An Air Canada flight carrying 61 passengers suddenly began jolting and swerving mid-air after the pilot suffered an 'incapacitating' medical emergency in the cockpit. 

Flight 7664, headed to Nova Scotia from New Jersey, was forced to make an abrupt diversion to Massachusetts on Wednesday afternoon.

The co-pilot was forced to take control of the aircraft, safely landing it at Boston Logan International Airport shortly afterward.

'When the plane swerved, I knew something was wrong. It was not turbulence,' passenger Rodney McDonald told WCVB News.

'It really felt like someone jolted the controls, and it happened over and over again.'

The flight, a De Havilland Dash 8-400 operated by regional partner PAL Airlines, took off from Newark Liberty International Airport just before 12.40pm on Wednesday.

About an hour into the flight to Halifax Stanfield International Airport, at around 1.37pm, Massachusetts State Police received a call that airport officials had been alerted to a severe medical episode suffered by the pilot mid-air. 

Authorities said Logan Airport received a harrowing alert that the plane would immediately divert to Boston, with control in the hands of the first officer. 

'Within a few moments one of the flight attendants went into the cockpit frantically, and a few moments later he was dragging one of the pilots out of the cockpit,' McDonald, who was traveling with his wife and two sons, told the outlet.

'You know, every thought goes through your mind, you start praying,' he added. 'My boys instantly started praying.' 

The father-of-two said it appeared the pilot was suffering a seizure, as he and four other passengers helped restrain him while the co-pilot jumped into action at the controls. 

'It was really horrifying,' McDonald told ABC News. 'I was sitting in the front row, my family was further back, just the way the seat arrangement worked out.'

'I went back to sit with them and then realized that the pilot was out of control physically, not violently, like it was clear that he was not in control of his faculties and needed to be restrained,' he added.

'We worked to get him under control, it was a fairly strenuous 40 minutes of keeping him down and using as many seatbelts as we could to restrain his legs, arms and chest.'

A registered nurse on board helped direct passengers and assist during the mid-air emergency - another hero McDonald pointed out alongside the flight attendants. 

'The flight attendants were stupendous,' he said. 'They stayed calm.'

The co-pilot successfully landed the plane at around 2pm, where swarms of ambulances, Massport Fire Rescue and Boston EMS officials swarmed the runway and 'responded to the crew as necessary.'

Dramatic photos captured a stretcher being rushed to the aircraft before the pilot was seen being brought from the plane and quickly taken to a waiting ambulance within just minutes of touching down.

Passengers said that the captain remained incoherent until the emergency landing, when he appeared to begin coming out of the medical episode, according to WABC News. 

The pilot, who remains unnamed, was taken to Massachusetts General Hospital for treatment, though the details of his condition and what unfolded in the cockpit also remain unclear. 

It is important to note, however, that someone who is potentially experiencing a seizure should never be held down due to the risk of causing further injury, according to the CDC.

The surrounding area should instead be cleared of sharp or dangerous objects, and the victim should be turned gently onto their side to allow the airway to remain clear.

Once the captain was removed, the Twin-Turboprop was towed to a gate in Terminal B where passengers were able to disembark, according to WCVB.

Around 7.30pm later that night, the flight ultimately left Logan Airport and landed safely in Halifax, Canada.   

'We're just grateful, grateful for how it all ended,' McDonald told ABC.

In a statement, Air Canada said: 'During the flight, the captain experienced a medical issue and was removed from the flight deck as per safety protocols. The first officer took control of the aircraft and diverted the flight to Boston, where it landed safely.'

Aviation expert Patrick Smith explained that this horror situation is the exact reason why every plane has more than one pilot, according to CBS News. 

'A lot of people have this idea that a co-pilot is kind of an apprentice or there to lend a helping hand. That's not really how it works. Both pilots are fully qualified and can operate the airplane,' Smith told the outlet.

'To do so with one of the two pilots incapacitated would have increased the workload for another pilot,' he added.

Former Massport CEO Tom Kinton echoed the same statement to WCVB, explaining that co-pilots are more than ready to step in during an unexpected emergency.

'I'm sure the passengers were alerted to this on board. I’m sure the co-pilot had to take command of the aircraft - he’s fully capable to do that,' he said.

'They don’t typically have as many hours as the left seat, but that doesn’t mean they’re not qualified to fly the plane.'