An expedited descent below minimum safe altitude
At night in IMC, a turboprop is being radar-vectored to an airport in mountainous terrain. The sector minimum safe altitude is 7,300 ft. The aircraft is at 9,000 ft, 22 NM from the airport. ATC issues: 'Descend to 4,000 ft, expedite, traffic at 7,000 ft, 3 miles, opposite direction.' The crew's terrain awareness display shows a ridge at 6,100 ft within 6 NM of the current position, and the GPWS has a terrain clearance floor. The controller repeats the instruction and adds that the opposite traffic has been told to maintain 7,000 ft. The aircraft is not on a published approach and is not cleared for a visual approach. The crew has 45 minutes of fuel before diversion, the destination weather is 600 ft overcast and 2 km visibility, and the destination's only approach is an RNAV LNAV to runway 09, 8 NM away. The PNF suggests descending to 6,000 ft to stay above the ridge but below the traffic.
1. What is the conflict, and what should the crew say to ATC?
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The conflict is between an ATC clearance to 4,000 ft and the sector MSA of 7,300 ft, with high terrain within 6 NM and opposite traffic at 7,000 ft. The crew is not established on a published approach and not visual, so it cannot treat the clearance as an approach clearance. Descending to 6,000 ft would put the aircraft below the MSA and only 1,000 ft above the ridge, with little margin for altimeter error, turbulence or an escape manoeuvre.
The crew should query, not comply blindly. A suitable call is: 'Unable 4,000 due terrain, maintaining 7,300, request 8,000 or vectors.' The request for 8,000 gives terrain clearance and 1,000 ft above the traffic at 7,000 ft. If the controller needs the aircraft lower for separation, the controller must provide an alternative that is terrain-safe, such as a different vector or holding the opposite traffic. The pilot should not accept a clearance that removes the terrain margin simply because the controller repeats it.
2. If the controller insists, what is the captain's authority and what should be done?
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The captain has final authority for the safety of the aircraft. A repeated ATC instruction does not transfer terrain responsibility to the controller in a way that obliges the pilot to fly below a safe altitude without a published procedure or confirmed terrain separation. The captain should state clearly that the clearance is unsafe and say what the aircraft will do: maintain 7,300 ft or climb to 8,000 ft, and request vectors.
If the situation becomes urgent, the crew may declare an urgency or emergency, deviate from the clearance and inform ATC. Use the terrain display, GPWS and MSA as independent checks. Do not descend to 6,000 ft as a compromise; that is still below the MSA and reduces the time available to react to a GPWS warning.
Record the exchange, the MSA, the terrain warning and the action taken. If the destination approach cannot be reached safely, divert before fuel becomes critical. The human-factors risk is an authority gradient: a junior crew may comply to avoid challenging the controller, and fatigue at night makes that more likely.
3. How should the RNAV approach and fuel state affect the decision?
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The destination has only an LNAV approach and the weather is 600 ft overcast, 2 km. The crew must know the LNAV minima and whether a 600 ft ceiling gives any meaningful margin. If the aircraft is not stabilised by the final approach fix or at minima, a go-around is required. The approach should not be used to 'get in' because the crew is low on fuel.
With 45 minutes before diversion and an 8 NM final approach, there is time for one approach and a diversion, but not for multiple attempts and holding. Set a fuel decision point before commencing the approach: if not established or if the weather is below minima, divert immediately. Declare minimum fuel if applicable rather than waiting until the final reserve is reached.
The combination of terrain, opposite traffic and a marginal approach means the safest plan may be to hold at 8,000 ft, obtain a revised clearance, and then either fly the approach with full monitoring or divert. The PNF's suggestion to descend to 6,000 ft is a symptom of pressure to make the approach work; it should be rejected.