Acey]

Great information on the ILS, but I think he was asking about the hold lines physically on the field.  For example, glancing down at the SFO chart I see "CAT 3" lines short of 28L as well as a "CAT 1" line on Foxtrot.  I think he's asking why the lines are in different places if they presumably use the same glideslope and localizer.  Back to the SFO diagram, the CAT 3 lines for 28L are closer than the CAT 1 line.

Adrian

[KSYR-pjr]

Great information on the ILS, but I think he was asking about the hold lines physically on the field.

LOL!  Are you serious?  Man, I really blew that interpretation.   Having never flown into San Fran, I was unaware that there were different categories of ILS hold short lines, too.  

For example, glancing down at the SFO chart I see "CAT 3" lines short of 28L as well as a "CAT 1" line on Foxtrot.  I think he's asking why the lines are in different places if they presumably use the same glideslope and localizer.  Back to the SFO diagram, the CAT 3 lines for 28L are closer than the CAT 1 line.

An educated guess suggests that this must be a testament to the type of receiving equipment on board the aircraft.  Aircraft certified to cat III approaches have redundant receivers, autopilots, and complex flight management systems and it appears from the differences in ILS hold short markings that this equipment is less susceptible to interference from ground based metal than the cat I ILS receivers.

I just did a Google search in the rec.aviation groups and it appears that no one there has ever discussed this so let me post this question over there, too, to see if the experienced jet aircraft pilots will chime in.  I'll post the results here, assuming a definitive answer does not arrive first.

[KSYR-pjr]

Without even posting the question, it just hit me (and I was wrong with the guess above):  

SFO only has one CAT III runway and that is RWY 28 R, whereas RWY 28 L is served by CAT I.    This would explain why the CAT I hold short line appears to be farther away when it really isn't.   If an ILS CAT III approach to RWY 28 R is in use, no one is going to be flying the CAT I approach to RWY 28 L and therefore that hold short line is not needed.

Food for thought while I await an answer in the newsgroups.

[KSYR-pjr]

I should also point out that the ILS critical area only applies to taxiways on the same side of the runway as the ILS transmitting tower.  

Perhaps the CAT I ILS RWY 28 L tower is located left of RWY 28 L, whereas the CAT III ILS tower is located between RWY 28 L and 28 R?

[Jason]

I should also point out that the ILS critical area only applies to taxiways on the same side of the runway as the ILS transmitting tower.  

Perhaps the CAT I ILS RWY 28 L tower is located left of RWY 28 L, whereas the CAT III ILS tower is located between RWY 28 L and 28 R?

Also note that ILS critical areas are only in effect when instrument approaches are being conducted with visibility less than or equal to 2 miles, and/or, ceilings are less than or equal to 800 feet or by request.

Jason

[SadWinter]

Great information on the ILS, but I think he was asking about the hold lines physically on the field.  For example, glancing down at the SFO chart I see "CAT 3" lines short of 28L as well as a "CAT 1" line on Foxtrot.  I think he's asking why the lines are in different places if they presumably use the same glideslope and localizer.  Back to the SFO diagram, the CAT 3 lines for 28L are closer than the CAT 1 line.

Adrian

I think that holding points are related with the size of the plane.....

[digger]

My semi-educated guess would take into account much of the information posted above.

The "ILS critical area" concept would require aircraft holding on taxiways to be a *greater* distance from a runway when CAT 3 approaches are being done, because of the greater degree of precision (specifically, the aircraft being guided by the ILS system *all* the way to the pavement), needed to insure a safe landing in zero visibility.

But that's just a guess too...

I should also point out that the ILS critical area only applies to taxiways on the same side of the runway as the ILS transmitting tower.

In some cases, the glideslope antenna is located on one side of a runway, while the localizer antenna is on the other side. This would result in a difference in the size and shape of the ILS critical are from one side of the runway to the other, but a critcal area on both sides.

[dmaviation]

Well...it is a bit more complicated than what appears here so far.  I have included an image to show the differences for these holding positions.  It was retrieved from FAA Advisory Circular 150/5340-18F dated Aug. 2010 (this is the current version for 2016). Below are a couple of paragraphs from the first page background reference the signs and markings for CAT II/III of this AC:

FAA Advisory Circular text:
The FAA is revising Terminal Instrument Procedures (TERPS) standards for the separation distance between a runway equipped for CAT II/III operations and the parallel taxiway that requires aircraft to hold, in certain circumstances, at a location other than the runway holding position.

Accordingly, the FAA has developed sign standards to assist airport operators in designating (1) the POFZ holding position in those instances where a taxiway, holding apron, or other movement area would result in an aircraft fuselage or tail penetrating, and (2) the alternative holding position on a taxiway during CAT II/III operations necessary to maintain adequate aircraft separation. The FAA has made a corresponding change to marking standards contained in AC 150/5340-1, Standards for Airport Markings.

Back to my commentary:
ILS holding positions are used to keep aircraft and vehicles out of the ILS glideslope beam and are always set up on the same side of the RWY as the glideslope antenna.  Localizer equipment is located at each end of the runway.  

An approach holding position (example sign:  15-APCH) has caused a great deal of confusion for some pilots because it has the same pavement markings as runway holding positions but is NOT at the RWY hold bar position (see attached diagram).  It is designed to keep taxiing aircraft out of the approach slope only while another aircraft is on approach.  

The CAT II/III holding position in the attached diagram I believe is used to protect missed approach surfaces should an aircraft in CAT II/III conditions decide to go around or be slightly off on their approach.  It simply provides more clearance from aircraft waiting on an adjacent taxiway - in this case, the AC details a position on a parallel taxiway to the runway.  

So there are essentially four RED potential holding positions signs (using the example RWY in the attached diagram).

1. A Runway Holding Position Sign 15-33 or just 15 if at that end of the runway (same for 33 if your aircraft is at the opposite RWY end)
2. A Runway Approach Holding Position Sign (protecting the Approach trapezoid for arriving aircraft)  15-APCH
3. An ILS Holding position sign (protecting the glideslope equipment) ILS
4. A RWY CAT II/III Holding position sign ( to protect a parallel taxiway to a runway for arrivals in CAT II/III minima)  15-CAT II/III

Hope this help folks...hope it doesn't complicate things and it is exclusively my personal understanding and interpretation.  Let me know if you see it differently.  Thanks!  Be sure to review the attached diagram!  Dave

P.S.  CAT I can be added to the CAT II/III sign.  It just depends on the approach, runway, and taxiway configurations. As minima become more restrictive there are more requirements to protect surfaces.  Perhaps a particular airport's signage/holding position requirements may not include a CAT I APCH if sufficient clearances are available for that apch category.

P.S.S.  Not every airport has all of these signs - they are as needed only based on that airport's individual configuration.

[dmaviation]

Brad G.   I think you mean potentially mandatory, right? If ATC didn't explicitly tell you to hold short of anything other than the runway, then it shouldn't be mandatory.

Absolutely correct...only if/when instructed by ATC to hold short. Thanks for that important clarification!  

And that difference between a holding position's purpose is what causes occasional confusion to commercial and GA pilots for, in particular, the APCH Hold Short Signage and Markings.  This combination has the same Pattern A pavement marking (two solid yellow and two dashed yellow lines) as a runway holding position.  If a pilot has not flown into an airport before with these types of APCH holding configurations and signage they sometimes confuse this APCH sign and marking combination with a runway holding position.  

I will edit my response to include the word "potential".  Thanks again!  Dave