Boeing realizes its new 737 MAX jetliner can't win back the trust of airline passengers, shaken by two crashes that killed more than 300 people, without first regaining the confidence of the pilots who will fly it.

That's why pilots from 90% of the 50 carriers that bought the single-aisle jet have already used flight simulations to test a patch to anti-stall software faulted in the disasters, Boeing CEO Dennis Muilenburg explained Wednesday.

Feedback has been excellent so far, he said, which is a pivotal step toward rebuilding faith once the Federal Aviation Administration and its regulatory counterparts around the world allow the plane back in the air.

"We have to earn and re-earn the trust of the flying public," Muilenburg told investors and reporters during the Chicago-based company's first quarterly earnings call since the 737 MAX was grounded. "The key voices in all of this will be the pilots of our airlines; their voice is very important. That bond between passenger and pilot is one that's critical."

Safely returning the jet to commercial service and alleviating the concerns of passengers worried about flying it is crucial for Chicago-based Boeing. The best-selling model in the company's history, the 737 MAX is the latest version of one of the world's most widely flown planes and a vital cash-flow generator.

"When it comes to resource questions and application of resources, our top priority is the safe return to service of the 737 MAX," Muilenburg said. The aircraft was sidelined in mid-March after flight data from a downed Ethiopian Airlines plane showed choppy ascents and descents during takeoff, mirroring those before a Lion Air crash in Indonesia on Oct. 29.

In the Indonesian crash, a malfunctioning sensor fed incorrect data on the airliner's ascent vector to a computer system that attempted to lower the angle to avoid a stall, officials said. That prompted a struggle between computer software known as the Maneuvering Characteristics Augmentation System, or MCAS, and the pilot, who ultimately lost control of the aircraft.

All 189 people aboard were killed. The fact that Boeing hadn't yet completed a software patch that the FAA mandated afterward was part of what prompted the high level of concern when the Ethiopian Airlines crash outside the capital of Addis Ababa in early March killed all 157 passengers and crew.

The agency initially expected to approve a software patch by April, but Boeing stretched out the timeline to ensure all "pertinent issues" were addressed.

The grounding has snarled airline schedules for the foreseeable future — a challenge that Boeing executives attempt to address in almost-daily conversations with customers — and prompted Transportation Secretary Elaine Chao to seek a review of the jetliner's initial approval for commercial flights.

In late March, senators questioned the original certification of the 737 MAX in an aviation subcommittee hearing that focused on the FAA's relationship with the planemaker.

Based on a plane flown since 1967, the 737 MAX underwent less-rigorous scrutiny beforehand than a totally new model would have. The FAA relied heavily on inspections done by Boeing employees themselves under a system that delegates some duties to nonagency employees and organizations, including companies, that have particular expertise.

Evaluation of the 737 MAX, from Boeing's first permit application to final certification in March 2017, took five years and included 297 flight tests, some of which involved the anti-stall software linked to the first crash and under examination in the second, acting administrator Randy Elwell said.

Boeing has garnered more than 4,600 orders for the single-aisle jetliner, though just 67 of the aircraft are flown in the U.S. and fewer than 400 worldwide. The planemaker only began delivering the plane in 2017 and was working to ramp production up to 57 a month, which would have netted a potential $30 billion in sales this year, prior to the Ethiopian crash.

[Related: American Airlines cancels 10,000 more flights as Boeing scrambles to fix 737 MAX]

After Boeing had to reduce the production rate to 42 a month instead, it booked a $1 billion charge. That helped dragged profit in commercial airplanes, Boeing's biggest business, down 17% to $1.17 billion in the three months through March.

Fewer deliveries, which is when Boeing receives the bulk of payment from buyers, and higher costs to store completed planes until they can be turned over to customers, also curbed cash flow, Chief Financial Officer Greg Smith said.

"We expect our financial results to continue to be adversely impacted until we safely return the 737 MAX to service, ramp up production rates, and resume deliveries to customers," he added. As a result, Boeing has temporarily paused repurchases of company stock, on which it spent $2.3 billion from January through March.

By repurchasing its own stock, a company can boost the price and return cash to shareholders. Many companies used extra money after GOP-led tax cuts in 2017 to buy back their shares, prompting criticism from Democrats who said that wouldn't boost the economy in the same way that increasing pay and building new factories might have.