Here's why:
* Spelling: The "a" in "plane" is followed by a consonant ("n") and then a silent "e" at the end of the word. This pattern often indicates a long vowel sound.
* Pronunciation: The "a" in "plane" is pronounced like the "a" in "say" or "name," which are long vowel sounds.
Let me know if you have any other vowel sound questions!