The heroic pilot who safely ditched a stricken airliner in a New York City river in 2009 has been diagnosed with Alzheimer's disease.

Captain Chesley 'Sully' Sullenberger III, 75, shared the update on his personal website, writing that he was recently diagnosed and it is at an early stage.

"For now, this means a name may not come easily to me, I forget a story I have recently told, or I don't sleep as well, but I am in the beginning of this long journey," he wrote.

US Airways Flight 1549 came down in the Hudson River on 15 January 2009, after both its engines were disabled in a collision with a flock of geese shortly after take-off. All 155 people on board survived.

Sullenberger's quick thinking and calm demeanour were credited with averting disaster.

The former US Air Force fighter pilot from California said in a post on his website on Tuesday that his Alzheimer's diagnosis "has challenged what it means to be of service" and that he has found "the answer is to speak up" about the disease.

"Over the years, when people would ask about the successful outcome of Flight 1549, I would say that 'courage can be contagious,' and on that day it helped everyone band together to get everyone off that airplane successfully," Sullenberger wrote.

"Now we need that courage to battle this disease. I am now part of a larger community with many of you, and we will be courageous together."

Sullenberger had been flying for 40 years before Flight 1549.

He retired as a pilot a year later in 2010, and has continued advocating for aviation safety.

In 2016, his exemplary emergency landing was dramatised in a film called "Sully", starring Tom Hanks.