Safety Pilot: Too close for comfort

Thunderstorms claim a Piper Comanche

August 3, 2009

The loss of Air France Flight 447, an Airbus A330, over the Atlantic—presumably because of a thunderstorm entanglement—is a stark reminder that thunderstorms are to be avoided at all costs. How could an airliner fall victim to such a fate? We’ll know eventually, or there will be endless speculation because the unrecovered data recorders reside in Davy Jones’ locker.

But, if you’ll indulge me, perhaps 228 souls are gone for the same reason that a Piper Twin Comanche pilot lost control over Maryland’s Eastern Shore in April 2007—by getting too close to a severe thunderstorm. If you fly often in any season except winter, chances are good that you’ll encounter boomers. For VFR pilots the job is simple—stay well away from CBs. For IFR pilots, depending upon equipment and experience, the game becomes much more intense.

Stormy morning

On April 3, 2007, the day before the Piper Twin Comanche flight, the pilot called Flight Service for a weather briefing for a trip on April 4 from White Plains, New York (HPN) to Southern Pines, North Carolina (SOP). It was to be a golf outing with two friends accompanying the pilot. The specialist advised the weather would be “IFR and marginal VFR” at White Plains. Thunderstorms were forecast at the destination airport, and a stationary front lay along much of the route. This was almost a guarantee that the weather would be interesting at least and possibly hazardous.

At 7:30 Eastern daylight time the next morning the pilot called for another briefing. The specialist noted a low pressure system in the area with a cold front approaching from the west. An airmet was issued for moderate turbulence below 8,000 feet, and embedded thunderstorms were observed with tops to Flight Level 340. A convective sigmet was in effect for embedded severe thunderstorms along a portion of the planned route. The pilot filed an IFR flight plan through the Direct User Access Terminal (DUAT) service but did not request any in-flight weather updates.

Everything was routine as the Piper flew south over Delaware Bay and down Maryland’s Eastern Shore. The flight was handed off to the Patuxent River Naval Air Station Approach Control, in the vicinity of Easton, Maryland. As shown in Figure 1, the flight was moving into an area with significant convective activity, which was traveling northeast rapidly.

At 9:29 a.m. the pilot reported that he would have to deviate “a little bit left here,” to which the controller responded, “Approved, report when able to go direct Richmond.” The controller then asked if the Comanche had weather radar, and the pilot reported he did, and that it “gave him weather every five minutes.” Technically, that wasn’t correct. The Comanche had weather datalink that was being displayed on a Garmin MX20 multifunction display, but not live onboard weather radar.

At 9:30 a.m., the controller noted that the “lightest weather” was “about a one-nine-five heading for seven miles and then it looks like you will be able to get back to Richmond.”

The pilot responded, “Yeah, that’s about right. That’s why I’m heading to the left.” The controller reported “some pretty heavy stuff” at the three o’clock position and “on that turn southbound it looks like some pretty heavy stuff about 12 miles now.” The controller then advised the pilot that the weather was moving northeast, but that in about five miles he would be able to turn back toward Richmond. The pilot responded, “Yeah, thank you very much, that’s exactly what, uh, we’re looking at. We are hoping to be able to turn right in the next 10 to 12 miles.”

At 9:33 a.m., the controller informed the pilot, “Looks like direct Richmond will work out for you now, and...should be exiting all of that weather that I am receiving in about two miles.” The pilot responded, “Yes, sir. That’s, uh, pretty much what we’re looking at.”

At 9:35 a.m. the pilot reported, “Echo Mike is turning direct Richmond.” He also mentioned that there was “a lot of lightning” in the area; however, the turbulence was light. The lightning was a clue that all was not well.

At 9:36 a.m. the pilot reported, “Echo Mike, we just, uh, we got a problem. Looks like we just lost...we lost attitude.” The radar track indicated the airplane had started a right turn, just prior to this transmission, and began heading north. The controller responded, “OK, uh, Five-Echo-Mike, roger. I’m showing you northbound right now and, uh, do whatever you need to, ah, the weather is off to your, uh, right from about your 12 o’clock back through your six o’clock on the right side and it’s about four miles east of you.” During the following minute, the airplane continued a gradual right turn with the last radar hit at 9:37 a.m. No further transmissions were received.

Aftermath

The Comanche sustained an in-flight break-up with no survivors. The failure signatures on the wing showed twisting and overstress fractures. The engines and flight instruments showed no pre-impact failures despite the pilot’s comment regarding a possible loss of the attitude indicator. The nearest weather station, 13 miles north, reported 600 feet overcast with five miles in rain and thunderstorms.

The private pilot was instrument- and multiengine-rated with more than 4,000 total hours. He had logged 167 hours of actual instrument experience and in the previous 90 days he’d flown 12 hours, with 1.5 hours in instrument conditions. He had completed an instrument proficiency check in October

Commentary

On the face of it this seems rather simple: Pilot flying with weather datalink and talking to ATC while near severe weather, zigs when he should have zagged. But there are some key lessons to be taken. The most important is that complex thunderstorms environments should be dealt with strategically, not tactically. That means if the area has more than about 30 to 40 percent coverage and there is no clear path, make an end run or land and wait until it passes. It’s more art than science to make that precise determination—but if there is no solid escape, don’t go there! In this case, had the pilot continued another 30 miles or so farther south he would have cleared the entire mess.

If the area was growing and pushing him offshore, then landing would have been the prudent option. Check if there is an airport before getting sandwiched between the ocean, mountains, or a hot restricted area and a nasty line. An off-airport landing will save your life and, if faced with the choice of flying into a storm or causing minor damage to the aircraft because you didn’t decide sooner, go for the minor damage! Declare an emergency if you need to.

This system was moving at more than 30 knots, and fast-moving storms pack a lot of punch. The band of weather may be only 10 miles wide, but chances are good that a light aircraft plowing into the front side will exit from the bottom in an unairworthy condition rather than happily flying out the back side. All of this discussion of avoidance means not just missing rain shafts either visually, on weather datalink, or using onboard radar, but missing them by 20 miles. Current weather detection devices show precipitation, which can also be a derivative of turbulence. Under the right conditions there can be heavy rain with light to moderate turbulence, and severe turbulence in areas with no rain at all. Lightning is also a derivative, and a good indicator of severe turbulence—this is the theory behind spherics, better known as Stormscope.

The system for precipitation detection is a little different with different types of equipment and they all have limitations. Onboard satellite weather datalink is prevalent in many cockpits and gives a great strategic view of the weather. It typically updates every five minutes but depending on processing time from the vendor, the actual data could be somewhat older. Most of the time, this won’t matter if you’re keeping your distance. But if the storm is moving at 30 knots, that’s one-half a nautical mile a minute. So in five minutes it’s gone 2.5 miles; if we add another five minutes, generously, for processing it’s gone five miles. Anything moving that fast deserves a wide berth so the gap is still adequate—but that requires discipline and the commitment to deviate farther to maintain the 20 miles. If the area is growing rapidly around you, get out or get down.

Onboard radar, which the Comanche did not have, provides instant updates but requires considerable interpretation and is subject to attenuation, or blind spots, because of heavy rain. ATC radar comes in two flavors—en route and tracon or approach control. To learn more about those and how controllers are supposed to communicate with you, see ASF’s free online course, Weather Wise: Thunderstorms and ATC (www.asf.org/courses). Of the two, approach control radars are near real time, but they also require ATC to communicate information to you—which may be a little problematic when they’re really busy.

The controller was being conscientious in providing guidance, but he or she can’t see out the window and may not be a pilot. As pilot in command the decision on where to go always rests with you. I’ve had several occasions, not unlike this one, where the controller suggested a heading. Based on my view out the window, what was being displayed in the cockpit, and my understanding of the weather system, I elected to go farther around.

In summary, the Twin Comanche sustained an in-flight breakup in thunderstorms over Maryland’s Eastern Shore. The 4,000-hour pilot had weather datalink on board and was receiving guidance from ATC, but got too close. The lessons:

  • Understand the nature of the particular convective weather system—some are slow moving and weak, some are isolated storms, some are explosive and they can mutate from one state to another.
  • Maintain 20 miles distance from severe thunderstorms—arbitrarily, anything taller than 30,000 feet or moving faster than 15 knots. Squeezing between cells is pulling the tiger’s tail.
  • ATC can provide guidance, but you remain pilot in command and must make the final decision regarding safe operations.
  • Weather datalink is a tool that can be misused. Hammers are used to drive nails or smash thumbs!

In most cases, the weather will improve in a few hours. Wreckage is usually cleaned up in good weather.

Bruce Landsberg is president of the AOPA Air Safety Foundation.