The Winds Aloft Forecast, the Four-Digit Code Every Cross-Country Student Decodes Wrong, and the Altitude Tool That Can Save Thirty Minutes and Half a Tank of Gas
Learn to decode the Winds Aloft Forecast correctly and use it to choose the best cruise altitude - saving time and fuel on every cross-country.
Flying into a headwind you already knew about and failed to account for is one of the most common ways a cross-country goes wrong. The Winds Aloft Forecast contains everything needed to avoid it - but most students open it, skim it, and don’t actually use it. Knowing how to decode it correctly and apply it to an altitude decision can save thirty minutes and half a tank of gas on a routine flight.
What Is the Winds Aloft Forecast and Where Do You Find It?
The Winds Aloft Forecast is produced by the National Weather Service Aviation Weather Center in Kansas City. It’s labeled the FB Winds (or on older systems, the FD Winds). In ForeFlight, it appears under Winds and Temperatures Aloft. On the Aviation Weather Center website at aviationweather.gov, it’s under the same heading.
The forecast is issued four times a day and is valid for six, twelve, or twenty-four hour periods. Always confirm which period covers your actual flight time. A forecast valid at 0600Z is useless for a flight departing at 2200.
Forecast levels include: 3,000 · 6,000 · 9,000 · 12,000 · 18,000 · 24,000 · 30,000 · 34,000 · 39,000 feet. Most general aviation cross-countries live in the 3,000 through 12,000 foot range.
Why Do Winds Aloft Use True Direction, Not Magnetic?
Winds aloft are reported in true direction - not magnetic. This is one of the most commonly missed points in weather briefing training, and it matters for every groundspeed calculation you make.
If you’re flying a magnetic course of 270° in an area with 10° east variation, your true course is 280°. Comparing the forecast wind direction against your magnetic course instead of your true course can throw off groundspeed estimates by a meaningful amount. Always convert first.
How Do You Decode the Four-Digit Wind Code?
A typical winds aloft entry at nine thousand feet might look like this: 27 34 −12
- The first two digits are the wind direction in tens of degrees true. 27 = 270° - wind out of the west.
- The next two digits are the wind speed in knots. 34 = 34 knots.
- The number after the space is the temperature in degrees Celsius. −12 = −12°C.
One structural note: the 3,000-foot level carries no temperature. Surface observations provide a more accurate low-altitude picture than the model does, so the temperature is intentionally omitted at that level.
What Does 9900 Mean - and How Do You Decode Winds Over 100 Knots?
Two special cases consistently trip up students on the checkride.
9900 is the code for light and variable winds - less than 5 knots. No reliable direction can be assigned. For a cross-country, that’s actually good news: no meaningful headwind or tailwind penalty in either direction.
When wind speeds exceed 100 knots, the forecaster adds 50 to the direction code as a signal. An entry of 7315 decodes as: 73 − 50 = 23, meaning 230°; and 15 + 100 = 115 knots. The encoding exists because the direction field is only two digits - borrowing fifty allows speeds over one hundred to fit the same format without changing the structure.
What Happens When an Altitude Level Is Missing for Your Departure Airport?
When a reporting station sits within 1,500 feet of a forecast level, that level is omitted entirely. Denver, at roughly 5,300 feet MSL, will not show a 6,000-foot winds aloft entry. The station is already too close to that level. This is not missing data - it’s the system working as designed. Use the next available level or interpolate from nearby stations.
How Do You Use Winds Aloft to Choose the Right Cruise Altitude?
Reading the numbers is half the job. Applying them to an altitude decision is where the skill lives.
Consider a flight from Springfield, Illinois to Kansas City, Missouri. Magnetic course: approximately 270°. With 3° west variation, the true course is approximately 267°. The winds aloft along the route show:
- 6,000 feet: 180° at 12 knots
- 9,000 feet: 220° at 22 knots
- 12,000 feet: 270° at 35 knots
At 6,000 feet, the wind from 180° is nearly perpendicular to a 267° true course. Minimal headwind component. Groundspeed will be close to true airspeed.
At 9,000 feet, the southwest wind from 220° puts roughly 10–15 knots on the nose.
At 12,000 feet, the 270° wind is almost exactly head-on at 35 knots. In a Cessna 172 cruising at approximately 100 knots TAS, that drops groundspeed to the mid-60s. The flight just became significantly longer and more fuel-intensive.
6,000 feet is the right answer on this leg - mild crosswind, no significant headwind penalty, lower fuel burn than a higher altitude with a brutal headwind component.
This is exactly what the Airman Certification Standards (ACS) for the private pilot practical test requires: select a cruise altitude based on winds aloft data and explain your reasoning. Not just the VFR altitude rules - the actual winds, compared and justified.
Is There a Quick Way to Judge Whether a Climb Is Worth It?
A useful rule of thumb: for every 1,000 feet you climb, plan on roughly four minutes in the climb and additional fuel burn. If the next altitude level offers less than approximately four knots of groundspeed improvement from more favorable winds, the climb likely doesn’t pay off on a leg shorter than two hours.
It’s approximate - it depends on the aircraft and climb rate - but it works as a quick sanity filter before running the full vector calculation.
What Does the Temperature Column Actually Tell You?
The temperature data in the forecast is in degrees Celsius. Standard atmosphere places 9,000 feet at approximately +4°C. If the forecast shows +6°C at that level in July over Kansas, you’re two degrees warmer than standard - density altitude at that level will be higher than indicated altitude, and aircraft performance will fall slightly below what the pressure altitude charts show.
This matters most in summer, at high elevations, and on hot days. If winds look favorable at 12,000 feet but the temperature puts your density altitude at 14,000, your climb performance and cruise power will reflect that. Don’t skip the temperature column.
How Do You Cross-Check Winds Aloft With PIREPs?
The Winds Aloft Forecast is a numerical weather prediction model output - not a report from an aircraft that was actually up there. When the atmosphere is rapidly changing or the model is having a bad day, the forecast can be meaningfully wrong.
Cross-check it with PIREPs (Pilot Weather Reports). A PIREP from an aircraft that flew your planned route and altitude within the past two hours carries significant weight. If the forecast shows a 20-knot tailwind at 9,000 but every PIREP along your route reports calm or a headwind, believe the PIREPs. The people who were actually up there know more than the model does.
How Do You Estimate Winds Between Forecast Stations?
The forecast covers a fixed network of stations - not every airport has an entry. Your route may pass between two reporting points, and the actual winds over that stretch will fall somewhere in between.
If both adjacent stations show consistent wind direction and speed at the altitudes you’re considering, interpolate linearly and move on. If they differ significantly, that’s useful information: there’s likely a wind shear zone along your route. Think carefully about which altitude puts you on the better side of it.
What Is a Complete Pre-Flight Winds Aloft Workflow?
Run through these seven steps before every cross-country. The total investment is roughly ten minutes.
- Pull up the Winds Aloft Forecast for stations closest to your departure, destination, and major waypoints.
- Identify the two or three cruise altitudes you’re actually considering - ones that meet VFR requirements and your aircraft’s performance envelope.
- Decode the wind direction and speed at each altitude. Those directions are true, not magnetic.
- Estimate the headwind or tailwind component at each altitude for your true course. Precision to the knot isn’t required - you need to know whether you’re gaining or losing twenty minutes.
- Check the temperature. Does it change your density altitude picture in a way that matters?
- Choose your altitude and know why. On a training flight, write it in your flight log - the examiner may ask.
- Cross-check with any available PIREPs along your route.
Ten minutes. The difference between a flight that goes as planned and one where you’re nursing the fuel and explaining an unplanned stop.
Key Takeaways
- Winds aloft are reported in true direction - always convert to your true course before calculating headwind or tailwind components.
- 9900 means light and variable (under 5 knots); speeds over 100 knots are encoded by adding 50 to the direction digits and 100 to the speed.
- The 3,000-foot level carries no temperature; any level within 1,500 feet of a station’s elevation is omitted - neither is a data error.
- Use the four-knots/four-minutes rule of thumb to quickly judge whether climbing to the next altitude is worth the fuel and time cost.
- Always cross-check the forecast against available PIREPs - the model can be wrong; pilots who were actually at altitude are authoritative.
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