The chopper, operated by Liberty Helicopters, was carrying five Italian tourists and was piloted by Ocean County, N.J., resident Jeremy Clark.
All nine people aboard the two aircraft perished. As of last night, seven bodies and the mangled helicopter had been recovered from the river. The plane was still missing.
"It's a horrible, horrible tragedy beyond belief - it's your worst nightmare coming true," said Arthur Wolk, a fellow pilot who knew Altman from their time together at Wings Field, a public airport in Blue Bell. "Once that collision occurred, there was no hope for anybody - no one."
Altman's plane hit the back of the helicopter, splitting both aircraft apart, according to witness reports.
Wolk said that in the space above the Hudson, small aircraft must fly below 1,100 feet, which creates a busy corridor in which the navigation rule is "see and avoid."
Negotiating the airspace is made more difficult by presence of choppers that ascend and descend rapidly, he said.
"You can't avoid what you can't see," he said. "I'm sure that neither of these pilots saw the other - positive, actually."
Altman and his father, David, also a pilot, both had "above-average" skills as aviators, Wolk said.
"They are a very well-regarded family in the flying community and that says a lot because pilots are very, very critical of their own," Wolk said. "So, for people to so highly regard them is a remarkable achievement."
Not only were the Altmans known for their aviation prowess, but also for their commitment to Angel Flight East, an organization of pilots who volunteer their time and aircraft to fly people in need of medical attention to medical centers where they can receive the best treatment.
Irving Altman, 84, uncle to Steven and Daniel and great-uncle to Douglas, said yesterday that all the men aboard were "very fine young people" and that the crash is a "terrible blow."
"It's very tragic, very tragic. Can't describe it. Very disturbing," he said. "The whole family is very sad and upset."
A Facebook memorial page in tribute to Doug Altman, a student at Upper Dublin High School in Fort Washington, had more than 800 members by last night. One poster just wrote "Y" 129 times; another wrote: "this all feels so surreal because when I saw the accident on the news, it felt normal to see people die from it but then I found out it was you . . . and it made everything change."
Daniel and Douglas Altman lived in Dresher. Steven Altman, who lived in Ambler, had flown to pick the two up at Teterboro Airport in New Jersey before they were all to head to Ocean City, N.J.
Both brothers worked in real estate investment and Steven headed the Altman Management Company, according to his uncle, Irving.
Steven Altman sat on the board of the Albert Einstein Healthcare Network.
"The Einstein family sends their deepest sympathies to the family and loved ones of Mr. Altman," the network said in a statement.
The Altman family requested privacy and police cars blocked reporters from the driveway yesterday.
In a statement from Liberty Helicopters about pilot Jeremy Clark, who was from Lanoka Harbor, N.J., the company said Clark had been flying with them for a year and a half and had logged more than 3,100 hours since he began piloting in 2004.
"Jeremy was loved and respected by everyone at Liberty Helicopters."
The crash happened in the same part of the river where, in January, a US Airways Airbus A320 made a smooth water landing in which all aboard survived.
The Associated Press contributed to this report.