我坐飞机去西安

Use subject + verb + 了 (le) + 吗 (ma) to ask whether an event has happened by now. This is not a general past-tense ending: the question checks whether the event occurred or reached its result. In a positive answer, repeat the verb and add 了 (le). In a negative answer, put 没有 (méiyǒu) before the verb and leave out 了 (le). When the verb is already clear from the question, 还没有 (hái méiyǒu) is a complete short answer meaning ‘not yet.’

Chinese can place connected verb phrases one after another when the same person performs the whole sequence. To say the travel method, use 坐 (zuò) + transport + 去 (qù) + destination. To add the purpose, use 去 (qù) + place + action + object, as in 我去西安饭店接她 (wǒ qù Xī’ān Fàndiàn jiē tā): first I go to the hotel, and the purpose is to pick her up. These are connected route-and-purpose chains, not unrelated actions separated by a comma.