Re­gion of re­versed com­mands

THEY DO THINGS DIF­FER­ENTLY THERE

Flying - - Contents - By Peter Garrison

In July 2016, a Cessna 150F tak­ing off from a grass strip in In­di­ana failed to climb, struck trees and crashed, killing both oc­cu­pants. A sin­gle wit­ness, him­self a pi­lot, who was op­er­at­ing a mower, ob­served parts of the take­off. He re­ported that the Cessna had be­come air­borne at mid­field, which was nor­mal, and that when it crossed the de­par­ture end it was fly­ing at a high an­gle of at­tack. He did not ex­pect the air­plane to have any trou­ble clear­ing the tall trees 1,100 feet from the end of the run­way, how­ever, and turned away. When he later saw smoke ris­ing from the woods, he as­sumed the prop­erty owner was burn­ing some­thing, and it did not oc­cur to him there had been an ac­ci­dent un­til he heard sirens.

The pi­lot, 44, was a novice. On his ap­pli­ca­tion for a third-class med­i­cal, four months be­fore the ac­ci­dent, he re­ported 60 hours of to­tal time, 25 in the pre­vi­ous six months. Ac­cord­ing to press re­ports, he had bought the 150 late in 2015.

The 150F, which has “omni-vi­sion” rear win­dows, a swept ver­ti­cal tail, elec­tric flaps and a 100 hp Con­ti­nen­tal en­gine, is ap­proved for auto fuel, and be­fore the flight the wit­ness ob­served the pi­lot adding fuel to the air­plane from plas­tic jugs brought in the trunk of his car. They spoke; the pi­lot said he and his com­pan­ion were fly­ing to an­other air­port for lunch. The wit­ness also re­ported that when the pi­lot first ac­quired the 150, he had come to the grass strip with an in­struc­tor to prac­tice short- and soft-field take­offs.

The wreck­age was largely con­sumed by fire, but the Na­tional Trans­porta­tion Safety Board’s in­ves­ti­ga­tor was able to per­form the usual con­trol sys­tem con­ti­nu­ity checks. He found noth­ing amiss. The flaps had been up at the time of im­pact. The en­gine, with 960 hours since over­haul, was ex­am­ined and borescoped, the spark plugs and mag­ne­tos checked. There were no con­tam­i­nants in the car­bu­re­tor bowl, and noth­ing else was found that would pre­clude nor­mal op­er­a­tion; nor had the wit­ness re­ported any­thing un­usual about the en­gine’s sound. The pi­lot’s tox­i­col­ogy re­port was clean.

The in­ves­ti­ga­tor con­cluded, there­fore, that the prob­a­ble cause of the ac­ci­dent was im­proper soft-field­take­off tech­nique.

The strip is 2,081 feet long, 65 feet wide and, to judge from a pho­to­graph in the ac­ci­dent docket that must have been taken the day af­ter the event, smooth and well-mowed. Usu­ally, soft-field tech­nique (or rough-field, which is the same) is used for tall grass, snow, slush, mud, bumps and ruts, and so on; it’s not clear why the pi­lot felt it was called for here.

The aim of the tech­nique is to get weight off the wheels as quickly as pos­si­ble, ei­ther to re­duce rolling re­sis­tance, in the case of grass, mud or snow, or to mit­i­gate rough-sur­face im­pacts on the land­ing gear — par­tic­u­larly the nose strut, which is the most vul­ner­a­ble and prone to dig in. The pro­ce­dure is to use par­tial flap

THE NTSB IN­VES­TI­GA­TOR CON­CLUDED THAT THE PROB­A­BLE CAUSE OF THE AC­CI­DENT WAS IM­PROPER SOFT-FIELD-TAKE­OFF TECH­NIQUE.

and to hold the nose up from the start of the roll, grad­u­ally re­duc­ing the an­gle of at­tack as the air­plane light­ens. This is nat­u­ral in a tail­drag­ger; in a tri­cy­cle-gear air­plane, it re­quires hold­ing the yoke all the way back early in the roll.

While the wing is close to the ground, the in­duced drag due to lift, which is nor­mally very high at low speed, is al­le­vi­ated. As the wing gets far­ther from the ground, the drag in­creases. For a low-power air­plane like the 150, this can be a prob­lem: The air­plane may be­come air­borne, mush­ing, but be un­able to climb un­less it ac­cel­er­ates first.

The name of this para­dox­i­cal sit­u­a­tion is “the re­gion of re­versed com­mands.” It means that an air­plane with­out much ex­cess power, fly­ing at min­i­mum speed, may have to lose alti­tude first, in or­der to gain enough speed to be able to go up. Pulling back on the yoke in­creases drag and makes the air­plane sink. Ground ef­fect re­moves the prob­lem, per­mit­ting the air­plane to ac­cel­er­ate with­out los­ing alti­tude. The se­cond phase of a soft­field take­off, there­fore, is to fly level, just off the ground, to pick up speed.

The 150 may have been slightly over­weight — this could not be con­firmed be­cause the amount of fuel in the tanks was un­known — and may have had a slight tail­wind. Nev­er­the­less, 3,000 feet is am­ple dis­tance for a 150 to be­come air­borne and clear any nor­mal tree.

If the pi­lot brought an in­struc­tor to this field to in­struct him in soft-field­take­off tech­nique, it’s likely that he used that tech­nique for what proved to be his fi­nal take­off. It’s prob­a­ble, there­fore, that he be­gan the roll with par­tial flap, prob­a­bly 20 de­grees. Flap al­lows the air­plane to be­come air­borne sooner but in­creases its par­a­site drag. Once air­borne, there­fore, he would need to raise the flaps to ac­cel­er­ate more rapidly, but he should not be­gin to raise the flaps un­til the air­plane has at least a small safety mar­gin above its clean stalling speed.

When the wit­ness glimpsed the air­plane over the de­par­ture end of the run­way, he guessed it was at 50 feet — well out of ground ef­fect. The “high an­gle of at­tack” he ob­served sug­gests that the pi­lot may al­ready have re­tracted the flaps, be­cause flap causes any air­plane to as­sume a more nose-down at­ti­tude at a given speed.

Ac­cord­ing to the POH, the 150’s stalling speed with flaps 20 is 49 mph (the POH was writ­ten be­fore the FAA’S adop­tion of the knot as the aero­nau­ti­cal unit of speed). Clean, it’s 55. As­sume that it be­came air­borne at 49 mph in ground ef­fect at the mid­point of the run­way. If it did not ac­cel­er­ate at all, it would take about 15 sec­onds to reach the end, but it pre­sum­ably did ac­cel­er­ate, and got there a lit­tle sooner. The trees at the end of a run­way al­ways ap­pear closer than they are, and the pi­lot prob­a­bly felt pres­sure to get cleaned up so he could see at least 60 mph on the air­speed in­di­ca­tor.

Be­fore the switch to elec­tric ac­tu­a­tion in the 150, you con­trolled the flaps with a lever be­tween the seats. That method, how­ever un­so­phis­ti­cated, gave you a di­rect feel of the ef­fect of flap an­gle on per­for­mance. Al­though it is pos­si­ble to bleed the flaps up a bit at a time with the elec­tric switch, a novice is more likely to sim­ply se­lect flaps up and let the mo­tor take care of the rest.

It seems likely to me that the pi­lot, af­ter be­com­ing air­borne, in­ad­ver­tently al­lowed the air­plane to climb out of ground ef­fect too early. It was still at or near its min­i­mum speed. In that con­di­tion, it could nei­ther ac­cel­er­ate nor climb. As the end of the run­way ap­proached, he must have re­mem­bered to re­tract the flaps. But the air­plane barely had fly­ing speed.

The 150 main­tained alti­tude, but the trees got taller. The pi­lot prob­a­bly suc­cumbed to the ir­re­sistible urge to pull back on the yoke. In the re­gion of re­versed com­mands, how­ever, things are back­ward: Pulling makes you go down, not up. And so, the 150 went down.

IN THE “RE­GION OF RE­VERSED COM­MANDS,” AN AIR­PLANE WITH­OUT MUCH EX­CESS POWER, FLY­ING AT MIN­I­MUM SPEED, MAY HAVE TO LOSE ALTI­TUDE FIRST, IN OR­DER TO GAIN ENOUGH SPEED TO GO UP.

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