While Boeing scrambles to determine the cause of the latest crash of its newest passenger jet, the 737 Max 8, nearly three dozen countries around the world, including Britain, Germany, France and China, have banned the plane from their airspace.

At least 29 individual carriers have grounded it. And the entire European Union — 28 nations — has suspended “all flight operations” of the plane.

The Associated Press reports two Turkish Airlines 737 Max 8s bound for Britain were forced to return to Istanbul on Tuesday, and the airline later said it is grounding the planes.

But in the U.S., the planes remain in service — so far.

Southwest, American and United Airlines all fly 737 Max series jets; Southwest, which has 34 of the planes, and American, with 24, both say they’re confident of their safety.

But airline workers and consumer advocates are worried. The Association of Professional Flight Attendants, representing more than 26,000 flight attendants at American, asked the carrier’s CEO, Doug Parker “to strongly consider grounding these planes.” And Consumer Reports is calling on both airlines and the FAA to ground the 737 Max 8 until an investigation is complete.

At least four U.S. Senators — Dianne Feinstein (D-CA), Richard Blumenthal (D-CT), Mitt Romney (R-Utah) and Elizabeth Warren (D-MA) are also calling on the FAA to ground the planes, according to the

The FAA does say it will recommend “design changes” to the 737 Max, and Boeing says it will update flight-control software in all existing aircraft.

All this was triggered by the crash of an Ethiopian Airlines 737 Max on Sunday, shortly after taking off from Addis Ababa. All 157 people on board were killed. It was the second fatal crash of a 737 Max in five months: 189 died when an Indonesian Lion Air jet crashed into the ocean last October.

Boeing’s 737 Max series was introduced just over two years ago, and quickly became a best-seller; about 350 are currently in service around the world.

The investigation of the Ethiopian crash is just beginning, but a preliminary report from investigators of the Lion Air crash suggest that “the pilots failed to respond correctly after a faulty sensor led the autopilot to put the plane into a steep dive,” says Slate.

The 737 Max 8 is not an entirely new aircraft, but the latest in a long series of upgrades, going back to the original 737, which first flew more than 50 years ago.

“Boeing swapped out the engines for new models, which, together with airframe tweaks, promised a 20 percent increase in fuel efficiency,” Slate reports. “In order to accommodate the engine’s larger diameter, Boeing engineers had to move the point where the plane attaches to the wing. This, in turn, affected the way the plane handled.

“Most alarmingly, it left the plane with a tendency to pitch up, which could result in a dangerous aerodynamic stall. To prevent this, Boeing added a new autopilot system that would pitch the nose down if it looked like it was getting too high.” This is the system suspected of triggering the Lion Air crash.