What happened
On 29 April 2011, an Airbus A321-211, registration G-NIKO, departed Manchester Airport for Heraklion, Crete. During the takeoff roll, the pilot flying noticed that the sidestick controls felt unusually heavy. As the aircraft rotated and lifted off, the Lowest Selectable Speed (VLS) indication on the Primary Flight Display shifted significantly up the speed scale. To prevent a potential tailstrike or loss of control, the pilot reduced the pitch attitude, allowing the aircraft to accelerate and resume a normal climb. During the subsequent cruise phase, the crew discovered that the takeoff performance calculations had been performed using the Zero Fuel Mass (ZFM) rather than the Actual Take Off Mass (ATOM), resulting in a takeoff with insufficient thrust and speed.
The investigation
The investigation established that the error originated during the pre-departure phase. When the co-pilot requested the takeoff weight to begin independent calculations, the commander mistakenly read out the ZFM of 69,638 kg instead of the ATOM. This incorrect figure was recorded on the navigation log. While the crew performed their required crosschecks, they compared the performance figures between their two laptop computers but failed to crosscheck the FMS-generated green dot speed against the laptop-generated speed.
Furthermore, a Last Minute Change (LMC) involving an additional passenger and baggage was added to the loadsheet after the initial calculations were completed. Because the FMS requires the entry of the ZFM to compute performance, the error was propagated into the aircraft's flight management system. The investigation found that the ATOM was actually 17,000 kg heavier than the figure used for the calculations, which directly impacted the required thrust and reference speeds.
Findings
- The commander misread the Zero Fuel Mass as the Actual Take Off Mass when responding to a request for the takeoff weight.
- The takeoff performance calculations were based on an incorrect mass, leading to the use of lower than required reference speeds and reduced thrust.
- The crew failed to identify the discrepancy because the crosscheck of the green dot speed between the laptop and the FMS was not performed.
- A late addition to the loadsheet (LMC) was not accounted for in a recalculation of takeoff performance.
- The use of the ZFM instead of the ATOM in the FMS prevented the system from computing the correct required speeds and thrust settings.