Accident Analysis: Late-ncy

A friend who’s a designated pilot examiner (yes, DPEs have friends!) reports an ominous trend: Too many candidates for the instrument rating still don’t understand that the imagery provided by Nexrad downloads does not portray the current situation. Rather, it depicts precipitation at the time the original radar sweeps were made. By the time multiple images at different elevations have been recorded, assembled into a composite image, uploaded to satellites, and broadcast to users, they can be anywhere from five to 20 minutes out of date—a latency period that’s not shown in the image’s time stamp, which indicates when it was broadcast rather than when the data were originally acquired.

Images can be anywhere from five to 20 minutes out of date—a latency period that’s not shown in the composite image’s time stamp.That may not sound like much of a time lag, but 20 minutes is a lot if you’re trying to stay clear of a storm cell that’s sweeping into your path at a groundspeed of 50 or 60 knots. The recommended 20-nautical-mile clearance you’ve promised yourself you’d maintain from any convective activity could easily be less than 10 if you mistake the picture of where the storm was then for where it is now. It could even be zero, making for a very unhappy flight.

The AOPA Air Safety Institute is aware of at least four airplanes that broke up in midair as their pilots attempted to use downloaded radar images to skirt around the edges of active cells. One of the first was the subject of ASI’s Accident Case Study: Time Lapse. All were fatal (as in-flight break-ups tend to be). Each pilot mentioned having “radar,” perhaps leading the controllers to believe those airplanes were fitted with their own real-time radar systems. Confusion between the lagged view available to the pilot and the current one on the controller’s scope led the former to request and the latter to assign vectors carrying the airplane directly into a cell.

Remember: Nexrad is a strategic, not tactical, tool. It’s great for identifying areas you want to stay away from while there’s still time to go somewhere else. It’s not suited or intended as a guide to working the edges of that 20-mile buffer. Which, in turn, raises another question: With other options available, why risk that in the first place?