Autorotation Judgement After a Low-Altitude Power Loss
You are at 400 ft AGL and 55 kt indicated airspeed in an R44, having just lifted off from a coastal aerodrome at sea level on a +30 °C day. At 400 ft the engine loses power. Gross mass is 1,050 kg. The aircraft is 0.8 NM from the runway threshold, with a clear grass area ahead and to the right; a built-up area lies 0.5 NM ahead. Wind is 10 kt onshore, aligned with the runway. The low rotor RPM horn has not sounded. The height-velocity diagram in the flight manual shows the avoid curve extends to about 10 ft at zero airspeed and to about 450 ft at 40 kt, narrowing above that.
1. Analyse your position on the height-velocity envelope and decide the immediate action.
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At 55 kt and 400 ft you are outside the shaded avoid region - the aircraft is in a configuration where a safe autorotative landing is achievable. Immediate actions: lower collective fully and promptly to preserve rotor RPM, maintain 55–60 kt (the autorotation speed for best glide and energy), and select a landing area. The horn not sounding means RPM is currently within limits, but the collective must still go down immediately - delay is the single largest cause of unrecoverable RPM decay. Turn into wind (10 kt onshore) to reduce ground speed and flare height. The grass area to the right is the primary candidate: it is clear, close, and away from the built-up area. Do not attempt to stretch the glide to the runway 0.8 NM away - at 400 ft you have roughly 30–40 seconds of useful altitude, and a 0.8 NM glide at best autorotation speed would require about 50 seconds. Attempting to reach the runway risks undershooting into the built-up area.
2. Explain how density altitude and mass affect your autorotative performance and the choice of aiming point.
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At sea level and +30 °C, density altitude is roughly 1,800–2,000 ft - the rotor must produce the same thrust in thinner air, so both induced power and the rate of descent in autorotation increase. Higher mass at 1,050 kg raises the disc loading: rate of descent increases and the flare and cushioning margin at the bottom decreases. The practical consequences are: (1) the glide ratio is worse than the flight manual's sea-level ISA figure, so the reachable area is smaller - pick a nearer aiming point. (2) The flare has less energy to convert, so the final cushioning is shorter and the touchdown firmer; aim for a run-on landing rather than a spot landing. (3) The height-velocity avoid curve itself shifts - the 'knee' moves up and right, so the region where a safe landing is doubtful is larger than the sea-level chart. This reinforces choosing the nearest clear area rather than a distant runway, and reinforces flying a precise 55–60 kt rather than a slower speed that would reduce stored energy.
3. After a successful landing in the grass area, what should you do and what might you have done differently pre-flight?
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Post-landing: secure the aircraft, account for all occupants, and check for fire or fuel leaks before approaching. Make a MAYDAY or PAN call to ATC or the aerodrome operator with position and status; the aircraft is off-aerodrome and may be a hazard. Do not attempt to restart and lift off unless you have a defined, clear departure path and have established the cause - a partial power loss may recur. Preserve evidence for the occurrence report. Pre-flight improvements: (1) brief a power-loss plan before every departure, including a nominated forced-landing area for each phase of flight; here the grass area should have been identified before lift-off. (2) Note that the departure path passed close to a built-up area at low altitude - a departure turn away from the built-up area would have reduced exposure. (3) Recompute performance for +30 °C at sea level, not just ISA, and consider a reduced mass or a cooler departure time. (4) Practise autorotations at the actual density altitude, since the height-velocity diagram and flare performance both degrade.