Incorrect weight calculations have again been cited by investigators examining a laboured take-off run, this time involving a Virgin Atlantic Airbus A340-600 at London Heathrow last December.

Instead of entering a weight of 322.5t in a request for take-off performance data, the crew used the much-lower expected landing weight of 236t.

In a serious-incident inquiry report, the UK's Air Accidents Investigation Branch states that the crew, pushed for time, skipped the normal procedure which required them to estimate the take-off weight, and use this to crosscheck actual data.

It also points out that the crew flew the smaller A340-300, and that the lower weight figure would have been acceptable as a take-off weight for this aircraft type.

"The operator believed that time pressure on the crew was likely to have contributed to the events in this incident," says the AAIB.

When the A340-600 carried out the take-off run its calculated rotation speed, 143kt, was 14kt below the correct value. Despite an unusually high 'flex' temperature - corresponding to a low take-off thrust requirement - the crew did not check the take-off calculation.

 Virgin A340-600
 © Adrian Pingstone

The subsequent rotation was "slightly sluggish" and the flying pilot lowered the nose to gain speed for the climb-out, noting that the climb rate was low. Only later in the climb did the crew realise the calculation error.

Nine independent crosschecks failed to prevent the incident, and the AAIB says that adding further crosschecks to the carrier's "robust" procedures would probably "complicate" them with "no guarantee" of preventing another occurrence.

Instead it has endorsed its previous backing for monitoring systems which would warn crews should take-off performance be inadequate for the aircraft's configuration and the local conditions. The AAIB highlights recent similar incidents involving a Thomas Cook Airlines A330 and a Thomson Airways Boeing 767.

But the AAIB also acknowledges that a technological solution is unlikely to be available for a "considerable time" and points out the benefits of Airbus' electronic flight-bag concept - not used by Virgin at the time of the A340 incident - in calculating take-off performance.

Source: Air Transport Intelligence news