The Winds Aloft Forecast, the Altitude Selection Decision Students Get Backward, and the FD Product That Has More to Tell You Than Most Pilots Ever Ask

Unexpectedly low groundspeed on a cross-country almost always traces back to one thing: the winds aloft forecast didn’t get the attention it deserved during planning. The FD forecast - Forecast Winds and Temperatures Aloft - gives pilots the wind direction, wind speed, and temperature at specific altitudes along their route. Learning to read and apply it correctly is what separates a plan that holds up in the air from one that falls apart two hours in.

What Is the FD (Winds and Temperatures Aloft) Forecast?

The FD forecast is produced by the National Weather Service and covers the contiguous United States through a network of reporting stations. It’s available through 1800wxbrief, ForeFlight, aviationweather.gov, and most electronic flight bag applications.

At each station and altitude, the FD provides three values: forecasted wind direction in degrees, wind speed in knots, and temperature in Celsius. Every altitude selection decision in cross-country planning flows from those three numbers.

How Do You Read the FD Forecast Code?

The coded text format looks cryptic at first glance, but the structure is consistent. A typical FD entry for a single altitude looks like this: 2135 -04.

The first four-digit group encodes wind direction and speed. The first two digits are the direction code; multiply by ten to get degrees. The second two digits are the wind speed in knots. The number following is the temperature in Celsius.

Breaking down 2135 -04: direction code 21 × 10 = 210° (wind from the southwest), speed 35 knots, temperature -4°C. That’s the complete read for that station at that altitude.

What Are the Special Cases in FD Encoding?

Two encodings require extra attention.

Light and variable winds are reported as 9900. When winds are forecast below roughly 5 knots, this code appears in place of direction and speed data. No calculation needed - winds at that level are negligible.

High-wind encoding applies when wind speeds reach 50 knots or more. To keep entries within the four-digit format, the Weather Service adds 50 to the direction code and subtracts 50 from the speed. The trigger: if the first two digits of an entry are greater than 36, winds are 50 knots or stronger.

To decode: subtract 50 from the direction code, then multiply by 10 for degrees. Add 50 to the speed code for the actual speed in knots.

Example - 7305: First two digits are 73 (greater than 36), so high-wind encoding applies. 73 − 50 = 23, direction is 230°. Speed code is 05, so 05 + 50 = 55 knots. Wind from 230° at 55 knots. Missing this in fuel planning will cause a significant error.

Which Altitudes Does the FD Forecast Cover?

The FD covers altitude levels starting at 3,000 feet and stepping upward: 3,000 - 6,000 - 9,000 - 12,000 - 18,000 - 24,000 - 30,000 - 34,000 - 39,000 feet.

For general aviation singles on most cross-country flights, the relevant range is 3,000 through 12,000 feet. The FD does not cover below 3,000 feet - that layer is handled by terminal area forecasts and surface wind observations. Do not extrapolate FD data down to pattern altitude.

How Do Pilots Use Winds Aloft for Altitude Selection?

The most common mistake in student cross-country planning is looking only at the winds at the planned cruise altitude to determine headwind or tailwind. That’s one step short of the real question: which altitude gives the best combination of winds, temperature, and aircraft performance for this specific route and airplane?

Consider a Cessna 172 on an eastbound leg, choosing between 8,000 and 10,000 feet. At 8,000, winds are from 270° at 20 knots - a direct tailwind. At 10,000, winds are from 250° at 42 knots - a stronger tailwind. Ten thousand looks better on paper.

But a 172 at max gross in summer conditions climbs slowly. On a 150-mile leg, the airplane might reach 10,000 feet and immediately need to begin descent, enjoying those upper winds for only a fraction of the trip. The 20-knot tailwind at 8,000 feet, where the airplane spends most of the flight actually at cruise altitude and power, may produce a faster door-to-door time.

The optimization requires thinking through the whole flight, not just the cruise segment.

Why Does Temperature Matter in the FD Forecast?

Temperature from the FD affects the altitude decision in two specific ways.

First, aircraft performance. Colder air is denser, which means better engine efficiency and propeller bite. In summer at 9,000 feet, density altitude may be effectively 11,000 or 12,000 feet in terms of engine output. The temperature column makes this visible before departure.

Second, icing risk. The FD temperature doesn’t show clouds or moisture - but it identifies the temperature at each altitude. Combined with AIRMET Zulu and the graphical freezing level forecast, it pinpoints where icing risk exists along the route. An FD showing -2°C at 8,000 feet in an area with reported moisture is a direct warning for a non-deiced aircraft to avoid that level.

How Do VFR Cruising Rules and Oxygen Requirements Narrow the Choice?

Regulatory constraints filter the list of candidate altitudes before wind and temperature analysis begins.

VFR cruising altitude rules: On a magnetic course of 0° through 179° (eastbound), fly odd thousands plus 500 feet. On a magnetic course of 180° through 359° (westbound), fly even thousands plus 500 feet. This immediately limits the realistic options to three or four altitudes.

Oxygen requirements under 14 CFR Part 91: Above 12,500 feet for more than 30 minutes, the pilot in command and required flight crew must use supplemental oxygen. Above 14,000 feet, required flight crew need it continuously. Above 15,000 feet, passengers need it as well. For most general aviation cross-countries without supplemental oxygen on board, 14,000 feet is the practical ceiling, and most trips in a Cessna or Piper operate comfortably below that.

How to Apply Winds Aloft in a Real Cross-Country Plan

A cross-country from central Ohio to the Virginia mountains - roughly 250 miles southeast, Cessna 172, mid-morning departure - illustrates the full decision process.

Pulling FD data for stations along the route at three candidate altitudes:

• 6,000 feet: Winds from the southwest at ~15 knots. Tailwind on a southeast course. Temperature +12°C, well above freezing.
• 8,000 feet: Winds from the west-southwest at 28 knots. Stronger tailwind. Temperature +6°C, above freezing.
• 10,000 feet: Winds from the northwest at 35 knots. A headwind on a southeast heading. Temperature -1°C - at the freezing level with any moisture nearby, icing risk for a non-deiced aircraft.

Ten thousand is eliminated: headwind plus icing risk. Six thousand has a tailwind but lighter winds. Eight thousand offers the best tailwind component, temperatures comfortably above freezing, and is reachable without spending the bulk of the trip in the climb.

On a southeastbound heading (even thousands plus 500 for VFR), the call is 8,500 feet. Cross-check with AIRMET Tango for turbulence and AIRMET Zulu for icing, confirm the freezing level sits above 8,500 on the graphical forecast, and file accordingly.

Text vs. Graphical FD Tools: Which Should You Use?

Both have a place in the workflow.

The coded text FD is what appears in a standard briefing from 1800wxbrief and in the Winds and Temperatures Aloft section on aviationweather.gov. The Airman Certification Standards expect pilots to decode text FD entries by hand. On the oral exam, an examiner may hand over a printed text FD and ask for an interpretation of a specific entry.

The graphical version in ForeFlight or on aviationweather.gov is the practical daily-use tool. ForeFlight’s winds aloft map layer displays wind barbs at selectable altitudes across the route. The profile view plots winds and temperatures against altitude as a graph, making the crossover point - where a headwind becomes a tailwind, or where temperature crosses freezing - immediately visible.

Learn both. The graphical view builds the conceptual picture; the text decode is the exam skill.

When Is the FD Forecast Issued and How Long Is It Valid?

The FD is issued twice daily by the National Weather Service, typically around 0000Z and 1200Z. Each issuance includes forecasts valid for 6 hours, 12 hours, and 24 hours from the valid time.

For a morning departure planned that day, the 12-hour forecast is generally the most useful - it covers conditions at cruise altitude during the flight. The 6-hour forecast is more precise for an imminent departure. The 24-hour outlook supports next-day planning.

Monitoring the Forecast Against Actual Conditions

The FD is a forecast. Actual winds at altitude can differ, particularly in the lower levels where terrain, inversions, and local effects introduce variability.

Within five minutes of leveling off at cruise altitude, check actual groundspeed against planned groundspeed. If the difference exceeds 10 knots, conditions have changed and fuel planning needs a real-time adjustment. Waiting until 40 miles from the destination to discover the math stopped working an hour earlier turns a surprise into a problem.

Key Takeaways

• The FD forecast provides wind direction, wind speed, and temperature at altitudes from 3,000 to 39,000 feet across the U.S., issued twice daily around 0000Z and 1200Z.
• To decode a standard entry, multiply the first two digits by 10 for direction; the second two digits are speed. If the direction code exceeds 36, high-wind encoding applies: subtract 50 from direction code, add 50 to speed.
• Altitude selection is an optimization problem - the best altitude balances tailwind component, temperature, and aircraft performance across the entire flight, not just the cruise segment.
• Temperature from the FD flags icing risk when combined with freezing level forecasts; it also reflects density altitude effects on engine performance.
• On the oral exam, the examiner expects a reasoned answer: which altitudes were compared, what the FD showed at each, and why the chosen altitude was the best combination of winds, temperature, and aircraft performance for that route.