Oceanic Crossing: Jet Stream, Tropopause and Clear-Air Turbulence
You are planning a westbound oceanic crossing at FL350. The 300 hPa chart shows a jet core of 130 kt at 55°N 30°W. The tropopause is at FL380 north of the core and FL320 south of it. The 500 hPa temperature is −32 °C and the 300 hPa temperature is −58 °C. A SIGMET warns of severe clear-air turbulence between FL300 and FL380 near the jet axis. The aircraft is a twin-aisle with a service ceiling of FL410; performance allows FL370 or FL390. Fuel burn increases about 3% per 2,000 ft above optimum. The destination requires arrival at FL350 or below. The crew's turbulence penetration speed is M0.80/280 kt IAS. The area forecast mentions a deepening low east of the core with a cold front.
1. Locate the jet stream and the most likely clear-air turbulence zone, and explain the role of the tropopause.
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The jet core is at 300 hPa near 55°N 30°W, with 130 kt. The 500–300 hPa temperature difference of 26 °C shows a strong thermal wind and a pronounced horizontal temperature gradient, which supports strong vertical wind shear. The tropopause break from FL380 north of the core to FL320 south of it marks a discontinuity in stability and is a favoured region for clear-air turbulence, especially where the tropopause folds or where the flow is ageostrophic near a trough or jet exit. The SIGMET places severe CAT between FL300 and FL380 near the axis, which is consistent with the shear zone on the cold side of the jet and around the tropopause break. The deepening low and cold front east of the core will tighten the gradient further, so the threat is not confined to a single point. Avoid the core and the tropopause break rather than relying on a single turbulence forecast.
2. Compare FL370 and FL390 and recommend a level and route. Justify the fuel penalty.
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FL370 lies inside the SIGMET block from FL300 to FL380, so it offers little or no vertical escape from the forecast severe CAT. FL390 is just above the SIGMET top and above the northern tropopause at FL380, so it gives a better chance of smooth air, although turbulence can extend above the tropopause in a folding situation. The fuel penalty for FL390 versus FL370 is one 2,000 ft step, about 3% of trip fuel. That is significant on a long crossing, but it is smaller than the risk of structural damage or injury from severe turbulence, and the aircraft performance permits FL390. Recommend FL390 with a lateral offset of about 60–100 nm south of the core, where the tropopause is lower and the wind weaker; this reduces both the CAT exposure and the headwind penalty. If weight or temperature limits prevent FL390, remain at FL350 only with a larger offset and active turbulence reporting, not directly in the core.
3. If severe clear-air turbulence is encountered, what flight technique and reports are required?
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Reduce to the turbulence penetration speed of M0.80/280 kt IAS and maintain it; do not chase altitude or airspeed with large control inputs. Keep the autopilot engaged only if it is handling the turbulence smoothly; if it disconnects or oscillates, hand-fly with small inputs and accept altitude deviations. Ensure all occupants are secured and loose items stowed. Report the encounter to ATC as soon as workload permits, giving location, flight level, intensity, duration and the effect on the aircraft. Request a level change or offset if a smoother area is known. After the encounter, inspect for structural damage and check engine and control indications. File a PIREP and update the SIGMET information so following aircraft can avoid the area. Do not descend into the SIGMET block without coordinating with ATC and checking terrain and traffic.