RMT offshore organiser Jake Molloy said the group would go through "every aspect of it".
"It's fantastic news that everyone involved in this incident is safe and well, but nevertheless we've had a very serious incident and we need to know why that happened," Mr Molloy said after the accident.
A spokesman for Eurocopter, the French company which manufactures the Super Puma EC225, said it was investigating the incident with the operator and authorities "with the highest priority".
A spokesman for CHC said: "We are delighted that everyone is back in Aberdeen and that no one was injured. Following a detailed debrief with the crew and technical experts, CHC is continuing to delay all scheduled commercial flights on Super Puma EC225 aircraft.
"An investigation team from the Air Accidents Investigation Branch, with CHC's full participation, has begun an inquiry into the cause of the incident."