S8·E2 Percy's New Whistle

Percy's loud whistle blowing to show that he can make as loud a sound as 'Arry & Bert's horns distracts some of the others and is told by the Fat Controller to only whistle at appropriate times. However, Percy isn't sure to whistle to warn Thomas from crashing some time later. Will he let his friend crash or will he make the right decision by blowing a warning whistle?