The FB Winds Forecast, the Altitude Equation, and the Four Thousand Feet That Turn a Headwind Into a Tailwind

The Forecast Winds and Temperatures Aloft - commonly called the FB Winds or winds aloft - is issued by the Aviation Weather Center and available at aviationweather.gov. It gives you predicted wind direction, wind speed, and air temperature at a series of standard altitudes along your route. It’s updated four times daily, with forecasts valid for 6, 12, and 24 hours out. For most piston pilots, it’s also the most underused tool in the preflight weather package.

What Altitudes Does the Winds Aloft Forecast Cover?

The product covers nine standard altitudes: 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, and 39,000 feet MSL. If you’re flying a normally aspirated piston aircraft in the lower airspace, your focus will be on 3,000 through 12,000 feet - possibly up to 18,000 if you’re in a turbocharged airplane working the top of its envelope.

How Do You Read the Coded Wind Data?

Each altitude entry uses a four-digit group to represent the wind. The first two digits give direction; the last two give speed in knots. To decode the direction, multiply the first two digits by ten. The result is degrees true north - not magnetic. This distinction matters on both the written exam and oral.

So “2534” breaks down as: 25 × 10 = 250 degrees true, speed 34 knots. Winds from 250° true at 34 knots.

Following the four-digit wind group, you’ll see temperature in degrees Celsius with a plus or minus sign. The full entry “2534-08” means winds from 250° true at 34 knots, temperature -8°C.

What Are the Special Codes to Know?

No temperature at 3,000 feet. Surface variations near the ground make that value unreliable, so it’s intentionally omitted. If you see the 3,000-foot row without a temperature, that’s correct.

9900 = calm or light and variable. When the wind group reads “9900,” it means winds less than 5 knots. It’s not a data error or missing entry - it’s a specific code for light wind conditions.

Speeds above 99 knots use a workaround. The forecaster adds 50 to the direction code and subtracts 100 from the speed. A direction code of “77” tells you to subtract 50 (getting 270°) and add 100 to the speed. “7720” decodes to 270° at 120 knots - jet stream territory. You won’t encounter this at 8,500 feet in a Cessna, but your examiner may ask about it.

How Do You Use This to Pick the Right Altitude?

This is where the product earns its place in your preflight routine. Work through it in steps.

Step 1: Pull the forecast for stations along your entire route - not just departure and destination. On a 300-mile flight, check two or three intermediate points. A tailwind that dies at the midpoint and reverses at the destination is not a tailwind flight.

Step 2: Match the forecast time to your en-route window, not your departure time. If you’re wheels-up at noon for a two-hour cruise, use the forecast that covers that period. Grabbing a 24-hour-out forecast for a flight leaving in four hours introduces unnecessary error.

Step 3: Compare wind components at each valid altitude for your heading. Consider a westbound leg from Wichita to Denver - roughly 300 miles, magnetic course around 290°, in a Cessna 172 at 110 knots true airspeed. A midpoint winds aloft check shows:

• 6,000 ft: 270° at 22 knots
• 9,000 ft: 260° at 15 knots
• 12,000 ft: 250° at 28 knots

All headwinds. But the difference between altitudes matters. At 9,000 feet, the wind is lighter and has shifted slightly south of west, meaning less of it is hitting you head-on. Nine thousand looks best - until you apply the regulatory constraint.

How Does FAR 91.159 Affect Your Altitude Choice?

FAR 91.159 governs VFR cruising altitudes when operating more than 3,000 feet AGL:

• Magnetic course 0°–179° (eastbound): Odd-thousand-foot altitudes + 500 (e.g., 3,500, 5,500, 7,500)
• Magnetic course 180°–359° (westbound): Even-thousand-foot altitudes + 500 (e.g., 4,500, 6,500, 8,500)

On a 290° heading, you’re westbound. That makes 9,000 feet an illegal VFR cruising altitude. Your legal options become 8,500 and 10,500 feet.

Interpolating from the data: 8,500 feet carries roughly 19–20 knots of headwind component; 10,500 feet comes in around 22 knots. Eight thousand five hundred wins - not by a large margin, but it wins.

That decision chain - pull the data, compare altitudes, apply the regulatory filter, check terrain, choose - is exactly what a checkride examiner wants to hear. Not a guess. An analysis.

Why Does the Temperature Column Matter?

Most pilots glance at the temperature column and move on. Two concrete reasons not to.

Icing risk. Structural ice forms in visible moisture when temperature is between approximately +2°C and -20°C. That’s the danger band. If the winds aloft show -4°C at your planned altitude and your route runs near cloud layers, you’re in prime icing conditions. For a VFR pilot in a non-deiced aircraft, that combination is a no-go. The winds aloft temperature gives you an early signal before you’re digging through AIRMETs and PIREPs.

Engine performance. Temperature drops approximately 2°C per 1,000 feet in the standard atmosphere. On a hot summer day in high-elevation terrain, the temperature at your cruise altitude directly affects density altitude and how your engine performs. You don’t need a full density altitude calculation for every level in the table - just know whether conditions are warmer or cooler than standard.

What Does a Large Speed Jump Between Altitudes Tell You?

When you see a significant increase in wind speed between adjacent altitude levels - say, 15 knots at 6,000 feet and 40 knots at 9,000 feet - that large change over a small vertical distance is a flag for wind shear. Wind shear at the boundary between those air masses can produce turbulence. It’s not a guarantee, but it warrants a cross-check against Pilot Reports (PIREPs) from aircraft already in that area.

For mountain flying, this analysis becomes essential. Strong winds aloft flowing perpendicular to a significant ridge can set up mountain wave turbulence that extends well downwind of the peaks and reaches surprisingly high altitudes. The winds aloft don’t confirm wave activity on their own, but speed and direction data across multiple altitude levels is the starting point for understanding what the atmosphere is doing near terrain.

Key Takeaways

• The winds aloft forecast is issued 4 times daily by the Aviation Weather Center and is available free at aviationweather.gov. Use the forecast time that covers your en-route window, not your departure time.
• Decode the four-digit wind group by multiplying the first two digits by 10 for direction in true degrees, and reading the last two digits directly as speed in knots.
• 9900 = calm or light and variable; no temperature appears at the 3,000-foot level; speeds above 99 knots use the +50/-100 encoding workaround.
• Always apply FAR 91.159 after comparing altitudes - some levels that look favorable may be illegal for your direction of flight.
• Use the temperature column to flag icing risk (visible moisture between +2°C and -20°C is a no-go for non-deiced aircraft) and to assess engine performance relative to standard atmosphere.