The Winds Aloft Forecast, the FD Winds Table, and How to Read the Four-Digit Code That Tells You Which Altitude to File Before You Start the Engine

The Winds and Temperatures Aloft Forecast - still widely called the FD forecast or FD winds - is a twice-daily prediction of expected winds and temperatures at specific altitudes, from 3,000 to 39,000 feet MSL. Reading it correctly lets you select the best cruise altitude for groundspeed, fuel efficiency, and icing avoidance before you push the throttle forward.

What Is the Winds Aloft Forecast and Where Do You Find It?

The National Weather Service issues the FD forecast twice daily based on numerical weather model output. It is a forecast, not a real-time measurement - it tells you what winds are expected at altitude, not what pilots are finding right now. For current conditions at altitude, pair it with Pilot Weather Reports (PIREPs) from pilots who have been up there recently. The two products are teammates.

You can access the FD forecast at the Aviation Weather Center (aviationweather.gov) under the Winds and Temperatures Aloft tab. It is also included automatically in a standard 1-800-WX-BRIEF briefing package.

How Is the FD Forecast Table Organized?

The table lists three-letter station identifiers down the left side and altitudes across the top. The forecast covers nine levels: 3,000 | 6,000 | 9,000 | 12,000 | 18,000 | 24,000 | 30,000 | 34,000 | 39,000 feet MSL. For VFR cross-country planning, focus on the 3,000 through 12,000-foot columns. Instrument students flying higher cross-countries will look further up the table.

Some cells will be blank. Any station whose elevation falls within 2,500 feet of a forecast altitude gets that cell left empty - the data isn’t meaningful that close to the terrain at that location. The information isn’t missing; it simply doesn’t apply.

How Do You Read the Four-Digit Wind Code?

Each cell contains a four-digit wind group. The decoding rule is consistent throughout the table:

• First two digits = wind direction in tens of degrees. “23” means 230° magnetic.
• Second two digits = wind speed in knots. “18” means 18 knots.

So “2318” = winds from 230° at 18 knots. That’s the complete encoding. Once that pattern is solid, the table reads as usable information rather than a string of random numbers.

What Gets Added at 6,000 Feet and Above?

At 6,000 feet and above, a temperature value is appended to the four-digit wind group, producing a six-digit entry. The entry “231823” means winds from 230° at 18 knots, temperature +23°C. The entry “2318-04” means the same wind with a temperature of −4°C.

The sign on the temperature matters - it tells you something critical about where the freezing level sits, which affects icing risk. Entries at 3,000 feet carry no temperature data.

What Does “9900” Mean in the FD Table?

“9900” means light and variable. Wind speed at that altitude is either below 5 knots or shifting enough that a single direction would be misleading. The practical takeaway: no meaningful wind correction needed. Those are generally good days to be flying.

How Do You Decode the Heavy Wind Encoding?

When forecast wind speed reaches 73 knots or higher, the table applies a special encoding to maintain the four-digit format:

• 50 is added to the direction digits
• 100 is subtracted from the speed digits

A wind of 230° at 110 knots appears as “7310” rather than the impossible “23110.” When you see first two digits between 51 and 86, that is the flag. To decode:

• Subtract 50 from direction digits: 73 − 50 = 23 → 230°
• Add 100 to speed digits: 10 + 100 = 110 → 110 knots

At typical VFR cruise altitudes this encoding is rare. At jet altitudes it appears regularly. Recognize the flag even if you have never needed to use it in a real briefing.

How Do You Use Temperature Data to Find the Freezing Level?

The temperature values in the FD table allow you to locate the freezing level. If the 6,000-foot entry shows +4°C and the 9,000-foot entry shows −8°C, the freezing level falls somewhere between those altitudes. Linear interpolation places it at approximately 7,000 feet.

For a VFR pilot without ice protection, this is a planning boundary. In clear air with no moisture, the freezing level is largely academic. On a day with any cloud layers, reduced ceilings, or moisture near cruise altitude, that temperature data tells you which altitudes carry risk and which don’t.

Structural ice is not a carburetor ice problem. It accumulates on wings and control surfaces, degrading lift, increasing drag, and altering stall characteristics. Aircraft without ice protection equipment should treat the freezing level as a hard limit whenever moisture is present.

How Do You Use Winds Aloft to Choose a Cruise Altitude?

Here is a practical example. You’re flying a Piper Cherokee from Wichita to Kansas City - roughly two hours. The altitude options are 5,500 feet or 7,500 feet eastbound. The FD table shows:

• 6,000 feet: “2224+07” - winds from 220° at 24 knots, temperature +7°C
• 9,000 feet: “2332+02” - winds from 230° at 32 knots, temperature +2°C

Both altitudes offer a tailwind component - the wind is from the southwest, pushing you northeast. At 9,000 feet, that component is stronger: 32 knots versus 24 knots. Over two hours, the difference in groundspeed and fuel burn is real.

But the temperature at 9,000 feet is +2°C - barely above freezing. Before committing, check the METARs, TAF, and any AIRMETs for icing. If skies are clear and no moisture is present, 7,500 feet is the right call: better tailwind component, no icing exposure, better efficiency for the flight.

That is the FD forecast doing exactly what it is designed to do.

What Does Winds Aloft Affect in Your Flight Plan?

The groundspeed entered on your flight plan comes directly from your winds aloft calculation. True airspeed plus or minus the wind component at cruise altitude equals estimated groundspeed. That number drives time en route, fuel stop planning, and your fuel reserve calculation.

Get the winds aloft wrong and everything downstream in your planning is off. Get it right and you arrive with the fuel and time you expected. The FD forecast is the foundation. Everything else builds on that one input.

What Do Checkride Examiners Expect You to Know?

The Airman Certification Standards (ACS) for private pilot require use of the FD forecast in cross-country planning. On a checkride, “I picked this altitude because it seemed reasonable” is not an answer. A complete response sounds like: “I checked the 6,000 and 9,000-foot entries, found a stronger tailwind component at the higher altitude with acceptable temperatures and no icing AIRMETs, and selected 7,500 feet to improve groundspeed and fuel efficiency.”

Practice accessing the FD on aviationweather.gov and decoding real entries for a practice route. Be able to explain what the temperature value implies about the freezing level. Know what “9900” means and what the heavy wind encoding flag looks like, even if you have never encountered one in a live briefing. The examiner is not trying to trick you - they want to see that you used the forecast with intention.

How Do You Know Which Forecast Period to Use?

The FD is issued twice daily with three valid time periods: 0–12 hours, 12–24 hours, and 24–36 hours. Always confirm you are looking at the period that covers your planned flight time.

A morning briefing issued at 0600 UTC that you access at noon may describe winds forecast eight to ten hours ago. After a frontal passage or significant pressure change, actual winds aloft may have shifted meaningfully. Cross-check with recent PIREPs - when the FD and PIREPs agree, confidence is high. When they disagree, something has changed since the forecast was issued, and knowing what that is belongs in your go/no-go decision.

Key Takeaways

• The four-digit FD wind code encodes direction (first two digits × 10) and speed in knots (second two digits). At 6,000 feet and above, a temperature value follows.
• “9900” = light and variable; no wind correction needed at that altitude.
• First two digits between 51 and 86 signal a heavy wind entry: subtract 50 from direction digits, add 100 to speed digits.
• FD temperature values let you interpolate the freezing level - a critical data point on any flight where moisture is a factor.
• The FD forecast is the foundation of your flight plan: altitude selection, groundspeed, time en route, and fuel planning all flow from it.