On August 1 last year, a Boeing 737-838 (VH-VZR) operated by Qantas performed a "tailstrike" while taking off from Sydney airport. Today, the Australian Transport Safety Bureau (ATSB) has concluded that the strike was caused by the co-pilot fat-fingering the plane's takeoff weight: instead of typing the plane's actual weight of 76,400kg into the iPad, he accidentally typed 66,400kg. As a result, the plane didn't have quite enough thrust to clear the runway without clipping the ground with its tail.

Before you get too panicky, don't worry: this was still very much a human error. The iPad wasn't directly hooked up to the plane; the tailstrike wasn't caused by some software glitch in iOS.

According to the ATSB investigation (PDF), two mishaps occurred "independently and inadvertently." First, when working out the plane's takeoff weight on a notepad, the captain forgot to carry the "1," resulting in an erroneous weight of 66,400kg rather than 76,400kg. Second, the co-pilot made a "transposition error" when carrying out the same calculation on the Qantas on-board performance tool (OPT)—an iPad app for calculating takeoff speed, amongst other things. "Transposition error" is an investigatory euphemism for "he accidentally hit 6 on the keyboard rather than 7."

The co-pilot also entered the wrong temperature into the OPT—51°C rather than 35°C—but the investigation doesn't discuss why that happened, instead focusing on the takeoff weight issue.





After entering the weight of the plane and temperature, the OPT calculates the necessary engine thrust and takeoff speed to get safely off the ground, which are in turn entered into the plane's flight management computers (FMC). For a weight of 76,400kg and temperature of 35°C, the engine thrust should've been set at 93.1 percent with a takeoff speed of 157 knots; instead, due to the errors, the thrust was set to 88.4 percent and takeoff speed was 146 knots.

Curiously, neither the pilot or co-pilot realised that anything was amiss with the final figures that were input into the FMC, despite both having over 10,000 hours of flying experience. The ATSB investigation said that this was probably the result of the OPT factoring in so many variables that it's hard for the flight crew to develop a "rule of thumb" for each plane's takeoff speed and thrust—i.e., if the numbers produced by the OPT vary dramatically every time it's used, it's hard for the flight crew to know when they're wrong.

Fortunately, in this case, the tailstrike was very minor: a cabin crew member at the back of the plane reported hearing a "squeak" during takeoff. The flight crew, suspecting a tailstrike, checked the various sensors and contacted mission control, but found no indication of a tailstrike. Later, when the plane landed, the captain checked the 737's tail: some paint had been scraped off, but that was it.

Tailstrikes can be very dangerous, though: way back in 1985, 520 people died when a Boeing 747 (Japan Airlines flight 123) suffered explosive decompression due to a previous tailstrike incident that hadn't been correctly repaired.

Following the incident last year, Qantas has since tweaked its launch procedures to ensure the same gaffe doesn't happen again. Now, after the pilot and co-pilot do their separate calculations, they'll also double-check the final takeoff speed against the plane's reference manual.