Tribune reported that after the altimeters had been cleaned of corrosion, water, and dirt, they had been tested in a pressure chamber. According to the article, both of the devices functioned normally down to 1,000 feet above ground pressure level, but below 1,000 feet they stuck or lagged considerably. However, in official testimony before the board, the manufacturer of the altimeters submitted a report to the board that said that their investigation concluded that the instruments did not have any mechanical failure or malfunction before the crash. When they had been salvaged from the water, the pilot's and co-pilot's instruments had indicated minus 1,500 feet and minus 1,640 feet respectively, reflecting damage to parts of the instruments caused by immersion pressure. When questioned, the investigator acknowledged that there was no means to determine what the altimeters showed at the time of impact. In testimony before the CAB, the American Airlines's director of flying said that the identical malfunction of two altimeters at the same time was "almost mathematically impossible". The CAB heard from experts from the Lockheed Aircraft Corporation who had attempted to reproduce a 500-foot error in the reported altitude caused by ice buildup in the air pressure lines that provide the input data to the altimeters. They flew an Electra plane behind an Air Force tanker that was spraying ice-producing water to see if they could cause the pressure line to clog. In other tests, they artificially plugged the line, used different tools to disrupt the airflow into the port, and even had a mechanic spray a stream of water directly into the pressure port. None of the tests produced the 500-foot error that was reported by the pilots, and produced a maximum error of only of 40 or 50 feet. In trying to determine why the crew of the Electra had not seen the runway ahead of them, the CAB heard from two pilots of a Northeast Airlines DC-3 that had landed at La Guardia Airport a minute or two in front of the Electra. They testified that they had had no problem coming in under the 400-foot cloud ceiling and that they could see the whole mile-long runway ahead of them. However, interviews with crash survivors and crew members of the rescue tug suggested that an isolated patch of low clouds and fog was hanging over the river at the time of the crash. Final report