Boeing 737-800 – CAT III approach

JohanAero

Nowadays, category III operations are made on a daily basis anywhere in the world when we find low ceiling and poor visibility. When you fly a CAT III approach, it looks pretty simple but in reality, the technology behind it is complex. It also requires a lot of concentration for pilots. Depending on how well is equipped an aircraft, it will be able to perform a specific type of category.

In this article, we will discuss:

  • Minimums for CAT approaches
  • Fail operational
  • Fail passive
  • Visual references needed to continue an approach

Minimums for CAT approaches

CategoryDecision HeightRVR requirement
CAT INot lower than 200 ftNot less than 550m
CAT IIBelow 200 ft but not lower than 100 ftNot less than 350m
CAT III ABelow 100 ft but not lower than 50 ftNot less than 200m
CAT III BLower than 100 ft or no DHLower than 200m but not less than 75m
CAT III C
*
No DHNone

*From what I know, CAT III C doesn’t exist anywhere in the world. The technology is good enough, it is possible to make an aircraft land with no visibility. The problem is for taxiing thereafter and also for fire services in case of incidents/accidents.

Fail operational

The definition given by the FAA is the following:

A Fail Operational System is a system which after failure of any single component, is capable of completing an approach, flare and touchdown, or approach, flare, touchdown and rollout by using the remaining operating elements of the Fail Operational system. The failure effects of single components of the system, airplane or equipment external to the airplane which could have an effect on touchdown or rollout performance must be considered when evaluating Fail Operational systems. Fail Operational systems may be used to touchdown for Category IIIA (e.g., without a rollout system) or Category IIIB through rollout to a full stop. Use of a fail- operational system to touchdown in conjunction with a rollout system that is not fail-operational is acceptable as long as a suitable minimum RVR is specified in the operations specifications, for rollout.

Fail-Operational Autoland Status Annunciations

The following annunciations provide the flight crew with autoland system mode and status:

  • LAND 3 – two autopilots, three inertial sources, and the associated sensors are operating normally for an automatic landing and rollout.
  • LAND 2 – a failure has occurred above *Alert Height and redundancy is reduced; but the autoland system is still capable of making an automatic landing and rollout.
  • NO AUTOLAND – the system is unable to make an automatic landing.

With a LAND 3 (fail-operational) indication, the autoland system level of redundancy is such that a single fault cannot prevent the autopilot system from making an automatic landing.

With a LAND 2 (fail-passive) indication, the level of redundancy is such that a single fault cannot cause a significant deviation from the flight path.

Boeing airplanes are certified to CAT 3B operations when LAND 3 is annunciated. They are certified to CAT 3A when LAND 2 is annunciated. The Autoland Status does not equate to the category number.  

*Alert Height and Decision Height are two different things. Alert Height is 200 feet on all Boeing models based on the system design. It doesn’t change. That’s when the Autoland Status won’t degrade from LAND 3 to LAND 2 due to a single system failure, below the 200 feet alert height. Decision Height (minimums) is selected by the pilots and can be as low as 0 ft in CAT 3B operations.

Fail passive

Again, from the FAA:

A Fail Passive System is a system which in the event of a failure, causes no significant deviation of aircraft flight path or attitude. The capability to continue the operation may be lost and an alternate course of action (e.g., a missed approach) may be required. A fail-passive system is the minimum capability system acceptable for Category III operation with a Decision Height not less than 50 ft. HAT.

In order to conduct an AUTOLAND (Fail Passive) the following equipment must be operative:

  • Dual Channel Autopilot
  • Low Range Radio Altimeter and display for each pilot;
  • Decision Height (DH) Display for each pilot;
  • Two Digital Air Data Computer Systems;
  • Windshield Wipers for each pilot;
  • ILS Receiver and display for each pilot;
  • Flight Mode Annunciator for each pilot;
  • Two ADIRUs (associated with the engaged autopilots) in NAV mode;
  • Dual Hydraulic Systems;
  • Two sources of electrical power (the APU generator may be used as an independent power source); and
  • Both engines operating.

The advantage of being Fail Operational is that the redundancy of the system allows you to land in case of problem when we cannot with Fail Passive. But the cost for Fail Operational is very high. For exemple, Ryanair uses Fail Passive because it has a very big fleet so it’s more economical to divert from time to time rather than equipping all the aircrafts.

Visual references needed to continue an approach

Category I Operations

A pilot may not continue an approach below the CAT I decision height … unless at least one of the following visual references for the intended runway is distinctly visible and identifiable to the pilot:

(i) Elements of the approach light system;
(ii) The threshold;
(iii) The threshold markings;
(iv) The threshold lights;
(v) The threshold identification lights;
(vi) The visual glide slope indicator;
(vii) The touchdown zone or touchdown zone markings;
(viii) The touchdown zone lights; or
(ix) FATO/Runway edge lights.

Category II Operations 

A pilot may not continue an approach below the CAT II Decision Height … unless visual reference containing a segment of at least 3 consecutive lights being the centre line of the approach lights, or touchdown zone lights, or runway centre line lights, or runway edge lights, or a combination of these is attained and can be maintained. This visual reference must include a lateral element of the ground pattern, i.e. an approach lighting crossbar or the landing threshold or a barette of the touchdown zone lighting.

Category IIIA Operations 

For CAT IIIA operations, and for CAT IIIB operations with fail passive flight control systems, a pilot may not continue an approach below the Decision Height … unless a visual reference containing a segment of at least 3 consecutive lights being the centreline of the approach lights, or touchdown zone lights, or runway centreline lights, or runway edge lights, or a combination of these is attained and can be maintained.

Category IIIB Operations 

For CAT IIIB operations with fail-operational flight control systems using a decision height a pilot may not continue an approach below the Decision Height … unless a visual reference containing at least one centreline light is attained and can be maintained.

Most common types of approach light systems:

  • ODALS: Omnidirectional Approach Lighting System
  • MALSF: Medium-intensity Approach Lighting System with Sequenced Flashing lights
  • MALSR: Medium-intensity Approach Lighting System with Runway Alignment Indicator Lights
  • SSALF: Simplified Short Approach Lighting System with Sequenced Flashing Lights
  • SSALR: Simplified Short Approach Lighting System with Runway Alignment Indicator Lights
  • ALSF-1: Approach Lighting System with Sequenced Flashing Lights configuration 1
  • ALSF-2: Approach Lighting System with Sequenced Flashing Lights configuration 2
  • RAIL: Runway Alignment Indicator Lights
  • CALVERT I/ICAO-1 HIALS: ICAO-compliant configuration 1 High Intensity Approach Lighting System
  • CALVERT II/ICAO-2 HIALS: ICAO-compliant configuration 2 High Intensity Approach Lighting System
  • SALS: Short Approach Lighting System
  • SSALS: Simplified Short Approach Lighting System

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