The 30 year old U.S. Navy F-14s are showing their age. In early June, it was discovered that the nose wheel had a defect and all 156 Tomcats in service were grounded until they could be checked. It was found that about 20 percent of the F-14s needed repairs on their nose wheels. Old carrier aircraft are a new phenomenon, and no one is entirely sure how much regular maintenance rebuilding will prevent something from wearing out unnoticed. Carrier aircraft, unlike warplanes that operate from land bases, have to worry about salt water corrosion and the heavy structural stress of carrier landings. For this reason, carrier aircraft are more expensive to build (they have to be more sturdy) and maintain (got to watch out for the corrosion and landing stress damage).