VNAV Disconnect and Traffic at 7,500 Feet
During arrival at a busy airport, ATC clears a twin-jet: 'Descend via the ROKAN TWO arrival, cross ROKAN at 7,000 ft, 250 knots.' The crew sets 7,000 ft in the MCP altitude window and arms VNAV. The first officer is heads-down in the FMC because the runway has just changed; the captain is flying. Passing 9,300 ft, the aircraft levels off at 8,000 ft and the FMA shows ALT HOLD; VNAV has disconnected. The captain says, 'It's doing it again.' ATC then calls traffic at 7,500 ft, two miles ahead. The crew is 14 minutes from the threshold, speed is 280 knots, and the approach clearance has not yet been received.
1. Explain what has most likely happened and why this is a human-factors threat rather than simply a technical fault.
Read the worked answer
The aircraft levelling at 8,000 ft with ALT HOLD and VNAV disconnected, while 7,000 ft is selected, indicates that the automation is no longer following the crew's intended vertical path. The likely causes are a route discontinuity or altitude constraint introduced by the runway change, an FMC reprogramming action that invalidated the VNAV path, or an inadvertent mode reversion. The exact cause matters less than the human-factors picture: the crew has lost mode awareness, and the captain's 'It's doing it again' shows that this has happened before without being resolved.
The threat is not just the autopilot behaviour; it is the breakdown of monitoring. The first officer is heads-down during a high-workload arrival, so neither pilot is watching the FMA and the flight path. The captain is flying and also trying to diagnose the automation, which divides attention. The result is automation surprise: the aircraft is doing what it was told, but not what the crew intended. The speed of 280 knots, the traffic at 7,500 ft, and the missing approach clearance add time pressure. This is a classic 'what is it doing now?' situation, and the answer requires the crew to move from diagnosing the box to flying the aircraft. The technical fault, if any, is secondary to the loss of shared situational awareness.
2. What should the crew do in the next 60 seconds? Prioritise the actions and explain the reasoning.
Read the worked answer
The immediate priority is aviate: ensure the aircraft is not descending into the traffic and is on a safe path. The captain should disconnect or override the automation if necessary, set a clear vertical speed or level-off, and reduce speed toward 250 knots or below. The first officer must come heads-up immediately and monitor the flight path, traffic, and FMA. The call should be specific: 'I have the traffic at 7,500, two miles; we are level at 8,000; MCP is 7,000.'
Next, communicate with ATC. The crew should state its actual level and intention: 'We are level 8,000, descending to 7,000, traffic in sight' or request vectors if separation is in doubt. If the traffic is not in sight, request immediate vectors and traffic information. The crew should not accept a complex clearance until the aircraft is stable.
Only after the flight path and traffic are under control should the crew navigate and diagnose. The first officer can check the FMC for a discontinuity or constraint, but not while the aircraft is in a high-workload descent. A simple mode such as vertical speed or flight-path angle may be safer than reprogramming VNAV in the terminal area. The approach clearance should be requested only when the aircraft is configured and the crew is ready. The captain should verbalise the mode change: 'VNAV off, vertical speed set, 7,000 selected.' This restores shared mode awareness.
3. How should the crew prevent a recurrence, and what should be reported after landing?
Read the worked answer
Prevention starts with the briefing. Before every arrival change, especially a runway change, the crew should brief the vertical path, altitude constraints, and the expected automation modes. After any FMC change, the PM should call out the active modes and the next constraint: 'VNAV path, next constraint ROKAN at 7,000, MCP 7,000.' Both pilots should look at the FMA after every mode change and after every altitude selection. The 'heads-down' period should be limited to a phase when the aircraft is level and the other pilot is monitoring; during a busy arrival, the PF should not be the one reprogramming.
The captain's 'It's doing it again' is a warning sign that the previous event was not debriefed. The crew should use a short 'automation review' after the flight: what mode was expected, what mode was active, why the disconnect occurred, and what action resolved it. If the same thing happens repeatedly, the company should be told so that training or procedure can be reviewed.
After landing, the crew should file an occurrence report describing the mode disconnect, the traffic conflict, and the workload. The report should avoid blaming the first officer; it should identify the systemic threat: runway change, heads-down FMC work, and insufficient mode monitoring. The operator can then reinforce the 'aviate, navigate, communicate' priority and the use of simpler modes during high-workload arrivals.