Construction work was not finished on time by the airport's owner, BAA, which meant IT testing and staff training were "compromised" before the opening on 27 March, Walsh said at the Transport Select Committee on Wednesday. Crucial IT errors were also missed.
"If I were to pick one issue I would have done differently, it is that, having recognised the importance of testing and having designed six months of testing, we subsequently compromised on that," said Walsh.
The problems included a software filter being left on the BAA baggage system. It was used to restrict baggage-system data to the BAA testing operation, and ensure it was not sent to other baggage systems. However, it was not taken off until 31 March, when BA IT staff discovered it. It interfered with messages coming into the system and meant a number of bags were not recognised.
In addition to this, the servers could not cope with the number of messages the baggage system generated. Walsh said the amount of data the servers had to cope with was greater than expected.
He said the airline was aware of some of the problems before the opening, but took a "calculated risk" and decided to go ahead.
BA lost £16m in the first five days of operation, but Walsh said losses would have been "many times that" if the opening had been postponed. More than 500 flights were cancelled and 23,000 bags lost as a result of the problems. All but 125 of these bags have been reunited with their owners.
In addition to the IT problems, there were issues with staff parking, with getting staff through security because they didn’t know which security gates were open, and with air bridge jetties. On opening, 28 lifts were not working - this is now down to 17.
MPs called the terminal opening a "national humiliation" and accused BAA's chief executive, Colin Matthews, and non-executive chair of the board, Nigel Rudd, of complacency when they could not answer several of their questions.
Matthews said he had spent his time trying to fix problems, rather than apportioning blame, but the BAA officials were told they would have to investigate further and provide written answers, or return to another evidence session.