Crossing the Jet: Why a Stronger Wind Aloft Is Not Always Better
You are flight-planning an eastbound North Atlantic sector at FL 340, TAS 470 kt. The upper-air chart shows the polar-front jet crossing your route at right angles, with a core of 160 kt at FL 380. The tropopause is analysed at FL 290 on the poleward side of the jet and FL 340 on the equatorward side. The OAT at FL 340 in the crossing region is −63 °C. A preceding aircraft reports moderate turbulence at FL 340 on the poleward side of the jet. Forecast winds: FL 340, 240/090; FL 380, 250/140. If you climb to FL 380 the TAS falls to 450 kt because you are closer to the tropopause and the engine is at its ceiling. Your alternate on the European side is 90 minutes beyond destination; the aircraft has weather radar and no EGPWS uplink for turbulence.
1. What does an OAT of −63 °C at FL 340, with the tropopause analysed at FL 290 and FL 340 on either side, tell you about your position relative to the tropopause and to the jet core?
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ISA at FL 340 is 15 − 2 × 34 = −53 °C, so −63 °C is roughly 10 °C colder than ISA - far colder than anything in the troposphere below the tropopause would normally be at that level.
The tropopause is a discontinuity with a break across the jet: FL 290 on the poleward side, FL 340 equatorward. That break is the signature of the tropopause fold, where stratospheric air descends on the poleward (cyclonic) side of the jet. An indicated temperature of −63 °C at FL 340 is consistent with being in that intruding stratospheric air, i.e. above the local tropopause.
Two consequences:
- The jet core sits just below the tropopause level. Climbing towards FL 380 is therefore climbing towards and into the core region, not away from it. The maximum wind and maximum shear are found on the underside of the tropopause on the cold side.
- The reported moderate turbulence at FL 340 on the poleward side is textbook clear-air turbulence: horizontal and vertical shear is concentrated between the jet axis and the folded tropopause, on the cyclonic side. Your route crosses the jet, so you will spend time on that side during the crossing.
In short: you are in cold, stratospheric air, the jet is above/below you depending on which side of the crossing you are on, and the turbulence report is exactly where the structure of the chart predicts it.
2. Calculate the drift and groundspeed at FL 340 and at FL 380 for a track of 090°, and state whether the climb is beneficial.
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FL 340, wind 240/090, TAS 470. Relative angle = 240 − 090 = 150°, so the wind is from the rear right, blowing towards the front left. Crosswind = 90 × sin 150° = 45 kt; headwind = −90 × cos 150° = 78 kt.
- Drift = 45/470 rad ≈ 5.5°, and because the wind pushes you left of track, the heading must be 090 + 5.5 ≈ 096°.
- Groundspeed = 470 − 78 = 392 kt.
FL 380, wind 250/140, TAS 450. Relative angle = 250 − 090 = 160°. Crosswind = 140 × sin 160° ≈ 48 kt; headwind = −140 × cos 160° ≈ 132 kt.
- Drift = 48/450 rad ≈ 6.1°, heading ≈ 096°.
- Groundspeed = 450 − 132 = 318 kt.
Conclusion: climbing is strongly detrimental. The stronger wind at FL 380 has also backed/veered into a much more directly opposing direction, and the loss of 20 kt of TAS compounds it. Over a 2,000 nm leg the difference of 74 kt is about 3.4 hours. This is the standard trap: 'the wind is stronger higher, so climb' is only true if the wind direction stays the same and the wind is a tailwind. Eastbound into the westerly jet, the correct move is the opposite - descend below the core or route south of the axis, into weaker winds.
3. Given the turbulence report, what flight-level and routing decision do you take, and how does that interact with the fuel plan?
Read the worked answer
Decision: stay low and cross the jet at the shallowest practical angle, or step down rather than up.
Reasoning:
- The turbulence is on the poleward side at FL 340, exactly the altitude you are currently planned at, and it is associated with the tropopause fold. Climbing to FL 380 puts you nearer the core, where the shear is greatest, and would also cost about 74 kt of groundspeed. So FL 380 is excluded on both grounds.
- Request FL 300–FL 320 for the crossing if traffic and terrain allow. Wind there will be lighter and the shear weaker; the temperature is further from the fold. If that level is not available, ask ATC for a route offset of 10–20 nm south of the jet axis for the crossing, which puts you on the equatorward side where the tropopause is higher and the CAT risk lower.
- Cross the jet as close to 90° to the axis as the route allows - the crossing is where the shear is experienced, so minimising the time in it reduces exposure. If the route already crosses at right angles, keep it; if it runs along the axis, that is the worst case and a re-route should be considered.
- Use the radar and the turbulence-penetration speed from the flight manual if it is encountered; report the encounter to ATC in the standard format.
Fuel interaction. The plan at FL 380 would show a much higher headwind and a lower TAS; the flight plan must be rebuilt at FL 300–320 rather than assumed to be an improvement. With the European alternate 90 minutes beyond destination and a headwind of this magnitude, the fuel penalty of an unplanned level change must be checked before the oceanic entry point, not after. The decision point is the last point at which a level change is still compatible with the alternate fuel - and if the recalculated figures do not work, the correct answer is to reduce payload or delay, not to plan to 'find better winds' en route.