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Autoland describes a system that fully automates the landing phase of an aircraft's flight, with the human crew supervising the process. The pilots assume a monitoring role during the final stages of the approach and will only intervene in the event of a system failure or emergency and, after landing, to taxi the aircraft off of the runway and to the parking location.
Some autoland systems require the pilot to steer the aircraft during the rollout phase on the runway after landing, among them Boeing´s fail passive system on the 737-700 NG, as the autopilot is not connected to the rudder.
On the A-320 series and A330, the autoland system steers the aircraft on the runway, initially through the rudder and, as the aircraft slows via the nose wheel steering (NWS). In conjunction with the autobrake, a full stop can be made on the centre line without pilot intervention. If the NWS is not available, the QRH dictates that the autopilots must be disconnected immediately on touchdown and the pilot control the aircraft through the rollout.
Autoland systems were designed to make landing possible in meteorological conditions too poor to permit any form of visual landing, although they can be used at any level of visibility.
The Autoland System
The autoland system incorporates numerous aircraft components and systems such as the autopilot(s), autothrust, radio altimeters and nose wheel steering. Although not an integral component of the autoland system, the autobrake system is often used in conjunction with an automatic landing.
The pilots must program the FMS (or tune the appropriate radio aids), configure the aircraft for landing and engage the autopilot and autothrust systems in the normal fashion. The Autoland system then provide inputs to the aircraft flight controls and adjusts the engine power settings in order to maintain the required approach profile and land the aircraft safely without pilot intervention. Some systems require the pilot to reduce thrust to idle when performing autoland. The Airbus requires the pilot to move the thrust levers to the idle positon when the autocallout calls "RETARD" at 10' RA. HOWEVER, the autothrust has already reduced the thrust to idle before this point - the retard call is to remind the pilot to match the thrust levers to the demanded thrust requirement. In all cases, the pilot must select reverse thrust settings. The autopilots will be disengaged after landing to taxi clear of the runway.
The approach can always be discontinued at any time by pressing the TOGA switches or in the case of an Airbus, by advancing the thrust levers to TOGA detent. Depending on the aircraft type or autopilot system installed, the auto pilot may or may not disconnect at this point. Most aircraft capable of an autoland also have the capability of performing a go-around with the autopilot engaged.
Autoland systems are normally designated Fail Operational or Fail Passive.
- A Fail Operational system must have at least two autopilots engaged for the approach. The failure of one autopilot will still allow an autoland to be carried out. This allows a “no decision height” approach to be conducted.
- A Fail Passive system is normally associated with a single autopilot approach. In this case, failure of the autopilot will not result in any immediate deviation from the desired flight path; however, the pilot flying must immediately assume control of the aircraft and, unless he has sufficient visual reference to land, carry out a missed approach. The lowest allowable decision altitude (DA) for a fail passive system is normally 50’.
Suitability of Instrument Landing Systems
Depending on aircraft type and installed autopilot system, autoland may be used in any weather conditions at or above published minima on any runway with an ILS installed. Operators should note however that some CAT I installations are not suitable for autoland due to offset localizers or to unstable localizer or glideslope signals once below published minima. CAT II and CAT III installations should be used with caution when LVP are not in effect as the localizer or glideslope signals may be compromised by ground traffic. The aircraft AFM and company SOP should be consulted for further guidance.
The aircraft operator must be approved by the authority and pilots must be suitably qualified and experienced in accordance with IR-SPA.LVO.120 (Appendix 1 to EU OPS 1.450.)
- IR-OPS SPA.LVO.120 - Low visibility operations - Training and qualifications
- Acceptable Means of Compliance and Guidance Material to IR-OPS SPA.LVO.120
- EU-OPS 1.450, Appendix 1: Low visibility operations — Training and qualifications.
- Flight Safety Foundation ALAR Briefing Note 1.2 - Automation
- Airbus: "Getting to Grips with CATII and CATIII"