Jim Martin, 49, and Jon Ker, 37, crew members of the Great North Air Ambulance, were taken to Newcastle General Hospital after their light aircraft smashed into trees at Bywell Farm, north of Morpeth, just after noon.
The men, who had taken off from Longframlington, had been heading to Eshott Airfield when the incident happened.
The craft fell into a ravine in the wooded area but the aircraft's landing had been cushioned by trees between a ploughed field and Burgham Golf Course.
Several golf club members witnessed the incident and reported that they heard the engine stall and saw something fall from the aircraft. They contacted emergency services, who were on the scene within minutes.
The men were cut free from the wreckage before being taken to Newcastle General Hospital, one by police helicopter, the other by air ambulance.
It is believed one of the men was being transferred to Newcastle RVI yesterday.
The crash scene was cleared on Monday by air crash investigators and police said a piece of the tail was found in a field a few hundred yards away from the rest of the wreckage.
It is currently unclear how the aircraft came down.
The plane, a Banbi, is a fairly new model and it is believed that it had no history of problems, nor did the pilot report anything wrong before the crash.
The Air Accidents Investigation Branch has launched an inquiry but investigations are expected to take a couple of months.
Spokesman David Stewart said: "We have launched an investigation. There is a team of two people working on it. A report will be published in due course but I expect that the inquiry will take a couple of months.
"We will look at everything which could have caused the crash.
"We will look at the wreckage, weather conditions and talk to the survivors if possible."
A spokesman for Newcastle General confirmed yesterday that both men were in critical condition.
Sarah Booth, of the Great North Air Ambulance, said: "Our thoughts are with the families at this very difficult time.
"They were taken to Newcastle General Hospital and have received the best possible treatment."
Despite the crash, Sarah added: "Service will not be affected in any way, shape or form and it will still be running as per usual."