Over, Under, Around or Through

Who’s the wisest pilot — the one who flies below the clouds, the one who flies above them, he or she who deviates around clouds, or the pilot who files instruments and flies through? To quote a wise, retired airline captain friend of mine, “it depends.” There are risks associated with each approach and the best decision depends on the best match for your abilities.

What sort of adverse weather should you fly over? If you’re instrument rated and flying an IFR-certified and equipped airplane, your answer will obviously be different than if you’re limited to visual flight only.

VFR: A Visual Flight Rules (VFR) pilot (or any pilot your flying an aircraft that is not IFR equipped) should not fly above a layer of overcast or very large area of fog. Different actions may be appropriate if the clouds are scattered or broken. When deciding on a VFR-limited “personal minimum” for flying over clouds or fog, you should ask yourself,

  • Is the fog widespread? and
  • Are cloud layers scattered, broken or overcast?

And act accordingly:

  • Overfly only if the layer is scattered or scattered-to-broken, and
  • Delay flight over clouds and fog (or deviate to another route or airport) if the layer is reported as broken or overcast.

IFR: If you’re instrument rated and flying an IFR-certified airplane on an instrument flight plan, questions you need to ask might include:

  • Am I likely to accumulate ice if I descend into the clouds? and
  • Are the cloud bases and surface visibility high enough to permit a landing at my destination, or if I have to land early for some other reason?

VFR and IFR: Use your experience, judgment and the FARs to decide what sort of overflight is both legal and comfortable for you. With time (and perhaps after a few trips with an experienced pilot or instructor riding shotgun) you can safely expand your personal weather envelope.

If you can’t or won’t fly over a layer of clouds, your next choice is whether you’ll let yourself fly under them. This can be a larger concern for the VFR pilot, but is also a factor on the IFR flight plan if it calls for a circling approach or a descent through the clouds followed by visual flight to an airport not served by an instrument approach. Be aware of four key variables:

  1. Cloud bases — how much airspace do you have to maneuver in?
  2. Visibility beneath the clouds — how far can you see and how does that affect your ability to deal with an emergency?
  3. Terrain features and man-made obstacles — do you know where they exist along the route and can you avoid them?
  4. Speed of the aircraft you are flying — with restrictions on visibility and altitude, have you allowed yourself enough reaction time to avoid obstacles or other traffic?

IMPORTANT: A Mooney will cover a lot more ground for a given amount of time than a Cherokee, for instance, so the Piper pilot may be more comfortable — and more safe — flying under a lower cloud deck or in poorer visibility. When setting your personal limits for flying under clouds, be sure to ask “can I see and avoid terrain and obstacles?” with respect to the visibility and the capability of your airplane. And, if precipitation starts, are you likely to lose outside visual references? Level 1 and 2 radar returns often are thin enough to see through, less so if the precipitation is snow and not rain.

In general, flying around weather hazards is always preferable to flying through them. After earning my instrument rating, one thing I could never find any specific guidance on was how to decide whether to penetrate a cloud, or deviate around it. Through the years I’ve asked innumerable pilots and instructors what they use as a rule of thumb. I’ve talked to military aviators and airline captains, asking if their OPS-SPECs or policy manuals give them specific rules for penetrating or deviating around clouds. They don’t. By consensus (and through my own experience), though, I’ve come up with the questions to ask before penetrating a cloud:

  • Is it a stratus cloud or a cumulus cloud? Cumulus clouds develop in vertical columns of air, which means turbulence. While stratus clouds may be bumpy inside, they’re nothing like the spar-jarring Cus.
  • Am I likely to accumulate ice in the cloud? If the answer is yes, I must deviate unless the airplane’s certified and equipped for ice. Even if the aircraft is certified for flight into known icing, I must limit my exposure.
  • Is turbulence inside the cloud likely to be of moderate intensity or greater? Look at pilot reports, forecasts, and the stability or lifted index — if the air is very unstable, then turbulence may be more extreme.
  • Is the cloud forming or rising rapidly? This indicates powerful, vertical air movement.
  • Is the top of the cloud more than 10,000 above ground level? In my experience, any cumulus cloud with a top more than 10,000 feet above the terrain is one I don’t want to be in.
  • Is virga or precipitation falling from the cloud? That means a downdraft.
  • Is lightning visible, or registering on a lightning detection instrument? Steer well clear!
  • Does the cloud “look bad?” Sometimes, it’s just that simple. If it’s dark, or has well-defined edges, chances are it’s dangerous inside. Trust your instincts — if it “looks bad,” avoid it.

CONSIDERATIONS: Before you fly through precipitation, you should be able to answer the following questions with a confident ‘No.’ Am I likely to ice up in the rain or snow? Are temperatures at or below freezing at my altitude? Are conditions ripe for carburetor ice? If your or the airplane is VFR-only, you will not be going through.

BOTTOM LINE: Minimums aren’t just for instrument pilots. No one can set your “personal minimums” for you. Ask yourself the questions and be honest with your answers, and you will have the tools you need to set those limits. The better you know yourself, the safer pilot you’ll be. Setting and adhering to your own personal minimums — yours and those laid down by the FARs — is just one step.