Weather Avoidance Techniques

The most fundamental tool is available to all pilots: It's the Mark I Eyeball. Most of us have two, and if you're flying with a copilot or passengers, you may have spares available. It's easy to use and has just one significant limitation: You have to look outside the airplane. Stay out of the clouds, and you're almost guaranteed to avoid the worst effects of weather. This is a useful technique not only for visual flight rules (VFR) pilots, but also for instrument flight rules (IFR)---operating on top or between the layers, where you can see, is an ideal way to avoid nasty surprises.

Another tool is available to any pilot with a working radio: Flight Watch, which you can reach on VHF frequency 122.0 (when calling, give the nearest VOR: "Flight watch, N12345, Fortuna"). This will connect you with a flight service specialist who has access to current weather, including radar imagery.

Tell the specialist where you are ("12 miles east of Fortuna on the 260 radial, 6,000 feet") and where you're going, and you'll get current conditions, Airmen's Metereological Information (AIRMETs) and Significant Metereological Information (SIGMETS) and an outlook for your destination. On cross-country flights, I generally try to contact Flight Watch at least once on each leg---hourly on long legs. More than once, I've been alerted to problems in time to take action before a situation became serious.

For IFR pilots, an additional (and very important) tool is whatever ATC facility you're in contact with---center, approach, departure or a local tower. Most ATC facilities have radar with some weather capabilities. They're also in contact with other airplanes, which may be flying ahead of you on the same route.

If it's getting rough, precipitation is increasing and you're starting to wonder if continuing on your current route is really such a good idea, ask ATC if anyone's ahead of you---and how it's working for them---or what the weather looks like up ahead on the radar. If an airliner ahead is reporting a rough ride, or the last couple of airplanes missed the approach, or a suspected thunderstorm cell is moving across your route, it would be nice to know ahead of time!

Beyond the Mark I Eyeball and helpful folks on the radio, a number of devices are available that give the pilot a picture of the weather. Of these, the most common (and least expensive) is data link weather, provided either by satellite or a ground-based system, such as the FAA's Flight Information Service (FIS), a component of Automatic Dependent Surveillance (ADS/B).

Data link systems put a wide range of weather products on the multifunction display (MFD) in your panel, or a portable display. Depending on the particular system, this can include the Next Generation Radar (NEXRAD) radar mosaics, or Aviation Routine Weather Report (ETAR). It might also include the Terminal Area Forecast (TAF), in text or graphical form, AIRMET/SIGMET and some very useful non-weather data, such as temporary flight restrictions (TFRs).

But, there's one big catch: Data link weather is never completely up to date. The service provider receives data from a variety of sources, puts it in a form appropriate to their system and transmits it (via satellite- or ground-based radio link).

The equipment on your airplane receives the signal, decodes it and displays it. The process takes a minimum of several seconds, and in many cases, the data can be tens of minutes old---something that should come as no surprise to any pilot who's called up the Automatic Terminal Information Service (ATIS) for an airport just before the hourly update.

This limitation (referred to as latency) is a particular problem with NEXRAD radar imagery. Any data link system that displays NEXRAD will show you the time that the mosaic was created, which typically will be about five minutes old---but that's misleading: The mosaic itself is based on data from a number of different radar systems, which have to be collated together. The actual data on which a NEXRAD mosaic is based can be as much as 20 minutes old, and there's no way to tell that from the display.

Twenty minutes may not sound like a long time---but thunderstorm cells can build at rates of several thousand feet per minute, and cells in a line can advance by miles in that time. What looks like a safe route between a couple of small cells on a data link display may turn out to contain weather that no airplane can successfully fly through (for more on this topic---including details on two fatal crashes---see "When Using NEXRAD Can Be Dangerous" from the September issue of Plane & Pilot, and the links in this article's sidebar.)

Because of latency, data link weather should never be used by itself for tactical decision making at short ranges. On the other hand, data link weather is an outstanding tool for strategic decision making at long ranges---if you're flying a piston single or light twin without onboard real-time weather avoidance gear (and flight into known icing equipment) the best way to deal with a line of thunderstorms is to change your route early and fly around the entire area, giving yourself at least 20 miles of space on the upwind side.

To fly through a line, you need equipment that will show you what the weather's like in real time: either onboard weather radar or lightning detectors. Each has limitations.

Onboard Radar
Before flying an airplane equipped with onboard radar into active weather, I urge you to take one of the available radar training courses, such as Sporty's excellent Airborne Radar Training Course, or find a copy of Airborne Weather Radar: A User's Guide by former Delta captain James C. Barr (it's out of print, but available from online bookstores). On-board weather radar is complicated.

To use it effectively, you'll need to understand antenna tilt, attenuation, how the manufacturer maps intensity to color, storm gradient and shape, range settings, gain, how to estimate radar tops and optional features like stabilization, turbulence detection, wind shear detection and sensitivity timing control. Failing to understand these features may result in misinterpreting the radar picture and flying into an area where you really don't want to be!

One key thing to understand is that any radar, whether airborne or ground-based (including NEXRAD) detects only water. It can't show clear air turbulence, and also can't detect ice or dry hail. Tops of thunderstorms at very high altitudes may not be detectable as they consist almost entirely of ice.

Lightning Detection
As a practical matter, onboard radar is available only on multi-engine aircraft and the largest singles. It's expensive, and radar performance improves with antenna size---the smallest available is generally around 10 inches in diameter. You're not going to fit something like that on a typical piston single. An alternative technology that can be used on any airplane with an electrical system is lightning detection.

Lightning detectors are essentially sophisticated directional radio receivers that listen for the radio noise emitted by lightning strokes. They estimate distance based on the strength of the noise pulse. The result is a display that shows approximately where the lightning is relative to your aircraft. It has two advantages over radar-based systems: It works whenever there's lightning, whether there's water present or not, and it gives you a 360-degree view, showing activity to the sides and even behind you (onboard radar generally is limited to a narrow arc directly ahead).

Just as with data link and onboard radar, onboard lightning detection has limitations. The most significant is that only cells generating lightning are shown. Lightning detectors can't warn you about heavy rain (or other precipitation) that isn't generating lightning. This equipment also has a range limit, and there are other issues that vary from one type of detector to the next---you'll want to study the manual before flying with one in heavy weather.

Practice And Plan
For all types of sensors, practice is essential whether you have radar, data link, lightning detection or some combination of different sensors, you should use them regularly---even on flights where you don't expect to encounter significant weather. You don't want to find yourself trying to figure out the fine points of your equipment while in the soup on a rough day!

Before departure on any flight where significant weather is expected, get a complete preflight briefing so that you know what to expect---and make sure whatever weather avoidance equipment you have is working: Preflight inspection of radomes/antennas and ground tests of each sensor are critical before takeoff.

If you're launching into active weather, turn on whatever sensor(s) you have, but bear in mind that data link weather can be old---if you're looking at cells, check the age of data and think through how they may have moved/developed.

For on-ship radar, turn the system on and tilt the antenna up to paint any calls on your departure path. If you have a lightning detector, turn it on before taxi. You may also want to advise ATC (ground or tower as appropriate) that you may need extra time on the runway to stabilize the radar or other sensor before rolling for takeoff.

There can be situations where takeoff is possible, but returning to the field won't be (cells on the approach path). In that case, you may want to identify a takeoff alternate in case problems develop during climbout.

Deviations around active cells can be worked out with ATC, based on information from whatever sensor you have ---but if you're displaying on a multifunction display (MFD) and operating over anything other than flat terrain, you may want to periodically switch between the radar/data link/lightning display and terrain. You don't want to deviate around a cell and fly into a mountain!

As soon as you think a deviation may be needed, advise ATC, so that they can identify any conflicting traffic and approve or deny your request early.

With on-ship radar or lightning detectors, it's important to periodically switch to maximum range while working around local cells---otherwise, you might succeed in getting around a small problem and wind up face-to-face with a bigger one.

Thunderstorms are dynamic, and can build at rates of 5,000 FPM or higher. Thus, old data---from any sensor---can be worse than useless, tempting you to fly into areas that look clear, only to encounter a rapidly developing monster. If you're using data link, the only way to avoid this is to give all active areas a wide berth. With onboard radar, periodically checking shorter ranges may help. Lightning detectors will show any cells developed enough to produce lightning, but may not show building cells that haven't had time to develop significant electrical activity.

As you plan your approach, use whatever sensor you have to inspect weather not only at the destination airport, but also on your missed approach. If there's a cell near the missed approach hold you'll want to know about it and discuss what to do with ATC before beginning descent.

Understand the capabilities and limitations of whatever weather avoidance equipment you have, and you can fly safely when conditions are well below CAVU!