Richard Westgate had a number of health issues but grew "angry, frustrated and disillusioned" when British medical professionals were unable to cure him.
The 43-year-old, who "lived for flying", then grounded himself from piloting planes when bosses refused to permanently sign him off.
He moved to the Netherlands where Dutch medical experts and scientists believed his claims, an inquest heard, and set about trying to cure him, according to Daily Mail.
He had suffered years of poor health including severe headaches, mental confusion, sight problems and insomnia before he died in December 2012.
His twin brother Guy Westgate, 47 and also a British Airways pilot, told Salisbury Coroner's Court that his brother was working with researchers to see if his claims could be true.
Prior to his death, Richard Westgate instructed lawyers to sue BA for health and safety breaches as he was convinced his problems were related to being exposed to toxic chemicals on board the planes he flew.
His claims over what caused them centred around warm air being pumped into jets from engines to provide a comfortable environment and chemicals in engine oil also being able enter cabins.
The aviation industry has argued there is no threat to passengers or crew.
The coroner today said he will not examine whether or not Mr Westgate was made unwell by toxic fumes he was allegedly exposed to in the course of flying BA's commercial planes.
Dr Simon Fox QC said the issue - known as aerotoxic syndrome - was not something the parties will address.
Opening the inquest, Dr Fox QC said he will not consider if, in the period leading up to his death, Mr Westgate was poisoned after "suffering from an exposure to organophosphates in the course of his employment as a commercial pilot".
He said: "That is not a proper issue to be examined by this inquest."
But Dr Fox QC will address other factors which may have caused Mr Westgate's death, including whether he intentionally took his own life by overdosing on pentobarbital - a group of drugs which helps slow the nervous system.
Another factor he will consider when determining how Mr Westgate died is whether his death was brought about by lymphocytic myocarditis - inflammation of the heart muscle.
The room today was packed with solicitors representing various interested parties, including other airline staff.
Mr Westgate, who lived in Marlborough, Wiltshire, was found dead at the Bastion Hotel in Bussum, the Netherlands, on December 12, 2012.
He had been on medical leave since September 2011 with his various conditions, for which he was seeking care.
Guy Westgate, who attended the inquest with his sister Pam Love and their mother Judith, today wept as he described the pain his brother was suffering.
He said: "He would describe the pain as if his brain was being sandpapered.
"He couldn't find any other way to describe the severity of the pain."
He said his brother, a keen and talented paraglider with over 20 years' experience as a commercial pilot, loved his job and got the bug while at university.
Mr Westgate said: "He was very passionate about flying. He lived for flying."
He described how the pair of them grew up to be competitive but his brother concealed his ill-health for a number of years until late 2011.
He battled anxiety with his health, struggled with complex thoughts while becoming ever more clumsy and irritable.
Mr Westgate told the inquest: "Richard believed he was unfit to work. He was trying to get signed off to be permanently unfit to work and kick start his insurance payouts.
"That whole process was stopped because of the current system. No one was prepared to give a diagnosis of what he had."
He went on: "During the latter half of 2011 he seemed to go from bad to worse. I would say he was debilitated by pain more frequently and the complaints of his symptoms increased in frequency and severity.
"Richard's personality was that he tried to conceal his illness but he no longer could.
"The Dutch team he found were the first people who believed in him. He had been let down by doctors, BA doctors, BALPA [British Airline Pilots Association] doctors.
"The specialists he found in Holland were the first group who finally gave him light at the end of the tunnel. He went there to be cured.
"He intended to come back as Richard Westgate. He was angry, frustrated, disillusioned. He couldn't believe he was let down by so many people.
"He couldn't believe the medical profession let him down so badly."
The inquest heard loss of licence insurance payouts were finally triggered as a result of a car crash he was in, and he was able to be signed off work.
But Guy Westgate said the explanation for this as the cause of his ill health was "convenient" as the reality was "inconvenient", and his symptoms predated the crash.
Mr Westgate added that he was sure his brother didn't commit suicide.
The inquest came about following a report to prevent future deaths issued by former senior coroner for Dorset Sheriff Stanhope Payne.
Sheriff Payne wrote to the Civil Aviation Authority and British Airways in February 2015 outlining his concerns that Mr Westgate may have been poisoned after "testing of samples taken both prior to and after death disclosed symptoms consistent with exposure to organophosphate compounds in aircraft cabin air".
He said the matters of concern were the "consequential damage" to the health of pilots, and whether the "impairment to the health of those controlling aircraft may lead to the death of occupants".
The inquest continues.