US safety investigators disclosed that the US Army Black Hawk involved in January’s fatal mid-air collision with an American Airlines regional jet near Ronald Reagan Washington National Airport was flying above the 200-foot ceiling of its Potomac River route because a faulty barometric altimeter understated the aircraft’s height by 80–100 feet. On the opening day of a three-day National Transportation Safety Board hearing, officials said flight-data records and subsequent tests on three other helicopters from the same Army unit showed similar instrument discrepancies. The crash killed 67 people, making it the deadliest US aviation accident since 2001. The board is scrutinising Army operating procedures, the Federal Aviation Administration’s oversight of Washington’s congested airspace and the widespread practice of military flights operating without full Automatic Dependent Surveillance-Broadcast (ADS-B) positioning signals. Senator Ted Cruz this week introduced legislation that would mandate comprehensive ADS-B capability for all aircraft, including military, and compel the FAA to review low-altitude helicopter routes near major airports. NTSB investigators said findings from the hearing, which continues through Friday, will inform recommendations aimed at preventing similar collisions; a final report is expected next year.