Misdirection is Often Part of Accident Investigations

I love watching Penn and Teller, they’re so entertaining; their presentation so streamlined; and their technique so obvious. Teller works alone while Penn monopolizes the audience’s attention; Teller works silently while Penn laughs it up; and Teller works openly while Penn misdirects.

In accident investigation, the same thing happens: if you focus on the obvious, you miss the subtle. In reviewing my first accident investigation: LAX01MA272, I got caught up with the conspicuous while ignoring the elephant in the room. But I wasn’t alone.

On August 10, 2001, a Papillon Grand Canyon Helicopters air tour Eurocopter AS350-B2 with a Turbomecca Arriel 1D1 engine, tail number N169PA, crashed near Meadview, AZ. There were no witnesses; the sole survivor could not provide reliable information. The accident’s probable cause: the pilot’s loss of control of the helicopter for undetermined reasons.

Onboard were six family members – all adults – and the tour guide/pilot. N169PA was one of two Grand Canyon tour helicopters running a mapped-out sightseeing excursion of the canyon. After a planned stop and refueling, N169PA preceded the sister following helicopter (FH) by a few minutes toward the Grand Wash Cliffs. When the FH entered the same valley, N169PA was burning, destroyed, laying on its side on a 40-degree sloping wall. The survivor’s burns were extensive, her memory limited from shock.

Both helicopters’ passengers were part of the same tour group, N169PA being the lead helicopter during the tour’s last leg. The FH saw the smoke before coming up on the wreckage and calling for help. It was believed that N169PA slowed to allow the FH to catch up, hovering just inside the Grand Wash Cliffs valley. The experienced pilot had no reason to set N169PA down on the 40-degree slope, the pitch of which would guarantee the helicopter’s main or tail rotor strike the valley wall.

Reconstructing the accident

With no reliable witnesses, the accident had to be reconstructed. A powerplant investigator, a structural investigator and I – the aircraft maintenance investigator – were asked to assist from DC. Almost two weeks passed before all the maintenance records arrived in DC. During this time what was left of the engine was torn down and the aircraft’s structure examined. However, too much time had passed – and this is important – before the fuel depot where N169PA refueled, was tested for contamination.

The engine was found to have operated correctly in all parameters. Despite the engine’s post crash condition, all settings were correct; all air and flame necessary for combustion were properly regulated and available. Unfortunately fuel from the engine and filters was consumed by the fire or was too contaminated from the surrounding terrain for decisive findings of impurities.

As the maintenance investigator, I looked into N169PA’s maintenance as far back as its construction in May 1991. After one month of inspections and modifications, N169PA entered service for Papillon on August 2, 2001. All maintenance was performed according to manufacturer’s recommended inspection programs. The 10-year-old helicopter was employed strictly as an air tour transport – no lifting or special functions, e.g. firefighting. It had no unusual history, except for a catastrophic event the year prior.

Ground Resonance

Its previous owner was a Japanese company that used N169PA (not its tail number at the time) for moving executives about the city. On one occasion, the helicopter experienced a ground resonance while landing; the event was so violent that the helicopter suffered extensive damage to the airframe.

Here my lack of helicopter experience worked against me as I focused on this event. Ground resonance occurs when a helicopter with rigid landing gear, e.g. skids, either attempts to touch down or rise without breaking ground contact; a quasi-flight/non-flight mode. The helicopter’s rotorhead is equipped with drag hinges, each blade oscillating in the plane of rotation at its own harmonic. When this type of helicopter contacts the ground in this fashion, the resulting vibrations have the potential to destroy the aircraft. My friend, a former Vietnam Huey driver, told me the helicopter experiences this event during any flight cycle, but the time in ground resonance is momentary, almost too short to matter. However, if the ground resonance is extended, the vibrations from the main rotor translate through the hull, receiving feedback from the ground, which, itself, has no vibration.

N169PA was written off by the company’s insurance agency before being sold to a New Zealand company where it was repaired; it lacked any export certificate of airworthiness (C of A) from Japan to New Zealand. Major repairs were conducted in New Zealand, including the replacement of the left and right center beams; in addition to a complete dismantling and rework, a “C” check was completed four months prior to the accident. The aircraft was sold, crated up, and shipped to Papillon in June 2001. N169PA operated eight days in August with no abnormal issues.

Complete Powerplant Teardown

A complete powerplant teardown and structural examination produced no findings, eliminating the ground resonance question. Unfortunately in the absence of credible findings, the NTSB too quickly assigns ‘Pilot Error’ as the probable cause; this also misdirects from more involved investigating. The pilot was experienced, his skills admired by his fellow pilots. In the absence of better proof, ground effect was suggested. According to Paul Cantrell at www.copters.com, ground effect, “is due to the interference of the surface with the airflow pattern of the rotor system, and it is more pronounced the nearer the ground is approached.”

But would an experienced pilot allow himself to enter ground effect with no escape? Would ground effect occur against a 40-degree sloping wall? Wouldn’t there be obvious blade strikes from main and/or tail rotors against the sloping wall from such an event?

More importantly, in the absence of a quality fuel sample was fuel too easily dismissed?

Misdirection. Complete facts and reliable analysis are vital to accident investigation. A maintenance investigator unfamiliar with helicopters; fuel evidence not preserved; and the push to put an accident investigation ‘to bed’; these all amount to probable causes and recommendations that are nothing more than sleight of hand.