Shaykh Bin Baz was asked the following question:

متى يجوز قصر الصلاة في السفر، أي: بعد كم من الكيلو مترات يتم ذلك، وهل هو الحال واحد سواء كان السفر في السيارة أو في الطائرة أو في البحر؟

Question:

When is it permissible to shorten the prayers when travelling?  Meaning: after how many kilometres would that be established in, and is it the same case whether the journey be by car or by plane or by sea?

نعم المسافر يقصر، السنة للمسافر القصر سواء من طريق البر أو من طريق الجو أو من طريق البحر، ومتى غادر البلد شرع في القصر، إذا غادر بلده بناء البلد تجاوز البناء صلى ثنتين خارج البلد سواء كان في المطار أو غيره، إن كان المطار خارج البلد أو خرج في سيارته خارج البلد أو انتقل من البلد إلى السفينة، أو الباخرة يصلي ثنتين ما دام في السفر،…

Answer:

Yes, the traveler should shorten his prayers. The Sunnah for the traveler is that he shortens his prayers whether he is travelling by land or by air or by sea, and whenever he departs from the town, he should begin shortening his prayers. Meaning: once he goes beyond the city borders, he should pray two units when he is outside of the city area, whether he is in the airport or somewhere else. (Notice however that this is the case) if the airport is not located within the city borders or if he were to leave with his car outside of the city area.

Reference

On a Side Note: This is also the fatwa of Shaykh al-Uthaymeen and others that if the airport is a local airport, then he should not begin shortening his prayers up until he makes his departure from the town.