The transonic speed spat between Cessna’s Citation Ten and Gulfstream’s G650 is likely to hit the stops at Mach 0.95 when it encounters not “the sound barrier” but required safety margins. With the Ten’s top speed now pegged at Mach 0.935, Gulfstream’s G650 could thus leapfrog the Ten only slightly, if the Savannah-based aircraft manufacturer even chooses to do so.
According to the FAA, “FAR 25.335(b)(2) requires at least Mach 0.07 [40 knots at 40,000 feet] difference between the design cruise Mach and the dive Mach,” where the design cruise Mach is effectively the Mmo. This provides a safety margin for upsets and gusts, the FAA said, though it can be reduced “to as little as Mach 0.05, if supported by analysis.” In practice, some applicants have been able to reduce the difference to about Mach 0.06 (34.4 knots), an FAA spokeswoman said.
The Ten’s predecessor, the Citation X, exceeded Mach 1.0 in dive testing, a knowledgeable source told AIN, so it would be possible for the Ten’s Mmo to reach Mach 0.95 if Cessna can get the maximum margin reduction. Meanwhile, the G650 reached a reported dive speed of Mach 0.995, so its maximum permissible limit (without further dive testing) would be Mach 0.945.
But the sub-Mach 1.0 speed crown might be a moot point–the Ten can fly a 2,500-nm trip at high-speed cruise in 5 hours 10 minutes, while under the same conditions the G650 can do it nine minutes more quickly, according to data from the respective manufacturers.