Qantas B738 at Sydney on Aug 1st 2014, data input errors result in tail scrape
Last Update: November 16, 2015 / 14:58:59 GMT/Zulu time
The ATSB released their final report concluding the probable causes of the incident were:
- During the take-off, the aircraft was rotated at a speed about 10 kt lower than required for the aircraft's actual weight, which was sufficient to overpitch the aircraft, resulting in a tailstrike.
- During the calculation of the take-off weight, both the captain and the first officer made independent inadvertent errors in their calculations, which resulted in the same, but incorrect, weight figure being used to calculate the take-off speeds.
- The Flight Crew Operating Manual procedure for crew comparison of the calculated Vref40 speed, while designed to assist in identifying a data entry error, could be misinterpreted, thereby negating the effectiveness of the check. [Safety issue]
The ATSB reported that following the landing and discovery of the paint scraped off the tail skid assembly the captain phoned the airline's duty captain and was asked during that phone call to check the takeoff figures used. The first officer (more than 10,000 hours total, more than 7000 hours on type), pilot monitoring, then discovered that they had entered performance data indicated 10 tons less than correct takeoff mass, 66,400kg of takeoff mass had been used to compute the takeoff performance rather than the correct 76,400kg, which resulted in a Vr of 9 knots less than needed, 146 KIAS instead of 155 KIAS.
The ATSB reported the crew had used the company provided iPad to compute the takeoff performance, the crew had about 4 years of experience with this Onboard Performance Tool (OPT). Both flgiht crew members are required to enter their data independently and compare the results with 1 knot difference in the resulting speed permitted. In this particular occurrence the captain inadvertently dropped the leading 1 from the fuel figure resulting in an incorrect takeoff mass of 66,400kg instead of 76,400kg. The first officer on the other hand had computed the takeoff mass correctly at 76,400kg but when transferring the takeoff mass to the OPT made a transposition error and keyed in 66,400kg. The results matched, hence the errors were not detected.
The ATSB analysed that a further line of defense against such errors was missed when the crew compared the OPT computed performance figures with the FMC computed figures. While the OPT computed a Vref40 of 139 KIAS, the FMC had computed the correct figure of 149 KIAS. The flight crew operations manual required the values to match within +/- 1 knot. During the testimony the captain, as well as the first officer, told the ATSB that as result of this procedure it was quite normal to compare the last digit of those figures only, and that last digit did match. The first officer was surprised to find out that a 10 ton mismatch in takeoff mass resulted in a 10 knot difference of the Vref40 speed.
The ATSB wrote: "This focus on the last digit, combined with an expectation that any error would be apparent in the last digit, reduced the effectiveness of this check."
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This article is published under license from Avherald.com. © of text by Avherald.com.
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