The "burning electrical smell" a passenger on a flight to Queenstown on Sunday reported after the plane was struck by lightning was likely to have been associated with air particles in the cabin burning or the strike itself, an aviation expert said.
The "sacrificial" static dischargers, or wicks, at an aircraft's extremities are "designed to melt and burn as the discharge goes through the plane", said David Reynolds, senior technical officer with the New Zealand Airline Pilots Association. However, as the dischargers are on the outside of the plane, passengers will not normally smell them inside the cabin.
Even if you have been flying for years, the smell - and "big bang" that precedes it when the lightning bolt hits - can come as a shock, he said.
"It's quite a show. There's a massive great bang almost like an explosion and a flash as the lightning goes through the aircraft. There's a mild ozoney smell as well... It's very disturbing for everybody."
Air New Zealand flight NZ621 from Auckland was one of two flights to Queenstown diverted on Sunday due to lightning. The other was Air New Zealand flight NZ605 from Wellington. Both were forced to make unscheduled landings in Christchurch.
Lightning strikes on aircraft are not unusual and modern airliners are built to handle them.
It has been many years since a lightning strike was implicated in a deadly crash by an airliner, and lessons learned in the past have been incorporated into the design of modern planes.
According to Reynolds, a large commercial aircraft is hit by lightning once or twice a year on average.
"It's quite a traumatic event for the aircraft as well as for the people on board," he said. "Up to a million volts pass through the aircraft, which is massive when you think most power lines have thousands of volts.
"The energy is dissipated through the aircraft, which is the secret these days. They used to explode because the fuel tanks were not protected."
On a modern aircraft, it is not possible for lightning currents to cause sparks in the fuel tanks and the fuselage. The body acts as a Faraday cage (a container that blocks electromagnetic fields).
Typically, lightning strikes one of the plane's outer extremities - such as the wingtip, nose or rudder - and the current exits via another extremity, such as the tail.
"An aircraft can withstand the million volts passing through it," said Reynolds. "There are metal strips between everything to make sure the electricity gets conducted. But there's always a little bit of damage, such as a burn mark or a little distortion of the metal."
While a lightning strike is "not a fatal blow" for an aircraft, it can knock out certain displays and systems. Planes will land as soon as possible as "a precautionary measure".
Most strikes occur following take-off or while the plane descends to a landing. For strikes that happen after take-off, planes usually return to the airfields they left from. However, as the two Air New Zealand flights on Sunday were closer to their destination, being Queenstown, they diverted to Christchurch Airport, which has more engineering facilities than Queenstown.
"[W]hile a lightning strike doesn't make an aircraft unflyable, it does set up a requirement to land as a precaution to make sure all systems are working ok," Reynolds explained, noting that some systems cannot be rebooted in the air.
WHAT LIGHTNING DOES TO A PLANE
One catalyst for research into lightning effects on aircraft was the crash of a Pan American Boeing 707 in Maryland US in 1963, which killed all 81 people on board. It was the last time lightning caused an airliner to crash in the US.
An investigation decided the likely cause of the crash was the lightning-induced ignition of the fuel/air mixture in a fuel tank. The crew lost control of the plane after a resulting explosion caused the left outer wing of the aircraft to disintegrate.
The aircraft had safety features available at the time but much less was known about the way lightning affected aircraft, the FAA said.
Another well-known crash occurred in the Peruvian jungle on Christmas Eve 1971. Of the 92 people on board the plane hit by lightning, only one survived - Juliane Koepcke, who was 17 at the time.
The Telegraph,London, which interviewed Koepcke in 2012 after she wrote a book about her ordeal, reported that a bolt of lightning hit one of the fuel tanks of the LANSA airline Lockheed Electra turboprop. The right wing of the plane was torn off and the aircraft went into a nosedive.
Koepcke, who was sitting in the window seat next to her mother, was suddenly falling through the air, still strapped to her seat. She lost consciousness then came to the next morning on the floor of the rainforest. Despite falling more than 3km, she was able to walk away with nothing more than concussion, a broken collarbone, a gash on her leg and a small cut on her arm.
According to Air & Space, the magazine of the Smithsonian Institution's National Air and Space Museum, air passengers today rarely know if their plane has been struck by lightning. This is because of the use of aluminium in a plane's hull, which mostly conducts the charge from lightning strikes from entry point to exit, the magazine wrote.
The use of composite materials in aircrafts, such as the Boeing 787, where the fuselage made predominantly of carbon fibre meant some additional design features were needed. This included putting some metal back into the fuselage for lightning protection.
The nose cone, which for decades had been made of composite material to avoid interference with the radar inside, had thin metal strips incorporated onto the surface to act as little lightning rods. This prevented lightning from puncturing the radome and damaging its electronics.
Conductive metals were used to bond lights to the wingtips, with the bonding protecting the lights by grounding them to the rest of the plane.
Skin around fuel tanks in the wings of the plane need to be thick enough to avoid a burn-through, and all joints and fasteners were tightly secured to prevent arcing or sparking in the airplane's fuel tanks.
Avionics and flight control systems had surge protection devices installed, while wiring throughout an plane was shielded. Redundant systems as a backup to primary flight control systems were added.
Conductive copper or aluminium meshes were incorporated into the hull of aircrafts with composite skins. The mesh spreads current to minimise damage to the skin when the plane is hit by lightning and keeps the current on the outside of the fuselage. The design helps to reduce voltages that might be induced inside the plane that could threaten the plane's electrical systems.
Live Science reported the US National Transportation Safety Board recorded 24 incidents caused by lightning strikes between 1962 and 2010, out of a total of 140,000 aviation accidents.
The 24 incidents included the 1963 Maryland crash, with majority of the incidents involving small private planes or helicopters, and in one case, a hot air balloon. Four of the crashes involved fatalities, with 11 people dying.
William Voss, a former FAA commissioner and former head of US aviation safety non-profit group Flight Safety Foundation, told CNN it was "pretty unlikely" lightning would cause a plane to crash nowadays.
"I can't say anything is impossible, but we certainly don't see that happening. It's pretty well down on our list of concerns, again because we have a lot of experience with this, and aircraft get hit by lightning every day," Voss said.
Most of the time lightning strikes dissipates. "Sometimes the lightning bolt is substantial enough that it will actually maybe punch a little hole in the skin [of the plane' as it goes out, but that's about all that it really does."