Triggered Lightning: Lost of Control & Tail Rotor Blades

Onboard:

The helicopter’s autopilot, flight director, and all 4 electronic flight instrument system (EFIS) displays momentarily turned off. All EFIS displays quickly turned back on; however, only the 2 displays on the left contained valid flight and navigation information. The autopilot and flight director did not automatically re-engage.

As a result, the FO, seated to the left, initially continued controlling the helicopter while still in instrument meteorological conditions (IMC) without SAS or ATT modes (see below) to assist. 

All electronic displays recovered quickly, but the air data computer (ADC) for the right side of the cockpit had failed and the related flight information was no longer valid on those displays. 

However, for reasons unspecified by TSB:

The captain took control of the helicopter 21 seconds after the lightning strike, at which point the helicopter’s altitude had increased by approximately 100 feet, the left roll had increased by 10°, its heading was 20° further east than before the lightning strike, and its ground speed had decreased by 14 knots.

At 0930:21, the helicopter entered an uncontrolled rapid descent from 4029 feet to 885 feet ASL in 36 seconds, reaching a maximum of 63° left roll and 44° pitch down.

As the helicopter emerged from the clouds, the captain regained visual reference to the ground and was able to reduce the helicopter’s roll angle and stop the descent. The vertical acceleration peaked at 2.48g before the helicopter climbed back up to 2073 feet ASL.

The helicopter descended to approximately 1300 feet ASL.

The flight crew attempted to reactivate the autopilots during the rapid descent, but both autopilots were not engaged until approximately 3.5 minutes following the lightning strike after the helicopter had already regained level flight.

…the flight crew evaluated the helicopter systems in level flight and did not detect any significant defects that would preclude safe flight to the intended destination. 

The helicopter landed safely at 09:44. 

On inspection it was found that one “tail rotor blade assembly” (two blades) had separated from the helicopter, striking three main rotor blades, the left horizontal stabilizer, the left side of the tail boom, and the left engine cowl.

S-76C++ tail rotor blade assembly exploded diagram that identifies each tail rotor blade assembly with parts of the pitch control system, the tail gear box output flange, and the outboard retaining plate (Credit: Sikorsky SA 4047-76C-4 IPC, with TSB annotations)

The…

…blue main rotor blade had burn marks on both sides of the tip cap that exposed the underlying Kevlar skin. The tip cap’s leading-edge abrasion strip had buckled slightly on the upper and lower aft edge as well as on the lower aft edge at the overlapping outboard seam, which is consistent with lightning attachment. The tip cap attachment doubler had buckled on the upper and lower skins.

Top side of the blue main rotor blade tip cap with damage from lightning attachment (Credit: TSB)

The blue main rotor blade damper, pitch control rod, and bonding jumper exhibited damage from the lightning strike. The damper inboard and outboard spherical bearings had minor pitting, flaking, and discolouration. The pitch control rod upper and lower spherical bearings and bearing races had significant discolouration with melted material over a portion of the bearing and bearing races.

Blue main rotor blade pitch control rod with the spherical bearing damage highlighted (Credit: TSB)

TSB Safety Investigation

Helicopter-induced Lightning aka Triggered Lightning

Triggered lightning is a phenomenon in which the helicopter itself triggers a lightning strike, often in areas where there is little natural lightning activity.  In flight aircraft, acquire a negative charge by the frictional contact with the air. Helicopter rotor blades generate the greatest concentration of negative charge.

When the helicopter encounters a positively charged region of a cloud and the potential difference is great enough between the opposing charges, the helicopter can trigger a positive lightning strike.

The UK Met Office has identified that such lightning strikes occur when polar air masses move over a warmer sea surface and especially when:

• Flight level static air temperature from −2 °C to −6 °C;
• Freezing level from 1000 feet to 3000 feet ASL; and
• Precipitation rates above 4 mm/h.

The temperature where the lightning strike occurred was estimated to be from −2 °C to −6 °C at 3900 feet ASL. [Radar] data indicated precipitation rates of 5 mm to 11 mm/h…at 09:30.

While accepting that the conditions were conducive of triggered lightning, the TSB claim that such conditions…

…are not readily identifiable with current weather assessment methods.

In fact the UK Met Office has been researching this subject many years (see CAA Paper 2000/02 and this 2013 presentation to the UK CAA HMRSC), leading to the implementation of triggered lightning forecasts.  While it is still difficult to predict reliably, at best the TSB statement should have been clarified as “not readily identifiable with current weather assessment methods employed in Canada“. 

Design for Lightning Strike Protection

The tail rotor blades…

…use the aluminum wire fabric in the blade skin and tip cap to allow the lightning strike charge to transfer to the pitch horn and tail gear box output flange and onwards to the tail gear box and fuselage.

To prevent damage to the graphite spar, the fabric remains separated from the tail rotor blade fasteners that attach the spar to the blade and an insulating cover is applied over the blade fasteners.

S-76 tail rotor blade showing the conductive path from the tip cap to the tail gear box output flange using the combination of the aluminum wire fabric, pitch horn, and graphite spar (Credit: Sikorsky, Lightning Protection for the S-76, with TSB annotations)

The inboard section of the spar is also protected with an aluminum wire fabric coating that transfers any flashover from the blade root to the output flange.

At the time the S-76 was designed the FAA had no requirements for lightning strike protection (those followed in 1984).  Sikorsky did at their own initiative apply their own requirements.  For the tail rotor they conducted lightning tests on three test articles (each “a single tail rotor blade and spar that was attached to an off-aircraft tail gear box output flange, outboard retaining plate, pitch change beam, and a pitch control rod”).  They applied the 200 kA peak current amplitude specification from the US military (MIL-B-5087B) and the Society of Automotive Engineers (SAE). After that they…

…conducted static load tests and analyzed the aerodynamic effects on the blade assembly that had sustained the most damage. It was concluded that the assembly can survive a severe lightning strike and maintain operational capability.

The characteristics of the strike detected by CLDN exceeded the specification voluntarily applied during the design of the S-76.

The AFCS

The helicopter’s digital automatic flight control system [AFCS] consists of 2 independent flight control computers that each contain an autopilot and a flight director function.  When turned on, the autopilot has a stability augmentation system (SAS) mode and an attitude retention (ATT) mode.

In normal operations, both autopilots are on, but only 1 flight director can operate at a time; while 1 actively steers the aircraft, the 2nd is in standby mode. Flight director operation is inhibited if only 1 autopilot is on.

According to Helijet SOPs, the flight director is normally coupled during the departure, climb, cruise, descent, and approach phases of flight, whereas the helicopter is hand flown during take off or landing, or anytime the pilot flying would prefer to hand fly.

The EFIS

The helicopter had an integrated EFIS that consisted of electronic displays including an electronic attitude director indicator (EADI) and an electronic horizontal situation indicator (EHSI). 

S-76C++ instrument panel highlighting how the smaller size and central location of the standby attitude indicator differ from the larger electronic attitude direction indicator (EADI) and electronic horizontal situation indicator (EHSI) displays that are directly in front of the flight crew (Credit: TSB)

If there is a complete failure of all electronic displays, the helicopter has a standby attitude indicator, a standby airspeed indicator, a standby altimeter, and a standby magnetic compass.