Questions about the radio system used by the Fire Department during the World Trade Center terror attacks are being raised anew by the 9/11 commission, which has grilled a hero FDNY chief whose brother died when the buildings crumbled.
Commission staffers in recent weeks asked Deputy Chief Joseph Pfeifer why he did not use the critical Channel 7 frequency on the Motorola "handie talkies" in favor of another channel on the morning of Sept. 11, 2001.
Channel 7 was the only radio channel capable of transmitting messages from commanders in the Twin Towers' lobbies to firefighters on the upper floors. The channel used a signal booster - a "repeater" - located on the roof of 5 World Trade Center.
Commission investigators learned Pfeifer switched frequencies when they were listening to a 78-minute recording of FDNY radio chatter at the trade center that showed the repeater and radios worked.
The FDNY says problems with the repeater and the digital Motorola radios prevented up to 121 firefighters in the north tower from hearing evacuation orders given an hour before the building collapsed.
Results of the commission probe into the city's handling of the terror attacks will be aired next Tuesday and Wednesday when members hold hearings at the New School for Social Research in Greenwich Village - where commission member Bob Kerrey is president.
The commission is also questioning why Pfeifer allowed a civilian Port Authority employee to initially turn on the repeater control console.
Investigators are trying to determine whether having a civilian set up the repeater for operation may have been a factor in difficulties Pfeifer says he later experienced trying to use it.
Pfeifer also said there was nothing unusual about him asking a Port Authority employee to initially operate the repeater.
"That happens all the time," he said. "It's the Port Authority's building, and I asked them to turn the repeater on."
Pfeifer, 48, was the first battalion chief to arrive at the north tower after the 8:48 a.m. attack and was responsible for setting up communications.
He tested radio transmission on Channel 7 with Battalion Chief Orio Palmer, who was also in the lobby of the north tower, but the two could not hear each other, FDNY spokesman Francis Gribbon said.
Pfeifer sent an aide to try a portable repeater kept in his vehicle but found that did not work, either. He then switched to Channel 2 to confer with other brass, while using Channel 1 to address responding units, Gribbon said.
Both those frequencies were capable of reaching about five floors, said retired FDNY Chief Albert Turi, who was questioned by probers three weeks ago.
"Channel 7 would be the one we had to listen to to hear transmissions coming through the repeater system," Turi said.
After Pfeifer gave up on Channel 7, firefighters were ordered to switch their radios to Channel 2, which became the command channel.
One result was many firefighters struggling to rescue office workers in the north tower had no idea the south tower was hit by a second jet at 9:03 a.m.
When the south tower was struck, Palmer moved to the lobby in that tower and found out the repeater and radios worked when he was able to speak with firefighters as high up as the 78th floor, Gribbon said.
Those exchanges were recorded by the repeater, along with the test conversations between Palmer and Pfeifer.
At 9:28 a.m., Chief Joseph Callan, who had relieved Pfeifer, broadcast on the two radio channels in use orders that firefighters leave the north tower.
"Everyone come down out of the building. Leave the building immediately," Callan said, according to an interview he gave retired FDNY Chief Vincent Dunn.
But few in the north tower heard his order - and when the building collapsed an hour later, Pfeifer's brother, Kevin, was among the dead.
With the Port Authority maintaining that the repeater was in working order, commission staffers questioned Pfeifer at length about what steps he took before giving up on the repeater system, which was installed after the 1993 trade center bombing.
When approached by The Post, Pfeifer said he told the investigators he believed the lack of communications between the different emergency organizations was a bigger issue than the FDNY radios.
"I told them the radios were not the No. 1 issue," Pfeifer said last week. "People in general don't understand the technical aspects of communications.
"I don't think the people on the commission understand, either," he added.
Pfeifer declined to detail to The Post his responses to investigators. But in an interview with Firehouse Magazine April 2002, he said:
"The tracer has a repeater radio which operates on Channel 7 on our handie-talkies and we attempted to use that. We tried to communicate to each other, since we were very familiar with the system, and that failed. It did not work."
He also downplayed the need for radio contact with the rank-and-file.
"There were messages, urgent messages of firefighters having chest pains as they went up, but there wasn't a lot of need for handie-talkie communication because there wasn't any information to be passed either way," he told the magazine.
Pfeifer is set to be questioned again by commission probers, Gribbon said.
The FDNY at first claimed communications were hampered when the repeater was knocked out by falling debris.
Then the department pinned blame on the new Motorola radios.
But no official explanation has emerged as to why the repeater and radios worked - except when they were tested by Pfeifer.
"That's the $64,000 question," Gorman said. "Maybe it was a flawed test. Maybe the right button wasn't pressed."
The FDNY defended Pfeifer.
"The fact of the matter is some people heard his evacuation order," Gribbon said. "He made a decision under the most trying conditions you could imagine."
Retired FDNY Chief Anthony Fusco, whose report on the 1993 trade center attack exposed some of the same problems encountered nine years later, defended Pfeifer.
"I saw the video footage," Fusco said. "I thought Pfeifer handled the incident excellently."
So far, the commission staff, lead by Executive Director Philip Zelikow, have interviewed 200 to 300 people whose testimony pertains directly to the attacks in New York and emergency response, including Mayors Bloomberg and Rudy Giuliani, as well as Police Commissioner Ray Kelly and Fire Commissioner Nicholas Scoppetta.
They have collected testimony from more than 50 members of the FDNY, more than 50 members from the NYPD, and more than 50 others from other emergency-management agencies - they are specifically looking at the breakdown in communication that tragic day.
The reports containing these interviews were set to be finalized last night, sources said. Sometime tonight, the 10 commissioners will get the reports to review.