Collective Jam on Short Final
You are on short final to a 20 m wide runway at 200 ft AGL, 60 kt, in a light single-engine helicopter. The collective suddenly becomes stiff and then jams at a mid-range position; the helicopter begins to sink. The engine is at 100% torque, Nr is decaying slowly, and the low-rotor warning has not yet sounded. You are 0.5 NM from the threshold, with a clear grass area to the left of the runway. The wind is calm. The aircraft has a fixed skid landing gear and no autorotation training in the last 6 months.
1. Explain the likely mechanical cause of the collective jam and its immediate aerodynamic consequences.
Read the worked answer
A collective that becomes stiff then jams at mid-range suggests a mechanical binding in the collective control system - for example a seized bearing, a foreign object in the linkage, or a hydraulic servo fault if hydraulically boosted. The jam prevents further collective input, so the pilot cannot increase pitch to arrest the sink or reduce pitch to accelerate.
Aerodynamically, with the collective fixed and the engine at high torque, the main rotor requires more power than the engine can deliver as Nr decays. The rotor is moving toward the low-Nr region of the power curve, where induced power rises and blade stall may begin at the tips. Without collective control, the only remaining control is cyclic, which can trade altitude for airspeed but cannot restore Nr directly. The sink rate will increase, and the low-rotor warning may follow.
2. Decide on the landing action and justify it.
Read the worked answer
The priority is to get the aircraft on the ground before Nr decays further. With the collective jammed, you cannot flare normally, so you must use cyclic to manage the descent and accept a firmer-than-normal landing. The grass area to the left is clear and the wind is calm, so a run-on landing there is preferable to the runway, which may have obstacles or other traffic.
Action: lower the collective if any movement remains (it may be jammed only in one direction), otherwise use cyclic to maintain airspeed and reduce the sink rate, aim for the grass, and land with a slight forward speed to avoid a hard vertical impact. Keep the skids level. After landing, shut down and evacuate.
Justification: continuing to fly with a jammed collective is not possible; the aircraft will descend regardless. A controlled run-on landing at low speed is the least-risk option. Do not attempt a autorotation, because you cannot lower the collective to enter it.