Ground school lesson
Weather and the decision not to fly
Most weather decisions are made badly at the airfield, with the aircraft fuelled and somebody waiting. The fix is to decide the numbers beforehand.
What you will be able to do
- Name the four weather numbers that decide a light aircraft flight.
- Understand why a hot, high afternoon hurts performance.
- Write down a personal minimum and know why it beats judgement in the moment.
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Step 1 of 3
Four numbers decide most flights
Number Why it decides the flight Wind and gusts The crosswind component is what you fly against. A gusty wind is harder than a steady one of the same speed. Visibility A sport pilot needs at least 3 statute miles and visual reference to the surface. Ceiling A low cloud base squeezes you between the cloud and the ground, and the squeeze is where accidents happen. Density altitude Heat and elevation together decide whether the rotor will actually lift you off. Everything else in a weather briefing exists to tell you what those four are about to do.
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Step 2 of 3
Density altitude, the quiet one
Field elevation is what the map says. Density altitude is what the rotor feels. On a hot afternoon at a high field the two can be thousands of feet apart, and only one of them decides your ground roll and your rate of climb.
The same runway performs like one over three thousand feet higher on a hot day. You do not have to estimate this. Use the density altitude calculator with your field elevation, the altimeter setting and the temperature.
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Step 3 of 3
Write the limit down first
A personal minimum is a number you decide while nothing is at stake, and then obey when something is. It is not a rule imposed on you; it is you protecting yourself from you, standing at the hangar with somebody waiting.
Set yours with your instructor
- A maximum steady wind, and a lower one for gusts.
- A maximum crosswind component, below the aircraft's demonstrated figure while you are new.
- A minimum visibility and ceiling that are more conservative than the legal minimum.
- A density altitude above which you do not attempt the flight at your current weight.
The decision not to fly costs you an afternoon. It is the cheapest thing in aviation, and no one has ever regretted it on the way home.
Related: the crosswind lesson for the component maths behind the second limit above.
Check yourself
3 questions. Pick an answer to see whether it is right and why.
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What is the minimum flight visibility for a sport pilot?
A sport pilot needs at least 3 statute miles of visibility and visual reference to the surface.
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A hot afternoon at a high-elevation airport does what to density altitude?
Heat and elevation both thin the air. Density altitude climbs above the field elevation, lengthening the ground roll and cutting the rate of climb.
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Why write personal minimums down in advance?
Deciding while nothing is at stake protects you from the pressure of the moment. They sit inside the aircraft limits, never replace them, and should tighten or loosen with experience.
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FAA Aviation Weather Handbook and Pilot's Handbook of Aeronautical Knowledge (public domain). Sport pilot weather minimums are in 14 CFR Part 61 Subpart J.