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Boeing Co. (BA - Get Report) shares drifted modestly higher Tuesday after the world's biggest planemaker lifted its forecast for aircraft demand in China, the world's biggest market, amid ongoing questions over the return of its troubled 737 MAX jet.

Boeing said Tuesday that China would need just under 8,100 new planes for its expanding fleet between now and 2038, an estimate that would bring around $1.3 trillion in sales at current list prices for both the company and its main European rival Airbus SA (EADSY) . A further $1.6 trillion in commercial services for China's fleet will also be required, Boeing said.

"An expanding middle class, significant investment in infrastructure, and advanced technologies that make airplanes more capable and efficient, continue to drive tremendous demand for air travel," said Boeing's vice president of commercial marketing Randy Tinseth.

Boeing shares were marked 0.31% higher in early Tuesday to change hands at $379.86 each, a move that would peg the stock's one-month advance at around 14.7%.

Boeing shares were held down by a report from the Wall Street Journal that suggested a panel of international regulators will criticize the U.S. Federal Aviation Administration's approval process for the 737 MAX aircraft, which remains grounded following two deadly crashes in March 2019 and November 2018 that took the lives of 346 passengers.

The Journal said the panel, comprised of regulators from major economies around the world, will ask for more transparency from the FAA over its certification procedures. 

Surging global crude prices in the wake of Saturday's attack on two Saudi oil facilities, which triggered the biggest-one day gain in more than three decades, is also taking its toll on the world's biggest planemaker as airline stocks plunge amid rising fuel costs and investors trim forecasts for new aircraft investments.  

Boeing is also reportedly working on a major fix to the flight software linked to the 737 MAX that involves both flight control computers and will address concerns for the system raised by the FAA in June.

The fix is still expected to allow Boeing to hold to an October timetable for FAA approval, but deeper concerns with respect to the planemaker's exposure to China, as well as its deteriorating credit metrics, have pressured shares for most of the summer.

Last month, analysts at Standard & Poor's cautioned they may lower their credit rating for the world's biggest planemaker owing to the ongoing crisis in its 737 MAX program. S&P said Boeing's credit metrics are likely to deteriorate over the next few quarters, and could fall below the threshold of cash flow to debt that normally triggers a downgrade.

Boeing plans to sell bonds later this year in a six-part offering that analysts think will take priority over share repurchases at least until the 737 MAX is brought back into full service. Moody's Investors Service rated the notes at A2, while Fitch gave them a single-A grade.