V-22's engines can potentially damage ships' flight decks and coatings. NAVAIR devised a temporary fix of portable heat shields placed under the engines and determined that a long-term solution would require redesigning decks with heat resistant coating, passive thermal barriers, and ship structure changes. Similar changes are required for F-35B operations. In 2009, DARPA requested solutions for installing robust flight deck cooling. A heat-resistant anti-skid metal spray named Thermion has been tested on . Propulsion The V-22's two Rolls-Royce AE 1107C engines are connected by drive shafts to a common central gearbox so that one engine can power both proprotors if an engine failure occurs. Either engine can power both proprotors through the wing driveshaft. However, the V-22 is generally not capable of hovering on one engine. If a proprotor gearbox fails, that proprotor cannot be feathered, and both engines must be stopped before an emergency landing. The autorotation characteristics are poor because of the rotors' low inertia. In September 2013, Rolls-Royce announced that it had increased the AE-1107C engine's power by 17% via the adoption of a new Block 3 turbine, increased fuel valve flow capacity, and software updates; it should also improve reliability in high-altitude, high-heat conditions and boost maximum payload limitations from . A Block 4 upgrade is reportedly being examined, which may increase power by up to 26%, producing close to , and improve fuel consumption. In August 2014, the U.S. military issued a request for information for a potential drop-in replacement for the AE-1107C engines. Submissions must have a power rating of no less than at 15,000 rpm, operate at up to at up to , and fit into the existing wing nacelles with minimal structural or external modifications. In September 2014, the U.S. Navy, who already purchase engines separately to airframes, was reportedly considering an alternative engine supplier to reduce costs. The General Electric GE38 is one option, giving commonality with the Sikorsky CH-53K King Stallion. The V-22 has a maximum rotor downwash speed of over , more than the lower limit of a hurricane. The rotorwash usually prevents the starboard door's usage in hover; the rear ramp is used for rappelling and hoisting instead. The V-22 loses 10% of its vertical lift over a tiltwing design when operating in helicopter mode because of the wings' airflow resistance, while the tiltrotor design has better short takeoff and landing performance. V-22s must keep at least