Stall Warning and GPWS Alerts on Approach
A pilot is flying a visual approach into a regional airport in Borneo at night in a twin turboprop. The weather is fine, but the terrain rises steeply to the north of the runway. On a 4 nm final at 1200 ft AGL and 130 kt, the stick shaker activates briefly. At the same time the GPWS sounds a 'SINK RATE' warning. The pilot has just selected flaps 20 and reduced power to idle to slow down. The EGPWS terrain display shows rising terrain ahead. The aircraft is not equipped with autothrottle. The pilot has 7 nm to the runway and the aircraft is 300 ft above the glideslope.
1. Prioritise the warnings and explain what each is telling the pilot about aircraft state.
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Two independent systems are alerting:
- Stick shaker (stall warning): The angle of attack has reached the stall warning threshold. The likely cause is the combination of idle power, flap selection and a high sink rate, with the aircraft slow and possibly in a turn. The shaker is a direct AoA warning and must be treated as the highest priority because a stall at 1200 ft AGL in a twin turboprop is immediately dangerous.
- GPWS 'SINK RATE': The radar altimeter has detected a descent rate that is excessive for the height above terrain. With rising terrain ahead and the aircraft 300 ft above the glideslope, the GPWS is warning of potential CFIT.
Priority: stall warning first, because loss of control is more immediate than terrain contact. The correct response is to reduce angle of attack by lowering the nose, apply power, and ensure the aircraft is not stalled. Once the shaker stops and a positive climb or level flight is established, address the GPWS warning.
Note that the two warnings can conflict: lowering the nose to stop the shaker increases the sink rate and may worsen the GPWS alert. The pilot must manage both by adding power and reducing AoA without descending into terrain.
2. Describe the pilot response sequence, including configuration and communication, and explain how you would avoid a false GPWS warning on a normal approach.
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Response sequence:
- Stall warning: Lower the nose, add power, level the wings, and confirm the shaker stops. Do not retract flaps abruptly.
- GPWS: If the sink rate warning continues after the stall is broken, initiate a go-around: apply go-around power, pitch to the go-around attitude, and climb.
- Configuration: Set flaps to the go-around setting, positive rate, gear up as appropriate.
- Communication: Advise ATC of the go-around and the reason; request vectors for another approach.
- Debrief: Review the approach profile, power settings and flap schedule.
Avoiding false GPWS warnings:
- Fly a stabilised approach: on profile, on speed, and configured by 1000 ft AGL.
- Avoid large power reductions close to the ground.
- Use the EGPWS terrain display for situational awareness, not as a primary navigation tool.
- Be aware that GPWS 'SINK RATE' can be triggered by high descent rates on a normal approach if the glideslope is intercepted from above.