Settling with Power on a Confined-Area Approach
You are conducting a steep approach to a confined mountain clearing at about 4,500 ft pressure altitude on a warm afternoon (OAT 24 °C). The helicopter is a light piston type. On short final at low airspeed, high power, and a descent rate of roughly 400 ft/min, the aircraft begins to buffet, yaw control becomes less effective, and the rate of descent increases despite an increase in collective. Density altitude at the clearing is calculated at approximately 6,200 ft. The clearing is small and surrounded by trees.
1. Identify the aerodynamic condition developing and explain the combination of factors that produced it.
Read the worked answer
The symptoms describe settling with power (vortex ring state). It requires three elements together:
- Low airspeed - insufficient translational lift.
- High power - a large induced downwash velocity.
- A descent rate of roughly 300–500 ft/min - the helicopter descends into its own recirculating wake.
The result is a toroidal vortex around the rotor disc: the inner blade sections stall and lift is lost. Buffet, reduced tail-rotor effectiveness, and a descent that worsens when collective is raised are classic signs. High density altitude (6,200 ft) compounds the problem because the engine and rotor produce less thrust for the same power, so more collective and power are needed to arrest the descent, making the vortex state more likely.
2. Describe the correct recovery, and how you would have planned the approach to avoid it.
Read the worked answer
Recovery: Lower collective to reduce the induced downwash and break the vortex, then gain forward airspeed by lowering the nose. Do not pull collective - that deepens the condition. Trade altitude for airspeed, accept the loss of height, and only arrest the descent once translational lift is re-established. If height is insufficient, a controlled landing into the clearing is preferable to an uncontrolled impact.
Prevention: Plan a steeper, faster approach so the helicopter stays out of the 300–500 ft/min band at low airspeed. Use a higher approach speed and a constant-angle descent, and keep power changes smooth. At 6,200 ft density altitude, plan for a higher approach speed and longer ground run, and consider a running landing rather than a vertical one. Recompute performance for the actual temperature before committing.