High-Density-Altitude Lift Decision
You are the duty line engineer at a PNG highlands helicopter base. Field elevation is 1,650 m, QNH 1008 hPa, and the shaded air temperature is 31 °C. A single-engine turboshaft helicopter is scheduled to carry three passengers and a 60 kg external load to a ridge pad 8 NM away. On the previous lift the pilot reported that torque could only be held at 88 % with ITT steady at 742 °C; the aircraft flight manual limit is 810 °C for take-off and 780 °C continuous, and maximum torque is 100 %. At sea level the same aircraft routinely lifted four passengers with margin. The pilot asks whether the aircraft can complete the next lift and whether the engine is deteriorating. No maintenance has been performed for 40 flight hours, fuel is Jet A-1, and the engine has no recorded exceedance. You must make a decision before the next departure.
1. Explain, using the numbers given, why the aircraft's power margin has fallen. Identify what density altitude and turbine temperature tell you and what a single reading cannot tell you.
Read the worked answer
Approximate pressure altitude: field 1,650 m (about 5,400 ft) plus roughly 135 ft for QNH 1008, so about 5,540 ft. ISA temperature at that level is near 4 °C; the actual 31 °C is about 27 °C above ISA. Using the standard lapse of about 120 ft per °C above ISA, density altitude is roughly 5,540 + (27 × 120) ≈ 8,800 ft. Air density is therefore about 25 % below sea-level ISA, so the compressor passes less mass flow and the engine produces less shaft power for the same gas temperature.
The engine is not necessarily deteriorating: ITT 742 °C at 88 % torque is consistent with a hot, high, heavy demand. A single snapshot cannot separate ambient effects from fouling or wear. You need trend data at comparable conditions, a compressor wash and borescope, and fuel-control rigging checks before condemning the engine.
2. State your decision for the next lift and the conditions or limitations you would impose. Include what you would record.
Read the worked answer
Do not dispatch the planned three-passenger plus 60 kg load. Recalculate hover and climb performance from the flight manual using density altitude about 8,800 ft. If the chart shows positive margin, reduce payload and fuel to a figure that gives at least 10 % torque margin and keeps predicted ITT below 780 °C continuous. Brief the pilot on abort criteria: ITT approaching 780 °C, torque at limit, or inability to maintain hover. Consider a cooler departure time.
Record the pilot report, your calculation, the decision and any limitation in the technical log. If a reduced load still leaves no margin, cancel the lift and arrange a compressor wash and performance check before further high-altitude operations.