Shooting the Gap
A Piper Cherokee Six was on a cross-country flight when the pilot flew into an area of heavy rain showers. The IFR-qualified pilot informed an air traffic controller that he was in “bad” weather and was going to try to get through it. He never reported in again. The aircraft wreckage was found about 450 feet from a residential structure, minus the left wing, vertical stabilizer, rudder, and the right wingtip fuel tank. Those parts were located 200 feet from the main wreckage. The pilot and his family were fatally injured.
A section of the left wing from a Cherokee Six that suffered an inflight breakup due to a weather encounter. Photo courtesy of the NTSB.
Post-accident examination of the left wing spar showed that the wing failed in positive overload. Conditions at the time of the accident indicated the potential for heavy rain showers, thunderstorms, winds in excess of 45 knots, clear air turbulence, and low-level wind shear. The pilot had a GPS unit with a current subscription for Next Generation Radar (NEXRAD) and likely used this information for flight planning and diversion purposes. The problem is that at the time of the accident, the depiction in the cockpit would most likely have displayed weather conditions that existed a couple of minutes earlier. By the time the pilot arrived, the gap he was shooting for had firmly closed.
The owner’s manual for the GPS unit in this accident stated that “NEXRAD data is not real-time” and in fact, NEXRAD data can be as much as 20 minutes older than the age indicated on the cockpit display. This time difference can be significant, especially if a pilot is using it to navigate through inclement weather that can change quickly, significantly, and without notice.
The term “gap analysis” in research typically means comparing the actual performance of something to its potential or desired performance. But for me, “gap analysis” is the very real concern that pilots are unaware that the weather depicted on displays in GA cockpits might not reflect what is actually going on outside, and that pilots are navigating based on that displayed information unknowingly.
My counterparts in the FAA’s Weather Technology in the Cockpit (WTIC) NextGen weather research program work hard to incorporate weather and human performance research into the standards and guidance documents that improve pilot decision making. One of the biggest issues they are tackling is keeping pilots informed about the inherent lag in weather dissemination and application depictions.
Information presented in the cockpit is delayed because the National Weather Service needs upwards of five to 15 minutes to create a mosaic of precipitation from NEXRAD radars and render the data as a graphic. It can take five more minutes for the graphic to reach the cockpit. A timestamp on the image may only refer to the most recent data contributing to the mosaic image and might not include the delay required to develop the graphic in the first place. If GA pilots are unaware of that key discrepancy, the consequences can be fatal. The display may result in a dangerously false sense of existing conditions, especially when the aircraft is already headed for inclement weather. Approaching pilots might think there is a gap they can scoot through, only to discover — too late — that the gap is long gone. The result can be inadvertent entry into IMC conditions.
Note: another risk in shooting a gap is that if you get “too close” to a storm, you can hit severe turbulence. FAA guidance recommends 20 miles as the minimum safe distance from a convective storm to avoid the risk of severe turbulence, which for GA can also be fatal.
See page 12-22 of the Pilot’s Handbook of Aeronautical Knowledge for more details on thunderstorm hazards.
According to WTIC researchers, cockpit displays of data-linked weather are meant to be used for strategic planning, with the goal of helping pilots keep a respectful distance away from thunderstorms. It is also critical for pilots to assess these big-picture weather issues before takeoff. Regardless of how technologically advanced weather forecasting and depiction applications have become, they are still best used as a planning tool, and not as an in-the-soup fire-fighting tool. By then, the damage has been done and the odds of it ending well are not good.