UK investigators have determined that a LOT Boeing 737-800 took off from London Heathrow with a weight error close to 1t, after a consignment of mail was inadvertently discarded from loading records.

The mail was initially recorded twice in the load management system while the aircraft (SP-LWA) was being prepared for flight on 20 May.

“It was not determined why this had occurred,” says the UK Air Accidents Investigation Branch, but the handling agent had advised that such glitches were normally resolved by the dispatcher. The dispatcher initially deleted the duplicate entry to produce a correct loading instruction report.

But the cargo company subsequently sent an electronic update which also corrected the entry, and this resulted in the consignment of mail, amounting to 953kg, being removed entirely from the system.

The dispatcher did not notice that the mail consignment was missing from the load sheet, even though it had been loaded on the aircraft.

As a result the load sheet presented to the captain indicated the aircraft was lighter than it was.

Although the captain had noticed the stabiliser trim was close to limits, and asked the dispatcher to check the load sheet, he accepted a final load sheet from the dispatcher and used it to calculate take-off performance.

The aircraft departed without incident but, once airborne, the crew received a new load sheet transmitted through an ACARS message showing an increase in zero-fuel weight of 953kg and a 3% change of trim with forward centre of gravity.

Investigators state that the handling agent has reminded its dispatchers of the importance of checking that the load sheet reflects the actual loading of the aircraft.

“It also recognised that its dispatchers work with many operators who each use slightly different IT systems and that it can be challenging for dispatchers to remember the subtleties of each system,” the inquiry says, adding that dispatcher work patterns have been changed to give personnel experience in various IT systems.