WASHINGTON, Aug. 5 (Xinhua) -- At 8:00 p.m. EDT Sunday night, the countdown clock for U.S. shuttle Endeavour began ticking backward on schedule for a launch at 6:36 p.m. EDT (2236 GMT) Wednesday, according to NASA updates.

All countdown preparations are proceeding as planned at Launch Pad 39-A at the Kennedy Space Center in Florida, NASA space shuttle managers said at the countdown news briefing.

The weather forecast for launch day looks promising. Currently, there's only a 30 percent chance that isolated showers or anvil clouds could prevent the launch. This prediction remains the same in the event of a 24-hour delay, reported NASA weather officers.

Endeavour was originally planned to launch on Aug. 7. Because of unexpected work to fix a leak in the crew cabin, NASA delayed the launch by 24 hours to Aug. 8.

The flight's seven astronauts are staying busy with final launch preparations. On Sunday morning, Commander Scott Kelly and pilot Charlie Hobaugh made several practice landings in the shuttle training aircraft, a modified jet that mimics the flying characteristics of a space shuttle.

Endeavour's STS-118 flight will be the 22nd flight to the International Space Station.