I have heard that jhânas are not necessary to achieve liberation.
However, arhants have removed bad thoughts (so I assume they have also removed the 5 obstacles to jhâna factors). Does this imply that even though jhânas are not necessary to be an arahant, it’s true that all arahants have the skill to attain jhanas? If so, it would seem to mean that a person who has never attained jhanic absorption, all of a sudden, by becoming an arhant, can attain jhanic absorption.
It is true that Arahants do not have nīvaranas (5 hindrances).
However, not all Arahants attain Jhāna (but amon Arahants most of them do easily attain Jhāna).
The reason is, Jhāna means Balavā Jhānanga, strong Jhāna factors, directed towards a conceptual object (sammuti āramanna), one of the tranquility meditation subjects (Samatha Kammatthāna) that bring Jhānas, namely, 10 Kasinas, 10 Asubha, 4 Brahmā Vihāras, 4 Arūpas, Ānapānassati, Kāyagatassati (32).
Arahatta Phala, Arahant Fruition, is attained through Vipassanā, starting from Delimitation of Mentaility and Materiality, Nāma Rūpa Pariccheda, up to Arahatta Phala.
The Indriyas (faculties) developed by Vipassanā are different because the workings on different Āramannas activate different mind functions (Kicca).
The five Jhānas factors when applied to Vipassanā are associated with Strong Wisdom.
The five Jhāna Factors when applied to Samatha are associated with strong Samādhi.
Therefore if any Arahant has strong Wisdom
and less Samādhi because of his past Pāramī and present experience, he may not be able attain Rūpavacara Jhāna.
How?
When applying His mind free from defilements to Samatha Kammatthāna, his Jhāna factors may not have the power to make Him attain Absorption (Appanā). So His mind may remain in Access Concentration (Upacāra Samādhi ).
However, such one may develop the mastery of Fruition Attainment, Phala Samāpatti, by directing his mind towards the Deathless or by doing Vipassanā and dwell in the Highest Jhāna, the Fruition- Attainment of Arahantship ( Arahatta Phala Samāpatti ).
This is how it could be explained according to Dhamma Vinaya.