# What are the differences between Cat I, Cat II, and Cat III as concerns aircraft requirements?

I understand that Cat I, II, and III ILS approaches have progressively smaller weather minimums. However, I am asking about the aircraft themselves. What are the differences between Cat levels as concerns aircraft equipment requirements and approvals?

• Why the downvotes? This is an excellent question! – J Walters Jan 30 '16 at 0:35
• We already have a question about the differences in the approaches, it makes sense to have one about differences for the aircraft as well. – fooot Jan 30 '16 at 0:45
• As seen in the question that @fooot referenced, these two FAA AC documents seem to have information that might provide the answers: see AC 120-29A for Cat I & II, and AC 120-28C for Cat III. – J Walters Jan 30 '16 at 2:19
• If you want to deep dive in the topic, this Airbus publication is a good one: Getting to grips with CAT II / CAT III operations, on SKYbrary. Look at chapter 4, Aircraft Requirements, page 99. – mins Jan 30 '16 at 12:02

The differences are in the demonstrated accuracy of the ILS or RNAV equipment. Not sure if the question can be properly answered without writing a book, but digging into the documents linked by Jonathan Walters I extracted the tables below. The document is very long and full of caveats, exceptions and specifics on testing requirements, etc. and frequently refers to parameters set forth in other documents.

## CAT III

Landing System Performance. All types of low visibility landings systems, including automatic flight control, guidance for manual control, and hybrid, shall be demonstrated to achieve the performance accuracy with the probabilities prescribed in this section. The performance values may vary where justified by the characteristics of the airplane.

The performance criteria and probabilities are as follows:

• (a) Longitudinal touch down earlier than a point on the runway 200 ft. (60m) from the threshold to a probability of 1 x 10-6;

• (b) Longitudinal touch down beyond 2700 ft.(823m) from threshold to a probability of 1 x 10-6;

• (c) Lateral touch down with the outboard landing gear more than 70 ft. (21.3m) from runway centerline to a probability of 1 x 10-6.

• (d) Structural limit load, to a probability of 1 x 10-6. An acceptable means of establishing that the structural limit load is not exceeded is to show separately and independently that:

• (i) The limit load that results from a sink rate at touch down not greater than 10 f.p.s. or the limit rate of descent used for certification under 14 CFR part 25 subpart C (see section 25.473), whichever is the greater.
• (ii) The lateral side load does not exceed the limit value determined for the lateral drift landing condition defined in part 25, section 25.479(d)(2).
• (e) Bank angle resulting in hazard to the airplane to a probability of 1 x 10-7. A hazard to the airplane is interpreted to mean a bank angle resulting in any part of the wing, high lift device, or engine nacelle touching the ground. 6.3.2. Speed Control Performance. Airspeed must be controllable to within +/- five knots of the approach speed*, except for momentary gusts, up to the point where the throttles are retarded to idle for landing. For operations flown with manual control of approach speed, the flightcrew must be able to control speed to within +/- five knots of the approach speed.

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