I just tried to Transwarp from the Sol system to Eta Eridani using my Advanced Heavy Cruiser Retrofit and once the countdown hit 1 the nacelles started sparking with energy discharges and the ship made a sound similar to the original Excelsior in Star Trek 3 and my transwarp failed. I still got a cooldown but no transwarp jump.
I believe it's a 1% chance of failure. I've been flying the Excelsior for months, and I've had one failed transwarp jump the entire time. It's funny when it happens though, apart from having to now fly all the way across the galaxy. XD
Vice Admiral Laurence Corrin - USS Trafalgar
Captain Takaram ch'Rihan - USS Khellian
Captain Korax joH'a' vo'poH - USS Hartnell
General Hothkar, Son of Ta'lon - IKS oDwI'