Is it permissible for a mature woman to go for Umrah without a Mahram, if she is accompanied by trustworthy women and it is an obligatory Umrah by plane?

The fundamental principle is that a woman is not permitted to travel except with a mahram (definition) or her husband. An exception to this is travel for an obligatory act of worship, such as the obligatory Hajj or Umrah (definition). In such cases, she may travel with a trusted group if she cannot find a mahram or her husband. This is the view of the Maliki school, one opinion among the Shafi'is, and the choice of Shaykh al-Islam Ibn Taymiyyah. The evidence for this is what is stated in Mawahib al-Jalil by Al-Hattab al-Maliki, quoting from Al-Muwatta' by Imam Malik. Shaykh al-Islam Ibn Taymiyyah also holds that a woman may travel for any act of obedience.