Washington mid-air collision: helicopter altimeters differ by tens of meters

The investigation into the fatal collision in Washington in late January between a passenger plane and a US military helicopter has revealed discrepancies of several dozen meters in the altitude displayed by the military aircraft's instruments, according to hearings conducted in Washington. The US National Transportation Safety Board (NTSB) held investigative hearings from Wednesday to Friday, with in-depth questioning of experts and representatives of the various parties (companies, regulators, air traffic controllers, etc.) involved in the accident. The collision, which left a total of 67 dead, occurred on January 29 near Ronald Reagan Washington National Airport between a Sikorsky Black Hawk military helicopter—which was conducting a training flight—and a Bombardier CRJ700 operated by a subsidiary of American Airlines.

As early as February 14, relying on flight recorder data, the NTSB had reported a "discrepancy" regarding the helicopter's altitude. Agency Director Jennifer Homendy revealed that, shortly before the collision, the helicopter pilot had reported an altitude of 300 feet while his instructor pilot indicated 400 feet. "Neither pilot commented on the altitude discrepancy," she noted. "At this point, we don't know why there was a discrepancy between the two." "At the time of the collision, the Black Hawk was at 278 feet. But I want to caution that this does not mean that (...) that's what the Black Hawk crew saw on the barometric altimeters in the cockpit," she said, citing "conflicting information in the data."

Skip the ad

As part of the investigation, the hearings revealed this week, tests were conducted with three examples of the same Sikorsky Black Hawk Lima model, belonging to the same battalion. They highlighted discrepancies between the altitude displayed by the radar altimeter and the barometric altimeter, installed on this aircraft. In a "controlled environment, the discrepancy was within the tolerated limit of 20 to 55 feet (but) , once the rotors were turning and producing lift and thrust, the (barometric) altimeter readings dropped significantly and remained so for the entire duration of the flight," explained Marie Moler, one of the investigators, specifying that a "difference of 80 to 130 feet," or 24 to 40 meters, had been noted. This is a "very significant" discrepancy in this case, Jennifer Homendy insisted to journalists. "A difference of 100 feet is significant," she added. In fact, in the section where the collision occurred, the helicopters were supposed to be flying at a maximum altitude of 200 feet. "I am concerned. There is a possibility that what the crew saw was very different from what the altitude actually was," she continued. "That is something we will continue to examine," she assured.

Furthermore, criticism arose after the accident, when it emerged that the same controller was managing both helicopter traffic and some airplane traffic in the tower that evening. According to Clark Allen, an FAA employee who until recently worked in the tower, this overlapping role—allowed by regulations—was "probably more common" than a separation of roles. He also answered affirmatively when asked whether the hierarchical supervision of tower personnel was sufficient that evening, also agreeing that the staffing levels were high enough if it had been necessary to separate the management of helicopters.