Tuesday, March 12, 2013
Weather Avoidance Techniques
Staying Out of Trouble When it isn’t CAVU
The timestamp in the upper left corner says this NEXRAD mosaic is only two minutes old—but it can be based on radar data more than ten minutes old.
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.
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.
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