The manoeuvre that W&CG just described is called a "Rudder Doublet" and is the standard test for Dutch Roll.
When it comes to testing, whether it's checking something's serviceable, or a device does what it should for certification what is important is the "apparent" characteristics; that is, what is seen from the cockpit. So, the DR test would be used in either case, but obviously for certification work, you'd do it at a far greater range of conditions to check it works at every corner of the envelope.
In the VERY high value end of certification work, such as something like 777 or Typhoon, then what you'd actually do is computer model it to show how it *should* work, then flight test is to prove that the computer model was correct. Necessary, but far less fun.
During certification testing, you'd also include what are known as "role relatable" tests. That is, specific manoeuvres would be flown (such as a late turn onto centreline when descending out of cloud for a final VFR approach, or flying a guns fight - the latter not being encouraged in airliner work) to determine what effect the aircraft's DR characteristics would have on the actual useability in it's intended role.
G
Last edited by Genghis the Engineer; 8th Sep 2002 at 10:52.