A skilled pi­lot, a rou­tine ap­proach, an un­ex­pected catas­tro­phe


Flying - - CONTENTS - By Pe­ter Gar­ri­son

Con­di­tions at the air­port were VMC, with a 1,700-foot ceil­ing, 10-mile vis­i­bil­ity and a 9-knot wind gusting to 14 and vary­ing from 240 to 330 de­grees. The sun had just set, and the tem­per­a­ture on the ground was 32 de­grees Fahren­heit.

The 2,000-hour air­line trans­port pi­lot, 33, de­scended from his cruis­ing al­ti­tude of 9,000 feet and used the GPS ap­proach for Run­way 7, in­tend­ing to cir­cle to Run­way 25. Pass­ing through 4,000 feet, he switched on the anti-ic­ing sys­tem, which on the Cir­rus con­sists of a por­ous metal lead­ing edge from which an eth­yl­ene gly­col fluid with a low freez­ing point, called TKS, bleeds onto the wing sur­face. Six miles from the air­port, in the clear just be­low the clouds, he can­celed IFR and switched anti-ice off. Pass­ing the fi­nal ap­proach fix, he slowed, se­lected half flaps and be­gan a de­scent.

The Cir­rus en­tered a left downwind leg for Run­way 25. Wit­nesses men­tioned that it ap­peared un­usu­ally low and un­usu­ally close to the run­way. It turned base, then fi­nal. The wings lev­eled only briefly be­tween turns. On the base to fi­nal turn, the Cir­rus stalled and spun, crash­ing 100 yards from the run­way thresh­old and killing the pi­lot.

The air­plane was equipped with a data-log­ging de­vice. It showed that in the sec­onds pre­ced­ing the loss of con­trol, the an­gle of bank ap­proached 50 de­grees and the in­di­cated air­speed was be­tween 87 and 90 knots.

The Na­tional Trans­porta­tion Safety Board noted that, ac­cord­ing to the air­plane hand­book, at 60 de­grees of bank the stall speed with half flaps was 95 knots, and could have been even higher had any ice been present on the wings. It at­trib­uted the ac­ci­dent to “the pi­lot’s fail­ure to main­tain ad­e­quate air­speed while turn­ing from the base leg to fi­nal, which re­sulted in the wing’s crit­i­cal an­gle of at­tack be­ing ex­ceeded and a sub­se­quent aero­dy­namic stall.”

It is dif­fi­cult to know the pre­cise stalling speeds of par­tic­u­lar air­craft, but we can as­sume that the full-flaps stall speed of the SR22T at gross weight is no more than 61 knots. This air­plane was not at gross weight, how­ever; it was on a po­si­tion­ing flight, and there­fore prob­a­bly car­ry­ing no cargo, and the pi­lot had mes­saged ahead that he would be re­quir­ing 48 gal­lons of fuel. The ac­tual weight of the air­plane was likely around 3,100 pounds, so its full-flaps stall speed would have been 57 kcas. The stall would prob­a­bly oc­cur at around 63 kcas, or 65 kias, with half flap. The max­i­mum an­gle of bank recorded did not ex­ceed 50 de­grees, which cor­re­sponds to a level-flight G-load­ing of 1.56 and a stalling speed of 81 kias. The NTSB’s men­tion of 95 knots is a mere rhetor­i­cal flour­ish, ir­rel­e­vant to the ac­ci­dent.

In cases in­volv­ing a spin out of a turn — a fairly com­mon type of ap­proach-to-land­ing ac­ci­dent — the NTSB al­most al­ways al­ludes to the in­crease in stalling speed that oc­curs when an air­plane is banked, but never men­tions that it ap­plies only when al­ti­tude is main­tained. It is ob­vi­ous when you per­form a wingover turn, for in­stance, that you can put an air­plane into a 90-de­gree bank and still not stall it. I’m not sure whether the prob­lem is that the NTSB ac­ci­dent an­a­lysts don’t know this fact — which would be hard to be­lieve — or that they merely feel that their anal­y­sis is more force­ful if they omit to men­tion it.

In any case, the air­plane did stall. Why?

The gusting wind could have been a fac­tor. It was not very strong to be­gin with. Ground­speed and in­di­cated air­speed tracked each other closely in the logged data, and the air­plane would in any case have been turn­ing into the wind. How­ever, a sud­den 14-knot gust from 330 de­grees when the air­plane was banked 50 de­grees could con­ceiv­ably have in­creased the wing’s an­gle of at­tack by a cou­ple of de­grees and caused it to stall. A gusty right cross­wind is a lit­tle-rec­og­nized haz­ard on the base-to-fi­nal turn of a left-hand pat­tern.

From one of the wit­ness state­ments in­cluded in the ac­ci­dent docket, some in­fer­ences can be drawn about the state of mind of the pi­lot. He had a busy sched­ule fly­ing the Cir­rus. Ac­cord­ing to the line su­per­vi­sor at I19, who knew the pi­lot, “the goal for him” was to get back to I19 be­fore the FBO closed, which was at 8 p.m. in sum­mer and 6 p.m. in win­ter. The ac­ci­dent took place at pre­cisely 6 p.m. It’s likely, there­fore, that the pi­lot was try­ing to get down as ex­pe­di­tiously as pos­si­ble. The facts that the air­plane was low and close-in on the downwind leg — he had to turn right to widen out the ap­proach be­fore he turned left from downwind to base — and that the turn to fi­nal took place very close to the end of the run­way are con­sis­tent with that the­ory.

It’s prob­a­ble that, be­gin­ning from an un­usu­ally low downwind and hav­ing to make steeply banked turns be­cause he had flown the downwind leg so close to the run­way, the pi­lot found him­self sink­ing and in­creased the an­gle of at­tack in the fi­nal turn to ar­rest his de­scent.

Ice re­mains a pos­si­ble, but un­known, fac­tor. The ef­fect of ice on the stalling be­hav­ior of wings is dif­fi­cult to pre­dict, but it is well-known that very small amounts of ice can some­times have dis­pro­por­tion­ately large ef­fects. The pi­lot se­lected half flaps two and a half min­utes be­fore land­ing, but he did not be­gin to slow down un­til a minute later. He may have been in the habit of get­ting the air­plane con­fig­ured for land­ing well in ad­vance of ar­riv­ing at the run­way — the essence of a “sta­bi­lized ap­proach” and a sta­ple of Cir­rus stan­dard­ized train­ing — but it is also pos­si­ble that he was con­cerned about ice and thought it would of­fer an ex­tra mar­gin of safety.

The fact that the pi­lot used anti-ice dur­ing his de­scent sug­gests that he saw some air­frame ic­ing; this would be con­sis­tent with the weather con­di­tions and with pi­lot re­ports. That he turned off the TKS bleed upon emerg­ing from the clouds is log­i­cal: In the clear, with no pre­cip­i­ta­tion, no fur­ther ic­ing would oc­cur. It was un­clear from wit­ness ac­counts which wing stalled, al­though one wit­ness stated cat­e­gor­i­cally that the air­plane rolled to the right. It is pos­si­ble that the pi­lot turned off the TKS when the left wing ap­peared sat­is­fac­to­rily clean to him, and that he over­looked some residue on the right wing.

It’s of­ten said that ac­ci­dents arise from the com­bi­na­tion of mul­ti­ple fac­tors that are harm­less enough by them­selves. Here you had a well­trained pro­fes­sional pi­lot, very fa­mil­iar with his air­plane, mak­ing a traf­fic-pat­tern ap­proach to his home field in VMC. You could hardly imag­ine a less threat­en­ing sit­u­a­tion. And yet it ended with an ac­ci­dent. To un­der­stand how that could hap­pen, you can­not ig­nore all of the el­e­ments of the ap­proach that were slightly un­usual: that it was flown close in and low, that the banks were un­usu­ally steep, that the plane had been through ic­ing con­di­tions and that the pi­lot was most likely hur­ry­ing to get to the FBO. To this add the fact that the pi­lot was very fa­mil­iar with and per­haps overly con­fi­dent in the air­plane, and you have a glimpse of how what should have been a com­pletely or­di­nary ma­neu­ver could turn into a dis­as­ter.


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