By Shankar Ramakrishnan, Echo Wang

(Reuters) -Boeing is closing in on a plan to raise around $15 billion with common shares and a mandatory convertible bond as the jet maker bolsters finances worsened by a crippling strike, four sources familiar with the matter told Reuters.

The company on Tuesday said in regulatory filings that it could raise as much as $25 billion in stock and debt with its investment-grade credit rating at risk. One of the sources cautioned that a $15 billion sale may not be enough for Boeing to fix its ongoing crises.

The aerospace giant has been dealing with increased regulatory scrutiny, production curbs and a loss of confidence from customers ever since a door panel blew off a 737 MAX plane in midair in early January. Shares are down more than 40% this year.

It has been burning through cash all year, leading to its Tuesday announcements that it will raise money in the capital markets and that it had also secured a $10 billion credit agreement with major lenders: Bank of America, Citibank, Goldman Sachs and JPMorgan.

Boeing did not immediately comment.    

Four investor and banking sources said representatives from those lenders were inquiring about appetite for a combined offering of new shares and a mandatory convertible bond - a hybrid bond that could convert into equity on or before a predetermined date. 

Roughly $10 billion in new shares are being contemplated to be sold by the company along with nearly $5 billion in mandatory convertible bonds, the sources said.

One of the four sources said the deal was scheduled to be priced shortly after Boeing's Oct. 23 third-quarter earnings report. But another investor source said the company was trying to avoid a raise during the month-old strike which analysts estimate is costing tens of millions of dollars per day.

"The timing of any equity raise is still unclear but market consensus is that it should be done after the labor strike is resolved and earnings provide some visibility of its impact on current and future cash flows," said Michael Barr, senior research analyst at Neuberger Berman.

While Boeing burned less free cash than expected during the third quarter, the planemaker may have no choice but to act before the end of the strike to protect its investment grade rating, two of the sources said.

Boeing shares have rallied since its refunding announcement, suggesting some investors think the trough has been reached. The stock was up 1.1% at $154.01 on Wednesday afternoon.

One investor source said a three-year mandatory convertible bond paying 7% to 8% in annual coupon that could convert into shares at a premium of 20% over the current share price could attract strong demand.

The funding is expected to soften the blow for existing shareholders whose equity ownership could decline when a company issues new shares. 

A mandatory convertible option was being pursued because such hybrid bonds can be treated as equity capital by rating agencies, which means issuing them would not add to debt to the same extent as selling bonds. They are friendlier to existing shareholders, as stock conversion is a couple of years away and at a premium.

Raising equity capital is the only funding option for debt-laden Boeing which is intent on protecting its investment-grade ratings. 

The top three rating agencies - S&P, Moody's and Fitch - have warned they will cut Boeing's ratings to junk if it raised new debt without retiring some $11 billion of debt maturing through Feb. 1, 2026.