But yesterday, Tan was charged with one count of voluntarily causing hurt over the same case.
Tan was sombre when the charge was read to her.
Her husband is Mr Wong Ngit Liong, 65, the chairman and chief executive officer of Venture Corp, a listed company.
Mr Wong, one of the richest men in Singapore, was not in court.
Why was Tan charged even though the civil suit had been settled?
Lawyers said the reason is that civil cases are separate from criminal ones.
Mr Sunil Sudheesan, from law firm KhattarWong, said: 'A civil suit can be settled out of court, but it has no bearing on the criminal aspect of the case.
'A person can still be charged if it involves a criminal offence.'
Cases of voluntarily causing hurt involve less serious assault. The victim is required to make a magistrate's complaint first.
This is because a magistrate's authorisation is needed before the police can make an arrest or exercise full powers of investigation.
It is not known if Miss Then made a magistrate's complaint.
But Mr Sunil said that where road rage and air rage cases are concerned, the authorities take a stern view because there is significant public interest.
It is not necessary for the victim to file such a complaint because the police can apply for an order to investigate and make arrests.
The law also allows certain offences - such as voluntarily causing hurt, outrage of modesty and wrongful restraint - to be compounded after a person has been charged.
This is the equivalent of the accused being acquitted of the charge.
Mr Sunil said: 'Either the victim or prosecution can ask for the charge to be compounded, but the judge is the one with the final say.
'It may also involve the accused paying compensation and apologising to the victim.'
If the judge grants permission for the matter to be compounded, the accused will be given a discharge amounting to an acquittal. This means that he will not have a criminal record.
EMOTIONAL AND MENTAL DISTRESS
Tan and her husband were in business class when the alleged incident took place at around 3pm.
Miss Then was serving passengers in business class at that time. Details of why she was allegedly slapped are not known.
The flight touched down at Narita International Airport in Tokyo 80 minutes later, but the Wongs did not get on their subsequent connecting flight that was bound for Los Angeles.
Miss Then is also understood to have missed that flight. She stayed on in Tokyo and lodged a police report. She filed another police report when she returned to Singapore.
The flight attendant, who has been with SIA for two years, also took a few days off work to rest.
Miss Then filed a civil suit against Tan when Tan did not accede to her requests for compensation and a statement of apology.
The stewardess sought unspecified damages for 'emotional and mental distress' as a result of the 'wrongful assault and battery', and for defamation.
While the details of the settlement were confidential, the ceiling for awards in a magistrate's hearing, where the case was filed, is $60,000.
Tan's lawyer, Mr Ravinderpal Singh, told the court yesterday that he has made representations to the Attorney-General's Chambers.
The prosecution then asked for the case to be adjourned for three weeks. It will be mentioned again on 3 Apr. If convicted, Tan can be jailed up to one year or fined up to $1,000 or both.