ACCIDENT RECOVERY By David Jack Kenny


Identifying Risks in Real Time Don’t mistake bad vibrations for business as usual.


time frame for responding affords the lux- ury of, say, consulting a checklist. In helicopters, many incidents fall into the second class: the pilot’s reaction must be both immediate and exactly correct to avoid balling up the machine. Losses of main rotor rpm (especially in low-inertia systems) or tail rotor control can escalate beyond hope of recovery if those relatively brief sequences of memory items aren’t executed in order and without delay. Ground resonance is another example. If a fully articulated rotor system becomes unbalanced, the resulting vibration can excite a sympathetic vibration in the air- frame. If its frequency is close to the air- frame’s natural harmonic frequency, the two vibrations amplify one another until the helicopter shakes itself to pieces. In one famous case in Utah’s Grand Staircase– Escalante National Monument, the aircraft was essentially destroyed within four sec- onds of the vibration’s onset. The most common cause is a rough touchdown that knocks one blade out of phase with the others, but significant vibration from any cause can have the same effect. The required response depends on the


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helicopter’s energy state. If the rotor is still at flying rpm, an immediate lift-off—adding power as necessary—allows the fuselage’s vibrations to dissipate while any out-of- phase blades realign themselves automati- cally. At low rpm, lowering collective and reducing power to idle may succeed in sav- ing the aircraft. Between those extremes, catastrophic damage is likely, whatever the pilot does—one reason they’re trained to maintain full rotor speed until the helicopter is fully down, settled, and secure.


HERE ARE EMERGENCIES … and then there are emergencies. The distinction lies in whether the


The Flight Shortly before 10:00 a.m. on February 15, 2018, an Airbus AS350 B2 landed on the timber pad of a telecommunications tower at Bear Rock, three miles west-northwest of Tulita in Canada’s Northwest Territories. On board were the pilot and one passen- ger. Photographs taken shortly after the accident show that the pad was mostly clear, with patches of ice covering perhaps 20 percent of its surface. The weather was seasonably cold at


-27°C (-17°F). With no preheat available on site, the pilot initiated an engine run about 30 minutes after landing in accordance


with the AS350 flight manual supplement, Instructions for Operations in Cold Weather. The pilot later acknowledged hav- ing noticed some vibration, which he described as “consistent with those felt over the previous three days, both on the ground and during flight.” At 11:08 a.m. he began a second engine


run. Start-up was normal, and the engine accelerated smoothly to 70 percent Ng (gas generator speed). However, when the pilot increased fuel flow to the flight posi- tion, the helicopter began to buck fore and aft on its skids. The pilot reduced fuel flow in response,


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