Scop­ing out storms: Is it worth the risk?

Flying - - CONTENTS - By Peter Gar­ri­son

A Mis­souri busi­ness­man, 54, and his dog, who ac­com­pa­nied him ev­ery­where, died when his Piper Chero­kee Six broke up in flight over Cuba, Mis­souri, in 2015.

The 1,200-hour pi­lot had filed an in­stru­ment flight plan from Bran­son, Mis­souri, where he had a va­ca­tion home, north­east­ward to St. Louis. He was cruis­ing at 5,000 feet. When he first checked in with Kansas City Cen­ter, the con­troller warned him of “mod­er­ate to ex­treme pre­cip­i­ta­tion from your 10 o’clock to 2 o’clock po­si­tion, begin­ning in 50 miles, about 30 miles in di­am­e­ter.” The pi­lot ac­knowl­edged, and added that he had “a scope on board.”

He could have flown around the weather; a 20 nm dog­leg would have added only a few min­utes to the 180 nm trip. In­stead, he chose to con­tinue on course.

Thirty min­utes later, the pi­lot was in the mid­dle of the area of pre­cip­i­ta­tion. He told the con­troller that the ride was smooth. His Storm­scope was show­ing some re­turns to his right, and he said he was see­ing “a lit­tle bit on Nexrad.” Less than a minute later, the Chero­kee be­gan a right de­scend­ing turn; then its radar re­turn dis­ap­peared.

Wit­nesses on the ground re­ported hear­ing the sounds of an air­plane seem­ingly in dis­tress, and then see­ing the plane come out of the clouds in pieces. De­bris was scat­tered over more than half a mile, begin­ning, cu­ri­ously enough, with pieces of the wind­screen.

A Cape Air Cessna 402 hap­pened to be at 7,000 feet, in the same vicin­ity, at the same time. The pi­lot later told a Na­tional Trans­porta­tion Safety Board in­ves­ti­ga­tor that his trip had been rou­tine, in solid IMC, with light to mod­er­ate tur­bu­lence, some mod­er­ate to heavy rain and some small re­turns on his weather radar — noth­ing that gave him cause for con­cern.

Radar imagery from St. Louis, north­east of the ac­ci­dent site, showed re­turns “con­sis­tent with con­vec­tive ac­tiv­ity” along the Chero­kee’s flight path, but no light­ning was recorded by lo­cal light­ning-de­tec­tion net­works. A three-di­men­sional re­con­struc­tion of weather at the time of the ac­ci­dent, based on radar data, showed the Chero­kee in a clear area, hav­ing just turned to the right, pos­si­bly to cir­cum­nav­i­gate an area of “mod­er­ate-high re­flec­tiv­ity” that had moved into its path from the north­west. That area would have been vis­i­ble to the pi­lot as a yel­lowand-red patch on his Nexrad dis­play.

(The pi­lot told his con­troller that he had Nexrad, but ac­ci­dent in­ves­ti­ga­tors did not find “any ev­i­dence of Nexrad ca­pa­bil­i­ties” in the wreck­age. Since de­bris from the dis­in­te­grat­ing air­plane was scat­tered over a large wooded area, how­ever, it seems more prob­a­ble that the pi­lot

Wit­nesses on the ground re­ported hear­ing the sounds of an air­plane seem­ingly in dis­tress, and then see­ing the plane come out of the clouds in pieces.

had a hand­held dis­play that was never found than that he gra­tu­itously re­ported a nonex­is­tent Nexrad ca­pa­bil­ity to cen­ter. The fact that he flew pre­cisely through a nar­row low-re­flec­tiv­ity cor­ri­dor be­tween two more ac­tive ar­eas sug­gests guid­ance from ground-based weather radar.)

The NTSB’s prob­a­ble cause was “the pi­lot’s con­tin­ued flight into thun­der­storm ac­tiv­ity.” To be sure, all that could be known from radar re­turns was that there was heavy rain, which was “con­sis­tent with” thun­der­storms, but also con­sis­tent with just plain heavy rain. The pi­lot’s Storm­scope showed no light­ning ahead, and the ride was smooth. At the time of the breakup, the Chero­kee was in an area of zero radar re­flec­tiv­ity. The only ev­i­dence that he was in dan­ger was the breakup it­self. But the NTSB of­ten falls into the log­i­cal fal­lacy of us­ing the ac­ci­dent as ev­i­dence that the pi­lot must have done some­thing reck­less.

The prob­a­ble cause adds that a con­tribut­ing fac­tor was “the pi­lot’s reliance on on­board weather equip­ment to nav­i­gate through se­vere weather.” Ac­tu­ally, that is what FAA-ap­proved on­board weather equip­ment is for. Pre­sum­ably, the pi­lot’s aim was to avoid se­vere weather by us­ing his on­board equip­ment, not to go “through” it. The Cape Air 402 pi­lot was ap­par­ently guilty of ex­actly the same be­hav­ior, but was not chided for it.

In the syn­op­sis of the ac­ci­dent, the re­port’s au­thor de­clares that the pi­lot “was at­tempt­ing to use on­board weather radar to ma­neu­ver through an area of storms that he should not have been fly­ing through in the first place.”

Never mind that the pi­lot was not us­ing on­board radar at all. What ac­tu­ally hap­pened here was that a con­troller called a pi­lot’s at­ten­tion to an area of pos­si­bly heavy weather ahead of him; the pi­lot ac­knowl­edged the warn­ing and said he would use his light­ning de­tec­tor and Nexrad to guide him. Not only was he within his rights to do this, but it was what many pi­lots would have done, con­sid­er­ing that weather is as you find it, not as a con­troller tells you it may be. There is no rea­son to sup­pose that the pi­lot, who pre­sum­ably had flown this route many times, would not have turned back or de­vi­ated if he had seen clear ev­i­dence of haz­ardous weather ahead. He didn’t, and so he con­tin­ued.

On the other hand, at the time of the first warn­ing from the con­troller 30 min­utes ear­lier, he would have seen on his Nexrad dis­play that large clear ar­eas sur­rounded the patch of heavy rain. This was not a case of feel­ing one’s way be­tween storms in a squall line too long to cir­cum­nav­i­gate. The in­con­ve­nience of by­pass­ing the weather would have been min­i­mal. Why did he choose to aim straight at the cen­ter of the bad weather? Was he so in love with his on­board equip­ment that he could not pass up a chance to use it?

Ev­i­dently, the pi­lot un­luck­ily stum­bled into an area of se­vere lo­cal tur­bu­lence, lost con­trol of the air­plane and over­stressed it while try­ing to re­cover. The tur­bu­lence could not have been iden­ti­fied or fore­cast; it just hap­pened to be there. The Cape Air flight, cruis­ing nearby, did not en­counter it.

At the time of the ac­ci­dent, the Chero­kee was mak­ing a ground­speed of 163 knots. The winds at his al­ti­tude were south­west at up to 30 knots, so it ap­pears that the pi­lot had taken the pre­cau­tion of slow­ing down to a few knots above his ma­neu­ver­ing speed, which would have been around 114 kias. Ma­neu­ver­ing speed, how­ever, is no guar­an­tee of safety.

Man­u­fac­tur­ers of trans­port-cat­e­gory air­craft are re­quired to de­fine a tur­bu­lence-pen­e­tra­tion speed, Vb, whereas gen­eral avi­a­tion air­planes like the Chero­kee Six are pro­vided only with a ma­neu­ver­ing speed, Va. While the two speeds are ar­rived at dif­fer­ently, the un­der­ly­ing prin­ci­ple is the same: There is a speed at which wings or con­trol sur­faces will stall be­fore reach­ing their limit load. In other words, if you are fly­ing slowly enough, no ma­neu­ver or gust will be able to tear your wings off.

It is cold com­fort, how­ever, that the only thing that stands in the way of los­ing a wing is a stall, which is nor­mally some­thing you would try to avoid. Un­for­tu­nately, tur­bu­lence or gusts pow­er­ful enough to break an air­plane are also pow­er­ful enough to up­set it. An up­set in IMC is likely to bring about a rapid in­crease in speed in a down­ward di­rec­tion, and that is al­most sure to be fol­lowed, in turn, by a des­per­ate and ter­ri­fied pi­lot over­con­trol­ling the air­plane.

This ac­ci­dent can be seen as a case of a pi­lot tak­ing a rea­son­able risk and the chances hap­pen­ing to turn out very badly for him. He gambled and lost. On the other hand, you can con­sider the gam­ble it­self — how­ever small the risk — to have been com­pletely un­nec­es­sary. Take your pick.

The prob­a­ble cause adds that a con­tribut­ing fac­tor was “the pi­lot’s reliance on on­board weather equip­ment to nav­i­gate through se­vere weather.”

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