En-Route Cruise - Hot Day, High Altitude and Reduced Ceiling
You are cruising a turboprop at FL240 with a cabin altitude of 8,000 ft. OAT is ISA +14 °C. The AFM cruise chart gives a maximum cruise altitude of FL270 at ISA and a single-engine service ceiling of FL180 at ISA. The aeroplane is at 5,400 kg, and the AFM shows that the single-engine ceiling decreases by 400 ft per °C above ISA. A passenger becomes hypoxic and you need to descend. The nearest suitable aerodrome is 90 NM away at 3,200 ft elevation, and the terrain along the route rises to 11,500 ft.
1. Determine the single-engine service ceiling at the actual temperature and assess the terrain clearance risk.
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The ISA deviation is +14 °C. The single-engine service ceiling decreases by 400 ft per °C, so 14 × 400 = 5,600 ft. The ISA ceiling of FL180 (18,000 ft) becomes 18,000 − 5,600 = 12,400 ft.
The terrain rises to 11,500 ft. The single-engine ceiling of 12,400 ft leaves only 900 ft of margin above the terrain, which is inadequate for a safe drift-down with an engine failure, especially since the ceiling figure is a gross-weight, ideal-condition value. The aeroplane at 5,400 kg is not at MTOW, so the actual ceiling may be slightly higher, but the margin is still thin.
The correct decision is to descend to a lower altitude before crossing the high terrain, or to route around it. Do not plan to cross 11,500 ft terrain at FL240 with a single-engine ceiling of 12,400 ft; a drift-down from FL240 to 12,400 ft takes many minutes and covers a long ground distance, during which terrain clearance is not assured.
2. Explain how the hot temperature affects cruise performance and the choice of cruise level.
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High OAT reduces air density, which reduces engine power and propeller thrust at a given TAS, and also reduces the lift at a given IAS. The aeroplane must fly at a higher TAS to achieve the same IAS, which increases fuel flow and reduces range. The maximum cruise altitude is also reduced because the available thrust falls below the thrust required at high altitude.
At ISA +14 °C, the maximum cruise altitude of FL270 at ISA may fall to about FL240 or lower, depending on the chart. If the aeroplane is already at FL240, it may be at or near its ceiling, which means little margin for a step climb and reduced manoeuvrability.
The practical response is to plan a lower cruise level (FL200–FL220) where the thrust margin is better, accept a slightly higher fuel burn, and carry extra fuel for the diversion. The pilot should also check the single-engine ceiling before crossing high terrain, as the previous task shows, because the hot-day penalty affects both the cruise ceiling and the drift-down profile.