By David J. Kenny
Thunderstorms show no respect for pilot experience, mission importance, proximity to the destination, or much of anything else. That’s what it means to describe something as “a force of nature.” They go wherever they’re going and flatten anything and everything in their paths that can’t stand up to 6,000-foot-per-minute downdrafts, extreme turbulence, and hail. That includes most aircraft, regardless of size. Racing a storm to a landing site can be an extremely perilous business if the timing’s tight enough to place the outcome in any doubt.
At 5:34 a.m. on March 25, 2010, a Eurocopter AS350 B3 medevac helicopter landed at the Jackson-Madison County General Hospital in Jackson, Tenn., to deliver a patient. Another helicopter occupied the lower pad, so the AS350 landed on the upper platform. As soon as the patient was unloaded, the pilot phoned back to his home base in Brownsville, where the pilot assigned to the day shift had just come on duty. The two discussed the weather, which was presently dark (dawn was still more than an hour away) and cloudy with light rain, but was expected to deteriorate quickly. A storm front was already passing through Memphis, about 40 nautical miles southwest of Brownsville, and moving northeast at an estimated 22 knots. The day-shift pilot recalled the Nexrad image on the computer showing “red” over Memphis and “yellow” for another 10 miles or so in front.
He asked whether they could just “park” in Jackson and wait it out, but the pilot flying wasn’t comfortable with leaving the ship on the elevated pad in the storm. He calculated that he had about 18 minutes to make it back to base, just 20 nautical miles to the west-southwest. Since the flight nurses hadn’t yet reboarded, he asked the day-shift pilot to call them and advise that he was leaving without them; they could be picked up later by car. As it turned out, one nurse hadn’t brought her phone, and by the time he reached the other, both were on board the helicopter and airborne, “seven minutes out from the base.” He raised the hangar door and took another look at the radar, which now showed the storm entering the southwestern corner of Haywood County about 18 miles away.
The helicopter was still not in sight, but he could make out the flashing lights of a radio tower six miles to the east, confirming that visibility was good. Rain was light but the wind had increased to about 20 knots. He made another call to the nurse, and in that conversation suggested that they “had the weather beat.” She guessed that they were only about 30 seconds away.
Almost as soon as he hung up, a clap of thunder made him jump. He still couldn’t see the helicopter, and another call to the flight nurse wasn’t answered. The wreckage was found two and a half miles east of the airport at about 6 a.m., nine minutes after the helicopter lifted off from Jackson. All three members of the crew had been killed before a post-crash fire consumed most of the aircraft. Investigators found no evidence of any mechanical problem prior to impact.
Instead, they concluded that the aircraft had been forced out of control by an encounter with a thunderstorm that produced “localized instrument meteorological conditions, heavy rain, and severe turbulence.” Weather radar showed that the oncoming line of thunderstorms had developed a bow echo and began moving to the northeast at 61 knots—even as the Eurocopter, which lists a 132-knot cruise speed, flew southwest. The aircraft and storm converged at a rate of more than three nautical miles per minute. The storm reached the heliport first, though not by much. The AS350 would have made it in about another minute. Given three, it would have been safely on the ground before the gust front hit.
Interviews with the surviving company pilots uniformly suggested that this operator has an outstanding training program and a strong safety culture. Pilots and flight nurses agreed that any member of a crew could cancel a flight with absolutely no repercussions. But when asked to estimate how long the XM radar display available in the cockpit lagged real time, their answers ranged from “30-60 seconds” to a more reasonable, but still low, “5-6 minutes.” In fact, images can be as much as 15 minutes old by the time they reach the pilot.
Given this, and the fact that the accident pilot was relying on second-hand reports of the radar picture when he chose to launch, he may well have thought he had more time than he actually did. But even assuming a 10-minute window between his estimated arrival time and the leading edge of the storm front, launching on the assumption that the storm would keep moving in the same direction at the same speed would have been a high-risk gamble.