(MONEY Magazine) – If you or an older relative are house-rich but cash-poor, you might want to consider a reverse mortgage, a loan that pays a homeowner a fixed monthly sum and defers repayment of both principal and interest until the house is sold. While such deals have been around for eight years and are now available in 13 states, they have never gained much in popularity. All that may be about to change. In a pilot program beginning next month and continuing until Sept. 30, 1991, the Federal Housing Administration will insure up to 2,500 reverse mortgages written by private lenders, primarily banks, mortgage banks and thrifts. Generally, borrowers, who must be over 62, will be assured of staying in their homes as long as they wish. If all goes well with the tryout, Congress will probably greatly expand FHA insurance. Predicts Ken Scholen, director of the nonprofit National Center for Home Equity Conversion, a research organization in Madison, Wis.: ''FHA insurance is a major step forward that will give the reverse mortgage market a nudge.'' The new FHA-insured loans will not have to be repaid until a borrower dies or decides to sell, even if the amount due eventually exceeds the property's worth. When the mortgage comes due, the lender recovers what is owed from the borrower or his or her heirs. If the total exceeds what the house fetches on the market, FHA insurance makes up the difference. Borrowers pay a two-part premium for this insurance: an annual fee of one-half of 1% on the loan's principal balance and 2% of the lesser of 1) the home's value or 2) the maximum FHA loan amount, which varies by county from $67,500 to $101,250. The monthly income paid to borrowers will depend on their age, the loan amount, the type of loan they choose and the interest rate the lender charges (see the table below). A borrower whose house is worth more than the FHA loan limit and who wants a higher monthly payment would have to do business with a lender that doesn't offer FHA insurance. The FHA will back the following types of reverse mortgages, but lenders probably will not be required to offer all of them: -- Term mortgages. Although borrowers won't be forced to sell their homes at the end of the term, these loans are still appropriate only if you need high monthly income and plan to move at the end of the term, typically five, seven or 10 years. -- Tenure mortgages. You get income as long as you remain in your house. But security has a price: your monthly check will be smaller than under a term loan. -- Line of credit. Instead of receiving a check each month, you make withdrawals when needed. Under FHA rules, a 75-year-old with a $100,000 house who takes out a 10% loan could immediately draw on a $39,900 line of credit. -- Shared appreciation. This option, a variation on all three of the above types of reverse mortgages, gives you a bigger monthly check or a higher line of credit. When you move, sell or die, you or your heirs owe the lender a portion of your house's appreciation as well as the monthly advances plus interest. The bigger the stake you cede to the bank, the fatter your check is. For example, in return for $400 a month for life, Janet Roscoe, a 66-year- old widow in Fallbrook, Calif., signed up for an uninsured shared- appreciation mortgage with American Homestead Mortgage Corp., a Mount Laurel, N.J. mortgage bank that has made 2,100 such loans since 1983. Terms: 11 1/2% interest on her borrowings, and she or her heirs will forfeit 80% of the future appreciation -- starting when the contract was signed -- of her five- room attached house, now worth $153,000. ''For a long time, I've had a desire to learn to fly,'' she says. ''This additional income will make it possible.'' A shared-appreciation loan may prove costly if you die shortly after taking it out or if your house leaps in value. But it will be a bargain if you are long-lived and your property appreciates modestly.

Insurance will make shared-appreciation mortgages less risky for borrowers. Reason: lenders can't take more than 25% of a home's appreciation or charge more than a 20% effective interest rate -- interest plus the lender's share of the appreciation -- in the year the house is sold. Because of these limits, however, monthly payments to borrowers will be lower than they are under uninsured plans such as American Homestead's. What's more, contends American Homestead president James Burke: ''It's hard to conceive how lenders will participate because they will not make any money.'' But William Texido, president of Providential Home Income Plan of San Francisco, which offers a program similar to American Homestead's, is bullish. ''The FHA's plan to back our loans will add credibility to the business of reverse mortgages,'' he says.

BOX: Check It Out HOW THE NEW FHA LOANS CAN PAY OFF This table shows the estimated monthly payments that an owner of a $100,000 house would receive under different types of reverse mortgages the FHA will insure. In this example, 10% interest is charged on all but the shared- appreciation loan, which costs 8.5%. (The lower rate allows the lender to claim up to 25% of the home's appreciation.) These loans are described in the accompanying article.

BOX: Help -- To find out which lenders in your area, if any, will offer FHA-backed reverse mortgages, call the agency at 800-245-2691. -- For a list of publications about converting the equity in your home into cash, send a stamped, self-addressed envelope to the National Center for Home Equity Conversion, 110 E. Main St., Room 605, Madison, Wis. 53703. -- If you have questions about insured or uninsured reverse mortgages, address them to the AARP's Home Equity Information Center, 1909 K St. N.W., Washington, D.C. 20049.

CHART: NOT AVAILABLE CREDIT: Source: U.S. Department of Housing and Urban Development CAPTION: HOW THE NEW FHA LOANS CAN PAY OFF