Autopilot Failure During a CAT III Approach
An aircraft is conducting a CAT III ILS approach to runway 27 with RVR 250 m, fog, and wind calm. The aircraft is fail-operational and both autopilots are engaged; autothrust is in SPD mode. At 1,200 ft RA, AP 2 disconnects with an aural warning; AP 1 remains engaged and the localizer/glidepath are within limits. The crew's company SOP requires an alert height of 100 ft RA for CAT III and CAT II minima of 300 m RVR. The aircraft is at 1,200 ft, 4 NM from touchdown, and the crew has not yet acquired visual reference. Fuel is 2.5 hours. The missed approach is a straight climb to 3,000 ft, and the airport has a precision approach radar. The crew is well rested and has recently completed CAT III training.
1. What are the crew's options at 1,200 ft RA, and what should they do?
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At 1,200 ft RA the aircraft is still well above the 100 ft alert height, so the crew has time to make a decision. The failure of AP 2 means the automatic landing system is no longer fail-operational. CAT III requires fail-operational redundancy, so a CAT III approach to 250 m RVR is not permitted. The available downgrade is CAT II, which requires at least one autopilot and an RVR of 300 m; the reported RVR is 250 m, so CAT II is also below minima. CAT I would require 550 m RVR. Since no visual reference has been acquired and the weather is below the minima for the degraded capability, the correct decision is to go around. The crew should follow the published missed approach, notify ATC, and consider holding or diverting. Continuing to CAT III minima with one autopilot would be a clear violation of the approach capability and a serious risk.
2. At 300 ft RA, the autothrust disconnects and thrust drops toward idle. What is the risk, and what should the crew do?
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At 300 ft RA the aircraft is still above the 100 ft alert height, but the loss of autothrust is a failure of a required automatic landing system. Because the aircraft is in IMC and the approach is CAT III, the loss of autothrust means autoland is not available. The crew should go around immediately: apply TOGA, set go-around thrust, pitch to the go-around attitude, and follow the missed approach. They should not attempt to re-engage autothrust or continue manually without visual reference. If the autothrust disconnect was caused by a fault, the engine parameters should be checked; if the thrust levers are not responsive, the crew must use manual thrust. The decision to go around is mandatory below the approach minima when the required systems fail.
3. Explain the purpose of alert height in this decision.
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Alert height is a height above the runway used to decide whether a failure in the automatic landing system requires a go-around. Above alert height, a significant failure means the approach must be discontinued because there is still enough height to go around safely. Below alert height, the approach may continue because the system is designed to be fail-operational, or because a go-around at very low height may be more hazardous than continuing. It is not a visual decision height; it is a system-failure decision height. In this case, the AP failure at 1,200 ft and the autothrust failure at 300 ft both occurred above the 100 ft alert height, so the crew should go around. If a failure had occurred below 100 ft, the QRH would specify whether to continue or go around. The alert height is therefore a key part of the approach briefing and must be known before commencing the approach.