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Understanding Latency Issues with In-Cockpit Weather Imagery

By Dr. Sabrina Woods

Source: FAA Safety Briefing March/April 2020

Iwas very fortunate to be stationed in England for several years while on active duty. Living abroad often brings about wonderful new experiences and at times, new challenges to overcome. I distinctly remember exploring the open-air markets as a treat; learning to drive on the opposite side of the road (and car) was a trial. Sunday roast and tea times were a delight, while 140-plus days a year of rain could be a mood dampener. London’s West End was a must do, football (aka soccer) was king ... and then there was the Tube.

The Tube, which is a 156-year-old, predominantly underground transit railway system connects all of London and several outlying cities. Due to changes in the shape and heights of the trains themselves, significant horizontal and vertical gaps now exist between the cars and some of the older legacy platforms when certain trains arrive for boarding. The technology got better, but it introduced new, unforeseeable hazards into the system that had to be accounted for. Instead of trying to find some one-size-fits-all device to fill the ever-changing gap distances, someone chose a "Mind the Gap!" announcement to advise passengers of the risk of getting hurt. It resonates with domestic and international passengers alike and no one ever forgets it. Indeed, the phrase has almost become synonymous with the London Underground, and with London herself.

The phrase also makes me think of a problem very near and dear to my heart: the continuing issue of GA accidents and incidents related to unintended flight into instrument meteorological conditions (IMC).

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.

inflight breakup

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.

Gap Analysis

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 NEX-RAD 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.

weather radar

Betting Your Life

So why do we keep going? Why do we end up in situations that, viewed from afar (and from the safety of the ground), we know we shouldn’t? It’s because we humans have an innate desire to continue pursuing an endeavor simply because we’ve already invested money, effort, or time in it. This is in spite of the fact that the pursuit might lead us into danger or worse. We do it because we have a very real, almost tangible understanding of what we have already invested, and we value it very strongly — definitely more than the unknown change in course!

In other words, if a pilot chooses to divert or abort a flight due to weather, he or she knows for sure that there will be wasted time, probably a significant loss of money, and possibly the disappointment of not reaching the intended destination. If, on the other hand, the pilot continues to fly, a successful outcome remains to be seen and is thus the “unknown” in this equation. In the end, the risk of diverting the aircraft feels higher than proceeding into that unknown. We go for the gamble.

This desire gets compounded if we have ever done this sort of thing before and gotten away with it. Each time we have a successful outcome, our risk-aversion needle slips a little further into the “eh, what could possibly go wrong?” category, despite the fact that the hazard does not change. I call this whole phenomenon the human factors version of Betting on the Come. We don’t have what we need at the moment but we hope that when the time arrives, we will get what we want! It’s a dangerous gamble, especially when weather is involved. Weather is the house and, just as in a casino, the house always wins.

Garmin avionics

Mind the Gap!

Here’s what I want you to remember: The weather infor-mation you access and use as a pilot might not always be exactly as it seems. Just like the London Underground, the technology has gotten better. But in so doing, it has introduced new, unforeseeable hazards into the system that have to be accounted for. Over-relying on your gadgets and gizmos and failing to appreciate their limitations might get you caught up in a bad situation. So, avoiding the bad is a matter of actively working to reconcile what your app is telling you with other pieces of information inside and outside the cockpit. One great (and cheap) tool is the Pilot Weather Report (PIREP). Passing pertinent info back and forth among your fellow flyers about “how it really looks out there” is indispensable (and did I mention cheap?!).

Recognize that if you are out flying, it probably means you have somewhere to go. It also means you are already biased to want to continue on your journey despite evidence indicating it might not be the best idea. This is the very definition of get-there-itis. To combat these natural tendencies, build a plan and stick to the plan. A good plan should have a Plan B (and possibly C) in it. Determine and adhere to your personal minimums. Respect the limitations of technology, Mind the Gap!, and navigate well away from weather.

Learn More

Air Safety Institute Case Study “Time Lapse” -

NTSB Safety Alert on In-Cockpit NEXRAD Mosaic Imagery -

Dr. Sabrina Woods is a guest writer for the FAA Safety Briefing. She is a human factors analyst of Office of Accident Investigation and Prevention. She spent 12 years as an aircraft maintenance officer and an aviation mishap investigator in the Air Force.

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