Don't mistake bad vibrations for business as usual.
There are emergencies … and then there are emergencies. The distinction lies in whether the time frame for responding affords the luxury 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 airframe. If its frequency is close to the airframe’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 seconds 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 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 automatically. At low rpm, lowering collective and reducing power to idle may succeed in saving 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.
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 passenger. 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 having 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 position, the helicopter began to buck fore and aft on its skids.
The pilot reduced fuel flow in response, only to have the bucking intensify, leading him to suspect ground resonance. He increased fuel flow but did not advance it fully or lock it into its flight gate before raising collective, and neither the engine nor main rotor rpm reached their flight-governing ranges before the helicopter lifted from the pad.
The helicopter yawed and drifted to its left as engine rpm spooled up while the main rotor rpm decayed. Two minutes after engine start, the ship descended into the hillside and tumbled down the slope. The pilot—who was wearing his four-point harness but no helmet—managed to extricate himself from the wreckage after the engine shut down. He walked back to the tower’s service building where his passenger administered first aid.
After the pilot reported the accident, a company helicopter dispatched from Fort Simpson arrived about 3:00 p.m. Both men were initially flown to Yellowknife. The pilot was subsequently airlifted to Edmonton for treatment of injuries including a badly broken arm. Six months later, he was back at work but had not yet returned to flight duty.
The 5,277-hour commercial pilot had 2,017 hours in AS350s, with 6.5 hours in the previous week and 11.7 in the preceding 90 days. He held a Category 1 medical certificate and had completed recurrent training in the AS350 the month before the accident. His age has not been reported.
The AS350 B2 has a fully articulated, three-bladed main rotor powered by a single 732-horsepower Turbomeca Arriel 1D1 turboshaft engine. Its Starflex rotor head provides full articulation without hinges or lead-lag dampers; instead, flexible thrust bearings at the inboard ends of the mounting sleeves allow the blades to flex, flap, and move in the lead-lag axis, while elastomeric frequency adapters at the sleeves’ outboard ends provide damping. The accident aircraft was manufactured in 1989 and had served for 46,214 cycles comprising 11,005 hours of flight time.
Its landing gear featured two vibration-absorbing systems: flexible steel strips extending downward from the aft ends of the skids, and hydraulic dampers between the front horizontal crosstubes and the fuselage. After the accident, the operator tested the damper assemblies. The right damper (which had seen 1,395 hours of service compared to the left damper’s 3,001) failed the initial functional test, then passed after overhaul. The history of the accident sequence, however, makes it seem unlikely that inadequate damping was a factor.
Four days before the accident, in order to hangar the aircraft overnight, all three main rotor blades had been removed by a technician with the assistance of the same pilot. After they were reinstalled the following morning, the pilot did a ground run and noticed increased vibration.
Although vibration analysis equipment was available at the site, vibration levels were not measured, nor were blade tracking and balance assessed as required by the aircraft’s maintenance manual. Furthermore, the removal and reinstallation of the main rotor blades weren’t recorded in the journey log, contrary to Canadian Aviation Regulations. Investigators learned that the maintenance shop routinely removed and remounted blades without making the required logbook entries.
The vibrations continued throughout the six hours the pilot flew the helicopter during the intervening three days. “During this time,” according to the Transportation Safety Board of Canada’s (TSB) report, “no action was taken to verify or rectify the vibration and no aircraft journey log entries were made.” With no measurements having been recorded, the preaccident tracking and balance status of the rotor could not be determined.
Following the accident, the operator’s parent company emailed its pilots and maintenance personnel to remind them of the requirement to document all removals and reinstallations of rotor blades in the journey logs. It also instituted an audit procedure to more systematically track those events. Recurrent training for company pilots also stressed the need to record any sudden changes in vibration levels. While the TSB’s report doesn’t state this explicitly, it’s hoped this training also reinforced the importance of investigating and resolving any sudden increases in vibrations before further intensification.
Professional pilots—particularly those operating in remote locations and extreme environments—can develop a tolerance for apparently benign aircraft anomalies. But discrepancies as seemingly trivial as a burned-out indicator lamp can become the kind of emergency that requires quick recourse to memory items if the wrong thing happens at the wrong time. It’s up to the certificate holder to establish operating procedures, backstopped by applicable national regulations, that remove those decisions and the accompanying temptations from its pilots’ hands. But written procedures count for little if company culture doesn’t identify and call out violations.
Students and low-time pilots might be taken aback by the notion of flying a helicopter that’s had its main rotor blades remounted without first checking blade track and balance. The rotational momentum of all that mass spinning hundreds of times per minute would seem to raise the prospect that any imbalance would quickly build toward catastrophe. But in the field, the need to shelter aircraft from a bitter climate in limited hangar space made this an unremarkable practice—in part, no doubt, due to the lack of adverse consequences up to that time.
In this case, a highly experienced pilot noticed increased levels of vibration without apparently finding them alarming. Over the course of six hours flight in the harsh conditions of a Northwest Territories winter, they presumably did not worsen enough for his survival instincts to command a return to the maintenance hangar. But while pilot-in-command authority should always admit grounding an aircraft in the interest of safety, it’s the operator’s responsibility to identify risks that can’t be left to pilot discretion.