A MD80 pilot questioned a policy of engaging the autopilot at 500 FT. The aircraft's autopilot is slow and non-GPS aircraft may not track RNAV procedures correctly. Passenger comfort is an issue with high body angles and aircraft yaw in responce to rudder commands.

Narrative

I am requesting clarification about the autopilot engagement policy on RNAV departures. Question: Is this an FAA mandate? What events precipitated this policy? Is it a permanent policy change? My concerns range from the obvious -- that engaging the autopilot at 500 FT does not solve the recurring issue of aircraft straying into a parallel runway's airspace due to programming errors or strong crosswinds. Other concerns: 1) The MD80 autopilot is slower than the Flight Director. Depending on wind; and the speed of the aircraft; my experience has been that the autopilot has been MORE prone to venture off the desired course. This is a 30 year-old autopilot that is being tasked with something that was not even conceived at the time. 2) On older FMS aircraft the PFD course deviation indicator is not the same as the GPS aircraft. How the FAA ever allowed the FMS aircraft (no GPS) to be approved for RNAV SIDS/STARS is beyond me because without the stepped up sensitivity of the CDI you are left with looking at a ND that the average pilot can't see most of the time because the yoke is in the way. 3) We (I) have not been trained in the simulator how the autopilot responds to an engine failure below engine out acceleration altitude. My reaction would be to turn off the autopilot. Guidanace? 4) Passenger comfort: In a light aircraft on a cold day the autopilot will command more than 20 degrees ANU and the transition to vertical speed at 1000 FT HAA is abrupt. Also; the transition from the parallel rudder mode to the yaw damper mode when selecting vertical speed will cause a noticeable yaw effect that will create a temporary sideslip condition. If the RNAV departure procedures are too complicated to hand-fly; I suggest we get them changed. We need to keep the big picture here -- maintaining diverging flight paths off parallel runways.