What are the lowest authorized minimums for each ILS category?

Question

I have been recently told that ILS minimums were changed some time ago. Could anyone tell me what the lowest authorized ILS minimums are for CAT I, CAT II, CAT IIIa, CAT IIIb and CAT IIIc? I'm interested in FAA, EASA and ICAO standards.

Answer 1

As Carlo mentioned in the comments, the minimums may vary from place to place to place. Here in the US you can find all that information here. Or you can buy a printed copy from a variety of aviation suppliers.

This order published in 2012 sets some minimum performance guide lines for minimums as well. For example,

(2) The following is the minimum class of performance (refer to Appendix B) required for an ILS to support a published Title 14 of the Code of Federal Regulations (14 CFR) part 97 CAT II or III SIAP:

• Class II/T/2 for operations not less than RVR 1200.

• Class II/D/2 for operations not less than RVR 1000.

• Class III/D/3 for operations not less than RVR 700.

• Class III/E/3 for operations not less than RVR 600.

• Class III/E/4 for operations less than RVR 600.

See appendix B in that order for more information.

This may also be of some interest.

Answer 2

Minimums are specific to the approach in question and vary based upon their specific location and circumstances.

However:

AIM 1-1-9 3i states that ILS minimums cannot be lower than as follows for specific categories.

CAT I - 200ft AGL, 2,400ft RVR.

Special Authorized CAT I - 150ft AGL, 1,400ft RVR, if equipped with a FD or HUD.

CAT II - 100ft AGL, 1200ft RVR, using autoland or HUD to touchdown.

Special Authorized CAT II - 100ft AGL, 1200ft RVR, using autoland or HUD to touchdown. ALSF-2 lighting not required on the field.

CAT IIIa - <100ft AGL, RVR greater or equal to 700ft

CAT IIIb - <50ft AGL, RVR greater or equal to 150ft

CAT IIIc - ZERO-ZERO. No DH or RVR limitations.

Keep in mind these are MINIMUMS. Follow published approach plate minimums for specific approaches. If you attempt an approach to minimums in hard IFR, you better:

• Make sure your aircraft is equipped with the required avionics for the approach, the airport in question is equipped for that type of ILS approach, and there are current approach plates for flying it.

• Check the aerodrome NOTAMS for notices that said approach equipment is not down for maintenance, etc.

• Be VERY proficient in flying instrument approaches and the specific approach in question.

• Have a back plan for your backup and fuel to spare.

Ignore that, and you're likely to star in your very own NTSB report.

Answer 3

In fact, CAT I, CAT II, and CAT III are not ILS categories but approach categories. They are described by minimum visibility and decision heights. In the past typically ILS was used for these kind of approaches, but there are other technologies available that support low visibility operations.

From ICAO Annex 6 Part I (International Commercial Air Transport, Aeroplanes), Ninth edition - July 2010:

• Category I (CAT I) operation. A precision instrument approach and landing with:

  • a) a decision height not lower than 60 m (200 ft); and
  • b) with either a visibility not less than 800 m or a runway visual range not less than 550 m.

• Category II (CAT II) operation. A precision instrument approach and landing with:

  • a) a decision height lower than 60 m (200 ft), but not lower than 30 m (100 ft); and
  • b) a runway visual range not less than 300 m.

• Category IIIA (CAT IIIA) operation. A precision instrument approach and landing with:

  • a) a decision height lower than 30 m (100 ft) or no decision height; and
  • b) a runway visual range not less than 175 m.

• Category IIIB (CAT IIIA) operation. A precision instrument approach and landing with:

  • a) a decision height lower than 15 m (50 ft) or no decision height; and
  • b) a runway visual range less than 175 m but not less than 50 m.

• Category IIIC (CAT IIIA) operation. A precision instrument approach and landing with no decisoin height and no runway visual range limitations.

In Europe, the EASA OPS (EU legislative website)(English PDF) define the categories as:

• (12) ‘category I (CAT I) approach operation’ means a precision instrument approach and landing using an instrument landing system (ILS), microwave landing system (MLS), GLS (ground-based augmented global navigation satellite system (GNSS/GBAS) landing system), precision approach radar (PAR) or GNSS using a satellite-based augmentation system (SBAS) with a decision height (DH) not lower than 200 ft and with a runway visual range (RVR) not less than 550 m for aeroplanes and 500 m for helicopters;

• (13) ‘category II (CAT II) operation’ means a precision instrument approach and landing operation using ILS or MLS with:

  • (a) DH below 200 ft but not lower than 100 ft; and
  • (b) RVR of not less than 300 m;

• (14) ‘category IIIA (CAT IIIA) operation’ means a precision instrument approach and landing operation using ILS or MLS with:

  • (a) DH lower than 100 ft; and
  • (b) RVR not less than 200 m;
• (15) ‘category IIIB (CAT IIIB) operation’ means a precision instrument approach and landing operation using ILS or MLS with:
  • (a) DH lower than 100 ft, or no DH; and
  • (b) RVR lower than 200 m but not less than 75 m;

There is no IIIC defined in the EASA OPS

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Whether an operation in low visibility is possible of not is depending on a number of factors:

• The quality of the aircraft's navigation/guidance system
• The quality of the ground or space based system providing the navigation signal
• The training / approval of the flight crew
• Local factors such as terrain around the airport, installed lighting etc.

All these factors have an effect on the minima for a specific approach.

Answer 4

This gets a bit fiddly, because it depends on not only the ILS equipment available, but on the airport approach and runway lighting environment, the avionics available on the aircraft, and whether the operator has the necessary qualifications for the approach. (All approaches listed as "SA" require a specific OPSPEC/MSPEC/LOA approval. Standard CAT II and III approaches are available through blanket OPSPEC/MSPEC/LOAs.)

For FAA-land, though, here are the general rules, assuming no modifications due to obstacles penetrating the TERPS approach surfaces (34:1 or 20:1):

• 200' DA with 3/4mile visibility (4000 RVR) -- basic CAT I ILS with no approach lighting available (or substandard approach lighting such as MALS, SALS, or ODALS) and no special avionics support.
• 200' DA with 1/2 mile visibility (2400 RVR) -- CAT I ILS with MALSR/MALSF, ALSF-1, or SSALR/SSALF available, but no special avionics support.
• 200' DA with 1800 RVR -- CAT I ILS with MALSR, ALSF-1, or SSALR available and the use of command or automatic guidance (FD/AP/HUD) to DA. This is not a SA procedure -- it is charted as a text note on the standard ILS approach, and is available to all suitably equipped operators.
• 200' DA with 1800 RVR -- CAT I ILS with MALSR or ALSF-2 + TDZ/CL. This is the standard ILS procedure minimum for a runway so equipped. This is also the absolute lowest minimum available for a single pilot operation -- single pilot operators are ineligible to use CAT II/III or SA CAT I/II ILS approaches.
• 150' DH with 1600 or 1400 RVR -- SA CAT I ILS with MALSR or ALSF/2 + TDZ/CL and the use of HUD guidance to DH. Most approaches like this are 1400 RVR -- 1600 RVR versions are only present when TERPS demands an adjustment for close-in obstacles.
• 100' DH with 1200 RVR -- CAT II ILS with ALSF-2 + TDZ/CL. This is the baseline CAT II procedure, and can be flown by any operator that meets the regulatory requirements for CAT II operations. Requires a Class II/T/2 ILS.
• 100' DH with 1200 RVR -- SA CAT II ILS with MALSR/MALSF and the use of HUD or autoland to touchdown. Requires a Class II/T/2 ILS. Close-in obstacles may require autoland operations to touchdown.
• 100' DH with 1000 RVR -- CAT II ILS with ALSF-2 + TDZ/CL and the use of Autoland or HUD to touchdown. Requires a Class II/D/2 ILS. Not denoted as a SA procedure, but requires OPSPEC, MSPEC, or LOA approval -- it is a chart note on the CAT II procedure instead.
• 700' RVR, no DH -- CAT IIIa ILS with the use of a fail-passive autoland or HUD system. Requires a Class III/D/3 ILS and SMGCS.
• 600' RVR, no DH -- CAT IIIb ILS with the use of a fail-operational autoland system. Requires a Class III/E/3 ILS and SMGCS.
• 300' RVR, no DH -- CAT IIIb ILS with the use of a fail-operational autoland system. Requires full TWY lighting with controllable stop bars, a Class III/E/4 ILS, and SMGCS. May require surface movement radar.
• Operations below 300' RVR currently do not exist, except through special approval.