Automation Surprise on a Managed Descent
You are in a glass-cockpit twinjet descending on a STAR with the autopilot in a managed descent mode. You have been discussing a roster issue for several minutes while the aircraft flies. ATC clears you to descend to 7,000 ft and you select the new altitude. Shortly afterwards the aircraft levels off unexpectedly at about 9,500 ft and the speed decays toward the minimum manoeuvring speed. The flight mode annunciator shows a mode change you did not anticipate, and the thrust levers are at idle. The first officer says the aircraft is not doing what was selected. Terrain is not a factor, traffic is 3 nm behind and faster, and you are 12 nm from the initial approach fix.
1. Diagnose what has happened in human-performance terms and state the first actions.
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This is an automation surprise caused by mode confusion. The crew selected a target altitude but did not verify which vertical mode was armed or active, and the aircraft captured an altitude constraint from the STAR rather than continuing to the selected level. The mode change annunciated but was not monitored because attention was on the conversation, a classic failure of active monitoring rather than a system fault.
First actions:
- Fly the aircraft: take positive control of pitch and thrust, or disconnect the automation, and arrest the speed decay before it reaches the minimum manoeuvring speed. Speed is the immediate threat, not the altitude.
- Announce and confirm: state the actual mode and target aloud, and have the first officer verify on their display. Rebuild the vertical profile deliberately: select the required mode, set the target, and confirm the annunciation changed as expected.
- Inform ATC of the deviation and the reduced speed, and check the traffic behind; 3 nm and closing requires a prompt call and a plan to restore speed and separation.
The underlying lesson is that automation must be supervised actively. Selecting an altitude is not the same as commanding the path; the mode annunciation is the confirmation, and it must be read every time.
2. What would you change in how the crew manages automation and monitoring for the rest of the approach?
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Re-establish the monitoring contract. Both pilots read the flight mode annunciator after every mode or target change, call out what is armed and what is active, and confirm the aircraft's response. Use a verbal challenge: 'selected 7,000, mode is X, confirm.' Any unexpected level-off or mode change is treated as a surprise until explained, never assumed benign.
Manage non-flight conversation. The roster discussion is a legitimate distraction, but it should be parked during high-workload phases such as the descent brief, the approach and any level change. If a conversation must happen, one pilot is explicitly designated as monitoring and is not part of it.
For the approach: brief the expected vertical path and the likely constraints, decide in advance when to use selected versus managed modes, and set a stable-approach gate. Given the speed excursion, be conservative: configure early, allow extra distance, and be prepared to go around if the approach is not stabilised by the gate. Report the event if it breached an altitude or speed clearance, and use it as a training point on mode awareness.