* Sees much bigger-than-expected Q3 adj. loss
* Expects full-yr comp sales to be flat at best
* Shares tumble 24 pct to record low
* Troubles are at Penney, not others-brokerage
* Still, shares of Macy's and Kohl's drop (Adds analyst comments, updates shares)
Oct 27 (Reuters) - J.C. Penney Co Inc lost about a quarter of its value on Friday as the department store chain slashed its full-year forecast after selling stagnant inventory at heavy discounts, indicating weak sales during the back-to-school season.
Penney said it no longer expects an increase in full-year comparable store sales and estimated it would just about eke out an adjusted profit for the year after posting a huge loss in the current quarter.
The grim forecast sent Penney's shares to a record low and weighed on its rivals Macy's Inc and Kohl's Corp, although at least one brokerage said the troubles seem to be specific to Penney and not across the sector.
Penney's heavy discounting was across its apparel lines, with the women's section singled out for an overhaul to expand its inventory of casual and contemporary merchandise. The clearance was during the back-to-school period, the second biggest sales period for retailers after the holiday season.
Gordon Haskett analysts said Penny's forecast implied its comparable-store sales likely fell about 3.5 percent in August, which was concerning given it was during the back-to-school period and in a month when weather was favorable.
They, however, said the troubles seem to be specific to Penney and not across the sector.
Still Macy's shares fell 5 percent in morning trading and Kohl's declined 4 percent. Penney's shares sank 16 percent to $3.05, easing slightly after hitting a record low of $2.76.
Penney, like Macy's and Kohl's, has been struggling with declining demand for apparel, the mainstay of department stores, due to tough competition from off-price retailers as well as Amazon.com Inc.
Department stores try to clear out leftover inventory ahead of the holiday-shopping quarter to stock up on fresh merchandise for their biggest selling season of the year.
The clearance "increases available funding to invest in new and trending merchandise categories," Chief Executive Marvin Ellison said.
Penney estimated it would post an adjusted loss of 40-45 cents per share in the quarter ending Oct. 28, much bigger than analysts average estimate of 18 cents, according to Thomson Reuters I/B/E/S.
The company also said it now expects full-year comparable sales to be flat at best, while it had earlier forecast sales could increase as much as 1 percent. It slashed its adjusted profit forecast to 2-8 cents per share, from 40-65 cents.
Penney also said Chief Financial Officer Jeffrey Davis would oversee the company's pricing and planning policies to improve its predictive analytics capabilities and get a better view of current sales trends. (Reporting by Sruthi Ramakrishnan and Gayathree Ganesan in Bengaluru; Editing by Sai Sachin Ravikumar and Savio D'Souza)