Flying - - DEPARTMENTS - By Peter Gar­ri­son

Some­times, what you don’t know will hurt you

The crum­pled Bo­nanza lay in a field of corn stub­ble. The east­ern Colorado ter­rain was flat for miles around. It was hard to see how even a com­plete loss of power should have led to any­thing worse than a lit­tle scraped metal, but ev­i­dently the air­plane was out of con­trol when it hit the ground. It had cartwheeled be­fore com­ing to rest, its cabin crushed. Its two oc­cu­pants died: the owner of the air­plane, a non­pilot who had bought it three days be­fore, and an in­struc­tor who was tak­ing him to a busi­ness meet­ing that day and in­tended, in the fu­ture, to teach him to fly his newly ac­quired ma­chine.

The log of the in­struc­tor pi­lot, 35, showed 467 to­tal hours. Pi­lots who had flown with him, in­clud­ing the seller of the ac­ci­dent air­plane, had only praise for his fly­ing. But his ex­pe­ri­ence level in Bo­nan­zas is un­clear: The Na­tional Trans­porta­tion Safety Board’s re­port on the ac­ci­dent states (at dif­fer­ent points on the same page) that he had two and a half hours or 131 hours in the make and model of the ac­ci­dent air­plane. In an­other lo­ca­tion, how­ever, it refers to his “lim­ited ex­pe­ri­ence” in Bo­nan­zas.

The ac­ci­dent oc­curred about 8 miles north of Weld County Air­port (GXY) at Gree­ley, Colorado. The wind was gusty, 10 to 20 knots, out of the north. There were pi­lot re­ports of mod­er­ate to ex­treme tur­bu­lence in the area. Ap­proach­ing from the south­east, the Bo­nanza had de­scended to 400 feet as it over­flew Run­way 35 at GXY, but then con­tin­ued north­ward, climb­ing back up to 1,100 feet agl and main­tain­ing around 90 knots ground­speed — that is, tak­ing the al­ti­tude and the north wind into ac­count, an in­di­cated air­speed of some­where around 95 to 110 knots. The pi­lot did not give a po­si­tion re­port or state his in­ten­tions to uni­com, but a per­son mon­i­tor­ing the fre­quency


heard an ex­cla­ma­tion of an ex­ple­tive. The ex­act tim­ing of the ut­ter­ance is un­clear. Per­haps the pi­lot ex­claimed over some on­board prob­lem that made him abort the land­ing and fly north­ward for sev­eral min­utes with­out any ap­par­ent rea­son, or per­haps the ex­cla­ma­tion co­in­cided with the loss of con­trol.

Sev­eral wit­nesses on the ground re­ported hear­ing the en­gine “sput­ter­ing” be­fore the Bo­nanza be­gan a left turn to re­verse course and then dived. The NTSB, hav­ing ex­am­ined the en­gine and what was left of the air­frame and find­ing noth­ing to ac­count for the power loss, con­cluded that, although there was am­ple fuel in the air­plane, the tank feed­ing the en­gine had run dry. The fuel-tank se­lec­tor was set to the right main tank. The tank, which had not been breached in the crash, was empty.

This anal­y­sis gained plau­si­bil­ity from the de­sign of the fuel quan­tity in­di­cat­ing sys­tem. The Bo­nanza, built in 1956, had two main tanks, two wing aux tanks and two tip tanks, but only a sin­gle fuel quan­tity in­di­ca­tor with its own sep­a­rate se­lec­tor switches. It was the pi­lot’s re­spon­si­bil­ity to en­sure that he knew which tank’s fuel level the in­di­ca­tor was re­port­ing. The set­ting of the in­di­ca­tor’s se­lec­tor switches could not be deter­mined from the wreck­age, but in­ves­ti­ga­tors hy­poth­e­sized that the in­di­ca­tor might have been set for the wrong tank.

Since the left turn co­in­cided with a steady loss of al­ti­tude, and both co­in­cided with wit­ness re­ports of what sounded like en­gine trou­ble, it seems ap­par­ent that the power loss oc­curred while the air­plane was still track­ing north­ward, into the wind.

Given the strong north wind and the fact that the ter­rain ahead con­sisted of a se­ries of farm fields all equally suit­able for a forced land­ing, the pi­lot’s de­ci­sion to turn back south­ward seems puz­zling. Prob­a­bly he in­stinc­tively turned to­ward the air­port with the hope he might be able to restart the en­gine or at least milk some power out of it. He prob­a­bly be­lieved he was feed­ing from a tank that had fuel in it, and so his first hy­poth­e­sis would have been that there was a me­chan­i­cal rea­son for the power loss. He had about a minute to fig­ure out what it was.

A pi­lot’s think­ing in such a sit­u­a­tion — loss of power at low al­ti­tude, far from an air­port — is sel­dom or­derly or sys­tem­atic. Un­like the pi­lot of the 1920s, who con­sid­ered forced land­ings in farm fields nor­mal, the mod­ern pi­lot is un­pre­pared. Un­less the Air Force has trained him to rat­tle off the “bold­face items” from mem­ory, he does not go down a list of pos­si­ble causes, weigh­ing the like­li­hood of each, as we do when we re­flect on such a sit­u­a­tion in the se­cu­rity of our arm­chairs. He is more likely to find him­self un­able to think an­a­lyt­i­cally at all.

The NTSB sug­gested that the air­plane stalled be­cause it was turn­ing down­wind in gusty con­di­tions. This is plau­si­ble, although the physics of tran­si­tory fluc­tu­a­tions of air­speed are rather com­plex be­cause it is an­gle of at­tack, not in­di­cated air­speed, that causes a wing to stall. The mere fact of turn­ing from up­wind to down­wind would not, in it­self, en­tail any risk. But the pow­er­ful vis­ual im­pres­sion of slid­ing side­ways and ver­tig­i­nously gain­ing speed dur­ing such a turn at low (and rapidly de­creas­ing) al­ti­tude can lead a pi­lot to cross con­trols and un­con­sciously raise the nose. The low al­ti­tude down­wind turn re­quires at­ten­tion to the in­stru­ments; a dis­tracted pi­lot who is search­ing for the cause of an en­gine fail­ure might mis­han­dle it.

I am in­clined to think that the pi­lot’s de­ci­sion — or per­haps I should call it an im­pulse, since it prob­a­bly lacked the de­lib­er­ate­ness of a de­ci­sion — to turn back to­ward the air­port, while un­der­stand­able, was the root cause of the ac­ci­dent. He could have con­tin­ued straight ahead while try­ing to iden­tify the en­gine prob­lem. If he fig­ured it out, he would be able to re­turn to Gree­ley at his leisure. If he didn’t, he would at least en­counter the ground with the least pos­si­ble for­ward speed. Turn­ing down­wind, even if he had not stalled the air­plane, put him at risk of

ar­riv­ing with twice the ki­netic en­ergy he would have had fac­ing north. By the time he had turned 180 de­grees, he was down to 350 feet agl; he would not have time to turn back into the wind.

There was, how­ever, an­other de­ci­sion that might have con­trib­uted to the ac­ci­dent. It was made at a time in the 1940s or ’50s, when some engi­neers who are prob­a­bly no longer with us weighed the ad­vis­abil­ity of sep­a­rat­ing tank se­lec­tion from quan­tity in­di­ca­tion. There was an ob­vi­ous and rel­a­tively easy so­lu­tion to the prob­lem: a cam on the fuel se­lec­tor han­dle, and some mi­croswitches and re­lays, to keep the in­di­ca­tor in sync with the se­lec­tor. Some­one prob­a­bly ar­gued — not with­out rea­son — that such a sys­tem, be­sides cost­ing a few dol­lars, would in­tro­duce var­i­ous new modes of fail­ure, in­clud­ing the pos­si­bil­ity that the pi­lot could be left with no fuel quan­tity in­di­ca­tion at all. Which is bet­ter, a fal­li­ble sys­tem or a fal­li­ble pi­lot? In the end, they shrugged and said, “If the guy is smart enough to be a pi­lot, he can keep track of his fuel.”

The gospel on turn-backs af­ter a power loss just af­ter take­off is to con­tinue straight ahead, no mat­ter how un­invit­ing the ter­rain looks. Though it has been demon­strated, at least the­o­ret­i­cally, that suc­cess­ful re­turns are pos­si­ble given enough al­ti­tude and a suf­fi­ciently skilled and dis­ci­plined pi­lot, it has also been demon­strated that in prac­tice the out­come is far more of­ten an un­re­cov­er­able, and un­sur­viv­able, low-al­ti­tude stall.

The ac­ci­dent at Gree­ley had many of the el­e­ments of a turn-back, even though it took place miles from an air­port, and at the end, not the start, of a flight. The same rule should have ap­plied: Af­ter a power loss at low al­ti­tude, land — or crash — as nearly straight ahead as you can.


Newspapers in English

Newspapers from USA

© PressReader. All rights reserved.