Following three fatal crashes in the past month, including the helicopter crash in Kailua, the National Transportation Safety Board is calling for greater safety measures to the federal safety regulations covering them.

The NTSB is still investigating the mid-air collision between two floatplanes in Alaska last week, followed by a Beaver floatplane crash on Tuesday. Those aircraft, along with the Robinson R44 helicopter that crashed in a residential neighborhood of Kailua, were operating under Part 135 of Federal Aviation Administration regulations.

“While these tragic accidents are still under investigation, and no findings or causes have been determined, each crash underscores the urgency of improving the safety of charter flights by implementing existing NTSB safety recommendations,” said NTSB Chairman Robert L. Sumwalt in a news release. “The need for those improvements is why the NTSB put Part 135 aircraft flight operations on the 2019–2020 Most Wanted List of transportation safety improvements.”

The FAA’s Part 135 certificate covers the operation of range of aircraft offering business and charter flights, including air medical service, air taxi, charter, or on-demand flights. The NTSB said these operators are not required to meet the same safety requirements as commercial airlines.

“Even without requirements, many such operators could be taking more initiative to ensure the highest level of safety for their aircraft and passengers,” said the NTSB.

The NTSB’s safety recommendations call on Part 135 operators to implement safety management systems — currently not required — in addition to flight data monitoring. Additionally, the NTSB recommends that Part 135 operators make sure their pilots receive controlled-flight-into-terrain avoidance training.

All of the above are available, the NTSB said, and can improve safety and prevent crashes.

Major passenger airlines, which operate under Part 121, have adopted these measures and have seen a great improvement in safety, the NTSB said.

“A customer who pays for a ticket should trust that the operator is using the industry’s best practices when it comes to safety,’’ Sumwalt said. “And it shouldn’t matter if the operator has one airplane or 100. Travelers should have an equivalent level of safety regardless of the nature of the flight for which they paid.”

On Monday, a Beaver floatplane also crashed during a commuter flight in Metlakatla Harbor, Alaska, killing the pilot and passenger.

On May 13, a fatal, mid-air collision occurred between two floatplanes — a de Havilland DHC-2 Beaver and de Havilland DHC-3 Turbine Otter — near Ketchikan, Alaska, killing one pilot and five passengers.

On April 29, a Robinson R44 helicopter crashed on Oneawa Street in Kailua, killing the pilot and two female passengers aboard.

In the preliminary report for the Kailua helicopter crash, witnesses saw pieces falling from the air before the aircraft impacted the street. They saw the helicopter descending rapidly, nose down, with none of the rotor blades moving.

The NTSB released the preliminary report of the May 13 mid-air collision in Alaska today, but does not discuss probable cause. The preliminary report for the second airplane crash in Alaska has not been released yet.

Determination of probable cause and issuance of safety recommendations comes at the end of an investigation, saidthe NTSB, but take one to two years to complete.